Cryogenic Near-IR Spectro-Polarimeter (Cryo-NIRSP) Instrument Science Requirement Document

Transcription

1 Project Documentation Document SPEC-0056 Revision D Cryogenic Near-IR Spectro-Polarimeter (Cryo-NIRSP) Instrument Science Requirement Document M. Penn, J. Kuhn, D. Elmore * Instrument Group 28 July 2010 Prepared By: Name Signature Date David Elmore Instrument Scientist D. Elmore 10 Nov 2010 Approved By: Approved By: Approved By: Released By: Steve Hegwer Instrument Engineer Rob Hubbard Systems Engineer Thomas Rimmele Project Scientist Jeremy Wagner Project Manager S. Hegwer 10 Nov 2010 R. Hubbard 12 Nov 2010 T. Rimmele 22 Nov 2010 J. Wagner 10 Mar 2011 * Responsible author Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ Phone atst@nso.edu Fax

2 REVISION SUMMARY: 1. Date: 9/24/2001 Revision: A By: H. Lin, M. Collados (original authors) Changes: Initial version. 2. Date: 5/29/2003 Revision: B By: M. Penn Changes: Combine NIRHRS with NIRSP. 3. Date: 3/15/2006 Revision: C By: M. Penn Changes: Divide Nasmyth and Coudé instruments. 4. Date: 3/9/2009 Revision: C1 By: M. Penn Changes: Specify 1.0 arcsec/pixel, FOV 3 arcmin, thermal IR priority, re-examine science drivers. 5. Date: 3/13/2009 Revision: C2 By: M. Penn Changes: Change several requirements based on instrument group meeting. 6. Date: 4/23/2009 Revision: C3 By: M. Penn Changes: Incorporate changes from instrument group meetings; rename Cryo-NIRSP. 7. Date: 3/18/2010 Revision: C4 By: M. Penn Changes: Added responsible author (D. Elmore) 8. Date: 7/28/2010 Revision: D By: D. Elmore, J. Kuhn, D. Mickey Changes: Revised science, general features, requirements. WAIVERS: The following waivers are applicable to this specification: 1. RFW-0048: Field of View Vignetting (15-Nov-2013) SPEC-0056, Revision D Page ii

3 Table of Contents 1. Introduction PURPOSE SCOPE RESPONSIBLE AUTHOR APPLICABLE DOCUMENTS ABBREVIATIONS AND ACRONYMS Mission Example Science Cases General Features IMAGE STABILITY AND OCCULTING FLAT-FIELDING AND WAVELENGTH CALIBRATION POLARIZATION CALIBRATION CONTEXT IMAGER IMAGE SCANNING SAFETY MECHANISM DATA DISPLAYS Requirements SPECTRAL COVERAGE SPECTRAL RESOLUTION TOTAL TRANSMISSION OF THE INSTRUMENT THERMAL EMISSION POLARIZATION MODULATION POLARIMETRIC ACCURACY CALIBRATION OPTICS POLARIMETRIC ACCURACY TEMPORAL, SPATIAL AND SPECTRAL MODULATION SPATIAL SAMPLING SPATIAL FIELD OF VIEW MULTI-WAVELENGTH OBSERVATIONS IMAGE STABILITY REQUIREMENTS IMAGE ROTATION COMPENSATION CONTEXT IMAGING, REGISTRATION AND GUIDING OCCULTING ABSOLUTE PHOTOMETRIC AND SCATTERED LIGHT CALIBRATION... 9 SPEC-0056, Revision D Page iii

4 1. INTRODUCTION 1.1 PURPOSE This document defines the requirements for the Cryogenic Near-IR Spectro-Polarimeter. It is one of the first-light instruments for the ATST. 1.2 SCOPE This document covers all requirements of the Cryogenic Near-IR Spectro-Polarimeter (Cryo- NIRSP), including science requirements and some requirements derived from the science. Designers of the CRYO-NIRSP optical, mechanical, electronic, and software systems subsystems should use this document. 1.3 RESPONSIBLE AUTHOR Changes to this document are to be coordinated with the responsible author. 1.4 APPLICABLE DOCUMENTS [1] ATST Design and Development proposal [2] ATST Science Requirements Document, ATST SPEC-0001 [3] ATST ViSP Instrument Science Requirements Document, ATST SPEC-0055 [4] Collados, M. 1999, in ASP Conf. Ser. 184, Third Advances in Solar Physics Euroconference: Magnetic Fields and Oscillations, 3-22 [5] Penn, M.J., Lin, H., Tomczyk, S., Elmore, D., Judge, P. Background Induced Measurement Errors in the Coronal Intensity, Density, Velocity and Magnetic Field Solar Physics, 2004, 222, p61. [6] Penn, M. and the ATST Coronal Working Group, ATST Background Induced Measurement Errors in the Coronal Intensity, Density, Velocity and Magnetic Field, ATST RPT ABBREVIATIONS AND ACRONYMS ATST AO Cryo-NIRSP FOV GOS I c Advanced Technology Solar Telescope Adaptive optics Cryogenic Near-IR Spectro-Polarimeter Field Of View Gregorian Optical System Continuum intensity SPEC-0056, Rev D Page 1 of 9

5 2. MISSION The primary purpose of the Cryogenic Near-IR Spectro-Polarimeter (Cryo-NIRSP) is the study of solar coronal magnetic fields over a large field-of-view at near- and thermal-infrared wavelengths. Cryo-NIRSP will measure the full polarization state (Stokes I, Q, U and V) of spectral lines originating on the Sun at wavelengths from 1000 nm (500nm goal) to 5000 nm. It is the only ATST instrument with the capability of sensitively imaging the relatively faint infrared corona and the thermal infrared solar spectrum. Cryo-NIRSP depends on the full coronagraphic capabilities of ATST to observe both the near-limb (using ATST s prime-focus and secondary occulting) and the more distant corona and heliosphere. Its thermal infrared capabilities allow sensitive study of the solar disk in the CO lines. Near-limb capabilities allow unique observations of spicules, prominences, flares, and eruptive events in the low corona. SPEC-0056, Rev D Page 2 of 9

6 3. EXAMPLE SCIENCE CASES Some examples of the science topics to be addressed with Cryo-NIRSP are found in the ATST SRD and are listed below. The primary topic is listed in bold, while some predictable secondary topics, which will benefit from Cryo-NIRSP observations are listed in italics. Cryo-NIRSP and ATST in coronagraph mode are qualitatively new solar research capabilities that are not available anywhere -- we expect non-incremental, discovery science to result. SRD number Title Spatial res. (arcsec) Coronal Magnetic Fields Requirement (Goal) Spectral res. (λ/δλ) Requirement (Goal) Cadence (sec) 1.0 (0.5) 3x10 4 Few Coronal Velocity & Density in Loops Coronal Intensity Fluctuation Chromosphere heating and dynamics (CO observed on the disk) Prominence and spicule formation and evolution (Halpha and HeI near limb) 1.0 (0.5) 3x (0.5) 5 x (0.15) 1x10 5 (2x10 5 ) (0.5) 5x10 4 (1x10 5 ) 60 An instrument with a spatial pixel size of about 0.15 arc seconds and a spectral resolution of 1x10 5 will satisfy all science requirements. For the low-resolution coronal observations, requirements are appropriately met by binning by 2 or 3 in either or both dimensions. Stabilized images with image scanning and a slit spectrograph can meet temporal cadence requirements. SPEC-0056, Rev D Page 3 of 9

7 4. GENERAL FEATURES The CRYO-NIRSP will consist primarily of (a) a polarimeter, which performs the separation of the polarization state of the incident sunlight, (b) a spectrograph, which disperses the incident sunlight into a spectrum, (c) an IR camera, which records the polarized spectra provided by the polarimeter and the spectrograph and (d) an IR context camera, which images the infrared coronal emission surrounding the field-of-view of the spectro-polarimeter. Sensitive instrument performance depends on achieving a relatively low-emissivity background from the instrument surroundings and from low-emissivity ATST mirrors. Coronal performance depends on effective two-stage occulting of out-of-field light from the ATST optical train. 4.1 IMAGE STABILITY AND OCCULTING The Cryo-NIRSP requires image stabilization at the level of its 0.5 arcsec pixels. This is essential for useful long integrations and for scanning the image across the slit. Image stability is also essential for near-limb observations, as small image shifts with respect to the Gregorian occulter can produce large brightness variations from scattered light. Observations far into the corona must depend on the telescope tracking stability. We assume the near-limb stability will be achieved with a Gregorian light pick-off (or dichroic) that provides a stabilized image using M2 steering. At the Gregorian focus 0.1 arcsec tip-tilt image stability is desirable. In most circumstances the prime-focus occulter will reduce the total energy in this field to less than 0.1% of the unocculted field. An articulated secondary occulter must provide final occulting of the M2- stabilized image at the Gregorian focus. The final optical relay from the telescope into the spectrograph must present a stable, translatable, image to the Cryo-NIRSP. 4.2 FLAT-FIELDING AND WAVELENGTH CALIBRATION The flat-field calibration unit will produce a homogeneous illumination (i) by using continuum and fixed-wavelength lamps at selected wavelengths, and (ii) by moving the grating to remove solar spectral lines. Stable fixed-wavelength flat-field illumination will provide wavelength calibration over the full wavelength range of the Cryo-NIRSP 4.3 POLARIZATION CALIBRATION The Cryo-NIRSP requires a polarization calibration unit. Both the polarimeter and the calibration units will be located upstream near the telescope secondary focus, before most of the strong polarizing optical elements in the optical path. 4.4 CONTEXT IMAGER A context imager will be required to establish the spectrograph pointing relative to coronal or other solar structures, and is the only ATST coronal imaging instrument. The FOV should match the spectrograph FOV of 3 arcmin, and spatially oversample the image. The imager should have a filter system to observe specific coronal emission lines or continuum channels. 4.5 IMAGE SCANNING There must be control provisions in the optical relay between the telescope and the Cryo-NIRSP for steering the input beam so that the occulted field-of-view of the telescope can be scanned across the slit. SPEC-0056, Rev D Page 4 of 9

8 4.6 SAFETY MECHANISM The instrument must have a hardware safety mechanism that protects the context imager and spectrograph from sudden intensity increases associated with unexpected telescope re-pointing onto the disk. 4.7 DATA DISPLAYS The instrument must provide real-time user feedback in the form of graphic data displays, including context images, raw spectrograph images, Stokes parameter images, and maps of a scanned region at a selected Stokes parameter and wavelength. SPEC-0056, Rev D Page 5 of 9

9 5. REQUIREMENTS 5.1 SPECTRAL COVERAGE The Cryo-NIRSP will operate across the continuous spectrum from 1000nm (to observe the corona at 1075nm and the chromosphere at 1083nm.) to 5000 nm (e.g., molecular lines of CO 4666nm). Though subject to relatively higher levels of scattered light, useful science could be obtained at visible, nm and nm, emission lines. Requirement: Spectral coverage from 1000 nm to 5000nm Goal: 500nm to 5000nm Source: ATST Proposal 5.2 SPECTRAL RESOLUTION To properly resolve high temperature and low temperature spectral lines from the corona different spectral resolutions (and consequently dispersions) are needed. A single resolution of 30,000 combined with detector binning in the spectral dimension should provide a compromise, which will adequately address all main coronal science topics. For on-disk observations of the CO lines, resolution of at least 100,000 is required. Requirement: Spectral resolution 30,000 for coronal observations, 100,000 for disk observations Goal: Spectral resolution of 200,000 for on-disk observations Source: SRD 3.2.2, 3.2.4, 3.2.5, 3.2.7, TOTAL TRANSMISSION OF THE INSTRUMENT The coronal faintness makes it of primary importance to maximize the photon throughput of the instrument (telescope + polarimeter). As a guide, a total transmission larger than 10% is desirable Requirement: Total transmission of the instrument 10% Goal: Total transmission of the instrument 30% Source: SRD THERMAL EMISSION The thermal emission from the telescope and instrument combination detected by the instrument spectro-polarimetric camera must be minimized to low levels, below the level of a coronal emission intensity of 10 millionths of disk center brightness. Requirement: Low background emission at 3934nm (order 10 millionths disk brightness) 5.5 POLARIZATION MODULATION Polarization modulation must be provided ahead of the spectrograph slits. Provisions for geometric beam splitter image registration and calibration must be provided. Rotating wave plates as necessary to cover the operating wavelength range must be integrated into the telescope and system control. SPEC-0056, Rev D Page 6 of 9

10 5.6 POLARIMETRIC ACCURACY The amplitude of each element M ij of the overall Mueller matrix M (telescope plus polarimeter) must be calibrated to a relative accuracy ij at all time, i.e. M ij (1 ± ij), to get an absolute polarimetric accuracy of 5x10-4 I c, regardless of whether M is constant with time or not. The matrix gives the maximum absolute values of the relative uncertainties in M, 10 5x10 5x10 5x x10 5x x10 2 5x x10 5x Requirement: better than 5x10-4 Goal: 5x10-5 Source: SRD CALIBRATION OPTICS POLARIMETRIC ACCURACY The calibration optics should allow for accurate measurement of the polarization property of the telescope and the instrument to reach the polarimetric accuracy expressed by the error matrix shown above. Requirement: better than 5x10-4 Source: SRD 5.5 Requirement: 5x10-4 I c polarimetric accuracy of the calibration optics Goal: 5x10-5 Source: SRD 5.8 TEMPORAL, SPATIAL AND SPECTRAL MODULATION The modulation frequency requirement is largely determined by the design of the polarimeter. For a single beam system, the requirement for the modulation frequency is in the range of khz to tens of khz. In a dual beam system, however, this requirement can be relaxed considerably. The polarimeter thus should include a polarizing beam splitter that would provide two orthogonally polarized fields-of-view, and the instrument will provide for an optical swapping of these polarized fields, through the use of a polarization modulator such as a rotating retarder, to address seeing-induced and other systematic polarization crosstalk. In any case, the modulation frequency will be the highest possible depending on the technology development, not being less than 10 Hz (full images per second). Finally the instrument should be able to switch wavelength with the spectrograph within a period of about 10 seconds. This operation might include changing a grating angle or changing a blocking filter. Requirement: Polarimeter: Dual beam with 10 Hz frame rate; <10 sec wavelength change Goals: Change wavelengths in 1 second, 80Hz frame rate SPEC-0056, Rev D Page 7 of 9

11 5.9 SPATIAL SAMPLING The Cryo-NIRSP science topics present a variety of desired spatial resolutions. A spatial dispersion of 0.5 arc second/pixel will address most of the coronal science goals. Disk mode sampling should be at the diffraction limit for 4.7 micron wavelength. Requirement: 0.5 arc second/pixel size for coronal observations, 0.15 arc seconds sampling for disk observations (at 4.7 micron) Source: SRD 3.2.2, 3.2.4, 3.2.5, 3.2.7, SPATIAL FIELD OF VIEW The NIRSP will have the possibility to observe large fields of view, 4 arc minutes parallel to the limb and at least 3 arc minutes perpendicular to the limb. The goal is access to a full 5 arc minute FOV. Disk mode observation may be limited to 1.5 arc minutes FOV. Requirement: Coronal mode 4 arc minutes parallel to the limb, 3 arc minutes perpendicular to the limb. Disk mode, 1.5 arc minutes square. Goal: 5 arc minutes Source: SRD 3.2.2, 3.2.4, 3.2.5, 3.2.7, MULTI-WAVELENGTH OBSERVATIONS Some observations would require multiple wavelength regions to be observed with this instrument, such as 1075nm, 1080nm, 1431nm, and 3934nm coronal observations and 1565nm and 4666nm for disk observations. This instrument must have the ability to observe these different wavelength regions in succession (with about 10 sec cadence as discussed in 5.8 above). The spectrograph must be designed to efficiently access the highest priority emission lines. Requirement: Multiple wavelengths observable at high efficiency Priority: IMAGE STABILITY REQUIREMENTS The Cryo-NIRSP, with 0.5 arc second/pixel, requires image stabilization. The instrument should be provided with a tip-tilt corrected beam for both on-disk and near-limb observations. For nearlimb observations, the image stabilization should occur before the final occulting, which should be located at the Gregorian focus, and the solar limb may be used to provide a tracking signal. Requirement: Image stability with < 1.0 arc second root mean square Goal: image stability < 0.1 arc second root mean square Source: SRD 3.2.2, 3.2.4, 3.2.5, 3.2.7, IMAGE ROTATION COMPENSATION Image rotation compensation at the Coudé Lab is required. Requirement: Image rotation must be limited to less than 0.25 degrees peak-to-peak over one hour. Priority: 2 Source: Source: 3.2.2, 3.2.4, 3.2.5, 3.2.7, SPEC-0056, Rev D Page 8 of 9

12 5.14 CONTEXT IMAGING, REGISTRATION AND GUIDING In order to verify the observed field of view (and for some direct science observations) Cryo- NIRSP requires a mechanism for directing the light beam ahead of the slit or reflected from the slit to an IR sensitive imager. This imager should adequately sample the field of view of the spectrograph and should allow for filtered observations at the particular wavelengths of coronal emission lines contained within the wavelength range of the Cryo-NIRSP system. Requirement: Context imager will sample the coronal field of view with 0.5 arc seconds per pixel Goal: To achieve 0.15 arc seconds per pixel spatial sampling across the full field of view of disk observations Priority: 2 Source: 3.2.2, 3.2.4, 3.2.5, 3.2.7, OCCULTING Partial occulting will be provided by the limited field of view at the prime focus aperture. An additional occulter is required at the stabilized Gregorian image plane. The occulter will intercept a segment of the solar disk; the segment is to have a chord length of 4 arc minutes coincident with the long edge of the instrument field of view, and a radius of 16 arc minutes. The occulter must be capable of rotation about the center of view (optical axis) to provide access to any position on the solar limb. Requirement: Occulter at Gregorian image plane consisting of a circular segment with radius equivalent to 16 arc minutes on the sky and chord at least 4 arc minutes. Center of arc will be offset by 82.5 arc seconds from the optical axis, along any azimuth. It must be possible to underor over-occult by 5 arc seconds. Source: 3.2.4, 3.2.5, 3.2.7, ABSOLUTE PHOTOMETRIC AND SCATTERED LIGHT CALIBRATION Cryo-NIRSP needs to be able to observe stellar objects in order to calibrate the photometric and scattered light characteristics of the instrument. Requirement: Ability to observe stellar objects. Source: 3.2.4, 3.2.5, 3.2.7, SPEC-0056, Rev D Page 9 of 9

13 Request for Waiver RFW Number: 0048 Date Requested: 7-November-2013 RFW Title: Cryo-NIRSP field of view vignetting Contract No: 22023S Requestor: David Elmore WBS: Document/Drawing Infringed Upon: SPEC-0056 Cryo-NIRSP ISRD Summary of Issue: Unable to span full field without vignetting Next Higher Item Level Affected: N/A Items External to Contract Affected: Science performance Proposed Change and Justification: The Cryo-NIRSP ISRD requirement 5.10 calls for: Requirement: Coronal mode 4 arc minutes parallel to the limb, 3 arc minutes perpendicular to the limb. Disk mode, 1.5 arc minutes square. The modulator for the Cryo-NIRSP is in the converging beam about 550mm in front of the slit and outside the cryostat for thermal and mechanical reasons. A slightly undersized aperture vignettes the field of view while still permitting observations to the field edges. The clear aperture was originally specified to be 118mm to match the diameter of the calibration optics at the GOS. After the vendor informed us that the largest crystal optics they had ever fabricated at this thickness was 113mm clear aperture, the calibration optics were moved from the 'retarder' stage to the 'modulator' stage at the GOS since there are no longer have modulators at the GOS and thereby reduced the clear aperture to 105mm for the GOS optics. The Cryo team was asked to evaluate the consequence of a 105mm clear aperture in place of 118 for the Cryo-NIRSP modulator. Here is the response from 22 July. David and Tom, I didn't realize Don was out of Africa, but he's back and fired up zemax. With a 105mm CA modulator we'd see 50% vignetting at the edge of a 5' diameter field and 14% light loss at the 4' edge of our slit field. At 3' there is no vignetting loss. Thus, I find it hard to argue from a science requirements viewpoint that we will qualitatively suffer with your smaller (105mm) modulator. Jeff The SWG looked at this vignetting issue and concluded: the SWG does not anticipate that the proposed field of view and wavelength restrictions will have any negative impact on the critical first- instrument generation ATST science 1

14 Request for Waiver RFW Number: 0048 Corrective Actions Already Attempted: Moving the rotator into the cryostat has the consequence of a redesign of the rotator for vacuum operation, redesign of the cryostat to accommodate the rotator, and increased thermal pumping required of the Cryo-NIRSP. Documents Attached: Waiver, if granted, Adversely Affects: Performance: yes Reliability: Cost: Dimensions: Safety: Software: Weight: Maintenance: Other Risk: Impact if waiver not granted: The project would incur risk and cost increase due to the requirement of polishing crystals of an aperture larger than ever attempted. Concessions Offered: If optical technology permits, a full aperture modulator could be implemented in the future using the current rotator. Though the clear aperture fully meeting spec will be 105mm, the full aperture will be at least 10mm greater so that the light loss will be less than the estimate though polarization modulation performance may not meet specification. Please note: Both parties must sign to acknowledge acceptance of this waiver. Contractor Project Manager Work Package Manager Signature Date Signature Date Change Control Board Decision: APPROVED ADMINISTRATIVE USE ONLY Approval Date: Rejected Date: 15-November