Instrument Science Report STIS 98–15
MAMA Flat-field Status and Plans
Dick Shaw, Mary Elizabeth Kaiser, and Harry Ferguson
1998 April 6
ABSTRACT
An on-orbit flat-field monitoring program was initiated for the MAMA detectors with the
goals of comparing the pre-launch flat-field characteristics with those observed on-orbit,
measuring the stability of the FUV- and NUV-flats, and creating updates to the reference
flats. We found that the NUV-MAMA flat-field images at nominal (1024x1024) resolution
are well corrected with pre-launch flats. That same comparison for the FUV-MAMA shows
features in the pre-launch flat that make it unsuitible for generic pipeline use, although it
resolves some important questions. The output of the flat-field lamps has declined ~15–
20% in less than 12 hours of on-orbit use, which is much larger than that expected. A plan
is described for obtaining further on-orbit flat-field exposures in view of potentially much
shorter lamp lifetimes, and for exploring alternatives for using the Kr and D2 lamps.
1. Introduction
The STIS calibration sub-system includes a krypton (Kr) lamp and a deuterium (D2)
lamp for uniformly illuminating the MAMA detectors so that flat-fields may be derived.
Exposures with these lamps have been obtained both in pre-launch testing, and subsequently on-orbit (see Kaiser, Lindler, & Bohlin 1997, hereafter KLB; and Kaiser et al.
1998). This ISR describes the methodology and goals of flat-field calibration programs
that were initiated in August, 1997 to construct and evaluate the ground- and on-orbit flats.
Section 2 of this report describes the status of two calibration programs that were implemented to obtain flat-field images and monitor the flat-field stability during Cycle 7.
Section 3 probes the applicability of the pre-launch P-flats to the flat-field data obtained
on-orbit, while §4 describes a strategy and implementation plan for new on-orbit flat-field
programs in the context of managing the remaining lamp life over the expected 13-year
operational lifetime of STIS.
The method for constructing flat-field reference files from the constituent images was
described by Bohlin, Lindler & Baum (1996). In brief, the approach was to divide the flatfield into three components: a P-flat, which characterizes the pixel-to-pixel sensitivity
1
variations at a signal-to-noise (S/N) ratio of at least 100:1 per resolution element (or 2x2
low-res pixels), an L-flat, which characterizes variations on longer spatial scales (such as
vignetting), and a D-flat, which characterizes any time variability of the flat at a S/N of at
least 100:1 per low-res pixel. The time variability is believed to be potentially important
for the MAMAs, as charge is extracted from the micro-channel plates preferentially at the
spectral locations during the course of normal science operations1.
At the end of the SMOV program in the summer of 1997, a NUV-MAMA P-flat had
been constructed, though with a slightly revised technique (Bohlin, Lindler & Kaiser
1997). This reference P-flat was shown to be independent of spectral mode and color, and
of good quality, with a S/N ratio of ~150 per low-res pixel. At that time, no P-flat had been
successfully constructed for the FUV-MAMA, as few pre-launch exposures were available
for constructing a flat-field of good quality. In the past few months an FUV P-flat was constructed from a combination of echelle mode and first-order flat-field exposures. The
evaluation of that reference flat is presented in §3 and in KLB.
2. On-Orbit Flat-Field Monitoring
STIS calibration programs 7644 and 7728 were initiated in early August, 1997 to
obtain flat-field exposures for the NUV- and FUV-MAMAs, respectively. The goals of
these programs were the following:
1. Create on-orbit D-flats periodically (initially, once every two months) for each of
the NUV- and FUV-MAMAs.
2. Create a baseline for monitoring changes in the flat-fields: tracking localized DQE
losses, changes in color dependence, etc. At the outset of program 7728, important
features of the FUV-MAMA flats were not known, including any possible color- or
spectral mode-dependence.
3. Accumulate the constituent flat-field exposures gradually to look for early signs of
lamp degradation, to monitor the short-term stability of the lamps, and to permit a
periodic optimization of slit width and illumination.
4. Derive low-order flats (L-flats) for comparison to those made from a separate
vignetting study.
5. Assess the need to obtain further on-orbit D-flats throughout the STIS lifetime.
Initial Strategy
The initial monitoring program called for obtaining one 78-min flat-field exposure per
week for each MAMA, to alternate between at least two wavelength settings, and to sub1. To ameliorate the effect of charge extraction, the first-order and echelle spectra are offset each
month to slightly different locations on the detector in the cross-dispersion direction. This offset strategy will be the subject of a future ISR (Walsh & Baum 1998, in preparation).
2
step the slit where necessary to ameliorate the obscuration by the fiducial bars. Approximately 200 hours of calibration time had been reserved per MAMA detector for this
purpose in Cycle 7. A log of the exposures in the initial program is given in Table 1 for the
FUV-MAMA, grouped by observing mode; Table 2 gives a similar log for the NUVMAMA. The “Reference Rate” in the tables is the global count rate expected from prelaunch testing; the “peak” rate refers to the highest illumination achieved during the monitoring program. Initially, the global count rate was kept below 200,000 counts/s until onorbit experience with the lamp illumination could be accumulated.
Table 1: Log of Program 7728 FUV-MAMA
Obs Date
Mode
Aperture
Count
Rate
Reference
Rate
Observed/
Reference
Observed/
Peak
o46l01e7m
4-Aug-97
31x0.05NDC
197526
187954
1.0509
1.000
o46l04q8q
26-Aug-97
G140L/
1425
179253
0.9537
0.904
o46l07ngs
17-Sep-97
170169
0.9054
0.859
o46l02qrm
13-Aug-97
0.8512
0.945
o46l05avq
1-Sep-97
105208
0.6455
0.922
o46l08hdq
22-Sep-97
101066
0.6201
0.800
o46l03dqm
18-Aug-97
0.7704
0.920
o46l06uvq
9-Sep-97
0.7422
0.882
Image
G140M/
1272
G140M/
1518
52x0.05
138736
52x0.05
106226
162982
137888
102343
Table 2: Log of Program 7644 NUV-MAMA
Image
Obs Date
Mode
Aperture
Count
Rate
Reference
Rate
Observed/
Reference
Observed/
Peak
o46o01dpm
4-Aug-97
52x0.2
199902
194645
1.0270
1.000
o46o03btm
18-Aug-97
G230M/
2095
186411
0.9577
0.933
o46o05atq
1-Sep-97
175285
0.9005
0.877
o46o07m9s
17-Sep-97
168187
0.8641
0.841
o46o04pzq
26-Aug-97
0.8277
0.900
o46o06mlq
8-Sep-97
104971
0.7909
0.851
o46o08gkq
22-Sep-97
101863
0.7675
0.820
G230M/
2659
52x0.05
109846
132716
Figure 1 shows the detector illumination from the lamps for each of the modes in
Tables 1 and 2. An average count rate of 0.286 counts/s/pixel over the whole detector cor-
3
responds to a global count rate of 300,000 counts/s. Note the non-uniform illumination for
the FUV-MAMA, especially for the G140L and G140M/1272 exposures. This non-uniformity is a result of the limited spectral extent (128–160 nm) of the Kr continuum lamp in
the ultraviolet and the selection of grating configurations at the edges of the Kr lamp’s
FUV continuum bandpass. If the flat-field characteristics had been found to vary strongly
with mode, the G140L/1425 setting would have had to be supplemented with two other
central wavelength settings, including one at 1575, to provide sufficient counts over the
whole detector.
Figure 1: Illumination in the dispersion direction from the Kr lamp (upper) and D2 lamp (lower) for the
modes indicated.
Degradation of Lamp Output
In addition to monitoring the stability of features on short and intermediate scales, the
output of both the Kr and D2 lamps was monitored for signs of early degradation. A plot of
the lamp output as a function of hours used is shown in Fig. 2. In this figure, the lamp output has been normalized to the start of the monitoring campaign. The engineering data
were used to monitor the lamp output, temperature, and other parameters during the
course of each exposure. These data showed certain systematic trends, including, e.g., a
4
change in lamp output with lamp temperature which was usually, but not always, anti-correlated. The points in Fig. 2 have been corrected to a common lamp temperature of 15 C,
which was the approximate temperature at mid-exposure. The discrepancy with one of the
points from G140M/1272 may result from a small change in lamp color. All of the points
(after those at 1.4 hours) are uncertain by a few percent, owing to uncertainties in the normalization between different gratings.
Figure 2: Decline of Krypton (triangles) and Deuterium (squares) lamp output since the start of the flatfield monitoring programs.
What is most striking in Fig. 2 is the decline of 15–20% in both the Kr and D2 lamp
output. From the limited post-launch data it is not clear if the decline in the lamp output is
the result of a long term stabilization or photo-polymerization of contaminants on the
lamp window or other optic in the path which then attenuates the lamp flux. Groundbased measurements with the internal D2 lamp during thermal vacuum calibration exhibited a qualitatively exponential decline over 13 sequential one-hour observations. This
decline during thermal vacuum appeared to be the result of a long term stabilization which
had not been observed in lab testing. Given the limited data, both pre-launch and postlaunch, the reason for the decline in the post-launch data is not yet understood.
If the appropriate physical model for the decline is contamination on the lamp window,
then the decline would be expected to follow a decaying exponential of the form
I = I o e – ( α ⋅ τ ) where α relates to the type of material deposited on the window (which in
principle could change with time), and τ is linearly related to the cumulative lamp on-time.
But given the uncertainty in the stabilization time constant for the lamp and the short time
span of the monitoring programs, it is difficult to predict the remaining lamp lifetime accu-
5
rately. A least-squares fit to the D2 lamp output with an exponential decay (excepting the
first point) would give Io = 0.9736 and α = –0.01794. Extrapolating this relation would put
the lamp output at 50% after 37 hours of use, at 20% after 87 hours, and at 5% after 165
hours. A simpler, but less physical, linear decline of lamp output (based upon the last 4
observations with the D2 lamp) would predict zero output after roughly 75 hours of use.
Both these results are much less than the predicted lamp lifetimes from ground tests.
Since the applicability of either model is unclear, and the extent of the extrapolation is
so large, the lifetime estimates are extremely uncertain. If a decaying exponential is indeed
the more appropriate model, then a lifetime based on a linear decline is a conservative
lower limit. The decline in the Kr lamp output is even harder to project, owing to the scatter in the measured lamp output, but it should be similar in form to the D2 curve if an
accumulation of contaminants is responsible for the degradation. Both estimates are well
below the 475 hour (Kr) and 740 hour (D2) lifetimes expected from pre-launch vacuum
testing. Assuming a useful lamp lifetime of between 75 and 170 hours, and approximately
10–20 hours of exposure time to produce a D-flat, it should be possible to obtain between
6 and 15 more flats of this quality.
3. Applicability of the P-Flats
We evaluated the applicability of the P-flats at the nominal (1024x1024 pixel) resolution.2 The degree to which a constructed P-flat actually normalizes the spatial variation in
the detector response can be measured in a few complimentary ways. Perhaps the most
familiar is a blink comparison between the raw image and the same image after applying
the flat-field. This technique is most useful for evaluating how well the major structural
features are removed, such as the hexagonal and moire patterns, and the various blemishes. This sort of comparison works best on images with fairly high, uniform
illumination, such as flat-field images.
A second approach is to examine the short-scale variance in images with fairly uniform illumination. The method is to calculate the mean and square-root of the variance in
small checkboxes over the entire image. This technique works best if applied to several
images that together span a large range in illumination. As the checkboxes are made
smaller, the effect of small slopes within the boxes decreases, and the majority of the
points fall on a curve that characterizes the noise model for the detector. (Those boxes that
subtend steep gradients in image intensity, such as those near the fiducial bars, are easily
distinguished from the characteristic curve.) The applicable noise model for the MAMA
detectors is shot noise with a gain of unity, and zero read noise; the effect of systematic
changes in pixel-to-pixel detector response is modelled with a noise term that scales lin2. The stability of the flat-fields in the high-resolution (2048x2048 pixel) format will be the subject
of a future report.
6
early with the signal. The STSDAS noisemodel task implements this technique, and
provides a comparison of the expected and realized noise at all illumination levels in the
images.
Another approach to evaluating the flat-fields is to measure the realized S/N ratio in
extracted spectra of stars both before and after applying the P-flat. This evaluation is perhaps the most relevant for spectral data, though it is harder in practise to quantify. The
technique is to measure the improvement in S/N in an area that is binned over the area of
the virtual extraction slit (2 spectral x 11 spatial pixels, typically); the measurements are
best performed over line-free continuum regions of >50 wavelength bins. Thus, this sort of
comparison is less sensitive to short-scale variations in the P-flat, and the limiting S/N is
most apparent (i.e., measurable) after co-adding several extracted spectra.
FUV-MAMA
The on-orbit flat-field images, while not optimally exposed, have sufficient counts to
understand some important properties of the FUV-MAMA flat-field. The best S/N ratio
that could in principle be expected from the sum of the on-orbit FUV flats is shown in Fig.
3, where the average S/N is taken to be the square-root of the accumulated counts. The distribution along a column is fairly uniform, except under the fiducial bars. While these
images could support the construction of a P-flat with S/N ~80 per low-res pixel over
nearly half the image, the S/N ratio would vary considerably with position in the image.
Figure 3: Average of rows in the summed, on-orbit FUV-MAMA flatfield images
Several FUV flat-field exposures were obtained before launch using an external lamp.
Concern that the FUV flat-fields might be dependent on the angle of incidence, and hence
be mode dependent, motivated acquisition of these flats in the both the medium resolution
7
(E140M) and high resolution (E140H) echelle modes. These flats were evaluated independently and in comparison with the post-launch monitoring flats acquired in the first order
long slit modes, G140L and G140M (Kaiser et al., 1998). Toward this end, two P-flats
were constructed for evaluation: one using the pre-launch echelle images alone, and the
other a hybrid of pre-launch and on-orbit flat-field images. The ratio of these two trial Pflats is shown in Fig. 4. The amplitude of the residual structure has a spatial scale of ~10–
20 pixels and an amplitude of ~6% peak-to-peak.
Figure 4: Ratio of the ground (echelle) FUV P-flat to that composed of both ground and on-orbit images.
The intensity scale is from 0.95 (black) to 1.05 (white)
The structure shown in Fig. 4 results in large measure from non-uniformities in the flat
field illumination. For the ground-based data this is a result of acquiring the flats in the
echelle modes and the masking of emission lines present in the external continuum lamps,
in the post-launch flats it is a result of the initial selection of optical modes and grating
positions for which the internal krypton lamp does not uniformly illuminate the detector in
the spectral direction (KLB, Kaiser et al., 1998). Briefly, the echelle flat-field exposures
were obtained with the 0.5X0.5 arcsec slit, which left gaps in the detector illumination that
were (largely) filled in by slightly tipping the cross-dispersor grating between exposures.
The structure on intermediate and large scales is a result of the great difficulty in normalizing the various illuminated patches (for the echelle flats) and the uneven spectral coverage
(for the first-order modes) to a common scale.
8
Clearly the hybrid flat is not suitable for general pipeline reductions. But it is nevertheless sufficiently applicable for evaluating the reduction of the pixel-to-pixel variations and
most major structural features, and for planning subsequent calibration programs to generate on-orbit reference flats. A single spectrum of GD153 yields a S/N ratio of ~130 per
spectral resolution element (the Possion limit is 165) when the hybrid flat is applied,
though the S/N would be <100 without the flat (KLB). The noisemodel analysis (see Fig.
5) shows that applying the hybrid flat to the the on-orbit FUV flat-field images yields a
substantial reduction in pixel-to-pixel structure, though of course some correlation is
expected since this P-flat is composed partly of the on-orbit flats. That same analysis, but
applying the echelle flat instead, yields about half the reduction in scale noise. That these
trial P-flats reduce the pixel-to-pixel variations (as well as the hexagonal pattern and most
of the moire pattern) suggests that the FUV-MAMA flats are in large measure independent
of spectral mode and color.
Figure 5: Mean vs. square-root of the variance (sigma) computed in 4x25 checkboxes over all post-launch
raw (upper) and flat-fielded (lower) G140L and G140M images. The expected relation (solid curve) is based
on a gain of unity and zero read- and scale-noise; the dashed curve is for a scalenoise of 3.7%, which gives
an indication of the amplitude of the artificial structure in the FUV hybrid P-flat Points falling well off the
general trend are from checkboxes that subtend strong illumination gradients near, e.g., fiducial bars.
9
NUV-MAMA
As mentioned in §1, the pre-launch P-flat for the NUV-MAMA has a S/N of 150 per
low-res pixel, and was found to be independent of mode and of color. Figure 6 shows that
applying this flat to the summed on-orbit lamp images (re-binned to 1024x1024) for
G230M/2659 almost completely removes the major structural features. Though the average counts from the summed, on-orbit images implies a S/N no greater than ~40, it is still
possible to quantify the degree of improvement in S/N ratio after the flat-field is applied. A
blink-comparison of the raw and flat-fielded G230M/2659 frames shows that certain structural features persist (mostly blemishes), though at a lower level than in the raw images.
This suggests either that certain features were imperfectly characterized in the construction of the pre-launch flat, or that there has been a small evolution in the flat-field
characteristics during the intervening year.
Figure 6: Portion of the on-orbit flats for G230M/2659 for the sum (left) of the images, and sum after applying the NUV-MAMA reference flat (right). The nearly horizontal stripes, which result from variations in the
52x0.1 slit transmission with position, are removed during construction of the reference flats. The cross-cuts
(lower plots) shows how the deviations from the mean are reduced in the flat-fielded image.
10
A noisemodel analysis using 10x10 checkboxes on the raw and the flat-fielded images
is shown in Fig. 7. At a mean of ~1000, the sigma in the raw images was ~42, but dropped
to ~33 in the flat-fielded images. This is only a bit below the limiting S/N ratio of 30 based
on the count statistics; that it lies below the limit is not surprising given the structure of a
few percent that is imposed by the imperfections in the 0.1 arcsecond slit.
Clearly, the pre-launch NUV flat works very well for on-orbit flat-field images. KLB
analyzed the realizable S/N ratio for first-order and echelle stellar spectra, with and without applying the flat-field. Their results also indicate that the pre-launch NUV P-flat
improves the realizable S/N significantly, and that it should be routinely possible to obtain
S/N ratios in spectra well in excess of 100:1 per resolution element, given sufficient counts
from the source. The question of temporal stability of the flats can be investigated once the
on-orbit NUV flat-field images are obtained.
Figure 7: Mean vs. square-root of the variance (sigma) computed in 10x10 checkboxes over all post-launch
raw (upper) and flat-fielded (lower) G230M images. The expected relation (solid curve) is based on a gain of
unity and zero read- and scale-noise; the dashed curve is for a scalenoise of 2.7%, which gives an indication
of the amplitude of the structure in the NUV P-flat.
11
4. The Plan for Future On-Orbit Flats
The substantial decline in lamp output caused us to halt the monitoring program pending a review of the detailed engineering data, and of the construction of a plan to manage
the limited lamp lifetimes over the projected 13 year on-orbit lifetime of STIS. The information needed to formulate such a plan includes the projected lamp lifetime, a measure of
the applicability of both the pre-launch and on-orbit flats to science data, and the frequency with which the flat-field stability should be monitored. It is also important to
explore alternatives to using the Kr and D2 lamps for flat-field monitoring and
construction.
The existing data show that the accumulated counts from the on-orbit exposures are, in
themselves, insufficient to construct flat-field reference files of uniformly high quality for
either MAMA detector, and that additional exposures are needed.
Strategy for Future Flats
The analysis performed so far indicates that both the FUV- and NUV-MAMA flats are
nearly independent of mode and of color. The plots of the lamp illumination in Fig. 1, and
of the spectral response of the MAMAs to the flat-field lamps in Fig. 8, show that the most
uniform detector illumination with high count rates can be achieved with the M-gratings.
Figure 8: Spectral response of the Kr and D2 lamps.
Tables 3 and 4 summarize the predicted count rates for a number of combinations of
slit width and central wavelength setting for the first-order M gratings. The reference rate
12
is based on pre-launch measurements, and the predicted rate assumes a 20% decline in
lamp output. The “Required Integration” is the total amount of lamp time required to collect 10,000 counts/pixel, assuming no additional degradation in lamp output. This quantity
is listed where the predicted global count rates are at least 150,000 counts/s, but below
300,000 counts/s. Certain combinations of slit and central wavelength will yield a detector
illumination near the optimum of 250,000—300,000 counts/s, but this optimum combination will change as the lamp outputs continue to decline with use. It will be possible to
obtain well illuminated flat-field exposures even when lamp output has declined to ~4% of
current values using progressively larger slits (up to the 52X2). Below that level, additional flats will require exponential increases in the exposure time to achieve sufficient
counts. .
Table 3: Predicted Kr Lamp Count Rates
Grating/
Central Wave
G140M/1371
G140M/1387
G140M/1400
G140M/1420
G140M/1470
G140M/1518
Slit
Relative Slit
Transmission
Reference
Rate
(counts/s)
Predicted
Ratea
(counts/s)
Required
Integration
(hr)
31X0.05NDA
0.4218
201700
161360
18.1
52X0.05
1.0000
...
382550
—
31X0.05NDA
0.4218
198385
158710
18.4
52X0.05
1.0000
...
376260
—
31X0.05NDA
0.4218
196929
157540
18.5
52X0.05
1.0000
...
373500
—
31X0.05NDA
0.4218
190044
152040
8.1
52X0.05
1.0000
...
360440
—
31X0.05NDA
0.4218
131941
105550
—
52X0.05
1.0000
...
250240
11.6
52X0.05
1.0000
137888
110310
—
52X0.1
1.0000
...
220620
13.2
a. Assumes 20% decline from reference rate in lamp output based on Fig. 2. Also
based on post-launch global rates where available.
Table 4: Predicted D2 Lamp Count Rates
Grating/
Central Wave
Slit
Reference
Rate
(counts/s)
Predicted
Ratea
(counts/s)
Required
Integration
(hr)
G230M/2095
52X0.2
194645
164820
17.7
52X0.5
...
412060
—
13
Table 4: Predicted D2 Lamp Count Rates
Grating/
Central Wave
Slit
Reference
Rate
(counts/s)
Predicted
Ratea
(counts/s)
Required
Integration
(hr)
G230M/2176
52X0.1
124382
99510
—
52X0.2
...
199010
14.6
52X0.1
166657
133330
—
52X0.2
...
266650
10.9
52X0.1
194390
155510
18.7
52X0.2
...
311020
9.4
52X0.05
121405
97120
—
52X0.1
...
194250
15.0
52X0.05
124909
99930
—
52X0.1
...
199850
14.6
52X0.05
132716
106170
—
52X0.1
...
212350
13.7
52X0.05
134056
107244
—
52X0.1
...
214490
13.6
52X0.05
128634
102910
—
52X0.1
...
205810
14.2
52X0.05
125849
100680
—
52X0.1
...
201360
14.5
52X0.05
124234
99390
—
52X0.1
...
198770
14.7
52X0.05
107705
86160
—
52X0.1
...
172330
16.9
G230M/2338
G230M/2419
G230M/2579
G230M/2600
G230M/2659
G230M/2739
G230M/2800
G230M/2818
G230M/2828
G230M/2898
a. Assumes 20% decline from reference rate in lamp output based on
Fig. 2. Also based on post-launch global rates where available.
The optimum choice for the next flat-field programs are denoted in the tables in boldface. Since there are no pre-launch or on-orbit exposures available with the flight D2 lamp
and G230M/2338, we used an on-orbit exposure for program 7105 obtained on 1997 May
1 with the G140M/1470 setting and the 31X0.05NDC aperture as the basis for a prediction
of the expected detector illumination, which is shown in Fig. 9. The count rate of 266,400
is scaled to account for the NDC throughput of 7.04%, the 3.6% obscuration from the
occulting bars (with the 52X0.05 slit), and the 20% decline in lamp output from Fig. 2.
14
Figure 9: Predicted illumination in the dispersion direction for the grating/central wavelength indicated.
Plans for Cycle 7 On-Orbit MAMA Flats
The analysis performed here and by Bohlin, Lindler, & Kaiser (1997) and KLB demonstrates that the NUV-MAMA reference flat is of high quality and normalizes the pixelto-pixel sensitivity very well. There is, however, evidence for some flat-field evolution in
the NUV flat from pre- to post-launch. The trial FUV-MAMA reference flat, though useful
for demonstrating the applicability and mode independence of the FUV flats for science
data, is not suitable for generic pipeline calibration.
It is very important, then, to establish a new flat-field program for each camera, with
the goal of obtaining the necessary exposures by summer of 1998, and of delivering new
flat-field reference files for use in the pipeline by the following fall. A repeat of the flatfield programs for both MAMAs will be planned for the spring of 1999; these plans will of
course be re-evaluated after the flats from the spring of 1998 have been acquired and
analyzed.
The following strategy will be employed for the Cycle 7 flat-field programs:
1. Optimize the illumination by adjusting the combination of slit and central wavelength such that the count rate is roughly 250,000 to 300,000 counts/s. For the next
flat-field campaign, use G140M/1470 with the 52X0.05 slit for the FUV-MAMA;
and use G230M/2338 with the 52X0.2 slit for the NUV-MAMA (see Tables 3 and
4).
2. Implement five slit-step positions in order to illuminate the pixels normally
obscured by the occulting bars. There are 35 SLIT-STEP encoder positions per
MAMA pixel, and the plate scale is 0.029 arcsec/pixel. The fiducial bars are 0.50
15
and 0.89 arcsec in size, so relative SLIT-STEP values of +/-1680 and +/-3360 will
leave a gap of approximately 18 pixels between the projections of the occulting
bars. These settings were used successfully for the ground flats.
3. Execute the first visit 4 weeks prior to the remaining visits in order to verify the
lamp illumination and to provide an opportunity to re-optimize the slit/central
wavelength combination. Note that the first visit for NUV will be a 10 min. test
exposure to validate the lamp illumination.
4. Obtain the remaining images for a given band at the rate of ~one per week. This
rate allows some flexibility for changing the choice of slit/central wavelength midway through the program if warranted by the decline in lamp output.
Given a global count rate of >250,000/s (>0.24 counts/s/pixel), summed flats can be
constructed with a S/N = 100:1 per low-res pixel with exposures totalling ~12 hours of
lamp use. The existing, on-orbit exposures provide about one-quarter to one-half of the
needed counts for an initial, on-orbit flat. Subsequent flats should therefore be gathered in
campaigns of ~3 month’s duration.
Alternatives to the Flat-Field Lamps
Evidence for flat-field evolution on a timescale of ~1 yr (at least for the NUV) suggests
that monitoring is needed more frequently than annually. The current estimate of <200
hours of remaining flat-field lamp life may be less than that needed to obtain D-flats annually for the 13-year on-orbit life of STIS; it certainly rules out monitoring the flat-field
stability on a more frequent basis with these lamps. (The projection for the lamp lifetimes
will of course be continually refined as new flat-field exposures are obtained.) It is therefore important to identify alternatives to the Kr and D2 lamps for monitoring and/or
making flat-field exposures. The first visit for the spring 1998 flat-field programs will
include a short test exposure with the HITM lamps in imaging mode. Normal wavecal procedures will be used for turning on the lamps. The details of these exposures are given in
Table 5. These exposures will serve as an illumination check, and are expected to yield
Table 5: Planned HITM Lamp Exposures
FUV
NUV
HITM-2
HITM-1
10 mA
3.8 mA
Spectral Element
MIRROR
MIRROR
Aperture
F25ND3
F25ND3
19,000
20,700
10 M
10 M
Lamp
Current
Predicted Global
Count Rate
Exposure Time
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global count rates of ~10% of the optimal illumination. If successful, a renewal of the
MAMA flat-field monitoring campaign will be revisited.
5. References
Bohlin, R., Lindler, D. J., & Baum, S. 1996, STIS Instrument Science Report 96-15.
Bohlin, R., Lindler, D. J., & Kaiser, M. E. 1997, Pre-Launch NUV-MAMA Flats, STIS
Instrument Science Report 97-07.
Kaiser, M. E., Lindler, D. J., & Bohlin, R. C. 1997, Proc. HST Calibration Workshop,
eds. S. Casertano, et al. (Baltimore: Space Telescope Science Institute), p. 29 (KLB).
Kaiser, M. E., Bohlin, R. C., Lindler, D. J., Gilliland, R. L., and Argabright, V. S.
1998, PASP, submitted.
Walsh, J., & Baum, S. 1998, STIS Instrument Science Report, in preparation.
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