THE DEEP LAMP PROJECT: AN INVESTIGATION OF THE PRECISION AND ACCURACY OF THE ECHELLE WAVELENGTH SCALES OF SPACE TELESCOPE IMAGING SPECTROGRAPH

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1 The Astrophysical Journal Supplement Series, 177:626Y644, 2008 August # The American Astronomical Society. All rights reserved. Printed in U.S.A. A THE DEEP LAMP PROJECT: AN INVESTIGATION OF THE PRECISION AND ACCURACY OF THE ECHELLE WAVELENGTH SCALES OF SPACE TELESCOPE IMAGING SPECTROGRAPH Thomas R. Ayres Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO Received 2007 December 20; accepted 2008 February 27 ABSTRACT The precision and absolute accuracy of the echelle mode wavelength scales of Space Telescope Imaging Spectrograph (STIS) are investigated. The method is to measure deep exposures of the onboard Pt/CrYNe hollow cathode calibration lamp. The standard deviation of emission spots from their laboratory wavelengths in a single image is a measure of the internal precision of the pipeline-assigned scales. The average shift of the image as a whole is a measure of the absolute accuracy. While systematic patterns can be identified in all four echelle modes ( E140M, E140M, E230M, and E230H), the overall precision (even without compensating for long-range trends with k) is excellent: of order one-tenth of the resolution element ( 600 and 300 m s 1, for medium- [M] and high- [H] resolution modes, respectively). Furthermore, the absolute accuracy and its repeatability (assessed in a time series of WAVECAL images) is of order a remarkable 100 m s 1, aside from one of the E230M modes (secondary tilt k2269) that shows a systematic offset 10 times larger. The excellent precision of the STIS echelle wavelengths could be improved by adding higher order terms to the biquadratic polynomial currently implemented in the CALSTIS pipeline. On the other hand, the existing small distortions might be resolved more naturally by a physical instrument model, currently under development by the Space Telescope European Coordinating Facility s STIS Calibration Enhancement Project. Subject headinggs: instrumentation: spectrographs methods: data analysis ultraviolet: general Online material: color figures 1. INTRODUCTION There is a long and storied history of high-resolution UV spectrographs in space, beginning with Copernicus in the mid- 1970s, the International Ultraviolet Explorer in the late 1970s, Hubble Space Telescope s Goddard High-Resolution Spectrograph in the early 1990s, and Space Telescope Imaging Spectrograph in The Far-Ultraviolet Spectroscopic Explorer joined the crowd in IUE was a big advance in its day, because it combined a novel cross-dispersed echelle optical design with large-format cameras albeit TV-style vidicons, primitive by today s standards to capture broad spectral regions in a single exposure. IUE also pioneered the use of platinum-neon (Pt-Ne) hollow cathode lamps to provide an on-orbit calibration of the wavelength scales, to mitigate systematic shifts from short-term thermal flexing of the optical bench, or longer term secular changes due to, say, outgassing from the graphite-epoxy structure (used in all major UV spectrographs from IUE onward). Although HST GHRS sported relatively small format Digicon detectors, and thus could not match the broad coverage of the IUE echelles, its highest resolution bested that of IUE by an order of magnitude and by a similar gulf in signal-to-noise ratio (S/N; in careful observations of the brightest objects). When STIS came along, UV camera technology finally had matured to the point that high-resolution cross-dispersed echelles could be reunited with large-format sensors to enable efficient broad spectral coverage even in the highest dispersion modes (Woodgate et al. 1998). And, unlike the IUE vidicons, the STIS Multi-Anode Microchannel Array (MAMA) cameras could achieve high S/N and dynamic range, with almost no detector background. 1 1 At least for the solar blind FUV MAMA; the NUV channel suffered from elevated background due to anomalous fluorescence in the camera faceplate. 626 As the most recent generation of high-dispersion UV spectrographs, STIS was the workhorse for many areas of observational space astronomy including cool and hot stars, the interstellar and intergalactic medium, planetary atmospheres and surfaces, supernova remnants, active galactic nuclei, protostellar disks, and so forth. Its failure in summer 2004 has been a devastating loss to contemporary ultraviolet cosmic research. In HST Cycle 13, I was awarded a calibration observing program TheDeepLampProject totestthedifferential stability of the STIS echelle formats under varying environmental conditions encountered during the routine operations of the HST. I had been surprised to learn that the type of deep lamp exposures used to populate the coefficients of the STIS dispersion relations have been taken only a handful of times during the mission, about annually. This compares with IUE, forwhich such calibrations were taken biweekly. It is true that STIS is a much more stable instrument than the IUE spectrographs, thanks partly to its advanced design and partly to a far better controlled thermal environment. Nevertheless, there might be small but systematic distortions of the echelle formats, say due to short-term thermal effects, which would not be easily recognized in the existing sparse, widely spaced calibrations. My interest in the question was partly historical: nearly two decades earlier I and my colleagues carried out an analogous characterization of smallscale geometrical distortions of the IUE SWP-HI echelle mode (1150Y2000 8), based on deep exposures of the onboard Pt-Ne lamp (Ayres et al. 1988). The Cycle 13 proposal was to use a hierarchical time sequence of long-duration lamp exposures in three representative echelle modes. Each deep observation would be accompanied by the normal WAVELINE (a brief lamp exposure routinely recorded with every motion of the STIS grating mode-select mechanism [MSM]). Measurements of the deep images, processed as if they

2 DEEP LAMP PROJECT 627 were science frames, would test for local distortions of the wavelength scales (precision) and how well the global equivalent velocity shift matched the null expectation (accuracy). Shortly before the first series of Deep Lamp observations, however, STIS suffered its second major electrical failure, putting the instrument out of commission and the Cycle 13 project in limbo. Yet the importance of the proposed calibration work still remained. Although STIS was dead, its archive was very much alive and provided a way to carry out most of the objectives of the original project. There were, in fact, a number of STIS calibration efforts that had obtained sequences of deep lamp exposures in many of the standard observing modes, such as program (STIS MAMA Cycle 12 Deep Wavecals; PI: A. Aloisi) in the 2003Y2004 time frame and 9618 (MAMA Dispersion Solutions; PI: J. Valenti) the previous year. There were other examples as well. The exposures of these programs were taken in many different echelle settings, rather than the small group of representative ones in the proposed Cycle 13 effort. Nevertheless, they did fully sample the hierarchical range of time displacements over which distortions in the echelle format might be anticipated, a key objective of the Cycle 13 project. And while it is more convenient to have a small uniform set of wavelength settings to ferret out such distortions, in practice any echelle WAVECAL exposure can be used for the purpose, since the fundamental diagnostic is the pattern of lamp spots on the detector. Each different pattern samples a unique set of physical locations in the focal plane, but nevertheless, any persistent geometrical distortions contributed by the optical path or camera should be traceable regardless of what echelle setting is employed (as long as the image field is well populated by a sufficiently dense pattern of emission spots; that is why the Cycle 13 proposal called for very deep lamp exposures and, hence, the name of the project). This is a report of results from the replacement Deep Lamp archival program. First, a related effort by the HST European Coordinating Facility ( ECF) is summarized; next, a selection of suitable lamp images from the archive is described; then, a discussion of the processing of the frames, the co-addition and concatenation of spectra to improve S/ N, and the measurement of the emission lines; and finally, a description of the findings. 2. THE ECF STIS-CE EFFORT The HST ECF is carrying out a program to refine the STIS dispersion relations using a powerful physical instrument model approach, rather than the empirical polynomial formula used in the existing CALSTIS pipeline. 2 The STIS Calibration Enhancement project (STIS-CE: Kerber et al. 2006a) builds on a successful upgrade of the dispersion relations of HST s Faint Object Spectrograph and includes a laboratory campaign to accurately measure the hollow cathode spectrum for the specific STIS lamp design. 3 An earlier experimental effort had been undertaken in the late- 1980s at the then National Bureau of Standards (Reader et al. 1990) using a modified Pt-Ne lamp (with a copper cathode to 2 The STIS dispersion relations are implemented in a biquadratic interpolation formula that has its roots in the IUE Spectral Image Processing System ( IUESIPS), and was used by GHRS as well. The scheme represents the dispersion coordinate S ( sample ) by a polynomial in order number m and wavelength k, with terms up to (mk) 2. 3 The STIS lamps differ from the earlier IUE light sources in incorporating a small admixture of chromium in the platinum cathode, to provide better emission spot coverage in the NUV and visible wavelengths (3000 Y5000 8) wherethe Pt I, II spectrum (and that of the carrier gas neon) is sparse. provide an absolute reference via the Cu ii spectrum), in anticipation of the much higher accuracy that would be required for the HST spectrographs compared with the lower resolution (R 10 4 ) IUE echelles (and considering that the best-available laboratory measurements of the far-uv platinum spectrum at the time dated back to the 1930s [Shenstone 1938]). The STIS-CE group has published an initial catalog of Pt/CrYNe wavelengths (Sansonetti et al. 2004), encompassing some 1200 features in the FUV interval 1132Y with an estimated uncertainty of 2 m8 (400 m s 1 in equivalent velocity units). An extension to the NUV region is in progress (Kerber et al. 2006b). 3. THE DEEP LAMP SAMPLE The standard WAVELINE, to accompany an MSM motion, is 10 s regardless of mode. This is long enough to ensure adequate S/N on at least one or a few bright lines in the echelle pattern at any FUVor NUV central wavelength setting, so the zero-point offset can be established accurately. However, the 10 s exposure is too short to permit a detailed evaluation of the rich Pt/CrYNe emission spectrum. Thus, I restricted consideration to lamp exposures of roughly an order of magnitude or more longer. A perusal of the STIS archive identified about 200 echelle mode data sets meeting this criterion. Among these were a number of very deep observations obtained in Cycle 12 program 10031, alluded to earlier, but which consisted of multiple subexposures. These are less convenient to treat in the modified CALSTIS processing to be described later, so for simplicity I further restricted the sample to single exposures. The selected data sets, numbering 158 total, are distributed among the E modes as follows: 34 E140M, 33 E140H, 21 E230M, and 70 E230H. All of the prime tilts are represented ( E140MY1425; E140HY1234, 1416, 1598; E230MY1978, 2707; E230HY1763, 2013, 2263, 2513, 2762, 3012), and several of the secondary settings as well. The sample is summarized in Table 1. Prime tilts are in boldface. Most of the exposures were acquired with the default high res aperture for that mode: 020 ; for the FUV and NUV M modes, 020 ; 009 for E140H and 010 ; 009 for E230H. I also included several WAVECALs taken through the 020 ; 020 photometric aperture (13 E140M), which commonly was used in stellar FUV observations owing to its higher throughput, and a few in the tiny 010 ; 003 Jenkins slot (two E140H and one E230M), which occasionally was exploited to achieve the highest possible resolution, say for bright interstellar line sources. These alternative aperture combinations potentially can test aspects of wavelength distortion that might not be accessible to a sample consisting of solely the default slots. For example, unlike a stellar point source, the Pt/CrYNe lamp image is diffuse, so the emission completely fills the 020 ; 020 aperture. The resulting line profiles have a more boxy appearance than the nearly Gaussian shapes recorded through the default aperture and, accordingly, have a larger spatial footprint on the camera; possibly sampling any local geometric distortions in a different way than the narrow slots. 4. PROCESSING AND MEASUREMENT Raw FUV and NUV MAMA frames were retrieved from the Multimission Archive at Space Telescope ( MAST). Examples of the four independent echelle modes are illustrated in Figures 1aY1d. The level 0 echellegrams were subjected to a series of processing, postprocessing, and measurement steps as described below. 4 Referring to the aperture of height perpendicular to the dispersion, and width in the dispersion direction.

3 TABLE 1 Pt/CrYNe Lamp Exposures in STIS Echelle Modes Dataset ModeYk cen Aperture a ( yr) U.T. Date t exp (s) Medium Resolution Far-UV Echelle (R 4.5 ; 10 4 ) I max (counts) o40103jnm... E140MY ; o40103jrm... E140MY ; o40103jvm... E140MY ; o40103jzm... E140MY ; o40103k3m... E140MY ; o40103k7m... E140MY ; o40102wnm... E140MY ; o40102wrm... E140MY ; o40102wvm... E140MY ; o40102wzm... E140MY ; o40102x3m... E140MY ; o40102x7m... E140MY ; o4bl12h2q... E140MY ; o4nr01dkq... E140MY ; o4nr51dcq... E140MY ; o5j201ivq... E140MY ; o67w02ixq... E140MY ; o69i01o8q... E140MY ; o5j251xcq... E140MY ; o6ij03nuq... E140MY ; o8gs09ymq... E140MY ; o47k01ctm... E140MY ; o47k04u1q... E140MY ; o47k04uvq... E140MY ; o47k04voq... E140MY ; o47k04wkq... E140MY ; o47k04x4q... E140MY ; o47k05kmq... E140MY ; o47k05lfq... E140MY ; o47k05lsq... E140MY ; o4bl12h4q... E140MY ; o4pg02qiq... E140MY ; o4pg02qmq... E140MY ; o4vt02ntq... E140MY ; High-Resolution Far-UV Echelle (R 1.1 ; 10 5 ) o4bl13hnq... E140HY ; o8gs01loq... E140HY ; o8gs01lpq... E140HY ; o4nr03i6q... E140HY ; o4nr53caq... E140HY ; o5j203iyq... E140HY ; o69i03olq... E140HY ; o5j253xxq... E140HY ; o6ij01lfq... E140HY ; o8gs02tzq... E140HY ; o47s01jrm... E140HY ; o47s01k7m... E140HY ; o47s01kbm... E140HY ; o47s01kfm... E140HY ; o8gs03m6q... E140HY ; o4nr01dmq... E140HY ; o4nr51ddq... E140HY ; o5j201iwq... E140HY ; o67w01o2q... E140HY ; o69i01o9q... E140HY ; o5j251xdq... E140HY ; o6ij02hpq... E140HY ; o8gs05vqq... E140HY ; o4bl16yjq... E140HY ; o4nr02neq... E140HY ; o4nr52j0q... E140HY ; TABLE 1 Continued Dataset ModeYk cen Aperture a (yr) U.T. Date t exp (s) I max (counts) o5j202meq... E140HY ; o69i02g6q... E140HY ; o5j252vpq... E140HY ; o6ij04htq... E140HY ; o8gs04mpq... E140HY ; o4ii06crq... E140HY ; o67w01o0q... E140HY ; Medium Resolution Near-UV Echelle (R 3.0 ; 10 4 ) o4ii01biq... E230MY ; o4nr04ngq... E230MY ; o4nr54mnq... E230MY ; o5j204ilq... E230MY ; o69i04khq... E230MY ; o5j254wcq... E230MY ; o6ij05mdq... E230MY ; o8gs20hoq... E230MY ; o4bl17xnq... E230MY ; o67w04juq... E230MY ; o8gs34gsq... E230MY ; o4ii01bkq... E230MY ; o4nr04niq... E230MY ; o4nr54moq... E230MY ; o5j204imq... E230MY ; o69i04kiq... E230MY ; o5j254wdq... E230MY ; o6ij11flq... E230MY ; o8gs36u5q... E230MY ; o4bl17xjq... E230MY ; o67w04jqq... E230MY ; High-Resolution Near-UV Echelle (R 1.1 ; 10 5 ) o4ii02rkq... E230HY ; o8gs27ogq... E230HY ; o8gs27oiq... E230HY ; o8gs28ouq... E230HY ; o8gs28owq... E230HY ; o4nr05kyq... E230HY ; o4nr55d5q... E230HY ; o5j205ihq... E230HY ; o69i05otq... E230HY ; o5j255siq... E230HY ; o6ij02hqq... E230HY ; o8gs25hwq... E230HY ; o8gs26woq... E230HY ; o6ij06faq... E230HY ; o8gs24htq... E230HY ; o8gs22h5q... E230HY ; o4bl18lkq... E230HY ; o8gs33zxq... E230HY ; o8gs19baq... E230HY ; o8gs35tiq... E230HY ; o8gs19bcq... E230HY ; o8gs18t4q... E230HY ; o4ii03s4q... E230HY ; o4nr06lwq... E230HY ; o4nr56dtq... E230HY ; o5j206mbq... E230HY ; o69i06g3q... E230HY ; o5j256xfq... E230HY ; o6ij03ntq... E230HY ; o8gs34guq... E230HY ; o4bl19dcq... E230HY ; o8gs32trq... E230HY ;

4 DEEP LAMP PROJECT 629 TABLE 1 Continued Dataset ModeYk cen Aperture a (yr) U.T. Date t exp (s) I max (counts) o67w03j8q... E230HY ; o8gs18t6q... E230HY ; o6ij07jdq... E230HY ; o8gs16daq... E230HY ; o8gs15pnq... E230HY ; o8gs15poq... E230HY ; o8gn10mpq... E230HY ; o8gn10n4q... E230HY ; o8gn10nfq... E230HY ; o8gn10nxq... E230HY ; o8gn10o2q... E230HY ; o8gs17sfq... E230HY ; o8gn50ljq... E230HY ; o8gn50ltq... E230HY ; o8gn50m0q... E230HY ; o8gn50m5q... E230HY ; o8gn50m9q... E230HY ; o8gs23g4q... E230HY ; o8gs20hqq... E230HY ; o6ij12hlq... E230HY ; o8gs21h0q... E230HY ; o8gs29phq... E230HY ; o4ii04sjq... E230HY ; o4nr56duq... E230HY ; o5j206mcq... E230HY ; o4nr06lyq... E230HY ; o69i06g4q... E230HY ; o5j256xgq... E230HY ; o6ij05meq... E230HY ; o8gs31t3q... E230HY ; o4ii05s2q... E230HY ; o8gs30ugq... E230HY ; o6ij11fmq... E230HY ; o8gs29piq... E230HY ; o8gs36u6q... E230HY ; o8gs36u8q... E230HY ; o4ii04slq... E230HY ; o8gs31t2q... E230HY ; Note. Values in boldface represent prime tilts (see text). a Aperture h ; w refers to the slot of height h (in units of 10 mas) perpendicular to the dispersion, and width w (in same units) in the dispersion direction. 5 See: HST Data Handbook for STIS online at /stis/ documents/ handbooks/currentdhb/stis_ longdhbtoc.html. 6 The procedure is described in more detail by Hodge et al. (1998) OTF CALSTIS Pipeline The zero-point registration, background subtraction, spectral extraction, and flux calibration of the level 0 raw two-dimensional counts images yielding a series of one-dimensional tracings of flux density, f k (ergs cm 2 s ), versus wavelength, k (8), for the individual echelle orders, m was accomplished by the CALSTIS 5 pipeline. The processing sequence is driven by keywords in the level 0 FITS header. In order to reduce a lamp exposure as a science image, 6 one first creates a companion oxxxxxxxx_wav.fits fileasacopyoftheoriginal oxxxxxxxx_raw.fits image, and invokes that file name for the argument of the keyword WAVECAL in the FITS header. One then modifies the argument of keyword ASN_MTYP to SCIENCE to trick the software into treating the image as a normal science exposure. Next, one alters several other control switches from the WAVECAL default of OMIT to PERFORM, specifically excluding DOPPCORR, which convolves the reference files with the average orbital Doppler shift and HELCORR, which converts to heliocentric wavelengths. In order to simulate the on-the-fly (OTF) processing routinely applied to STIS spectra dearchived from MAST, one next replaces key filenames in the FITS header with the most recent versions available from the calibration database (CALDB), which in some cases also are a function of observation date. The reference files used here were those available from the CALDB as of 2007 April 1. Initially, all of the lamp exposures were processed using the unadulterated raw image as its own WAVELINE, even though a normal science image ordinarily is calibrated with a default lightly exposed (10 s) lamp frame. To test whether that strategy might introduce any systematic errors, a subset of the deep lamp spectra were processed with a WAVELINE derived from that exposure, but for which the original counts were divided by the factor (t exp /10) and rounded to integer values, to simulate the default exposure. The image copying, FITS header modifications, and call to the CALSTIS executable cs0 were implemented in an IDL procedure, and the resulting x1d spectra were postprocessed using custom software, also in IDL, as described in the next section Post Processing Three layers of postprocessing were carried out in this study: (1) the creation of a one-dimensional tracing of flux versus wavelength for a single observation; (2) the co-addition of onedimensional spectra from data sets acquired with the same echelle mode, central wavelength setting, and observing aperture; and (3) the concatenation of spectra from the same dispersion mode (e.g., H or M), but with different central wavelength settings. Step (1) is the most basic postprocessing required to yield a finished spectrum suitable for measurement. Step (2) produces a co-added spectrum of higher S/N, in which there is a certain amount of averaging over the detector geometry, since the MSM does not always place the echelle pattern in exactly the same y-location. Step (3) yields a broad coverage spectrum in which further averaging is accomplished by co-adding overlapping segments of adjacent k cen settings, whose wavelengths in common nevertheless were recorded at very different positions on the detector. Because of the progressive averaging, each collection of measurements in the hierarchical set will tell a different story concerning the origin of any systematic distortions in the wavelength scales To Merge (or not to Merge) the Orders The main output of the CALSTIS pipeline is the x1d file, which contrary to its name is a series of two-dimensional arrays wavelength, flux, and ancillary quantities such as data quality and photometric error wherein the x-columns correspond to sample number (like the horizontal coordinate in the raw image) and the y-rows correspond to the individual echelle orders. The usual strategy is to merge the orders to produce a truly one-dimensional tracing covering the full wavelength grasp of the mode. One thereby recovers S/ N by co-adding the fluxes in common to the overlapping but lower sensitivity regions on either side of the echelle blaze peak and averages over any wavelength-scale distortions that might be present at opposite sides of the detector; but at the expense of introducing interpolations that can smooth the spectra and compromise resolution, particularly if the different sides of the format experience wavelength distortions of opposite sign. In practice, the overlapping regions of adjacent echelle segments were combined by interpolating fluxes of order m onto the wavelengths in common with m 1, then combining the f k weighted

5 630 AYRES Vol. 177 Fig. 1. (a) Raw image of deep E140M medium-resolution echelle spectrum. The y-coordinate LINE and x-coordinate SAMPLE labels were adopted from the IUE SIPS terminology, which has partially carried forward to the present day (although referring to the scanning vidicon devices used in the earlier era). Higher echelle orders are at the bottom. Wavelengths increase from left to right in each order, and from bottom to top overall. Note that the lower orders (near the top) are more widely spaced. (b) E140H high-resolution echelle spectrum, taken through the super high-resolution Jenkins aperture. Again, the higher echelle orders are at the bottom, and wavelengths increase in the same pattern as for E140M. (c) E230M medium-resolution echelle spectrum. Now, the higher echelle orders are at the top, and wavelengths increase from top to bottom overall, although still left to right within each echelle order. Scattering halos appear around the brighter features. (d) E230H high-resolution echelle spectrum. The general pattern is the same as for E230M. by the monochromatic sensitivities s k (which vary smoothly with k according to the echelle blaze function, a bell-shaped curve peaking at the order center). A photometric error for each co-added flux was carefully propagated through the co-addition process. Of course, the order averaging defeats the objective to assess spatial distortions that might be present on opposite sides of the camera (such as the dislocations in the fiberoptic coupler of the IUE vidicons implicated in the Ayres et al. [1988] study), so a more primitive form of order merging was implemented in parallel; a one-dimensional spectrum was constructed simply by interleaving the flux-wavelength points in ascending k order, regardless of m, but assigning the appropriate m value to each entry to preserve its identity. Because this represents a trivial rearrangement of the original x1d order tracings, there is no loss of information Co-adding Similar Data Sets Data sets from the same echelle mode and central wavelength setting were co-added to produce a spectrum of higher S/N and to

6 No. 2, 2008 DEEP LAMP PROJECT 631 average over small-scale geometrical distortions (since the independent exposures naturally would cover a albeit small range of y-positions on the detector, owing to imperfect repeatability of the MSM). Deviations of lamp lines from their laboratory wavelengths in such averaged spectra must then result from stable moderate-scale geometrical distortions (in the optical train or camera), not compensated for by the dispersion model. The co-addition ordinarily would be preceded by a step to register each tracing to a common velocity zero point, say by crosscorrelation against a template. An initial examination of the processed WAVECAL spectra showed, however, that those fromthesamemodeandk cen setting displayed only very slight relative shifts, much smaller than the line widths. In fact, the self-wavecal step in the pipeline processing accomplishes an equivalent cross-correlation against a reference lamp template, but in the raw image instead of in the processed wavelength domain. If such deviations represent random uncorrelated offsets, then blindly co-adding a set of n such spectra p ffiffi should suppress any systematic shift in the final average by n. Since the apparent shifts were much smaller than the line widths, there would be no adverse impact on the point-spread function. In practice, each of the component spectra was interpolated onto an evenly spaced wavelength grid, which oversampled the pipeline scale by a factor of 2, then was weighted in the average according to the sum of the net counts in that spectrum. (The lamp output is somewhat erratic, and the total net counts is a fairer measure of the true exposure level than the exposure time.) The interpolation of all the spectra onto a more finely spaced reference grid ensured that each would be treated in an identical way, and would more cleanly capture any small relative shifts of the component spectra. For the order-resolved x1d spectra (retaining the m identities of the fluxes), the co-addition was carried out order by order, again adopting a uniform wavelength scale that oversampled the original grids. Following co-addition, the two-dimensional flux versus order spectra were interleaved into a one-dimensional spectrum as before, again associating each f k with its m of origin Concatenating Spectra The final step, mainly cosmetic, was to concatenate the coadded order-merged spectra from specific mode settings into a global trace of flux density versus wavelength. This was trivial for E140M, because the single prime setting (k1425) captures the full FUV band (1150Y1700 8), aside from a few small gaps between the lowest orders at the longer wavelengths. All the other mode combinations, however, had several to many segments that collectively covered either the FUVor NUV interval. Table 2 summarizes the concatenation schemes for each of the four modes, listing the wavelength ranges retained from each co-added segment, the full wavelength span of the final product, and the uniform wavelength grid spacing (corresponding to a dispersion of 5:0 ; 10 5 for the H modes, and 2:5 ; 10 5 for the M modes; i.e., 5 points per resolution element). The weighting of the f k values of the individual co-adds in the concatenation was according to the exposure time of the segment relative to the total exposure time at that wavelength in the sum. 7 Again, photometric errors were carefully propagated through the co-addition sequence. 7 Although I argued earlier that t exp can be a poor indicator of the true exposure level for an individual observation, owing to inconsistencies in the lamp output, the cumulative (t exp ) tot of a co-added spectrum is a fairer measure of the effective exposure level of the sum, thanks to the averaging over the somewhat variable lamp output in the subsamples of typically 8frames.Using(t exp ) tot as a weighting factor avoids the issue of trying to accurately match integration bandpasses of the partially overlapping segments if net counts were adopted as a relative weighting factor. TABLE 2 Co-addition/Concatenation Schemes ModeYk cen Aperture t exp Wavelength Range Medium Resolution FUV Echelle: 1150Y [k = 5 m8] E140M ; Y1730 High-Resolution FUV Echelle: 1150Y [k = 2.5 m8] E140H ; Y1320 E140H Y1350 E140H Y1390 E140H Y1510 E140H Y1680 Medium Resolution NUV Echelle: 1610Y [k = 10 m8] E230M ; Y2330 E230M Y2670 E230M Y3070 High-Resolution NUV Echelle: 1640Y [k = 5 m8] E230H ; Y1893 E230H Y1943 E230H Y1990 E230H Y2040 E230H a 1845Y2096 E230H Y2142 E230H a 1942Y2190 E230H a 1985Y2240 E230H a 2034Y2291 E230H a 2083Y2338 E230H Y2389 E230H Y2434 E230H Y2486 E230H Y2538 E230H Y2590 E230H Y2638 E230H a 2440Y2693 E230H a 2494Y2748 E230H Y2791 E230H a 2586Y2830 E230H Y2875 E230H Y2927 E230H Y2985 E230H a 2792Y3040 E230H a 2830Y3087 E230H Y3145 a Single exposure. Figures 2a and 2b illustrate examples of the co-added/ concatenated lamp spectra based on the highest resolution echellegrams available for the FUV ( E140H series) and NUV (E230H series). The S/N at the peaks of the brightest features can exceed several hundred Measurements Spectra from the three different levels of co-addition/ concatenation were subjected to either fully automated or semiautomated line-fitting software, depending on specific objectives as described below. A semiautonomous system was applied mainly to the broad coverage, concatenated spectra of the four distinct echelle modes ( E140M/ H and E230M/ H), and to representative individual order-resolved mode/settings. The procedure was driven by a line list consisting of the Sansonetti et al. (2004) catalog for k < , based on laboratory measurements of Pt/CrYNe

7 632 AYRES Vol. 177 Fig. 2. (a) Example of merged co-added high-resolution FUV lamp spectrum, 1550Y The dashed curve is a long-range continuum fit. The lower dotted curve is the 1 photometric error; the upper curve is 5. Thin vertical lines mark lamp features whose laboratory wavelengths have been reported. Lighter shading marks members of the auto-fit line sample; these and the additional darker shaded features constituted the full sample measured semiautonomously. (b) NUV region, 2700Y The many unidentified emissions in this region mainly are Cr lines present in the STIS lamps, but absent from the hollow cathode sources measured in the original NIST laboratory work. [See the electronic edition of the Supplement for a color version of this figure.] lamps essentially identical to the STIS flight units; and the earlier Reader et al. (1990) study for longer wavelengths, using a Pt/CuY Ne light source and a Westinghouse Pt-Ne lamp similar to that flown on GHRS. Although the STIS lamps emit many Cr lines, which are not present in the Reader et al. list (e.g., Fig. 2b), the density of cataloged Pt and Ne lines in the NUV proved satisfactory for the current study. Improvements could be made, however, when the Sansonetti et al. extension to k > is available. For each entry in the list, the software examined the spectrum in the close vicinity of the laboratory wavelength, and attempted to fit a Gaussian profile to a set number of points on either side of the local flux maximum. If the software detected a close-by neighboring line, it adjusted the distribution of points to avoid the rise into the adjacent peak. The software displayed the fit and the operator decided whether or not to save it. When the procedure was applied to co-added spectra that preserved the m identity of the fluxes, a double fit was displayed for lines falling in the order-overlap zones, and the operator had the choice of saving the pair. Although in principle the procedure could have been run in an entirely automated mode, the human operator is very effective

8 No. 2, 2008 DEEP LAMP PROJECT 633 Fig. 2 Continued in recognizing subtle defects, especially faint blends affecting the line centroid (indicated by slightly asymmetric line shapes). Examples of lines flagged for retention by the semiautonomous procedure are shaded in Figure 2. Since the operator personally must evaluate several thousand line fits, there always is the question of the personal equation in the operator s choices ( here, erring on the side of including the maximum number of measurements). To guard against any influence of a small admixture of questionable fits, the large samples were screened post facto against criteria described later, prior to conducting statistical tests. Measurement errors were assigned to the line centroids according to the photometric associated with the local fluxes, and the Gaussian profile scaling law for the Poisson noise case described by Lenz & Ayres (1992). The rule of thumb is that the precision of the centroid measurement in critically sampled data is the line width (FWHM; the resolution in the case of the unresolved lamp features) divided by the peak S/N. Thus, a bright line can be centroided, in principle, to a small fraction of the resolution. The absolute accuracy of the laboratory wavelengths cited by Sansonetti et al. (2004) is 2 m8 (about 400 m s 1 ), excellent for the ultraviolet. Even so, any lamp line recorded at S/N > 10 in the H modes a common occurrence will best that limit. The software was run in a fully automated mode to fit the entire sample of 158 individual level 1 order-merged spectra.

9 634 AYRES Vol. 177 Fig. 3. (a) Wavelength displacements, expressed as equivalent Doppler shifts (in km s 1 ), of calibration lines measured in the order-merged, co-added, concatenated spectra of the four STIS echelle mode combinations. (b) Wavelength displacements as a function of measurement errors, v. Results in the region enclosed by horizontal lines and to the left of the vertical dot-dashed line, were retained for subsequent statistical evaluations. Clusters of at the lowest values represent very high S/ N measurements at the 0.01 m8 discretization limit of the Gaussian fits. However, below a 2 m8 threshold (400 m s 1 at ), uncertainties in the laboratory wavelengths will dominate the errors. [See the electronic edition of the Supplement for a color version of this figure.] A special line list was devised by scanning through the merged high-resolution FUV and NUV spectra, and selecting the bestquality cataloged features: bright, isolated, symmetric, and narrow. The highly culled list consisted of about two dozen reference lines in the FUV region and about three dozen in the NUV interval. These high-quality profiles (lighter shaded in Fig. 2) were trivial for the automated procedure to model in all of the echelle mode combinations. For the set of E140M-1425 exposures taken through the wide 020 ; 020 slot, the diffuse lamp lines display more rounded profiles than the sharply peaked lineshapes recorded through the narrow full-resolution aperture. These spectra were fitted with the function x ¼ f 0 e x4, where x (k k 0 )/k, k 0 is the line center wavelength, k is the characteristic profile semiwidth, and f 0 is the peak flux; compared with the Gaussian profile appropriate to the narrow apertures, x ¼ f 0 e x 2. In both cases, the modeled lineshapes were close enough to the true point-spread functions to permit an accurate recovery of the centroid, the fundamental factoid of the present analysis. 5. RESULTS Figures 3a and 3b summarize measurements of the merged, co-added, concatenated spectra of the four STIS echelle mode combinations. In each panel of Figure 3a, the deviations of the calibration lines from their laboratory wavelengths, expressed as the equivalent velocity shift v [i.e., c(k obs k lab )/k lab ], are depicted as a function of wavelength. In the matching panels of Figure 3b, thev are displayed as a function of the measurement

10 No. 2, 2008 DEEP LAMP PROJECT 635 Fig. 3 Continued error v. Most of the points cluster at small values of v, reflecting the high S/N of the majority of features. To avoid the few outliers, the sample was screened against v, using the cutoffs listed in Table 3 ( Fig. 3b, vertical dot-dashed lines). Any jvj > 2kms 1 also was eliminated regardless of v, since the discrepant value likely was caused by fitting a strong feature close to a cataloged line that was, in fact, not the correct identification (especially a problem longward of where the rich Cr spectrum was not represented in the merged line list). The mean velocity shift, and standard deviation, of each screened sample are displayed in the top right corner of the panel (and in Table 3), and depicted by the shaded rectangle bisected by the dot-dashed line. Because there are hundreds of measurements in each average, the standard error of the mean would be smaller than the reported by an order of magnitude or more. The, nevertheless, are a measure of the average internal precision of Mode TABLE 3 Cutoffs and Average Velocity Shifts cutoff (m s 1 ) v crit (m s 1 ) Global Spectra (m s 1 ) v ave Individual Images (m s 1 ) E140M E140H E230M E230H

11 636 AYRES Vol. 177 Fig. 4. Offsets of individual lamp exposures as function of epoch. The lighter points in the E140M panel represent the larger aperture (020 ; 020) images. The discrepant lighter points in the E230M panel are for secondary tilt k2269. Sequences of observations taken close together are stretched along the time axis for visibility, maintaining the time order in all cases. [See the electronic edition of the Supplement for a color version of this figure.] the wavelength scales, and the values in all cases are close to the accuracy quoted for the laboratory wavelengths. Furthermore, the v ave all are very close to zero, indicating that there are no significant systematic offsets in the merged, co-added, concatenated spectra, which would be the desired outcome given the substantial amount of averaging inherent in the different layers of postprocessing. Thick solid curves in Figure 3a are running filtered means, for which each of the 11 consecutive measurements were combined, throwing out the three most extreme points in each group (Olympic filter). The heavily filtered averages show subtle, but systematic, trends. For example, E140M displays a noticeable decline below and an overall shape reminiscent of a shallow cubic function. E140H is much smoother, although with a small drop below and a possible shallow dip near E230H displays a sharp drop at its shortwavelength end, more prominent than that of E140H, but the flattest behavior overall. E230M generally is similar to E230H, although more erratic in detail. Note that the most conspicuous deviations mainly are confined to extremes of the mode ranges: spectra at interior wavelengths were accumulated from several different locations on the camera, tracing different optical paths, so any distortions would be averaged. In addition, since these are order-merged spectra, any feature in the order overlap zones would have its average profile contributed from different sides of the field, thereby partially randomizing even moderate-scale geometrical distortions. Figure 4 depicts velocity offsets of individual WAVECALs over the time span of the full sample. Recall that the offset is based on a few dozen of the best-quality bright features captured by that mode. The comparison focuses on images taken with the default slot for each mode, although also included is the group of

12 No. 2, 2008 DEEP LAMP PROJECT 637 Fig. 5. (a) Spatially resolved velocity shifts of lamp lines from the co-addition of a set of twelve 140 s E140M-1425 exposures, through the default 020 ; 006 aperture, taken very close together in time. Largest symbol in lower panel is 2000 m s 1, smallest is 300 m s 1. (b) Sum of nine deep 663 s E140M exposures, but spread over time from 1997 to Largest symbol is 1600 m s 1, smallest is 300 m s 1. (c) Sum of 13 E140M spectra taken through the 020 ; 020 photometric aperture; nine 85 s exposures on the same day in 1997, and four 350 s exposures spread over the subsequent year. The largest symbol is 2000 m s 1, smallest is 300 m s 1. [See the electronic edition of the Supplement for a color version of this figure.] large-aperture E140M-1425/020 ; 020 exposures. For clusters of exposures, their spacing in the diagram is expanded for visibility s sake, retaining the time order. For each mode, except E140M, several different kcen settings are represented. One sees that the long-term repeatability of the wavelength calibration is superb the standard deviations mostly are less than 150 m s 1 and the offsets display no strong secular trends. The one exception is E230M-2269 ( lighter points in the E230M panel of Fig. 4). It shows a consistent deviation of 1000 m s 1 for all three exposures of this type. Apparently there is a small error in the dispersion coefficients, which likely has not been noticed because the setting is a rarely used secondary tilt and the offset admittedly is small (although clearly anomalous compared to the other examples). The E140M-1425/020 ; 020 exposures (lighter points in the E140M panel of Fig. 4) track the average as well as the observations with the default slot. Furthermore, the echellegrams that were processed using the pseudo-10 s WAVELINEs (not displayed) were not significantly different from those that tagged the full exposure for the zero-point offset. If anything, and counterintuitively, the former showed less of a dispersion in v than the latter. Perhaps the zero-point cross-correlation works better on the lightly exposed images, for which only a few bright lines are visible, than on the deeper ones with their richer, more complex spectral distributions. Figure 5 illustrates spatially resolved velocity shifts of lamp lines from the co-addition of deep exposures from the same mode and setting, where the m identities of the measurements have been preserved. Figure 5a is for the set of 140 s integrations in the E140M-1425 mode, through the default aperture, all taken on the same day in The top panel illustrates a representation of the echellegram assembled from the x2d geometrically rectified file. Wavelengths on the right side ordinate correspond to the center of the particular order. Circles indicate the specific lamp lines measured by the semiautonomous procedure. The bottom panel depicts the velocity shifts of a subsample of the lamp lines, screened according to cutoff from Table 3 and jvj < 2 km s 1, and displayed as circles whose area is proportional to r v/vcrit. The limiting velocity in the denominator is listed in Table 3. For absolute values of the ratio smaller than unity, an open circle with r ¼ 1 is plotted. Positive shifts r > 1 are indicated by dark circles; negative shifts r < ( 1) are lighter circles. Hatched areas depict the overlap between adjacent echelle orders. Systematic patterns are clearly visible in the diagram: a band of blueshifts at the bottom, a band of redshifts at the top, and isolated patches of either sign scattered about the central zone. Figure 5b is an analogous map for the 663 s E140M-1425 exposures, where now the underlying sample was collected over a span of several years. The pattern mirrors that of the shorter exposure sum, although the patches are better defined thanks to the larger number of valid measurements captured in the deeper cumulative exposure. Figure 5c is for the set of large-aperture (020 ; 020) E140M1425 exposures. Because of blending problems owing to the 3 times broader profiles, fewer lines could be measured. Nevertheless, the general pattern seen in the previous two maps is repeated. The persistence of the deviations over the different types of apertures, time, MSM shifts, etc., strongly implicates a stable

13 Fig. 5 Continued Fig. 6. (a) Similar to Fig. 5, but for seven E140H-1271 exposures, mainly 822 s (one at 767 s), spread over 4 years. The default 020 ; 009 aperture was used. The largest symbol is 1400 m s 1, and the smallest is 150 m s 1. (b) Sum of four 600 s E140H-1307 exposures taken on a single day in The largest symbol is 1200 m s 1, and the smallest is 150 m s 1. (c) Sum of eight 265 s E140H-1416 spectra spread over 4 years. The largest symbol is 1700 m s 1, and the smallest is 150 m s 1. (d) Sum of eight E140H-1598 spectra, mainly 400 s (one at 361), over a 4.5 year span. The largest symbol is 1100 m s 1, and the smallest is 150 m s 1. [See the electronic edition of the Supplement for a color version of this figure.] 638