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Instrument Science Report STIS 2004-04

Time-Dependent Sensitivity of the CCD and MAMA FirstOrder Modes
David J. Stys, Ralph C. Bohlin, and Paul Goudfrooij September 14, 2004

ABSTRACT Analysis of the STIS Sensitivity Monitor observations from 1997 through March 2004 continues to show sensitivity trends correlated with time for all first-order modes, as well as temperature dependence in both the FUV MAMA and CCD detectors. The well-known sensitivity of the FUV MAMA to temperature has been reaffirmed at -0.265 %/degC, while new sensitivities to temperature of +0.33 and +0.20 %/degC for G230LB and G430L have arisen since the switch to the Side-2 electronics in July 2001. After these temperature corrections are applied, rates of sensitivity loss are seen to have slowed down significantly in the last epoch of observation beginning in early 2002. During this time, sensitivity losses range from 0.2-1.3 %/yr for G140L and 0-1.0 %/yr for G230L. Previously, CCD sensitivity trends were confused by CTE losses, an effect that is now well characterized. CCD sensitivity losses in the last epoch range from 0.2-0.7 %/yr for G230LB to 0.1-0.3 %/yr for G430L to 0-0.2 %/yr for G750L. Selected wavelength settings of the medium-resolution (M) gratings G140M, G230M, G230MB, G430M, and G750M have also been followed by this monitoring program. Sensitivity trends measured for the limited M-mode wavelength coverage are similar to those observed in the L-modes at corresponding wavelengths. On this basis, the STIS pipeline processing software properly accounts for time-dependent sensitivity (TDS) trends for the MAMA and CCD L- and M-modes. Additionally, SYNPHOT and the STIS exposure time calculators (ETCs) have implemented TDS trends for firstorder spectroscopy.

Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.


Instrument Science Report STIS 2004-04

Introduction
In this report, we provide an update of the analysis of sensitivity monitoring observations of the STIS first-order modes and include a description of the time-dependent sensitivity (TDS) corrections that are currently implemented in the STIS data reduction pipeline, SYNPHOT, and the Exposure Time Calculators (ETCs). Previous STIS Instrument Science Reports (ISR) on this topic were published by Walborn and Bohlin in ISR 98-27 (WB1998), by Bohlin in ISR 99-07, by Smith, Stys, Walborn, and Bohlin in ISR 2000-03, and by Stys and Walborn in ISR 2001-01 (SW2001). Information regarding CTE losses for the STIS CCD modes can be found in ISR STIS 2003-03 by Bohlin and Goudfrooij.

Observations
A flux standard, either the white dwarf GRW+70D5824 or the sub-dwarf AGK+81D266, has been monitored with each STIS grating to detect changes in sensitivity due to contamination or other causes. Observations included in this report are from the STIS Sensitivity Monitor calibration programs (program IDs are listed in Table 1) and include data from mid-1997 through March 2004. Tables 2 and 3 list observations taken since SW2001. Each observation uses the 52x2 slit and has been uniformly processed with the On-TheFly Reprocessing (OTFR) pipeline Starting in Cycle 12 the MAMA L-modes are monitored once every three months, while the M-modes are checked once per year. This represents a slight decrease from previous cycles. The monthly shifting of the mode select mechanism (MSM), which normally occurs for MAMA spectra, is disabled for these observations in order to minimize variations due to the spatial displacement of the spectra. The CCD modes are monitored more frequently in Cycle 12 than in the past, i.e., once every 1.5 months for the L-modes and once per year for the M-modes, in an effort to better constrain the dependence on CCD temperature during Side-2 operations. CCD observations are taken using CRSPLIT = 2 and CCDGAIN = 1. G750L and G750M observations include contemporaneous fringe flat exposures which are used for fringe removal. Spectra representative of those produced by each mode in the program appear in Figure 1-10 of WB1998. Table 1. Sensitivity Monitor Observing Program IDs
MAMA 7673 8424 8857 8919 9628 10033 CCD 7672 8418 8856 8914 9627 10030

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Instrument Science Report STIS 2004-04 Table 2. Recent MAMA Observations
Dataset Mode Central- (е) Date Time Proposal Exptime (sec) Target

G140L (1425е) o69s03010 o69s04010 o69s05010 o69s06010 o69s07010 o69s08010 o69s09010 o69s12010 o6i801010 o6i802010 o6i803010 o6i805010 o6i806010 o6i807010 o6i808010 o6i809010 o6i810010 o6i812010 o8ia01010 o8ia02010 o8ia04010 o8ia05010 o8ia06010 o8v501010 o8v502010 G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L G140L 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 1425 2000-11-09 2000-12-06 2001-01-09 2001-02-07 2001-03-19 2001-04-04 2001-05-05 2001-08-12 2001-09-10 2001-10-07 2001-11-10 2002-01-10 2002-02-16 2002-03-01 2002-04-16 2002-05-18 2002-06-05 2002-08-03 2002-10-17 2003-01-05 2003-05-06 2003-06-28 2003-09-19 2003-12-01 2004-02-18 05:53:34 12:34:48 04:11:31 13:57:54 14:41:12 06:25:43 08:56:02 18:11:30 03:03:37 05:51:03 13:26:37 14:35:17 17:05:11 14:07:39 10:15:17 21:10:00 13:58:47 09:57:35 12:44:47 02:37:33 13:27:54 12:52:07 11:02:38 22:17:22 13:22:45 8857 8857 8857 8857 8857 8857 8857 8857 8919 8919 8919 8919 8919 8919 8919 8919 8919 8919 9628 9628 9628 9628 9628 10033 10033 201.000 201.000 201.000 201.000 201.000 201.000 201.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824

G140M (1173е) o69s14030 o69s15030 o69s16030 o69s17030 o6i813030 o6i814030 G140M G140M G140M G140M G140M G140M 1173 1173 1173 1173 1173 1173 2000-11-07 2001-01-02 2001-03-02 2001-05-11 2001-09-04 2001-11-07 07:06:20 00:01:50 21:47:34 10:01:41 04:33:56 21:05:02 8857 8857 8857 8857 8919 8919 210.000 210.000 210.000 210.000 210.000 210.000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

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Instrument Science Report STIS 2004-04

Dataset o6i815030 o6i817030 o6i818030 o8ia07030 o8ia08030 o8ia09030 o8ia10030 o8v520030

Mode G140M G140M G140M G140M G140M G140M G140M G140M

Central- (е) 1173 1173 1173 1173 1173 1173 1173 1173

Date 2002-01-05 2002-05-02 2002-07-15 2002-10-15 2003-01-16 2003-05-02 2003-07-14 2004-01-19

Time 14:23:00 21:58:28 10:16:12 12:34:19 21:41:49 15:39:02 02:42:17 06:24:36

Proposal 8919 8919 8919 9628 9628 9628 9628 10033

Exptime (sec) 210.000 210.000 210.000 210.000 210.000 210.000 210.000 210.000

Target AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G140M (1567е) o69s14020 o69s15020 o69s16020 o69s17020 o6i813020 o6i814020 o6i815020 o6i817020 o6i818020 o8ia07020 o8ia08020 o8ia09020 o8ia10020 o8v520020 G140M G140M G140M G140M G140M G140M G140M G140M G140M G140M G140M G140M G140M G140M 1567 1567 1567 1567 1567 1567 1567 1567 1567 1567 1567 1567 1567 1567 2000-11-07 2001-01-01 2001-03-02 2001-05-11 2001-09-04 2001-11-07 2002-01-05 2002-05-02 2002-07-15 2002-10-15 2003-01-16 2003-05-02 2003-07-14 2004-01-19 06:53:15 23:48:45 21:34:29 09:48:36 04:20:51 20:51:57 14:09:55 21:45:23 10:03:07 12:21:14 21:28:44 15:25:57 02:29:12 06:11:31 8857 8857 8857 8857 8919 8919 8919 8919 8919 9628 9628 9628 9628 10033 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 300.000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G230L (2376е) o69s03020 o69s04020 o69s05020 o69s06020 o69s07020 o69s08020 o69s09020 o69s12020 o6i801020 G230L G230L G230L G230L G230L G230L G230L G230L G230L 2376 2376 2376 2376 2376 2376 2376 2376 2376 2000-11-09 2000-12-06 2001-01-09 2001-02-07 2001-03-19 2001-04-04 2001-05-05 2001-08-12 2001-09-10 06:03:50 12:45:04 04:21:47 14:08:10 14:51:28 06:35:59 09:06:18 18:21:49 03:13:56 8857 8857 8857 8857 8857 8857 8857 8857 8919 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824

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Instrument Science Report STIS 2004-04

Dataset o6i802020 o6i803020 o6i804020 o6i805020 o6i806020 o6i807020 o6i808020 o6i809020 o6i810020 o6i812020 o8ia01020 o8ia02020 o8ia04020 o8ia05020 o8ia06020 o8v501020 o8v502020

Mode G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L G230L

Central- (е) 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376 2376

Date 2001-10-07 2001-11-10 2001-12-01 2002-01-10 2002-02-16 2002-03-01 2002-04-16 2002-05-18 2002-06-05 2002-08-03 2002-10-17 2003-01-05 2003-05-06 2003-06-28 2003-09-19 2003-12-01 2004-02-18

Time 06:01:22 13:36:56 11:06:11 14:45:36 17:15:30 14:17:58 10:25:36 21:20:19 14:09:06 10:07:54 12:55:06 02:47:52 13:38:13 13:02:26 11:12:57 22:27:41 13:33:04

Proposal 8919 8919 8919 8919 8919 8919 8919 8919 8919 8919 9628 9628 9628 9628 9628 10033 10033

Exptime (sec) 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000 204.000

Target GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824 GRW+70D5824

G230M (2818е) o69s14040 o69s15040 o69s16040 o69s17040 o6i813040 o6i814040 o6i815040 o6i817040 o6i818040 o8ia07040 o8ia08040 o8ia09040 o8ia10040 o8v520040 G230M G230M G230M G230M G230M G230M G230M G230M G230M G230M G230M G230M G230M G230M 2818 2818 2818 2818 2818 2818 2818 2818 2818 2818 2818 2818 2818 2818 2000-11-07 2001-01-02 2001-03-02 2001-05-11 2001-09-04 2001-11-07 2002-01-05 2002-05-02 2002-07-15 2002-10-15 2003-01-16 2003-05-02 2003-07-14 2004-01-19 07:17:35 00:13:05 21:58:49 10:12:56 04:45:11 21:16:17 14:34:15 22:09:43 10:27:27 12:45:34 21:53:04 15:50:17 02:53:32 06:35:51 8857 8857 8857 8857 8919 8919 8919 8919 8919 9628 9628 9628 9628 10033 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 480.000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

5


Instrument Science Report STIS 2004-04 Table 3. Recent CCD Observations
Dataset Mode Central- (е) Date Time Proposal Exptime (sec) Target

G230LB (2375е) o69l01020 o69l02020 o69l03020 o69l04020 o6i901020 o6i902020 o6i903020 o6i904020 o8jj01020 o8jj02020 o8jj03020 o8jj04020 o8u201020 o8u202020 o8u203020 o8u204020 G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB G230LB 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2375 2000-10-24 2001-01-29 2001-04-01 2001-08-06 2001-10-04 2002-01-01 2002-04-03 2002-06-25 2002-10-06 2003-01-16 2003-03-24 2003-06-24 2003-09-08 2003-11-01 2003-12-21 2004-03-14 21:39:18 01:42:16 00:58:06 07:47:55 18:10:10 20:26:28 23:01:36 17:55:42 12:09:19 06:57:42 01:34:33 20:45:51 12:38:18 03:27:09 02:13:42 14:06:21 8856 8856 8856 8856 8914 8914 8914 8914 9627 9627 9627 9627 10030 10030 10030 10030 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G230MB 1995 (1995е) o69l08010 o69l10010 o6i910010 o8jj05010 o8jj06010 o8u220010 G230MB G230MB G230MB G230MB G230MB G230MB 1995 1995 1995 1995 1995 1995 2000-11-25 2001-05-01 2002-05-08 2002-11-08 2003-04-25 2003-11-03 04:59:10 09:19:37 14:01:37 11:04:47 22:01:13 01:45:49 8856 8856 8914 9627 9627 10030 240.00000 240.00000 240.00000 240.00000 240.00000 240.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G230MB (2416е) o6i910020 o8jj05030 o8jj06030 o8u220040 G230MB G230MB G230MB G230MB 2416 2416 2416 2416 2002-05-08 2002-11-08 2003-04-25 2003-11-03 14:11:58 11:21:18 22:17:44 02:07:30 8914 9627 9627 10030 180.00000 180.00000 180.00000 180.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G430L (4300е) o69l01030 o69l02030 G430L G430L 4300 4300 2000-10-24 2001-01-29 21:48:33 01:51:31 8856 8856 172.80000 172.80000 AGK+81D266 AGK+81D266

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Instrument Science Report STIS 2004-04

Dataset o69l03030 o69l04030 o6i901030 o6i902030 o6i903030 o6i904030 o8jj01030 o8jj02030 o8jj03030 o8jj04030 o8u201030 o8u202030 o8u203030 o8u204030 o8u2200b0

Mode G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L G430L

Central- (е) 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300 4300

Date 2001-04-01 2001-08-06 2001-10-04 2002-01-01 2002-04-03 2002-06-25 2002-10-06 2003-01-16 2003-03-24 2003-06-24 2003-09-08 2003-11-01 2003-12-21 2004-03-14 2003-11-03

Time 01:07:21 07:57:10 18:19:25 20:35:43 23:10:51 18:04:57 12:18:34 07:06:57 01:43:48 20:55:06 12:47:33 03:36:24 02:22:57 14:15:36 03:27:28

Proposal 8856 8856 8914 8914 8914 8914 9627 9627 9627 9627 10030 10030 10030 10030 10030

Exptime (sec) 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 172.80000 50.000000

Target AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G430M (3165е) o69l08030 o69l10030 o6i910030 o8jj06050 G430M G430M G430M G430M 3165 3165 3165 3165 2000-11-25 2001-05-01 2002-05-08 2003-04-25 05:19:22 09:39:49 14:21:49 23:18:31 8856 8856 8914 9627 120.00000 120.00000 120.00000 120.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G430M (4194е) o45a02040 o45a07040 o45a08040 o45a09040 o45a10040 o45a11040 o5ig08040 o5ig09040 o5ig10040 o69l08040 o69l10040 G430M G430M G430M G430M G430M G430M G430M G430M G430M G430M G430M 4194 4194 4194 4194 4194 4194 4194 4194 4194 4194 4194 1997-09-01 1998-01-01 1998-04-18 1998-08-24 1999-01-13 1999-05-01 1999-09-11 2000-01-15 2000-05-14 2000-11-25 2001-05-01 12:24:00 09:05:24 01:12:34 16:03:11 06:47:38 02:30:08 13:59:41 18:15:38 06:27:39 05:27:23 09:47:50 7672 7672 7672 7672 7672 7672 8418 8418 8418 8856 8856 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

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Instrument Science Report STIS 2004-04

Dataset o6i910040 o8jj05070 o8jj06070 o8u220080

Mode G430M G430M G430M G430M

Central- (е) 4194 4194 4194 4194

Date 2002-05-08 2002-11-08 2003-04-25 2003-11-03

Time 14:29:50 12:34:18 23:30:42 03:11:09

Proposal 8914 9627 9627 10030

Exptime (sec) 120.00000 120.00000 120.00000 120.00000

Target AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G750L (7751е) o69l01040 o69l02040 o69l03040 o69l04040 o6i901040 o6i902040 o6i903060 o6i904060 o8jj01060 o8jj02060 o8jj03060 o8jj04060 o8u201060 o8u202060 o8u203060 o8u204060 G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L G750L 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 7751 2000-10-24 2001-01-29 2001-04-01 2001-08-06 2001-10-04 2002-01-01 2002-04-03 2002-06-25 2002-10-06 2003-01-16 2003-03-24 2003-06-24 2003-09-08 2003-11-01 2003-12-21 2004-03-14 21:57:48 02:00:46 01:16:36 08:06:25 18:28:40 20:44:58 23:32:50 18:26:56 12:40:33 07:28:56 02:50:00 21:17:05 13:09:32 03:58:23 02:44:56 14:37:35 8856 8856 8856 8856 8914 8914 8914 8914 9627 9627 9627 9627 10030 10030 10030 10030 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 432.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

G750M (7283е) o69l08060 o69l10060 o6i910060 o8jj050b0 o8jj060b0 o8u2200g0 G750M G750M G750M G750M G750M G750M 7283 7283 7283 7283 7283 7283 2000-11-25 2001-05-01 2002-05-08 2002-11-08 2003-04-25 2003-11-03 05:39:42 10:00:09 14:42:09 12:55:39 23:52:03 03:48:15 8856 8856 8914 9627 9627 10030 120.00000 120.00000 120.00000 120.00000 120.00000 120.00000 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266 AGK+81D266

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Instrument Science Report STIS 2004-04

Temperature Dependence
In addition to the sensitivity changes with time that are discussed below, there is a significant sensitivity change with instrumental temperatures, as shown in Figures 1-5 for the low dispersion modes. The correlations use the charge amplifier temperature keywords OM1CAT and OM2CAT for the FUV and NUV MAMAs, respectively, while the CCD housing temperature (OCCDHTAV) is used for the three CCD modes. The procedure used to define the temperature dependencies is iterative. First, the time dependencies discussed below are determined and data are corrected for these time changes. Second, dependence on the temperature is determined. Data are then corrected for temperature and a new timedependence is calculated. This process repeats until convergence is achieved. The G140L response variation was originally quoted as -0.25 +/-0.05%/degC by Bohlin (1999). This has now been refined to -0.265+/-0.035 using all 64 observations of GRW+70D5824 in the photometric 52x2 arcsec slit since the start of STIS operations, as summarized in Table 4. Before the death of the STIS Side-1 electronics and the switch to the backup Side-2 in July 2001, no significant dependence of sensitivity on temperature was observed for the NUV-MAMA or the CCD modes. However, the lack of thermal control has now made the CCD susceptible to significant changes with temperature. The amount of change in %/degC also depends on wavelength, as itemized in Table 4 and illustrated in Figures 1-5. The changes averaged over coarse wavelength bins decline from +0.33 %/degC for G230LB to ~0 for G750L. Over the full 16 to 22.5°C temperature range observed so far with G230LB, the correlation implies a 2.2% change in photometric sensitivity. Even though this G230LB correlation is based on only 18 observations obtained since July 2001, there is a 17-sigma level of confidence in this preliminary result; and the rms scatter of the 11 observations is reduced by a factor of 4, from 0.52% with no temperature correction to an rms scatter of only 0.12% after correcting for the preliminary 0.33%/degC temperature dependence. More observations are required to refine the precision of the CCD correlation coefficients. [At the time this report is issued, temperature corrections for CCD spectra are not yet included in the pipeline data processing.] Table 4. Coefficients of Sensitivity Change with Temperature
Mode G140L G230L G230LB G430L G750L Range(A) 1300-1650 1900-2900 1900-2900 3300-5300 5900-8700 %/degC -0.265 0 +0.332 +0.20 0 Uncertainty(%/degC) 0.035 0.015 0.019 0.05 0.020 RMS Before, After(%) 0.75, 0.50 0.18, 0.18 0.52, 0.12 0.40, 0.26 0.14, 0.14 N 64 66 18 22 18

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Instrument Science Report STIS 2004-04

Figure 1: G140L Temperature Sensitivity. The two parameter time and temperature fit is to the 64 diamonds representing the GRW+70d5824 observations.
G140L: GD153(filled) GRW(diam) GD71(triang)

REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)

1.01

1.00

0.99

Corrected for time changes Average over 1300.-1650. Sigma(%) BEFORE= 0.75 Resid. %/yr=-0.01 +/- 0.04 %/dC=-0.265 +/- 0.035, SIGMA= 0.50

32
Fit 64 DIAMONDS only

34 36 38 OM1CAT = (dgC) M1 charge amplifier temp

40

Figure 2: G230L Temperature Sensitivity
G230L: GD153(filled) GRW(diam) GD71(triang)

REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)

1.01

1.00

0.99

Corrected for time changes Average over 1900.-2900. Sigma(%) BEFORE= 0.18 Resid. %/yr=-0.01 +/- 0.01 %/dC=-0.013 +/- 0.015, SIGMA= 0.18

34
Fit 66 DIAMONDS only

36 38 OM2CAT = (dgC) M2 charge amplifier temp

40

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Instrument Science Report STIS 2004-04 Figure 3: G230LB Temperature Sensitivity
G230LB: GD153(filled) G191B2B(open) AGK(diam) GD71(triang) HZ43(cir)

REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)

1.01

1.00

0.99

Corrected for time changes Average over 1900.-2900. Sigma(%) BEFORE= 0.52 Resid. %/yr= 0.01 +/- 0.03 %/dC= 0.332 +/- 0.019, SIGMA= 0.12

16
Fit 18 DIAMONDS only

17 18 19 20 21 22 OCCDHTAV = average CCD housing temperature (degC)

Figure 4: G430L Temperature Sensitivity. The significance of the the two parameter fit is low, so that the more conservative single parameter temperature fit written near the top of the plot is adopted.
G430L: GD153(filled) G191B2B(open) AGK(diam) GD71(triang) HZ43(cir)

Dots: %/dC= 0.204 +/- 0.048 from temp. fit only
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)

1.01

1.00

0.99

Corrected for time changes Average over 3500.-5500. Sigma(%) BEFORE= 0.40 Resid. %/yr=-0.15 +/- 0.06 %/dC= 0.231 +/- 0.043, SIGMA= 0.26

16
Fit 22 DIAMONDS only

17 18 19 20 21 22 OCCDHTAV = average CCD housing temperature (degC)

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Instrument Science Report STIS 2004-04 Figure 5: G750L Temperature Sensitivity
G750L: GD153(filled) G191B2B(open) AGK(diam) GD71(triang) HZ43(cir)

Dots: %/dC= 0.047 +/- 0.021 from temp. fit only
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)

1.01

1.00

0.99

Corrected for time changes Average over 5900.-8700. Sigma(%) BEFORE= 0.14 Resid. %/yr=-0.07 +/- 0.03 %/dC= 0.064 +/- 0.020, SIGMA= 0.11

16
Fit 18 DIAMONDS only

17 18 19 20 21 22 OCCDHTAV = average CCD housing temperature (degC)

Changes in Sensitivity with Time
After correcting the FUV MAMA and CCD counts for non-linearities, temperature, and CTE, each STIS mode is examined for any variation in sensitivity with time. The mean of the NET counts for each observation is divided by the mean of the NET counts for the first observation. These relative sensitivities are plotted versus time and are fitted with linear segments to obtain a good fit at any given time. These fits are extrapolated back in time to 1997.38, the date of MAMA activation. Relative sensitivities and the linear segments are then normalized to unity at this date. The slope, i.e. percent-per-year change in sensitivity, and 1 uncertainty in the fits are printed at the bottom of each plot (see Figures 6-18). The 1 RMS(%) of the data residuals from the linear fits is also shown. For modes where more than one linear segment is necessary, each segment is forced to connect with another for continuity, which is necessary for pipeline implementation. The number of distinct time segments needed for a good fit depends on the grating in question. For G140L, 3 segments, or epochs, were selected. The transition from the first to the second epochs represents the shift of the default aperture from the +3" spectral position on the detector prior to March 15, 1999 to the -3" location. The G230L (four epochs) and G230LB (three epochs) sensitivities increase in their first epoch and decline steadily thereafter. The G430L and G750L trends show the slowest rates of sensitivity losses over time and can be fit with one linear segment each. Sensitivity trends measured for the limited M-mode wavelength coverage are similar to those observed in the L-modes at corresponding wavelengths; the pipeline corrections use the measured low-dispersion changes. During the last epoch, the rate of sensitivity decline for the UV-modes slowed down significantly.

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Instrument Science Report STIS 2004-04

Figure 6: G140L (1150-1700е)

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Instrument Science Report STIS 2004-04 Figure 7: G230L (1600-3100е)

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Instrument Science Report STIS 2004-04 Figure 8: G230LB (1700-2800е)

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Instrument Science Report STIS 2004-04 Figure 9: G430L (2900-5500е)

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Instrument Science Report STIS 2004-04 Figure 10: G750L (5500-9900е)

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Instrument Science Report STIS 2004-04 Figure 11: G140M (1173е)

Figure 12: G140M (1567е)

Figure 13: G230M (2818е)

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Instrument Science Report STIS 2004-04 Figure 14: G230MB (1995е)

Figure 15: G230MB (2416е)

Figure 16: G430M (3165е)

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Instrument Science Report STIS 2004-04 Figure 17: G430M (4194е)

Figure 18: G750M (7283е)

TDS Corrections Applied in the Pipeline and in Synphot
Corrections for the time dependencies of the sensitivities of all MAMA and CCD firstorder configurations have been incorporated into the STScI data-reduction pipeline, so that all OTFR retrievals of such data will have corrected fluxes regardless of their acquisition epoch. Corrections as a function of wavelength for the low-resolution modes are applied to medium-resolution observations as well. The header keyword TDSTAB points to a reference file (*_tds.fits) that stores the time-dependent sensitivity coefficients. Figures 19-23 show relative fluxes that have been corrected for TDS. These corrections tend to be less certain near the very beginning and end of the wavelength span of each configuration. Additionally, forcing the continuity between linear segments (as described above) introduces a small (<0.5%) error to the TDS corrections for the G140L grating near the switch-over from the +3" to the -3" default position at epoch 1999.2. Despite these issues, the TDS corrections are accurate to better than 1%. In Figures 21-23, corrected CCD fluxes show an increase in scatter about the linear fit beginning in 2001 (which is when the switch to Side-2 electronics occurred) due to a poorly constrained temperature depen-

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Instrument Science Report STIS 2004-04 dence. Future calibration observations would be needed to determine more accurate temperature corrections, which should reduce this scatter. As of April 2004, SYNPHOT (and, therefore, the ETC) also takes into account the changes in sensitivity with time for all first-order modes. The default date for sensitivity calculations with SYNPHOT and the ETC is currently July 1, 2004, i.e., the start of Cycle 13. The default date will be updated annually. SYNPHOT has the added capability of calculating sensitivities at any specified Modified Julian Date (MJD). This capability is illustrated in the inset below for an example where one is determining the total counts in a G140L spectrum of a 30,000 K blackbody with a V-band magnitude of 18 at the default date and at an MJD of 51000 (i.e., in July 1998):

Sensitivity changes for the imaging configurations are similar to the L-modes and will be incorporated into the pipeline data reduction in the first upcoming pipeline build (currently estimated to occur in January 2005). The time-dependence of echelle mode sensitivities is being investigated and should be available soon.

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Instrument Science Report STIS 2004-04

Figure 19: G140L Corrected Flux

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Instrument Science Report STIS 2004-04 Figure 20: G230L Corrected Flux

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Instrument Science Report STIS 2004-04 Figure 21: G230LB Corrected Flux

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Instrument Science Report STIS 2004-04 Figure 22: G430L Corrected Flux

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Instrument Science Report STIS 2004-04 Figure 23: G750L Corrected Flux

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Instrument Science Report STIS 2004-04

References
Bohlin, R. 1999, Instrument Science Report, STIS 99-07, (Baltimore:STScI) Bohlin, R. & Goudfrooij P. 2003, Instrument Science Report, STIS 2003-03, (Baltimore:STScI) Smith, E., Stys, D., Walborn, N., and Bohlin, R. 2000, Instrument Science Report, STIS 2000-03, (Baltimore:STScI) Stys. D., & Walborn, N. 2001, Instrument Science Report, STIS 2001-01, (Baltimore:STScI), (SW2001) Walborn, N., & Bohlin, R. 1998, Instrument Science Report, STIS 98-27, (Baltimore:STScI), (WB1998)

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