TECHNICAL REPORT Title: Observing scenarios for NIRSpec: Initial release Authors: D. Soderblom, T. Beck, K. Gordon, D. Karakla, T. Keyes, D. Long, J. Muzerolle, J. Tumlinson, and J. Valenti Phone: 410-338-4543 Doc #: Date: Rev: JWST-STScI-002270, October 6, 2010 - Release Date: 24 February 2011 1.0 Abstract This report summarizes test cases created to examine and verify the capabilities of the JWST ground system for NIRSpec, particularly the planned NIRSpec templates. These observing scenarios were created to represent a broad range of potential science programs likely to be undertaken by NIRSpec users as a means of testing existing template concepts and potential needs for user info0rmation and software. The lessons learned are summarized, with recommendations for potential changes and additions to the templates. 2.0 Introduction JWST is being designed and built with several key scientific goals in mind and it is also intended to be a general-purpose observatory for infrared astronomical observations (see JWST-RQMT-002558, James Webb Space Telescope Project, Science Requirements Document, J. Mather). The telescope and its instruments have been specified and built to be able to achieve those key scientific goals. Several years ago an effort was undertaken to draft a Science Operations Design Reference Mission (see JWST-STScI-000373, Science Operations Design Reference Mission (SODRM), Phase 1 Proposals, L. Petro). The primary purpose of the SODRM was to examine realistic scientific uses of JWST and to see how they could be accommodated within the design constraints then considered and to estimate the overall observing efficiency of the observatory. Since the SODRM was completed, the ground system for JWST has been specified and is now being created and tested. The capabilities of JWST s instruments are made available to observers via templates in APT, the Astronomers Proposal Tool. These templates combine instrument settings and constrain user selections; with the goal of simplifying construction of Phase II programs and simplifying the software that supports that. These templates can either support or inhibit the kinds of observations that can be performed with the observatory, and those capabilities also affect significantly the efficiency of how observations are carried out. Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration under Contract NAS5-03127
The Webb Instruments Team recently began an effort to understand better the capabilities and limitations of the templates in order to verify that critical capabilities are present and to help set priorities for any additional capabilities that may be required. This Technical Report summarizes results for JWST s Near-Infrared Spectrograph (NIRSpec). The scenarios outlined here are not completely defined observing programs yet, in part because of the effort needed to do that and in part because not enough detailed information yet exists on the usage of JWST. A major goal of creating these scenarios is, in fact, to help guide choices in making the observatory and its capabilities more fully defined. This report is being issued in its current form to provide the basis for further discussion and decisions, with the expectation of significant revisions as time goes on. For example, these scenarios could form the basis for other user studies, such as how observer-supplied information gets fed to the data management system, or how documentation can better meet user needs. 3.0 Observing scenarios In undertaking this study, the WIT NIRSpec group first discussed the general science topics to be covered and the methods to be used in these studies, and individuals then volunteered to work on specific scenarios. The primary aim is to ensure that necessary capabilities are in the ground system to enable NIRSpec to meet its science goals. A secondary goal is to identify areas where changes could lead to significant efficiency improvements. It is also important to minimize the usage of the mechanisms on JWST, particularly reconfigurations of NIRSpec s Micro-Shutter Arrays (MSAs), and so the use cases have been examined with a view to efficiency in a broad sense. The procedure used included these steps: Describe the science goals sufficiently to determine the observing parameters that would be required, such as number of targets and grating-filter combination. Create a candidate source list with flux estimates or ranges of fluxes. Estimate exposure times needed, based on existing knowledge of sensitivity, and select a dither pattern based on the science requirements. Lay out the likely exposures in detail, including preferred usage of MSA shutters for targets and backgrounds. The details of the MSAs (closed and open shutters) were not considered at this stage. Describe verbally the considerations used in creating the scenario as reference. List the pros and cons of the existing template structures in building the observations and what changes to the templates could lead to better observing efficiency or better science capabilities. For these scenarios we did not go as far as choosing guide stars because that seemed unnecessary at this stage. The observing scenarios that have been created are detailed in the Appendix. Here we provide short summaries so as to provide a listing of what has been treated. - 2 -
Program Title Author Mode Features 200 Kinematics of stars in the Galactic Center Valenti MSA crowded field; multiple pointings 201 Evolution of ices in star-forming environments Beck MSA 202 YSO jets near IRS-1 in NGC 2264 Karakla MSA extended objects; bright sources 203 Massive star-forming regions in the Milky Way 204 First-light galaxies in the Hubble Ultra-Deep Field 205 Carbon abundances in Omega Centauri 207 MSA spectroscopy of a very extended object Muzerolle MSA very crowded region; large brightness range Soderblom MSA very long integrations; very faint sources Tumlinson MSA extremely crowded field Keyes MSA coverage efficiency 230 NIRSpec follow-up of Gamma-ray burst afterglows Tumlinson Fixed Slit Quick turnaround TOO 231 Exoplanet atmospheres Valenti Fixed Slit high signal-to-noise; critical timing 261 Atomic hydrogen filaments in Perseus A (NGC 1275) 502 MIRI/NIRSpec IFU observations of extragalactic H II regions Beck IFU mosaic, large field Gordon IFU multi-instrument 4.0 Lessons Learned The scenarios described here were intentionally chosen to exercise the full range of NIRSpec s functional capabilities, and that means that as an ensemble they are probably not typical of what will be executed with the instrument on-orbit. At the same time, the range of modes used and the science topics covered means that these scenarios should test the bounds of the ways in which general observers will use the instrument for scientific purposes. Also, some of the observations described here are based on some of the scientific motivations for building JWST in the first place, and the capabilities needed to execute them are among the highest-level requirements on the observatory. The scenarios were constructed with the goal of identifying obstructions or observational inefficiencies in the systems that observers will use. Some inefficiencies result when more overall time is required to execute a series of observations than would be the case with a modest change to the software.. Other inefficiencies lead to more reconfigurations - 3 -
of the MSA a limited lifetime item than would be the case if other options were available. 4.1 Templates: Each of the scenarios described includes concerns identified with the current templates. They are listed here for conciseness. In many cases each concern identified applies to several of the scenarios. Single visits need to be able to carry out multiple target acquisitions. This need arises if the total visit length exceeds ~10,000 sec or if a dither of more than ~5 arcsec is needed. Both situations are likely to be common. A single visit may now include multiple target sets, which aids efficiency significantly. However, current capabilities only allow for a single acquisition confirmation image to be obtained, after the first target set is centered. Observers will require confirmation images of all target sets, and so such a capability would be used frequently. The work-around is to schedule separate target sets as individual visits, but that entails unnecessary guide star acquisitions and MSA reconfigurations that can be avoided. Some science observations need to ensure that there are no interruptions in the data taking for a single target set. A NON-INT special requirement may be needed. The onboard scripts now assume that exactly the same sequence of actions occur at every dither location in a visit. At each dither location, the scripts can loop through a list of gratings each with their own exposure time. However, the scripts (currently) do not have a mechanism for specifying different MSA configurations or different exposure times at different dither locations. The assumed mode of operation for NIRSpec is an observation planned well in advance and with a preliminary NIRCam image so that precise source positions can be measured as part of preparing for the target acquisition. Targets of opportunity (TOOs) cannot be observed that way (with possible rare exceptions) and so an alternative acquisition scheme is needed. In general, it should be possible to acquire reliably TOOs if they are observed in the 1.6 arcsec square fixed slit and if there is a peak-up algorithm available to tune the pointing. Planet transit data-taking is likely to place unusual demands on scheduling the observatory. It may be necessary, for example, to tolerate high overhead times so that a visit can execute reliably at the correct time and so that an instrument is in a predictable and stable configuration in order to obtain very high signal-to-noise data. These overheads need to be accounted for, at least statistically, in evaluating proposals. Exoplanet observations are likely to need sub-arrays because of the source brightness. For exoplanet studies it is necessary to obtain a very large number of short integrations to both reach the needed S/N and to cover the time period in question. Such a situation may require splitting the observation into several exposures to work around observatory constraints, but those separate exposures must be timed to ensure continuous data during the eclipse. To achieve very high S/N it may be advantageous to suppress re-acquisition of guide stars. - 4 -
4.2 Documentation and user information (including APT): Users will need better information than is now available for the overheads associated with changing guide stars, or with dithers to cross the detector gap. A program that both dithers (as virtually all will) and uses multiple gratings can be carried out in several ways, and it is not clear which method is most efficient or most conservative of resources. Some simulations may be beneficial and better information is needed. Some types of observations involve high dynamic range scenes in which faint objects are near to bright ones. This may cause problems if bright objects fall on closed shutters of low contrast, and there is a need for such shutters, when known, be identified. In addition, in such scenes it may be necessary to take detector persistence into account in constructing the flow of observations. Despite the many shutters available in the MSAs, the potential for overlapping spectra, the distribution of target centers relative to shutter centers, and the limited dynamic range of a single exposure mean that multiple target sets are needed for even a modest list of objects. Some guidance to users in this area would be helpful. The currently available information on detector readout (NRSRAPID versus NRS) is not adequate to make an informed judgment. A quantitative calculator may be needed. The software that helps to optimize centering of targets may benefit from a user being able to note whether individual targets are point sources or not because extended objects generally do not need precise centering in a shutter to get good data. Measuring the positions of faint objects on a preliminary NIRCam image that are near bright objects is compromised by the saturation present. Exoplanet hosts are likely to be very bright and so it may be difficult to measure their positions relative to reference stars well on a preliminary NIRCam image. The shortest possible full-frame NIRCam image will saturate. The peak-up procedure used for centering a TOO would also work well for bright targets. It is unclear to a user how much overhead time is associated with changing guide stars, as when a large-scale dither is used to cross the detector gap. The interplay of dithering and changing gratings can cause confusion. Is there a preferred hierarchy or order? Do we wish to prefer one method over another to reduce mechanism usage? In some cases observers may wish to obtain preliminary NIRCam images at more than one wavelength so that backgrounds can be evaluated. Information on this will be needed. Very crowded fields are problematic. Some simulations of such observations may be helpful in guiding users. Observations of very bright sources may be challenging for reliably rejecting cosmic rays because of the smaller number of groups obtained. It is not clear if APT can handle the construction of IFU mosaics. The relation of such mosaics to guide stars is also unclear, and that then means overhead times are undefined. Observers will need guidance in comparing different modes for obtaining spectra over large areas with either the MSA or IFU. - 5 -
5.0 Conclusions We anticipate further development of these scenarios as useful test particles for various aspects of STScI systems. The above lessons learned will affect decisions on how commanding for NIRSpec is written, for example. These scenarios can be expanded and refined to provide input to a test Call for Proposals as a test of those systems, too. Additional scenarios are being written for additional science cases, and, particularly, to include moving targets and how those will be acquired and observed. Some aspects of observation planning mentioned above such as NIRCam preliminary images will require detailed examination so that we understand such things as the lead time required and necessary precisions. These scenarios also give us a view to the kinds and methods of information needed by users in preparing proposals and programs, another effort that will be undertaken in the near future. 6.0 References Garcia Lopez, R., Nisini, B., Eislöffel, J., Giannini, T., Bacciotti, F., & Podio, L. 2010, A&A, 511, 5. Lu, J. R., Ghez, A. M., Hornstein, S. D., Morris, M. R., Becklin, E. E., and Matthews, K. 2009, ApJ, 690, 1463 Simon, T., & Dahm, S. E. 2005, ApJ, 618, 795 Stiavelli, M. 2009, Observational Cosmology with the ELT and JWST, in Science with the VLT in the ELT Era, Ap. & Sp. Sci. Proc. Tanvir, N. R. et al. 2009, Nature, 461, 1254. Wang, H., Yang, J. Wang, M. & Yan, J. 2002, A&A, 389, 1015 Ward-Thompson, D., Zylka, R., Mezger, P.G., & Sievers, A. W. 2000, A&A, 355, 1122. - 6 -
Appendix: Details of the programs 7.0 Program 200: Kinematics of stars in the Galactic center Project title: Author: Purpose: SI usage keywords: Instrument modes: Kinematics at the Galactic Center J. Valenti A test case of observing many point sources in a crowded region to obtain radial velocities of giant stars in the Galactic Center to measure the mass of the central black hole and to separate populations kinematically. Finding valid background shutters may be difficult. Multiple pointings are needed to cover a sufficiently wide field to achieve the science goals. Crowded field; multiple pointings NIRSpec MSASPEC G235M+F170LP NRSRAPID 7.1 Scientific background: Lu et al. (2009) have recently published an image of the Galactic Center obtained with Laser Guide Star Adaptive Optics (LGS AO). Their Figure 5 is reproduced below. The two panels show the same field at different scales, with Sgr A* located at the black cross; this is the presumed location of the Galactic Center. The arrows show the direction and magnitude of measured proper motions, with red arrows denoting objects that Lu et al. believe to be members of a disk, and blue arrows are for non-members. The objects range from 9 to 15 in K magnitude, and coordinates are good to 2 mas precision. The green slitlets are to scale but are otherwise schematic to illustrate the difficulty of locating unoccupied background in this crowded region. - 7 -
This second image has additional slitlets added to represent a second configuration and pointing so as to observe additional stars in the same field. Slitlets shown in green constitute one target set and slitlets shown in purple constitute a second target set. The two target sets are observed sequentially, not simultaneously. In general, a given field of view may have multiple target sets. By default, the MSA planning tool will select slitlets in a single target set such that spectra formed by the slitlets will not overlap. This scientific constraint means that a horizontal line anywhere in the figure intersects at most one green slitlet and at most one purple slitlet. Note that the targets in the green slitlets are all in the top shutter of each slitlet. This is one of the two dither locations required for the green target set. The second dither location is offset by 0.45 arcsec along the vertical axis of the figure, such that the targets move to the same relative position in the bottom shutter of each slitlet. The MSA configuration does not need to be changed between these two dither locations. The visit break down table below shows that each target set will also have a third and fourth dither location (not shown in the figures), offset by one shutter along the horizontal axis. These are dithers in the sense that the targets are identical, but the MSA configuration for dither positions 1 and 2 is different from the MSA configuration for dither positions 3 and 4. When switching from the green target set to the purple target set, an MSA reconfiguration is required and a small target offset is required to position the new set of targets in the top shutter of the purple slitlets. Between the first and second exposure of the purple targets, a dither of 0.45 arcsec along the vertical axis is required. Two more dither positions (not shown in the figure) are required, as with the green target set. 7.2 Required MSASPEC template parameters: o Spacecraft pointing and orientation: - 8 -
o Five sets of [MSA RA, MSA Dec, MSA Orient] values; one per FOV. o Target acquisition: o Filter, MSA configuration, and readout pattern (N group = 3, NINT = 1, subarray = FULL). o Filter = F110W, readout = NRSRAPID. o Acquisition reference stars list (RA and Dec): 5 sets. o Ten MSA configurations for acquisitions; two per target set. o Dither pattern: x-offset, y-offset, MSA configuration. o 40 sets of [x-offset, y-offset] and 20 MSA configuration values. o Grating+filter list, for each dither position: o G235H+F170LP. o Confirmation image for each grating+filter combination: Readout pattern = [NRS, NRSRAPID], NGROUP = [0,3] (NINT = 1, subarray = FULL). o Science spectrum parameters for each grating+filter. Readout = [NRS, NRSRAPID], NGROUP = [3, 6, 8, 10, 12, 13, 15], NINT = [1, 3]. 7.3 Observation notes: o Observations: o Visits are very short, so group all visits into a single observation o Target sets (distinct sets of targets): o Some target sets are too bright for a confirmation image o Initial pointings for each target set are offset by a small amount Fraction of a shutter to align target set in shutters Integer offset to minimize the impact of failed shutters o Four dither positions: 2 x 2 block of adjacent shutters Measuring velocities, so no need to fill wavelength gap o DMS will process each target set as a separate association o Target acquisitions: o Pointings for all target sets and all dithers in a visit are within 5 Only one target acquisition is needed for a visit o Configure the MSA during target acquisition to block bright stars o Exposures: o One grating for all exposures o NRSRAPID is used when NRS would yield fewer than 6 groups o Targets in set 3 are so bright that only 3 groups are possible Set NINT=3 to allow cosmic ray rejection - 9 -
7.4 Visit breakdown: 7.5 Observing concerns identified: o Rules need to be established to allow the insertion of multiple target acquisitions within a single visit. There are several scenarios that require that capability: o There is a need for a re-acquisition to be made after a significant time has elapsed (nominally 10,000 sec). o A re-acquisition is also needed after a dither slew of more than ~5 arcsec. o Different science programs may have different requirements on pointing tolerance. o There is a need to obtain a confirmation image for each target set within a single visit, not just the first. This is a capability that would be used very frequently, in fact as a default. A potential work-around exists if separate visits are specified, but at a significant cost in additional time for guide star acquisitions that are not really necessary. o The Data Management System (DMS) requires observers to specify details of data reduction in their Phase II programs. However, as far as the observatory it self is concerned, these are only comments. o It is not clear if a single set of acquisition parameters is sufficient for all the target sets in a visit. - 10 -
8.0 Program 201: Evolution of ices in star-forming environments Project title: Author: Purpose: Instrument modes: 8.1 Scientific background: Evolution of ices in star-forming environments T. Beck NIRSpec MSASPEC G235H+F170LP; G395H+F290LP NRSRAPID JWST-STScI-002270 This program would observe NGC 2024, an embedded star-forming region (SFR) in Orion. NGC 2024 has about half a dozen B-type stars in the central region and the extinction seen has a large gradient, from A V ~ 5 at the periphery to A V ~ 30 at the center. The B stars in the center are optically invisible because they are obscured by a tongue of dense cloud material. The goal is to get accurate continuum measurements of point sources to measure ices in the cloud material. 8.2 Observation notes: o The observations use G235H+F170LP and G395H+F290LP with NRSRAPID readout. o A confirmation image is required to verify target centering. o There are four target sets that require 4 pointings and 4 guide stars. o There are 4 exposures and MSA configuration per target set: o Two spatially distinct configurations for dithering, with possible reconfiguration of the MSA. o The wavelength gap needs to be filled with an additional dither of about 20 arcsec. o Observations within a target set should not be interrupted. o The acquisition needs to take account of very bright sources in the field, both as a protection risk and because those objects will saturate in the preliminary NIRCam image. Guide star Target set Exp. (grating 1) Exp. (grating 2) 1 1 30 30 1 2 60 60 1 3 200 240 2 4 30 40 2 5 100 160 3 6 30 40-11 -
Guide star Target set Exp. (grating 1) Exp. (grating 2) 3 7 100 160 4 8 20 30 4 9 50 60 4 10 200 240 8.3 Observing concerns identified: o Like other scenarios, this program requires a confirmation image to be obtained after each MSA reconfiguration, not just the first one that follows the target acquisition. o It is not clear to a user how much overhead time is associated with changing guide stars, as when a large dither is made to cross the detector gap. o It is not clear how to make the program most efficient given that multiple dithers are needed for two different gratings. Is it best to dither first and then change gratings? o It is strongly desired to not be interrupted during the observations for a given target set. Should there be a Special Requirement? o The brightest targets in the central cluster have K = 5 and will saturate in a NIRCam image. This may cause problems if such bright objects fall on low-contrast shutters. - 12 -
9.0 Program 202: Jets from young stars JWST-STScI-002270 This example of an observing scenario by D. Karakla helps to illustrate the process and results: Project title: YSO jets near IRS1 in NGC 2264 Author: Purpose: SI usage keywords: Instrument modes: 9.1 Description: D. Karakla Test case of observing highly extended objects. These are also emissionline objects that cover a broad range of flux levels. Extended objects; bright objects; point sources NIRSpec MSA G140M+G235M (extended sources) G140M+G235M+G395M (point sources) NRS+NRSRAPID YSO (Young Stellar Object) jets are emission-line sources found in star-forming regions. Many are also designated as Herbig-Haro (HH) objects. One well-studied star-forming region is the ~5-Myr cluster NGC 2264, which is about 760 pc distant. HH objects are jets emanating from very young stars and as such are extended primarily in one direction along an axis. Molecular hydrogen (H 2 ) emission (1.121 micron and more) is often seen in jets with lower- to intermediate energy levels, excited by shocks in outflows, along with [Fe II] lines (1.257 microns and more), although not necessarily in exactly the same spatial locations. The knots in these emitting regions have velocities of 100 200 km s 1. The illustration below is from Simon and Dahm (2005) and it shows the region of interest and several features. This image was made from J-, H-, and K-band images using QUIRC on the 2.2-m telescope on Mauna Kea. EXS-1 is an x-ray flare star, and the contours shown are from an observation made with XMM-Newton. IRS-1 is a very strong infrared source discovered with the IRAS satellite. Note the faint knots. - 13 -
The next illustration (below) shows infrared data for jets in NGC 2264 from Wang et al. (2002). These were obtained at the Okayama Observatory in J, H, K, and H 2 (2.121 microns). The five sources marked with a cross are likely to be high-mass proto-stars (Ward-Thompson et al. 2000). The goals of this program are to study the emission knots of YSO jets as well as the potential YSO sources in this region. The knots are emission line sources, and the brightest knots appear as red objects in 2MASS K-band images and in Spitzer IRAC 3.6- micron images. Spectra are to be obtained of not only the jets but also the YSOs presumed to drive the jets. It may also be possible to find the infrared counterparts to the strong millimeter sources in this area. - 14 -
9.2 Exposure time estimation: JWST-STScI-002270 No fluxes or spectra were found in the literature for these sources. However, the brightest knots are seen in 2MASS K-band images and the limiting K magnitude in those is ~16.5. Similar sources were found and scaled to the distance of NGC 2264. Likely exposure times were estimated as follows: For the knots in the jets, the emission-line spectrum shown in Garcia-Lopez et al. (2010) was used, taken to extend over 10 MSA shutters, with 2 additional shutters used to measure background. The Exposure Time Calculator (ETC) of J. Valenti was used with these flux values to estimate an exposure time of 212 sec for an integrated (total line flux) S/N = 4 in one of the weaker [Fe II] lines at 1.6 μm in Band 1. About 170 sec is needed to achieve S/N = 4 in a weak H 2 line at 2.248 μm in Band 2. Final exposure times were adjusted upward. The jets may arise from some fainter YSOs in the field which are also to be observed, as are near-infrared counterparts to 5 known millimeter-wave (MM) sources (Ward- Thompson et al. 2000). 9.3 MSA planning: It is assumed that a preliminary image with NIRCam will be obtained and from that precise source positions will be measured. The HH objects (jets) are expected to be extended primarily in one direction (along their axis). However, these jets also extend in the cross direction as well, and that may require additional MSA configurations to provide full spatial coverage. A test case of MSA usage was based on existing astrometry. It was found that several separate target sets were needed to achieve the science goals and to achieve optimal centering of sources in the shutters; also they were organized hierarchically by brightness so that any one target set can be observed well with a single exposure time. Version 17.0.3 of APT includes a prototype MSA planning tool. The figure below shows this field in Aladin, as taken at 3.6 microns with IRAC on Spitzer. It would be necessary to close shutters during the target acquisition in order to avoid saturation by the brightest objects. The NIRCam image would probably best be done in the narrow-band H 2 filter in order to minimize saturation and to locate the knots. - 15 -
9.3.1 Observing summary: A narrow-band NIRCam preliminary image is needed to obtain precise astrometry of sources. Additional NIRCam images may be needed to estimate background fluxes. NIRSpec confirmation images are required. The operating mode is MSASPEC: o G140M+F100LP (Band 1) to observe [Fe II] lines at 1.257 and 1.644 μm at R = 1000. o G235M+F170LP (Band 2) to observe H 2 at 2.121 μm at R = 1000. o Both use NRS readout mode, except use of NRSRAPID when the number of NRS groups falls below 6 (for better cosmic ray rejection). Three target sets are needed to cover all extended sources in this scenario: o One guide star may suffice for all 3 target sets, but an additional target acquisition is needed to dither objects across the detector gap. o At the same time, the durations of exposures prevent placing all exposures into a single visit. o Each target set needs 12 exposures plus 2-4 MSA configurations to achieve the needed spatial coverage of knots: There are 3 spatial offsets of the MSA pattern, using slitlets that are 16 shutters high on the larger knots (10 source shutters and 2 for - 16 -
background, with 2 shutters closed between the end of source and the background at top and bottom). A large dither is done to fill the wavelength gap. This is all done for both gratings. For the point sources, a 3-high by 1-wide slitlet is adequate for each. A specific setup was not tested, but the total number of point sources is small and it is assumed that they can be accommodated in one or possibly two target sets. o The K-band flux of ISR1 (Simon & Dahm 2005), another YSO showing a bipolar outflow, was scaled to K-band fluxes of sources as measured by Ward- Thompson et al. For the three bands: 201 sec to get S/N > 10 in Band 1. 53 sec to get S/N > 10 in Band 2. 53 sec to get S/N > 10 in Band 3. NRSRAPID is used because the sources are bright. Gratings G140M, G235M, and G395M are used at R = 1000. Complete wavelength coverage is desired, and so dithering is used to cover the wavelength gap between the detectors. There are 36 exposures per target set. Confirmation images are required. 9.4 Summary for extended sources: Exposures per dither point for extended objects, using a 16-shutter slitlet. Pattern uses 3 spatial and 2 spectrum dither points for 3 target sets. Guide star Target set Grating+filter Exposure (sec) 1 1 G140M+F100LP 296 (NRS, 7) 1 1 G235M+G170LP 296 (NRS, 7) 1 2 G140M+F100LP 222 (RAPID, 21) 1 2 G235M+G170LP 222 (RAPID, 21) 1 3 G140M+F100LP 466 (NRS, 11) 1 3 G235M+G170LP 466 (NRS, 11) Note that for target sets 2 and 3 an attempt was made to vary the exposure times up and down to allow for sources both 50% brighter and fainter. 9.5 Summary for point sources: Exposures per dither point for point sources, using a 3-shutter slitlet. Pattern uses 3 spatial and 2 spectrum dither points for 1 to 2 target sets. Guide star Target set Grating+filter Exposure (sec) 1 1 G140M+F100LP 212 (RAPID, 20) 1 1 G235M+G170LP 212 (RAPID, 20) - 17 -
Guide star Target set Grating+filter Exposure (sec) 1 1 G395M+F290LP 63 (RAPID, 6) 1 2 G140M+F100LP 212 (RAPID, 20) 1 2 G235M+G170LP 212 (RAPID, 20) 1 2 G395M+F290LP 63 (RAPID, 6) 9.6 Observation concerns identified: The existing NIRSpec templates are minimally sufficient to support this kind of observation, but significant efficiency improvements could be made. A significant and unanticipated number of separate target sets were found to be necessary for a given pointing, even with only a few dozen total objects. This was needed to cover the full area, to be able to deal with a broad range of source brightnesses, and to optimize source centering in shutters and also to avoid overlapping spectra. It is not always easy to judge when to trade off use of NRS versus NRSRAPID. NRSRAPID is preferred when there is a risk of losing too much data due to cosmic rays (below about 250 sec exposure time), but the default use of NRSRAPID raises data volume concerns. It may be helpful to allow the optimization code to take account of the type of object, based on the NIRCam pre-image. Point sources must be centered in the sweet spot of a shutter, while extended sources do not, but the extended sources must still avoid bad shutters. Overlapping spectra can be a problem in even a moderately-crowded field. The MSA Planning Tool will need to provide a capability that avoids placing bright object spoilers in low-contrast shutters. The optimization of the MSA configuration will depend critically on the quality of the NIRCam pre-image. Measuring the positions of sources near bright objects will be affected by saturation. It is not now possible to obtain more than a single confirmation image in a visit. Multiple target sets can be observed, but observers will need the means to obtain a confirmation image for each. Currently, the onboard scripts assume that exactly the same sequence of actions occur at every dither location in a visit. At each dither location, the scripts can loop through a list of gratings each with their own exposure time. However, the scripts (currently) do not have a mechanism for specifying different MSA configurations or different exposure times at different dither locations. 10.0 Program 203: Formation of massive stars Project title: Massive star-forming regions in the Milky Way Author: J. Muzerolle Purpose: Very crowded regions with a wide brightness range. - 18 -
Instrument modes: NIRSpec MSASPEC G140M, G235M, G395M NRSRAPID 10.1 Scientific background: The figure below shows a NICMOS image (F110W+F160W+F222M) of the region W3. Enlargements of an area at two longer wavelengths are also shown. The goal of this program would be to determine star formation rates and the initial mass function (IMF) in a cluster containing massive stars, where there is an environment of extreme radiation and very high densities. The evolution of circumstellar disks would also be studied. The information to be obtained would include spectral types, measures of spectrum veiling or continuum excess, and signatures of accretion and/or outflows, such as atomic and molecular emission features. The stars to be observed number in the hundreds per cluster, and range in K magnitude from about 11 to 18 at a nominal distance of 2 kpc. - 19 -
10.2 Operations notes: 0.0 The figure below shows examples of MSA fields for the bright (left) and medium (right) targets. The stars in red actually get observed, and blue objects would be saturated. o A confirmation image is required. o There are 4 exposures per target set so as to obtain spatial dithers: o One dither is done to cover the wavelength gap, and that requires two MSA configurations per target set. o There would be two open shutters per target in each configuration, with a dither performed between the two shutters. o There would be multiple target sets to deal with the range in brightnesses, from bright (11<K<14) to medium (14<K<16) to faint (16<K<18), with exposure times of ~30, 100, and 1000 sec for S/N > 50 in G140M and G235M, and S/N = 30 in G395M. o To estimate exposure times a catalog of known members of the Orion Nebula Cluster was used, scaled to a distance of 2 kpc. This is a typical distance for the nearby massive star-forming regions, and the resulting field of view is well matched to the MSA. There would be ~1500 potential targets in the desired brightness range. o An optimization code was used to determine MSA configurations. This included the dither to cover the wavelength gap. Three configurations were used for the three ranges of brightness noted above to provide spectra of 136/319 bright stars, 215/674,medium, and 157/477 faint stars, which means about 30-40% of potential targets are observed. o Note that if no gap dither were used then only one configuration per target set would be needed and it would be possible to observe 40 to 50% of the potential targets. o The total time needed for 9 target sets in one cluster is about 9 hours. - 20 -
10.3 Exposure breakdown: Guide star Target set targets/cand idates G140M (sec) G235M (sec) G395M (sec) 1 1 59/319 30 30 30 1 2 41 30 30 30 1 3 36 30 30 30 1 4 79/674 100 100 100 1 5 72 100 100 100 1 6 64 100 100 100 1 7 56/477 1000 1000 1000 1 8 54 1000 1000 1000 1 9 47 1000 1000 1000 10.4 Observing concerns identified: o The organization and scheduling of target sets: o Faint stars may need different acquisition break points. o The observations may need to be chained in such a way that bright sets are done first since they can accommodate the initial slew tax and still stay within the maximum visit size of 10,000 sec. o There are concerns over background subtraction in this crowded region, particularly at longer wavelengths (above 3 microns). Pre-imaging at multiple wavelengths will be needed. o Dealing with source crowding in order to get reliable background subtraction, as well as guide star and reference star selection. o There are ~20 sources in the field that will saturate. In some cases it may be possible to detect them through closed shutters, particularly for the longer exposures to study the faint stars. A mechanism for tracking such objects may be needed. - 21 -
11.0 Program 204: First-light galaxies in the Hubble Ultra-Deep Field Project title: Author: Purpose: Instrument modes: 11.1 Description: First-light galaxies in the Hubble Ultra-Deep Field D. Soderblom JWST-STScI-002270 A few extremely faint galaxies have been found in the Hubble Ultra-Deep Field with large photometric redshifts. Spectroscopic redshifts of these objects are needed to confirm their distances. These objects may be the faintest observed by NIRSpec and so test the ability to schedule repeated visits with the same MSA configuration so as to build up the needed signalto-noise. NIRSpec MSASPEC G235M+F170LP; G140M+F100LP NRSRAPID This scenario is based on SO-DRM program 402 as written by M. Stiavelli in January, 2006. The goal is to obtain medium-resolution spectra (R = 1000) to confirm the nature of first-light sources in the distant universe. For a brief discussion of the science, see Stiavelli (2009). The objects to be observed are extremely faint galaxies in the UDF. These objects are marginally resolved, with sizes of 0.2 arcsec or so, requiring careful placement in the NIRSpec MSA shutters for good throughput. The z850 magnitudes of the objects of interest (i-band drop-outs) range from about 27 to 29.5, with most fainter than 28 (see Fig. 1 in the above ref.). The J110 magnitudes are about 27 to 29. Using the conversion provided by Jakobsen ( Calculating the nominal sensitivity of NIRSpec ), which is ABmag = 31.43 2.5 log(f(njy)), then J110 = 27 corresponds to a flux of about 60 njy, and 29 to about 10 njy. The exposure time calculator (ETC) rendered in IDL by Tumlinson predicts that one needs 6.4 Msec for the 10 njy sources at 1.2 microns, or nearly 1800 hours. The brighter (60 njy) sources would need about 300 hours. By comparison, the SO-DRM allowed for two sets of exposures: 300 hours at R = 1000 and the medium wavelength setting (G235M + F170LP). 100 hours at R = 1000 and the short wavelength setting (G140M + F100LP). For this exposure time, the scenario in the SO-DRM only reaches the brighter objects shown in Stiavelli s paper. Whether this is a problem or not (in reaching a JWST primary science objective) requires judgment by experts in the study of first-light galaxies. Note that these galaxies are presumed to be largely continuum sources and not necessarily strong sources of Lyman-alpha line emission. The goal in obtaining spectra is - 22 -
to detect the Lyman break and so confirm photometric redshifts so that the galaxies can be more precisely set into the cosmic time-scale. The anticipated density of the primary targets is about one per square arcmin, or ~10 over the NIRSpec FOV. These objects are extremely faint, and In addition, other relatively faint objects will be observed, presumed to be galaxies at z > 6. The density of these objects is taken to be several per square arcmin and they are assumed to be brighter, needing 10 hours integration each. Because of the very long integration times, this program is broken into 10-hour visits. Each visit will always observe the same 10 first-light objects, plus an additional set of other objects that changes for each visit. In addition, this program may execute at varying roll angles, requiring additional MSA configurations. A total of 40 visits is needed. 11.2 Visits and sub-visits Each 10-hour visit is 36 ksec long and so is broken into four 9 ksec units. Each visit uses a different MSA config. Each 9 ksec unit (sub-visit) starts with a target acquisition and consists of 9 x 1 ksec exposures. This then allows for nine dither positions. Some dithers are sub-aperture and use the same MSA config, others go to different apertures. 11.2.1 Middle-wavelength sub-visit Parameter Opsmode Optical elements Readout Conf. image? Value MSASPEC G235M + F170LP NRS Yes MSA configs 3 Exposure Dither 1,000 sec per dither 3x3 11.2.2 Short-wavelength sub-visit Parameter Value Opsmode MSASPEC Optical elements G140M + F100LP Readout NRS Conf. image? Yes MSA configs 3 Exposure 1,000 sec per dither Dither 3x3-23 -
11.3 Scheduling of observations The proposed observations of the HUDF pose a distinct challenge that few other programs will match: their extended duration. The total time needed is 400 hours, or 40 visits of 10 hours each, which means a total of roughly 20 days of on-target time. Is it possible to observe the HUDF at a single roll angle for 20 days? A single roll would be preferred because then one could be certain of getting the same faint, high-redshift galaxies. A change in roll angle is likely to mean some object would no longer fall within the sweet spot of a shutter. The plot below, prepared by W. Kinzel, shows the maximum permitted time at a given angle in ecliptic coordinates. For the HUDF it turns out that the maximum at one orientation is 13 days. However, one can achieve the same roll six months later, and so a total of about 26 days should be available at a single roll angle for the HUDF pointing. If such a strategy is used, differential velocity aberration may lead to positional shifts within the MSA fields that cause objects to fall outside their sweet spots. In other words, it may not be good enough to execute two separate pointings six months apart. Another possibility is to execute this program over two years. - 24 -
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12.0 Program 205: Carbon abundances in Omega Centauri Project title: Author: Purpose: SI usage keywords: Instrument modes: Carbon abundances in Omega Centauri J. Tumlinson JWST-STScI-002270 Test case of obtaining observations of an extremely densely populated field. MSA NIRSpec MSASPEC NRSRAPID 12.1 Description: This scenario has been constructed to test the feasibility of NIRSpec MSA observations in an extremely crowded field, in this case near the center of the Galactic globular cluster Omega Centauri. The full description of this scenario is appearing in a separate report and so will only be briefly summarized here. This field offers more than 24,000 stars brighter than R = 20, a brightness at which the high-resolution grating in NIRSpec yields signal-to-noise = 20 in 400 sec. About half of those stars randomly fall within the acceptable sweet spot of MSA shutters. The observing strategy used in this study is to keep the telescope fixed at a single position and to then repeatedly reconfigure the MSAs to move across the field to useful targets. This strategy assumes no spectrum overlap and no need for background measurement at these bright levels. 12.2 Operations notes: o A total of 40 separate MSA configurations and exposures are needed, for a total of 16,000 sec or 4.4 hours, before overheads are included. o Some shutters with good targets must be avoided because they end up with more than one good target. 12.3 Observing concerns identified: o Low-contrast shutters can potentially spoil observations for other objects in the same row, but this appears to not be a significant problem in this study. o NIRSpec can successfully observe this challenging field. 13.0 Program 207: MSA spectroscopy of a very extended object: NGC 6302 Project title: MSA spectroscopy of a very extended object: NGC 6302 Author: Purpose: T. Keyes Test case of obtaining NIRSpec observations of a very large object (3 x 3 arcmin) without information on background or target acquisition. The efficiencies of alternate scenarios are compared. - 26 -
SI usage keywords: Instrument modes: Extended object; mosaics NIRSpec MSASPEC G140H+F100LP NRSRAPID 13.1 Description: NGC 6302 is a Galactic planetary nebula with a large physical extent on the sky (>~3 x 3 arc min). This program is intended to study nebular abundances and the thermal and ionization structure at high spatial scale using diagnostic emission lines in the NIRSpecaccessible IR. The scenario considered uses NGC 6302 as a proxy for observations of any object whose physical extent is comparable in size to the MSA field, and for which no contemporaneous background and target acquisition observation is possible. The observing strategies considered in this study compare two alternatives: Keep the telescope fixed at a single position and repeatedly reconfigure the MSAs to move across the field. Use a single column of shutters and move the telescope in a sequence of slews to move the observation sampling across the field. These strategies assume no spectrum overlap will occur and that there is no need for background measurement. The image on the left below is an HST ERO image issued recently. Narrow-band WFC3 images are available using filters at H-alpha, [N II], [O II], and [O III]. These reveal numerous shocks, filaments, and outflows extending nearly a parsec from the central star. Near-infrared emission lines arising from a broad variety of species are also present. The right-hand image shows NGC 6302 overlaid with the outline of the NIRSpec 3 x 3 arcmin field of view, with the yellow line representing a full-length MSA column of shutters. - 27 -
13.2 Observing notes: o An initial goal is to obtain spectra that can reveal a wide range of ionization states and species, particularly high-ionization lines, as well as [Fe II] and H 2 features. o No target acquisition is required (and none may be possible), but there is a need to be able to identify positions accurately after the fact. Hence a confirmation image with the MSA open would be obtained and is probably preferable to a NIRCam image. o The grid of points on the nebula would be obtained by using an entire vertical column of open shutters, stepped 480 points at 0.25 arcsec each. Thus the entire field would be about 180 x 120 arcsec. Once the 4-point dithers are counted, a total of 1,920 exposures is obtained in the spatially-scanned direction. o The exposure taken at each step is short, roughly 3 min, with dithers included. However, the effects of saturation for this bright source have not been considered and some special exposures may be needed in some cases. - 28 -
o The total time needed is estimated at ~24 hours, but that is before guide star acquisition times are included. The number of guide stars needed to cover this region is not known. 13.3 Use of the MSA versus the IFU: For this large object the intent is to obtain a full set of spectra parallel to the disk axis. This could be done using either the MSA or the IFU, with these considerations (see also program 261): The MSA provides poorer spatial sampling in the dispersion direction: about 0.2 arecsec compared to 0.1 arcsec for the IFU. At the same time, the IFU would need about 6 times the number of pointings. MSA failed-open shutters can cause significant problems for this or similar programs, effectively making an entire row useless. Use of the MSA means the need for fewer guide stars compared to the IFU. Both modes require dithers to treat detector effects, but failed-open shutters may require additional dithers to work around them. 13.4 Observing concerns identified: Information on the need for preliminary and/or confirmation images is insufficient at this point to judge their need. For this program, it would be ideal to have both before and after images, even during the raster sequence. It is not clear how mosaic tiles will be registered for later combining. Will FGS information be adequate for this purpose? Will it is necessary to obtain a NIRCam image? This object and ones like it have inherently have a very large range of brightnesses. The problem of detector persistence may require division of the observations into stages defined by dynamic range. This may exacerbate registration concerns, and is it possible to keep such a sequence of operations within a single visit? As noted with program 261, it is not yet clear how mosaics and the need for guide stars interact. Will this be automated? - 29 -
14.0 Program 230: NIRSpec follow-up of Gamma-ray burst afterglows Project title: Author: Purpose: SI usage keywords: Instrument modes: 14.1 Description: JWST-STScI-002270 NIRSpec follow-up of Gamma-ray burst afterglows J. Tumlinson Test case of obtaining NIRSpec observations on a quick-turnaround basis. Tests ability of the system to handle TOOs. Quick-turnaround TOO; faint sources; fixed slit NIRSpec FIXEDSLIT G235M+F170LP NRS JWST is required to be able to respond to targets of opportunity (TOOs): Regular TOOs do not interrupt the current Observation Plan and need only be executed within two weeks. Rapid turn-around TOOs require much faster response, with this requirement: o The S&OC is required to be able to update the executing Observation Plan to incorporate a Rapid Response TOO within 24 hours after submission of the updated TOO Observation (MR-293); this depends on the availability of a DSN contact to intercept the executing Observation Plan. (JWST-STScI-000648) o The 24-hour requirement breaks down into 22 hours for the PPS to process the new Phase II submitted by the PI and to perform the necessary safety and feasibility checks, plus 2 hours for the new Observation PLan to be staged by the FOS for upload when the next ground contact occurs. o This implies the following time budget for response to a Rapid Response event: User completion of new Phase II program using APT: 2 4 hours. Processing by PPS: <22 hours. Staging and upload: < 2 hours. Wait for next ground contact at two per day: <12 but typically ~6. o This makes the likely shortest time to be 32 hours (2 + 22 + 2 + 6) and the maximum about 40 hours (4 + 22 + 2 + 12). These net turn-around times (32 to 40 hours) allow for an adequate response to a gammaray burst event. For example, GRB 090423 still had a flux of ~5 mjy that late after outburst. However, events at higher redshift will presumably be fainter, and it would make success much more likely if the 22-hour processing time the single biggest item in the time budget could be reduced. In addition, the above calculations do not really take account of the limited on-shift hours for JWST operations. An event triggered on, say, Friday evening could be delayed much more. - 30 -
14.2 Scientific background: JWST-STScI-002270 Gamma-ray burst (GRB) afterglows can be used as distant light sources to reveal absorption from gas in the host galaxy or in the intervening inter-galactic medium (IGM). The host galaxy gas may contain tracers of molecular gas or diagnostics of ionization, temperature, and metallicity. The spectrum from a GRB is flat and that of a synchrotron source resulting from a reverse shock in the ejected material. GRBs are now being detected and announced rapidly and automatically by the Swift satellite. This rapid-turnaround capability has proved to be so important that it is likely that successors to Swift will be operating in the JWST era. The illustration below shows the evolution of the spectral energy distribution (SED) of GRB 090423, taken from Tanvir et al. (2009). The estimated redshift is 8.2. The three bands distributed in frequency on the left of the figure correspond to K, H, and J in going from left to right. The observations are in time sequence from top to bottom. Note that Tanvir et al. obtained S/N ~ 3 over 400 Å bins in 2.5 hours using VLT with SINFONI. The NIRSpec ETC indicates that NIRSpec can obtain S/N = 15 per resel at R = 1000 in ~10 min. 14.3 Operations notes: In most respects a GRB afterglow is an ordinary single-object, fixed-slit observation. The goal is to obtain a reduced one-dimensional spectrum over 1 to 5 microns at good signal-to-noise. However, GRBs offer a significant challenge because of their transitory - 31 -
nature. In particular, it is not feasible to obtain a preliminary image with NIRCam, and so the target acquisition must be done with less-precise coordinates. Several questions arise: Can a satisfactory acquisition be achieved in, say, the largest fixed aperture? Would that require additional software similar to the ACQ/PEAKUP operation performed with COS on HST? How are TOOs handled by the ground system and is that system adequate to meet science needs? Will there need to be observing taxes levied on observers seeking rapid-turnaround observations such as the 15-orbit penalty assessed on HST observers of quickturnaround TOOs. Regarding acquisitions, the default target acquisition strategy for NIRSpec has been developed with the MSA in mind, given that it is anticipated that MSA observations will dominate NIRSpec usage. That TA strategy places targets within an acceptance zone of an MSA shutter with an accuracy of 20 mas, given coordinates of sufficient precision. This process requires the use of 8 to 10 reference stars with relative astrometry good to ~5 mas, based on NIRCam or other equivalent imagery. In comparison, a GRB s position can be good to ~100 mas, based on immediate ultraviolet or optical follow-up, but such positions do not come with any reference stars. An alternative, as noted, is to use the 1.6 arcsec square fixed aperture in NIRSpec, together with flight software that can scan a somewhat larger area of the sky than the aperture itself. Another possibility is to use reference stars but to allow coarser requirements on their relative astrometry, perhaps using information from 2MASS, VISTA, or SASIR. 14.4 Observing concerns identified: The assumed mode of operation for NIRSpec is an observation planned well in advance and with a preliminary NIRCam image so that precise source positions can be measured as part f preparing for the target acquisition. TOOs cannot be observed that way (with possible rare exceptions) and so an alternative acquisition scheme is needed. In general, it should be possible to acquire reliably TOOs if they are observed in the 1.6 arcsec square fixed slit and if there is a peak-up algorithm available to tune the pointing. - 32 -
15.0 Program 231: Exoplanet atmospheres Project title: Authors: Purpose: SI usage keywords: Instrument modes: Exoplanet atmospheres J. Valenti, D. Long JWST-STScI-002270 Test the ability of the system to handle observations that must be executed at a specific time in order to capture an event. 15.1 Scientific background: High signal-to-noise; critical timing NIRSpec FIXEDSLIT NRSRAPID Exoplanet observations have yielded some of the top science results from Hubble and Spitzer, and the same is likely to be true of JWST. JWST will observe exoplanets transiting in front of the host star and half an orbit later being eclipsed by the host star. For astrophysical context, the table below gives median and extreme characteristics for 66 known transiting planets. The number of planets known to transit stars brighter than K=12 will increase by a factor of a few between now (mid-2010) and the launch of JWST. Property Minimum Median Maximum Ks magnitude (2MASS) 5.5 9.85 14.7 Orbital period (days) 0.8 3.2 111 Transit duration (hours) 0.8 3.1 12 Transit depth (%) 0.035 1.1 2.8 Eclipse contrast ratio at 3.9 microns 1E 6 8E 4 2E 2 Observers will use two complementary observing strategies to characterize exoplanet atmospheres. The eclipse spectroscopy strategy subtracts spectra obtained during eclipse from spectra obtained immediately before and after eclipse. The subtraction removes direct emission from the star, leaving only starlight reflected by the planetary atmosphere and thermal emission from the planet. The transit spectroscopy strategy divides spectra obtained during transit by spectra obtained immediately before and after transit. The division yields the fraction of starlight not blocked by the planet, which is a function of wavelength because opacity in the planetary atmosphere is a strong function of wavelength. Eclipse and transit spectra will yield planetary radii, composition (e.g., water, methane, carbon monoxide, carbon dioxide), vertical temperature structure (including stratospheric temperature inversions), and horizontal heat distribution. Planet atmospheres have spectral features that are typically 0.01% (transit) to 0.1% (eclipse) of the stellar signal, so observations must achieve a S/N ratio of 10,000 per hour - 33 -
and per wavelength bin. JWST has more than enough sensitivity to achieve this S/N ratio for dozens of transiting exoplanets. The challenge will be to minimize systematic errors due to mechanical, thermal, and electronic transients on timescales of minutes to hours. 15.1.1 Observing description: Transiting planets have well-defined ephemerides. Most known exoplanets have orbital periods of a few days, implying tens of scheduling windows per year. In general, visits will not have an orientation constraint. Given the flexibility of event-driven observations, the observatory may be ready to execute an exoplanet visit before the nominal start time. The observer should be charged (at least statistically) for the time spent by the observatory waiting for the exoplanet transit or eclipse to occur. As soon as the preceding visit completes, the observatory should slew from the old attitude to the exoplanet-observing attitude. [N.B. A dummy visit could perhaps be used to force an early slew, if the standard visit execution logic is not sufficient.] Slewing immediately allows slew-related mechanical and thermal transients to decay as much as possible before the high-precision exoplanet observation begins. If it is not too difficult to implement, extra exposures of the target should be obtained after target acquisition and before the nominal visit start time. Starting exposures immediately allows the detectorrelated electrical and thermal transients to decay as much as possible before the highprecision exoplanet observation begins. Observations should use the large (1.6 x 1.6 arcsec) fixed slit. For a well-centered star, this large aperture is relatively insensitive to target drift (due to ISIM thermal changes and/or rotation about the FGS guide star) because the wings of the PSF have dropped roughly symmetrically to relatively low levels at the edge of the aperture. [N.B. What level of displacement is acceptable without affecting photometric precision?] The baseline NIRSpec target acquisition procedure requires precise coordinates for the science target, relative to a set of reference stars distributed across the NIRSpec field of view. NIRCam pre-imaging is used to obtain precise relative coordinates. However, exoplanet hosts are bright enough to saturate NIRCam, even in the shortest possible fullframe exposure. In JWST-STScI-1751, Beck describes an alternate strategy for acquiring bright objects (particularly exoplanet hosts) in the large aperture. This strategy should be implemented and used for exoplanet observations. Exoplanet observations with any of the gratings will use the (yet to be defined) S1600A1 subarray, which spans 32 rows and 2048 columns per detector (0.66 seconds per frame). Observations of the brightest exoplanet hosts may require fewer columns (less wavelength coverage) or fewer rows (less precise photometry) to avoid saturation. Exoplanet observations with the PRISM and F070LP filter will use a subarray that is 32 rows by 512 columns (0.17 seconds per frame) because the prism spectrum does not fill the detector. [N.B. Smaller subarrays may also be needed for the other fixed slits.] Because exoplanet hosts are bright, integrations will saturate after only a few frames. The NRSRAPID detector pattern (one frame per group, i.e. NFRAMES=1) will be used to record multiple groups before saturation. Because the data rate for a subarray is 25% the data rate for full-frame, NRSRAPID can be used indefinitely without exceeding the NIRSpec data volume allocation. - 34 -
Whenever possible, integrations will have at least 4 up-the-ramp measurements per integration (NGROUPS), so that the pipeline can use the standard algorithm to detect cosmic rays and fit ramps. For the brightest sources, NGROUPS will need to be less than 4 to avoid saturation. Cosmic rays can still be detected by comparing count rates in successive integrations. Even NGROUPS=1 may yield valuable data for bright sources, despite a lack of information about the bias level after each pixel reset. Exoplanet observations will typically be twice as long as the transit duration plus extra time for slew, setup (e.g., guide star acquisition, NIRSpec target acquisition), and detector stabilization. An exoplanet with a 3 hour transit would be observed with the following sequence: Slew: 45 min Set-up 30 min Stabilization: 30 min Pre-eclipse: 90 min Eclipse: 180 min Post-eclipse: 90 min. From the perspective of photometric stability, the data gathered during stabilization through post-eclipse should be a single 390 minute exposure with 390*60/0.66 = 35454 integrations. This may exceed the maximum length for a single exposure and/or the maximum number of integrations for an exposure. This means that the observation may need to be split into multiple exposures. To minimize the effect on the photometry of exposure boundaries, exposures will preferentially be started well out of eclipse or at the midpoint of the eclipse. In the example above, a plausible set of four exposures would have durations of 60, 150, 150, and 30 minutes with 5455, 13636, 13636, and 2727 integrations, respectively. Nominally, a new target acquisition is required after 10,000 seconds to compensate for target drift. This implies a target acquisition before each of the four exposures described above. However, for very precise photometry of exoplanets, the mechanical, thermal, and electronic disturbances associated with a target acquisition may create larger errors than the drift between target acquisitions. Depending on actual observatory performance, exoplanet observers may need to suppress target acquisitions. Note that splitting out of eclipse observations into a pre-eclipse and a post-eclipse segment corrects to first order the effects of target drift. 15.2 Observing concerns identified: Because of the need for exact timing of the observations, planet transit data-taking is likely to place unusual demands on scheduling the observatory. It may be necessary, for example, to tolerate overhead times so that a visit can execute reliably at the correct time and so that an instrument is in a reliably stable configuration in order to obtain very high signal-to-noise data. These overheads need to be accounted for, at least statistically, in evaluating proposals. Exoplanet hosts are likely to be very bright and so it may be difficult to measure their positions relative to reference stars well on a preliminary NIRCam image. The shortest possible full-frame NIRCam image will saturate. The peak-up procedure used for centering a TOO would also work well for bright targets. - 35 -
Exoplanet observations are likely to need sub-arrays because of the source brightness. Observations of very bright sources may be challenging for reliably rejecting cosmic rays because of the smaller number of groups obtained. For exoplanet studies it is necessary to obtain a very large number of short integrations to both reach the needed S/N and to cover the time period in question. Such a situation may require splitting the observation into several exposures to work around observatory constraints, but those separate exposures must be timed to ensure continuous data during the eclipse. To achieve the needed S/N it may be advantageous to suppress re-acquisition of guide stars. - 36 -
16.0 Program 261: Atomic hydrogen filaments in Perseus A (NGC 1275) JWST-STScI-002270 Project title: Atomic hydrogen filaments in Perseus A (NGC 1275) Author: Purpose: SI usage keywords: Instrument modes: 16.1 Description: T. Beck Test case of obtaining observations of a large object at high spatial resolution, requiring a large number of mosaicked exposures. IFU; mosaic pattern NIRSpec IFUSPEC G140H NRSRAPID Perseus A is a cd galaxy at the heart of the Perseus cluster of galaxies. This program is intended to study the kinematics and energetics of the neutral hydrogen filaments. The figure below shows an HST/ACS image of Perseus A, together with an indication of the 3-arcsec square NIRSpec IFU field of view. 16.2 Operations notes: This program will use the IFU on the (assumed) need for 0.1 arcsec spatial resolution, and also for better rejection of unwanted light from other parts of the object. The goal is to observe both Paschen-alpha and Paschen-beta in a single setting, with additional features from [Fe II] and H 2 appearing. - 37 -
The overall mosaic is 484 p[ointings in a 22 x 22 pattern on 2.7 arcsec centers in order to get 10% overlap between pointings. Each IFU position in the mosaic will be done with a 4-point small pattern. At 30 sec each, each position should need about 3 min, including overheads. The total integration time is about 23 hours, and that does not include the time needed for guide star and target acquisitions. The pattern is shown below. It is estimated that 5 guide stars will be needed for 5 sets of targets. 16.2.1 The IFU compared to the MSA: This program s goal is to obtain full 3D spectra of the inner 60 x 60 arcsec region. The MSA could be used instead of the IFU, but with these consequences: Poorer spatial sampling in the dispersion direction because of the 0.2 arsec-wide shutters. Dithering in the cross-dispersion direction would be needed to work around the MSA bars. A full 60 arcsec of slit is needed with no failed shutters. About 300 pointings would be needed in the dispersion direction to sample the full 60 arcsec. At the same time, use of the MSA would likely mean that fewer guide stars would be needed because stepping would be done in only one direction, not two. - 38 -