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
CCD
7673
7672
8424
8418
8857
8856
8919
8914
9628
9627
10033
10030
2
Instrument Science Report STIS 2004-04
Table 2. Recent MAMA Observations
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
G140L (1425Å)
o69s03010
G140L
1425
2000-11-09
05:53:34
8857
201.000
GRW+70D5824
o69s04010
G140L
1425
2000-12-06
12:34:48
8857
201.000
GRW+70D5824
o69s05010
G140L
1425
2001-01-09
04:11:31
8857
201.000
GRW+70D5824
o69s06010
G140L
1425
2001-02-07
13:57:54
8857
201.000
GRW+70D5824
o69s07010
G140L
1425
2001-03-19
14:41:12
8857
201.000
GRW+70D5824
o69s08010
G140L
1425
2001-04-04
06:25:43
8857
201.000
GRW+70D5824
o69s09010
G140L
1425
2001-05-05
08:56:02
8857
201.000
GRW+70D5824
o69s12010
G140L
1425
2001-08-12
18:11:30
8857
204.000
GRW+70D5824
o6i801010
G140L
1425
2001-09-10
03:03:37
8919
204.000
GRW+70D5824
o6i802010
G140L
1425
2001-10-07
05:51:03
8919
204.000
GRW+70D5824
o6i803010
G140L
1425
2001-11-10
13:26:37
8919
204.000
GRW+70D5824
o6i805010
G140L
1425
2002-01-10
14:35:17
8919
204.000
GRW+70D5824
o6i806010
G140L
1425
2002-02-16
17:05:11
8919
204.000
GRW+70D5824
o6i807010
G140L
1425
2002-03-01
14:07:39
8919
204.000
GRW+70D5824
o6i808010
G140L
1425
2002-04-16
10:15:17
8919
204.000
GRW+70D5824
o6i809010
G140L
1425
2002-05-18
21:10:00
8919
204.000
GRW+70D5824
o6i810010
G140L
1425
2002-06-05
13:58:47
8919
204.000
GRW+70D5824
o6i812010
G140L
1425
2002-08-03
09:57:35
8919
204.000
GRW+70D5824
o8ia01010
G140L
1425
2002-10-17
12:44:47
9628
204.000
GRW+70D5824
o8ia02010
G140L
1425
2003-01-05
02:37:33
9628
204.000
GRW+70D5824
o8ia04010
G140L
1425
2003-05-06
13:27:54
9628
204.000
GRW+70D5824
o8ia05010
G140L
1425
2003-06-28
12:52:07
9628
204.000
GRW+70D5824
o8ia06010
G140L
1425
2003-09-19
11:02:38
9628
204.000
GRW+70D5824
o8v501010
G140L
1425
2003-12-01
22:17:22
10033
204.000
GRW+70D5824
o8v502010
G140L
1425
2004-02-18
13:22:45
10033
204.000
GRW+70D5824
G140M (1173Å)
o69s14030
G140M
1173
2000-11-07
07:06:20
8857
210.000
AGK+81D266
o69s15030
G140M
1173
2001-01-02
00:01:50
8857
210.000
AGK+81D266
o69s16030
G140M
1173
2001-03-02
21:47:34
8857
210.000
AGK+81D266
o69s17030
G140M
1173
2001-05-11
10:01:41
8857
210.000
AGK+81D266
o6i813030
G140M
1173
2001-09-04
04:33:56
8919
210.000
AGK+81D266
o6i814030
G140M
1173
2001-11-07
21:05:02
8919
210.000
AGK+81D266
3
Instrument Science Report STIS 2004-04
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
o6i815030
G140M
1173
2002-01-05
14:23:00
8919
210.000
AGK+81D266
o6i817030
G140M
1173
2002-05-02
21:58:28
8919
210.000
AGK+81D266
o6i818030
G140M
1173
2002-07-15
10:16:12
8919
210.000
AGK+81D266
o8ia07030
G140M
1173
2002-10-15
12:34:19
9628
210.000
AGK+81D266
o8ia08030
G140M
1173
2003-01-16
21:41:49
9628
210.000
AGK+81D266
o8ia09030
G140M
1173
2003-05-02
15:39:02
9628
210.000
AGK+81D266
o8ia10030
G140M
1173
2003-07-14
02:42:17
9628
210.000
AGK+81D266
o8v520030
G140M
1173
2004-01-19
06:24:36
10033
210.000
AGK+81D266
G140M (1567Å)
o69s14020
G140M
1567
2000-11-07
06:53:15
8857
300.000
AGK+81D266
o69s15020
G140M
1567
2001-01-01
23:48:45
8857
300.000
AGK+81D266
o69s16020
G140M
1567
2001-03-02
21:34:29
8857
300.000
AGK+81D266
o69s17020
G140M
1567
2001-05-11
09:48:36
8857
300.000
AGK+81D266
o6i813020
G140M
1567
2001-09-04
04:20:51
8919
300.000
AGK+81D266
o6i814020
G140M
1567
2001-11-07
20:51:57
8919
300.000
AGK+81D266
o6i815020
G140M
1567
2002-01-05
14:09:55
8919
300.000
AGK+81D266
o6i817020
G140M
1567
2002-05-02
21:45:23
8919
300.000
AGK+81D266
o6i818020
G140M
1567
2002-07-15
10:03:07
8919
300.000
AGK+81D266
o8ia07020
G140M
1567
2002-10-15
12:21:14
9628
300.000
AGK+81D266
o8ia08020
G140M
1567
2003-01-16
21:28:44
9628
300.000
AGK+81D266
o8ia09020
G140M
1567
2003-05-02
15:25:57
9628
300.000
AGK+81D266
o8ia10020
G140M
1567
2003-07-14
02:29:12
9628
300.000
AGK+81D266
o8v520020
G140M
1567
2004-01-19
06:11:31
10033
300.000
AGK+81D266
G230L (2376Å)
o69s03020
G230L
2376
2000-11-09
06:03:50
8857
204.000
GRW+70D5824
o69s04020
G230L
2376
2000-12-06
12:45:04
8857
204.000
GRW+70D5824
o69s05020
G230L
2376
2001-01-09
04:21:47
8857
204.000
GRW+70D5824
o69s06020
G230L
2376
2001-02-07
14:08:10
8857
204.000
GRW+70D5824
o69s07020
G230L
2376
2001-03-19
14:51:28
8857
204.000
GRW+70D5824
o69s08020
G230L
2376
2001-04-04
06:35:59
8857
204.000
GRW+70D5824
o69s09020
G230L
2376
2001-05-05
09:06:18
8857
204.000
GRW+70D5824
o69s12020
G230L
2376
2001-08-12
18:21:49
8857
204.000
GRW+70D5824
o6i801020
G230L
2376
2001-09-10
03:13:56
8919
204.000
GRW+70D5824
4
Instrument Science Report STIS 2004-04
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
o6i802020
G230L
2376
2001-10-07
06:01:22
8919
204.000
GRW+70D5824
o6i803020
G230L
2376
2001-11-10
13:36:56
8919
204.000
GRW+70D5824
o6i804020
G230L
2376
2001-12-01
11:06:11
8919
204.000
GRW+70D5824
o6i805020
G230L
2376
2002-01-10
14:45:36
8919
204.000
GRW+70D5824
o6i806020
G230L
2376
2002-02-16
17:15:30
8919
204.000
GRW+70D5824
o6i807020
G230L
2376
2002-03-01
14:17:58
8919
204.000
GRW+70D5824
o6i808020
G230L
2376
2002-04-16
10:25:36
8919
204.000
GRW+70D5824
o6i809020
G230L
2376
2002-05-18
21:20:19
8919
204.000
GRW+70D5824
o6i810020
G230L
2376
2002-06-05
14:09:06
8919
204.000
GRW+70D5824
o6i812020
G230L
2376
2002-08-03
10:07:54
8919
204.000
GRW+70D5824
o8ia01020
G230L
2376
2002-10-17
12:55:06
9628
204.000
GRW+70D5824
o8ia02020
G230L
2376
2003-01-05
02:47:52
9628
204.000
GRW+70D5824
o8ia04020
G230L
2376
2003-05-06
13:38:13
9628
204.000
GRW+70D5824
o8ia05020
G230L
2376
2003-06-28
13:02:26
9628
204.000
GRW+70D5824
o8ia06020
G230L
2376
2003-09-19
11:12:57
9628
204.000
GRW+70D5824
o8v501020
G230L
2376
2003-12-01
22:27:41
10033
204.000
GRW+70D5824
o8v502020
G230L
2376
2004-02-18
13:33:04
10033
204.000
GRW+70D5824
G230M (2818Å)
o69s14040
G230M
2818
2000-11-07
07:17:35
8857
480.000
AGK+81D266
o69s15040
G230M
2818
2001-01-02
00:13:05
8857
480.000
AGK+81D266
o69s16040
G230M
2818
2001-03-02
21:58:49
8857
480.000
AGK+81D266
o69s17040
G230M
2818
2001-05-11
10:12:56
8857
480.000
AGK+81D266
o6i813040
G230M
2818
2001-09-04
04:45:11
8919
480.000
AGK+81D266
o6i814040
G230M
2818
2001-11-07
21:16:17
8919
480.000
AGK+81D266
o6i815040
G230M
2818
2002-01-05
14:34:15
8919
480.000
AGK+81D266
o6i817040
G230M
2818
2002-05-02
22:09:43
8919
480.000
AGK+81D266
o6i818040
G230M
2818
2002-07-15
10:27:27
8919
480.000
AGK+81D266
o8ia07040
G230M
2818
2002-10-15
12:45:34
9628
480.000
AGK+81D266
o8ia08040
G230M
2818
2003-01-16
21:53:04
9628
480.000
AGK+81D266
o8ia09040
G230M
2818
2003-05-02
15:50:17
9628
480.000
AGK+81D266
o8ia10040
G230M
2818
2003-07-14
02:53:32
9628
480.000
AGK+81D266
o8v520040
G230M
2818
2004-01-19
06:35:51
10033
480.000
AGK+81D266
5
Instrument Science Report STIS 2004-04
Table 3. Recent CCD Observations
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
G230LB (2375Å)
o69l01020
G230LB
2375
2000-10-24
21:39:18
8856
172.80000
AGK+81D266
o69l02020
G230LB
2375
2001-01-29
01:42:16
8856
172.80000
AGK+81D266
o69l03020
G230LB
2375
2001-04-01
00:58:06
8856
172.80000
AGK+81D266
o69l04020
G230LB
2375
2001-08-06
07:47:55
8856
172.80000
AGK+81D266
o6i901020
G230LB
2375
2001-10-04
18:10:10
8914
172.80000
AGK+81D266
o6i902020
G230LB
2375
2002-01-01
20:26:28
8914
172.80000
AGK+81D266
o6i903020
G230LB
2375
2002-04-03
23:01:36
8914
172.80000
AGK+81D266
o6i904020
G230LB
2375
2002-06-25
17:55:42
8914
172.80000
AGK+81D266
o8jj01020
G230LB
2375
2002-10-06
12:09:19
9627
172.80000
AGK+81D266
o8jj02020
G230LB
2375
2003-01-16
06:57:42
9627
172.80000
AGK+81D266
o8jj03020
G230LB
2375
2003-03-24
01:34:33
9627
172.80000
AGK+81D266
o8jj04020
G230LB
2375
2003-06-24
20:45:51
9627
172.80000
AGK+81D266
o8u201020
G230LB
2375
2003-09-08
12:38:18
10030
172.80000
AGK+81D266
o8u202020
G230LB
2375
2003-11-01
03:27:09
10030
172.80000
AGK+81D266
o8u203020
G230LB
2375
2003-12-21
02:13:42
10030
172.80000
AGK+81D266
o8u204020
G230LB
2375
2004-03-14
14:06:21
10030
172.80000
AGK+81D266
G230MB 1995 (1995Å)
o69l08010
G230MB
1995
2000-11-25
04:59:10
8856
240.00000
AGK+81D266
o69l10010
G230MB
1995
2001-05-01
09:19:37
8856
240.00000
AGK+81D266
o6i910010
G230MB
1995
2002-05-08
14:01:37
8914
240.00000
AGK+81D266
o8jj05010
G230MB
1995
2002-11-08
11:04:47
9627
240.00000
AGK+81D266
o8jj06010
G230MB
1995
2003-04-25
22:01:13
9627
240.00000
AGK+81D266
o8u220010
G230MB
1995
2003-11-03
01:45:49
10030
240.00000
AGK+81D266
G230MB (2416Å)
o6i910020
G230MB
2416
2002-05-08
14:11:58
8914
180.00000
AGK+81D266
o8jj05030
G230MB
2416
2002-11-08
11:21:18
9627
180.00000
AGK+81D266
o8jj06030
G230MB
2416
2003-04-25
22:17:44
9627
180.00000
AGK+81D266
o8u220040
G230MB
2416
2003-11-03
02:07:30
10030
180.00000
AGK+81D266
G430L (4300Å)
o69l01030
G430L
4300
2000-10-24
21:48:33
8856
172.80000
AGK+81D266
o69l02030
G430L
4300
2001-01-29
01:51:31
8856
172.80000
AGK+81D266
6
Instrument Science Report STIS 2004-04
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
o69l03030
G430L
4300
2001-04-01
01:07:21
8856
172.80000
AGK+81D266
o69l04030
G430L
4300
2001-08-06
07:57:10
8856
172.80000
AGK+81D266
o6i901030
G430L
4300
2001-10-04
18:19:25
8914
172.80000
AGK+81D266
o6i902030
G430L
4300
2002-01-01
20:35:43
8914
172.80000
AGK+81D266
o6i903030
G430L
4300
2002-04-03
23:10:51
8914
172.80000
AGK+81D266
o6i904030
G430L
4300
2002-06-25
18:04:57
8914
172.80000
AGK+81D266
o8jj01030
G430L
4300
2002-10-06
12:18:34
9627
172.80000
AGK+81D266
o8jj02030
G430L
4300
2003-01-16
07:06:57
9627
172.80000
AGK+81D266
o8jj03030
G430L
4300
2003-03-24
01:43:48
9627
172.80000
AGK+81D266
o8jj04030
G430L
4300
2003-06-24
20:55:06
9627
172.80000
AGK+81D266
o8u201030
G430L
4300
2003-09-08
12:47:33
10030
172.80000
AGK+81D266
o8u202030
G430L
4300
2003-11-01
03:36:24
10030
172.80000
AGK+81D266
o8u203030
G430L
4300
2003-12-21
02:22:57
10030
172.80000
AGK+81D266
o8u204030
G430L
4300
2004-03-14
14:15:36
10030
172.80000
AGK+81D266
o8u2200b0
G430L
4300
2003-11-03
03:27:28
10030
50.000000
AGK+81D266
G430M (3165Å)
o69l08030
G430M
3165
2000-11-25
05:19:22
8856
120.00000
AGK+81D266
o69l10030
G430M
3165
2001-05-01
09:39:49
8856
120.00000
AGK+81D266
o6i910030
G430M
3165
2002-05-08
14:21:49
8914
120.00000
AGK+81D266
o8jj06050
G430M
3165
2003-04-25
23:18:31
9627
120.00000
AGK+81D266
G430M (4194Å)
o45a02040
G430M
4194
1997-09-01
12:24:00
7672
120.00000
AGK+81D266
o45a07040
G430M
4194
1998-01-01
09:05:24
7672
120.00000
AGK+81D266
o45a08040
G430M
4194
1998-04-18
01:12:34
7672
120.00000
AGK+81D266
o45a09040
G430M
4194
1998-08-24
16:03:11
7672
120.00000
AGK+81D266
o45a10040
G430M
4194
1999-01-13
06:47:38
7672
120.00000
AGK+81D266
o45a11040
G430M
4194
1999-05-01
02:30:08
7672
120.00000
AGK+81D266
o5ig08040
G430M
4194
1999-09-11
13:59:41
8418
120.00000
AGK+81D266
o5ig09040
G430M
4194
2000-01-15
18:15:38
8418
120.00000
AGK+81D266
o5ig10040
G430M
4194
2000-05-14
06:27:39
8418
120.00000
AGK+81D266
o69l08040
G430M
4194
2000-11-25
05:27:23
8856
120.00000
AGK+81D266
o69l10040
G430M
4194
2001-05-01
09:47:50
8856
120.00000
AGK+81D266
7
Instrument Science Report STIS 2004-04
Dataset
Mode
Central-λ (Å)
Date
Time
Proposal
Exptime (sec)
Target
o6i910040
G430M
4194
2002-05-08
14:29:50
8914
120.00000
AGK+81D266
o8jj05070
G430M
4194
2002-11-08
12:34:18
9627
120.00000
AGK+81D266
o8jj06070
G430M
4194
2003-04-25
23:30:42
9627
120.00000
AGK+81D266
o8u220080
G430M
4194
2003-11-03
03:11:09
10030
120.00000
AGK+81D266
G750L (7751Å)
o69l01040
G750L
7751
2000-10-24
21:57:48
8856
432.00000
AGK+81D266
o69l02040
G750L
7751
2001-01-29
02:00:46
8856
432.00000
AGK+81D266
o69l03040
G750L
7751
2001-04-01
01:16:36
8856
432.00000
AGK+81D266
o69l04040
G750L
7751
2001-08-06
08:06:25
8856
432.00000
AGK+81D266
o6i901040
G750L
7751
2001-10-04
18:28:40
8914
432.00000
AGK+81D266
o6i902040
G750L
7751
2002-01-01
20:44:58
8914
432.00000
AGK+81D266
o6i903060
G750L
7751
2002-04-03
23:32:50
8914
432.00000
AGK+81D266
o6i904060
G750L
7751
2002-06-25
18:26:56
8914
432.00000
AGK+81D266
o8jj01060
G750L
7751
2002-10-06
12:40:33
9627
432.00000
AGK+81D266
o8jj02060
G750L
7751
2003-01-16
07:28:56
9627
432.00000
AGK+81D266
o8jj03060
G750L
7751
2003-03-24
02:50:00
9627
432.00000
AGK+81D266
o8jj04060
G750L
7751
2003-06-24
21:17:05
9627
432.00000
AGK+81D266
o8u201060
G750L
7751
2003-09-08
13:09:32
10030
432.00000
AGK+81D266
o8u202060
G750L
7751
2003-11-01
03:58:23
10030
432.00000
AGK+81D266
o8u203060
G750L
7751
2003-12-21
02:44:56
10030
432.00000
AGK+81D266
o8u204060
G750L
7751
2004-03-14
14:37:35
10030
432.00000
AGK+81D266
G750M (7283Å)
o69l08060
G750M
7283
2000-11-25
05:39:42
8856
120.00000
AGK+81D266
o69l10060
G750M
7283
2001-05-01
10:00:09
8856
120.00000
AGK+81D266
o6i910060
G750M
7283
2002-05-08
14:42:09
8914
120.00000
AGK+81D266
o8jj050b0
G750M
7283
2002-11-08
12:55:39
9627
120.00000
AGK+81D266
o8jj060b0
G750M
7283
2003-04-25
23:52:03
9627
120.00000
AGK+81D266
o8u2200g0
G750M
7283
2003-11-03
03:48:15
10030
120.00000
AGK+81D266
8
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
Range(A)
%/degC
Uncertainty(%/degC)
RMS Before, After(%)
N
G140L
1300-1650
-0.265
0.035
0.75, 0.50
64
G230L
1900-2900
0
0.015
0.18, 0.18
66
G230LB
1900-2900
+0.332
0.019
0.52, 0.12
18
G430L
3300-5300
+0.20
0.05
0.40, 0.26
22
G750L
5900-8700
0
0.020
0.14, 0.14
18
9
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.
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)
G140L: GD153(filled) GRW(diam) GD71(triang)
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
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)
G230L: GD153(filled) GRW(diam) GD71(triang)
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
10
40
Instrument Science Report STIS 2004-04
Figure 3: G230LB Temperature Sensitivity
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)
G230LB: GD153(filled) G191B2B(open) AGK(diam) GD71(triang) HZ43(cir)
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)
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)
Dots: %/dC= 0.204 +/- 0.048 from temp. fit only
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)
11
Instrument Science Report STIS 2004-04
Figure 5: G750L Temperature Sensitivity
G750L: GD153(filled) G191B2B(open) AGK(diam) GD71(triang) HZ43(cir)
REL NET SENS. (Fits AFTER 1997.38=MAMA turnon)
Dots: %/dC= 0.047 +/- 0.021 from temp. fit only
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 1σ uncertainty in the fits are printed at the bottom of each plot (see Figures 6-18). The
1σ 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.
12
Instrument Science Report STIS 2004-04
Figure 6: G140L (1150-1700Å)
13
Instrument Science Report STIS 2004-04
Figure 7: G230L (1600-3100Å)
14
Instrument Science Report STIS 2004-04
Figure 8: G230LB (1700-2800Å)
15
Instrument Science Report STIS 2004-04
Figure 9: G430L (2900-5500Å)
16
Instrument Science Report STIS 2004-04
Figure 10: G750L (5500-9900Å)
17
Instrument Science Report STIS 2004-04
Figure 11: G140M (1173Å)
Figure 12: G140M (1567Å)
Figure 13: G230M (2818Å)
18
Instrument Science Report STIS 2004-04
Figure 14: G230MB (1995Å)
Figure 15: G230MB (2416Å)
Figure 16: G430M (3165Å)
19
Instrument Science Report STIS 2004-04
Figure 17: G430M (4194Å)
Figure 18: G750M (7283Å)
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-
20
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.
21
Instrument Science Report STIS 2004-04
Figure 19: G140L Corrected Flux
22
Instrument Science Report STIS 2004-04
Figure 20: G230L Corrected Flux
23
Instrument Science Report STIS 2004-04
Figure 21: G230LB Corrected Flux
24
Instrument Science Report STIS 2004-04
Figure 22: G430L Corrected Flux
25
Instrument Science Report STIS 2004-04
Figure 23: G750L Corrected Flux
26
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)
27