SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
Instrument Science Report STIS 2013- 05
STIS CCD spectroscopic P-flats
E. Mason
October 3, 2013
ABSTRACT
The analysis of the cycle 17 STIS CCD spectroscopic flats (Niemi 2010) pointed to the need
to create a new dust mote image for the low resolution grating (L-mode) P-flats. This ISR
describes the creation of the new L-mode P-flat from low resolution exposures taken during
cycles 18 and 19. The document also analyzes the time dependence of the dust mote images
as well as that of the intrinsic flat noise, comparing flat exposures collected since the start
of operations. The effectiveness of the flat field correction is also tested and verified against
high signal to noise observations of spectrophotometric standard stars. Recommendations
are given for future observing strategies, flat fielding correction and future tests of the flat
frames.
1. Introduction
Flat fields are needed in order to correct the instrumental signatures and the pixel
to pixel response of the STIS CCD detector. There are 2 potential ways to remove those
via flat-fielding: 1) by dividing the science frames by a high signal to noise ratio (SNR)
flat field image taken with exactly the same instrument setup (this corrects at a time for
both the spectrograph response and the CCD pixel to pixel variations); 2) by creating 2
c 2008 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Copyright ⃝
separate calibration images: one which contains only the pixel to pixel variation (i.e the high
frequency variations: the P-flat) and the other which describes the instrumental response for
each instrument setup (i.e the low frequency variations: the L-flat). While the P-flats require
high SNR observations (typically obtained with internal/external quartz lamp exposures),
the L-flats do not and can be obtained by different types of observations (e.g. low SNR flat
filed, dithered/offset images/spectra of a standard stars). Method 2 is the one adopted at
STScI as it requires far less calibration time and HST visits than method 1. However, in
order to be applied it must be assumed that the P-flat is independent of the wavelength and
CCD gain.
Bohlin (1999) analyzed a number of flat field frames taken with a multitude of different
instrument setups (grating, aperture, gain and lamp) during the commissioning phase and
the first year of operations. He showed that P-flats are wavelength independent so that flat
field data taken with different medium resolution (MR) gratings can be combined. He also
showed that, for the same reason, P-flats built from MR grating exposures (M-mode P-flat)
can be used to correct science observations taken with the low resolution (LR) gratings,
provided that the different anamorphic magnification of the MR and LR gratings is properly
taken into account (see Bohlin 1999 and Niemi 2010). Last but not least, Bohlin’s analysis
also showed that the P-flats are independent of the CCD gain. However, that same analysis
showed that the flat fielding is not perfect and the ability to correct any science exposure
for pixel-to-pixel variations depends on the time elapsed between the science and flat-field
observations. Very high SNR data can be obtained only with contemporaneous science and
requires high SNR flat-field observations with at least 1 million electrons/pixel (Bohlin 1999).
2. The flat monitoring plans and delivered reference files
Following Bohlin’s STIS CCD spectroscopic flat analysis, the instrument calibration
programs continued collecting a number of flat exposures taken with different instrument
setups (see Table 1). The goals of each calibration program were to i) produce a new reference flat and ii) continue monitoring of the individual modes. At the end of each cycle
only the g430m exposures were used to create a new P-flat (see, e.g. Diaz-Miller & Goudfrooij 2001, Proffitt & Davies 2003, Dressel & Davies 2004). More specifically, the MR flat
frames obtained with the g430m grating and the apertures 50ccd and 52x2 are separately
processed combining (with calstis) the exposures and normalizing the resulting stacked
images via row-by-row and column-by-column spline fitting. The 52x2 exposures are taken
with the aperture wheel rotated to 5 different offset positions in order to both illuminate the
bottom and top rows of the CCD normally masked by the 50ccd aperture and mitigate the
2
Cycle
Prog ID
8
8411
9
8845
10
8907
11
9613
12
10023
17
11852
18
12405
19
12766
20
13135
Instrumental Setup
# of orbits
G430M/5216/50CCD/T/1
G430M/5216/50CCD/T/4
G430M/5216/52x2/T/4
G750M/6581/52x2/T/1 G230MB/2557/50CCD/D/4 G230MB/3115/50CCD/D/4
G230LB/2375/50CCD/D/4
G430L/4300/52x2/D/4
G430L/4300/52x2/T/4
G750L/7751/52x2/T/4
G430M/5216/52x2/T/4
G430M/5216/50CCD/T/4
G430M/5216/50CCD/T/1
G750M/6581/52x2/T/1 G230MB/2557/50CCD/D/4 G230MB/3115/50CCD/D/4
G230LB/2375/50CCD/D/4
G430L/4300/52x2/D/4
G430L/4300/52x2/T/4
G750L/7751/52x2/T/4
G430M/5216/52x2/T/4
G430M/5216/50CCD/T/4
G430M/5216/50CCD/T/1
G750M/6581/52x2/T/1 G230MB/2557/50CCD/D/4 G230MB/3115/50CCD/D/4
G230LB/2375/50CCD/D/4
G430L/4300/52x2/D/4
G430L/4300/52x2/T/4
G750L/7751/52x2/T/4
G430M/5216/50CCD/T/4
G430M/5216/50CCD/T/1
G430M/5216/52x2/T/4
G750M/6581/52x2/T/1 G750M/6581/50CCD/T/1 G230MB/2557/50CCD/D/4
G230MB/3115/50CCD/D/4 G230LB/2375/50CCD/D/4 G430L/4300/52x2/D/4
G430L/4300/52x2/T/4
G430L/4300/50CCD/T/4
G430L/4300/50CCD/T/1
G750L/7751/52x2/T/4 G750L/7751/50CCD/T/4
G430M/5216/50CCD/T/4
G430M/5216/50CCD/T/1
G430M/5216/52x2/T/4
G430L/4300/52x2/T/4
G430M/5216/50CCD/T/4
G430M/5216/50CCD/T/1
G430M/5216/52x2/T/4
G430M/5216/52x2/T/4
G430L/4300/50CCD/D/4
G430L/4300/50CCD/T/4
G430L/4300/52x2/T/4 G750L/7751/50CCD/T/4 G750L/7751/52x2/T/4
G430M/5216/50CCD/T/4
G430M/5216/52x2/T/4
G430L/4300/50CCD/T/4
G430L/4300/52x2/T/4 G750L/7751/50CCD/T/4
G430M/5216/50CCD/T/4
G430M/5216/52x2/T/4
G430L/4300/50CCD/T/4
G430L/4300/52x2/T/4 G750L/7751/50CCD/T/4
G430M/5216/50CCD/T/4 G430M/5216/52x2/T/4
79
79
79
68
21
50
37
37
19
Table 1: STIS CCD spectroscopic flat monitoring programs performed at each cycle of
STIS operations, together with the adopted instrument setup and the number of used HST
(internal) orbits.
effect of the “occulting bars” which characterize the slit apertures and shade several rows of
the CCD. The normalized flats are then co-added (with proper error and DQ flag propagation) into the final reference P-flat. Because of the gain and wavelength independence, this
new reference flat is delivered to the archive for the calibration of all the offered spectral
modes, including the LR grating observations. However, as the LR and MR gratings have
different magnifications, the P-flat created from the MR flat observations cannot be blindly
applied to LR observations. The blemishes and window dust motes which appear on the
CCD have to be “recreated” at the correct position and size in the P-flat intended for the
LR observations. This is done by copying the image of the dust motes from the LR P-flat
(L-mode P-flat) created during the first cycle of operation (cycle 7) and pasting it into the
new MR/M-mode P-flat. As the dust motes appear smaller in the LR frames, than in the
MR ones, their image is pasted together with that of their surrounding pixels. All these
steps in the creation of the reference P-flat were performed through a set of IDL scripts
3
created by Bohlin (1999). However, since STIS’ return to operations (cycle 17), they have
been re-coded in a python script by Niemi (2010), which has then been slightly revised by
Mason (2013, see TIR 2013-02 for a detailed comparison of the two versions of the python
codes).
3. Time evolution
3.1 Evolution of the actual and intrinsic noise
In order to check for the time evolution of the CCD flats across the life of STIS, we downloaded from the archive the flat-field frames taken with the g430m/50ccd, g430l/50ccd
and g430l/52x2 setups and processed them with calstis for bias and dark correction,
co-addition, and cosmic-rays rejection. In particular, we stacked together all the images
taken with the exact same instrument setup, producing what we called a “superflat”. We
note, however, that only the g430m/50ccd superflats reach very high SNR at any time and,
therefore, we limited our analysis to these frames. Similarly to Bohlin (1999) we computed
the standard
deviation, the Poisson noise and intrinsic noise, where the latter has been de√
2
fined as stddev2 − Pnoise
). Figure 1 shows the evolution of the detector noise with time
as computed from a defect-free, uniformly illuminated area of each cycle’s superflat.
We also computed the actual, Poisson and intrinsic noise in “monthly superflats” and
individual imsets/frames (see Figure 2, left and right panels respectively). We call monthly
superflats the calstis processed and stacked g430m/50ccd exposures which were taken
within each visit. In fact, these visits are typically spaced by 30-40 days from each other, for
monitoring purposes. The imsets, instead are the single exposures which are taken within a
visit. The comparison of Figures 1 and 2 shows that despite the fact that monthly superflats
and imsets have significantly lower count levels than the cycle superflat (see top panel of
Figures 1 and 2), they all share similar noise characteristics. This suggests that the intrinsic
noise cannot be reduced and is not averaged out by indefinite co-addition of frames. At
the same time it appears that the actual and intrinsic noise levels have increased after the
first year of operations but have been relatively stable since cycle 8. Figures 1 and 2 also
show that the post-SMOV4 flats have actual and intrinsic noise levels slightly larger than
their corresponding imset. This instead might suggest systematics effects within the data
processing, possibly a non optimal setting of the cosmic ray-rejection parameters (Bohlin,
private communication).
We note however, that the actual and intrinsic noise levels discussed here are not directly
comparable to tables and figures in Bohlin (1999) as these are not normalized frames (i.e.
4
0.01
0.005
0
10
15
Fig. 1.— The relative noise of the STIS CCD spectroscopic flats across the life of STIS, as
measured from the g430m/50ccd superflats from each cycle. Solid black circles represent
the detector standard deviation, empty squared symbols represent the Poisson noise and
the diagonal crosses represent their difference in quadrature (aka intrinsic noise). All noise
values have been computed after converting the DN into electrons.
they have not yet been fit along the rows and the columns with a spline function). The
normalization process will reduce the actual and intrinsic noise as it remove the low frequency
variations which are not insignificant in the case of the g430m flat exposures. In fact these
are characterized by a steep illumination gradient along the dispersion direction. In order to
minimize the bias of the low frequency variations, we selected the flattest and most uniformly
illuminated area in the g430m/50ccd frames, which we identify in [875:925,140:330]. We
should also note that, once normalized, i.e. once the P-flat has been created, the actual
noise is relatively constant with values in the range 0.009-0.011 in all the archived reference
p-flats. This was verified by checking the dispersion in the central 100x100 pixels of each
reference P-flat.
We also divided each cycle’s superflats by its average count level (in order make them
5
0.01
0.01
0.005
0.005
0
0
0
1000
2000
3000
4000
0
1000
2000
3000
4000
Fig. 2.— The relative noise (in electrons) of the STIS CCD spectroscopic flats across the life
of STIS, as measured on the monthly g430m/50ccd superflats (left panel) and the individual imsets (right panel). The solid black circles represent the detector standard deviation,
empty squared symbols represent the Poisson noise and the diagonal crosses represent their
difference in quadrature (aka intrinsic noise). Note (left panel) how the Poisson noise varies
with the exposure time (and therefore the incoming photon flux) adopted at the various
epochs. The fact that it is constant for a given exposure time implies that the lamp flux is
constant. The vertical scatter of the Poisson noise in the right panel reflects the variation
of the lamp’s output within each visit: the lamp takes some time to warm-up so that the
first exposures have smaller fluxes than the last. The deviant points in the right panel are
explained with cosmic rays (and SAA passages), which have not been removed in the single
imsets.
directly comparable to each other) and then divided by the cycle 8 superflat1 . The residuals
in the central area as well as the whole detector are reported in Table 2. It is noticeable
that the residuals of the ratio images increase with time as already reported by Bohlin
(1999). In particular, the residuals increase from ∼0.3% to 0.7% when consecutive or 10
year apart superflats are considered, respectively. The residuals vary from 0.3 to 1.1% when
the whole sensitive area of the detector is considered. It should be also noted that the post
SM4 superflats display different vignetted/shaded areas (on the top and the bottom of the
1
The cycle 7 superflat has low count levels and therefore lower SNR and for this reason was not considered.
6
Superflat Ratio
Cycle
Prop ID
9/8
10/8
11/8
12/8
17/8
18/8
19/8
8845/8411
8907/8411
9613/8411
10023/8411
11852/8411
12405/8411
12766/8411
σ
[575:675,460:560]
[9:1013,49:973]
0.003183
0.004072
0.004512
0.005124
0.005795
0.006819
0.007202
0.003115
0.004144
0.004617
0.005626
0.006417
0.01103
0.01114
Table 2: Standard deviation in the central area and the whole detector as computed from
the ratio of each cycle’s superflat to cycle 8, created with the g430m/50ccd exposures.
detector)from those produced before the STIS failure. This affects the residual computed
on the whole detector area.
Superflat Ratio
Cycle
[64:93,632:660] ⋆
[140:169,515:543]
σ
[190:226,1000:1021] ⋆
[550:586,580:601]
9/8
10/8
11/8
12/8
17/8
18/8
19/8
9/8
10/9
11/10
12/11
17/12
18/17
19/18
0.003127
0.004091
0.004924
0.005769
0.006847
0.007948
0.008284
0.003127
0.002857
0.002126
0.002654
0.005703
0.003176
0.002417
0.00326
0.004095
0.004321
0.005108
0.005456
0.00686
0.007262
0.00326
0.00281
0.002065
0.002898
0.005261
0.002946
0.002338
0.003158
0.004806
0.005526
0.006434
0.007839
0.1168
0.1121
0.003158
0.003326
0.002362
0.002626
0.005986
0.1159
0.009212
0.002932
0.003564
0.004149
0.004849
0.005862
0.007184
0.007384
0.002932
0.002706
0.00205
0.00266
0.00543
0.003493
0.002444
Table 3: Standard deviation in the two areas affected by the shift of the dust motes (columns
1 and 3, marked with the ⋆) and in the comparison area of equal size that is free from low
frequency variations (columns 2 and 4) as computed from the ratio of each cycle’s superflat to
cycle 8 (top) or the previous cycle (bottom). Superflats were created from the g430m/50ccd
exposures.
3.2 Evolution of the dust-motes
In order to verify the time scale for the variation of the dust motes we visually inspected
each cycle’s superflat ratio images as well as the ratio of the cycle’s superflat with respect to
7
cycle 9 superflat. It is apparent that a minor shift of a few dust motes has occurred during
cycle 9 as all ratio images show the same structures when their ratio is computed adopting
the cycle 8 superflat as denominator; while no structures are apparent when the ratio is
computed with respect to the cycle 9 superflat.
We have computed the residual of the ratio images in two areas covering the shifted
motes and in two areas of equal size which are free from low frequency variations. We used
both the ratio images of each cycle’s superflat to the cycle 8 superflat and the ratio of each
cycle’s superflat to the previous cycle’s superflat. The residuals in the 4 areas are reported in
Table 3, for each ratioed image. The table shows that there is no or just minimal difference
in the residuals computed at the dust mote position and the “clean” detector area, when
ratio images made of consecutive superflats are considered. However, the residual noise in
the dust mote position increases faster with time than in the clean area, when the analyzed
ratio images are those created dividing each cycle’s superflat with the cycle 8 superflat. This
implies that, once a change has been detected, the dust mote image should be updated
immediately otherwise noise is added in the flat-field corrected frame, although in very
localized areas. Note also that the larger values of the residual noise recorded for the cycle
19 and 18 ratios, at the position of the top blemish, are explained with the change of the
“vignetting” shadow and its steeper slope. Hence, it should not be ascribed to the actual
dust mote.
At last we repeated the same exercise on the LR g750l/50ccd flats taken since SM4
(i.e. within the flat monitoring program during cycles 17, 18 and 192 ). We focused on the
g750l/50ccd setup for its larger un-vignetted area in the cross-dispersion direction and
its uniform (i.e. flat) response curve in the dispersion direction. In addition, since the dust
motes are better focused and sharper in LR flats than in MR ones, small shifts are possibly
more easily noticeable. Taking the ratio of the g750l/50ccd superflats of cycle’s 18 and
19 to that from cycle 17 we notice no low frequency features of any type down to the 0.3%
level. The fact that no variations at the position of the blemishes are noticeable between
one cycle and the next or across two cycles (i.e. 2 years) implies that it is safe to use old
L-mode P-flat dust mote images for the creation of the L-mode P-flats from the M-mode
ones. However, variations as large as >2%
have been reported between the cycle 17 LR
∼
flats dust mote images and those in the L-mode reference P-flat created back in 1999 (Niemi
2010).
From this and the results in Table 3 it seems that the dust motes map should be re-done
every fourth cycle at the very most, or even less frequently: the baseline is across only 3
2
These are the only flats with sufficient and comparable signal.
8
cycles in the case of the LR flats, and up to 10 cycles in the case of MR flats.
4. The new L-mode/LR P-flat and dust motes image
Following the need to obtain a new dust motes image with the LR grating magnification,
cycle 18 and 19 flat monitoring programs collected a number of flat exposures with the low
resolution gratings g430l and g750l. The observing strategy was identical to that adopted
from the MR grating flats: 50ccd aperture flats are taken to collect most of the light and
52x2 aperture flats are taken at offset positions in order to extend the signal in the upper
and lower part of the CCD (otherwise “vignetted” by the 50ccd aperture). The procedure
to process the LR flats and create the corresponding P-flat is also identical to that for the
MR grating exposures, with the caveat of using different masked areas when necessary. The
raw imsets were individually bias and dark subtracted and then combined together within
calstis. Images taken with the same instrument setup were further combined (ocrreject)
and then columns and rows were normalized via spline fitting, producing one P-flat per
instrumental setup. Finally the various P-flats were combined in a single reference P-flat
(eventually delivered to CDBS).
We note that among the LR (/L-mode) exposures, those taken with the g750l grating
are far more luminous and uniform than those taken with the g430l grating. However,
g750l/slit flat exposures are not suitable for the creation of the L-mode P-flat due to
fringing (see STIS handbook). We also note that the g430l flat exposures show a steep
gradient from blue to red wavelengths which is enhanced when the 52x2 aperture is used.
The SNR in the bluer part (∼ first 300 columns) of the g430l/52x2 exposures is so low
that they could degrade (rather than increase) the SNR of the combined reference P-flat.
We verified this by creating two different L-mode P-flats: one using only the LR grating
exposures taken with the 50ccd aperture and one using all the available LR grating frames
(i.e. including the g430l/52x2 exposures) The standard deviation in the P-flat created
using only the 50ccd exposures is 0.0109, 0.0087 and 0.0102 in the first 300 columns, the
central 100x100 pixels and the whole detector, respectively. The standard deviation in the
P-flat obtained combining also the 52x2 exposure/P-flat is, instead, 0.086, 0.012 and 0.047 in
the first 300 columns, the central 100x100 pixels and the whole detector, again, respectively.
There is a difference in the masked area in the two P-flats: the bottom 17 rows are masked
(i.e. =1) in the “50ccd P-flat” while only the bottom 3 rows are masked in the p-flat
created combining both 50cdd and 52x2 exposures3 . However, the gain of the 14 bottom
3
The top 26 rows remain masked in both reference frames.
9
Fig. 3.— The ratio image of the cycle 18 L-mode reference P-flat to the cycle 7 L-mode
reference P-flat. The latter was used from cycles 8 to 17 as source of the dust mote images
to be pasted in the M-mode P-flat for the creation of the L-mode P-flats. The ratio image
shows the shift and changes in several dust motes with differences up to ∼8% (see cuts
sub-window in the bottom left corner of the figure). The wavy pattern on the right side of
the detector is just an effect of the different spline fitting and is ≤0.5%, consistent with the
residuals in the other “clean” area of the detector.
rows is not worth the increased noise in the final reference P-flat. Hence, we conclude that
the use of the 52x2 exposures is not recommended and that they should be dropped from
any flat field monitoring program which include LR g430l flat observations. This would
save about 3540 s per cycle or 10 internal orbits. However, by creating the L-mode P-flat
using only the 50ccd aperture exposures, we loose information about the dust motes on
the very bottom part of the detector. For this reason the complete set of LR flat exposures
taken during cycles 18 and 19 were combined to create a “reference dust mote image” to
be used to copy the dust mote and blemishes images that have to be pasted in the M-mode
P-flat when creating the L-mode P-flat. This image should not be used as L-mode reference
P-flat (because of its large noise).
10
We compared the new (cycle 18) L-mode reference P-flats with the one created in 1999.
The ratio image of the new P-flat to the old one is shown in Figure 3. It is noticeable that
a number of dust motes have shifted, showing differences up to ∼8%, thus justifying the
creation of a new L-mode P-flat.
We also created a P-flat for LR observations using the old strategy, i.e. using the Mmode P-flat into which we pasted the new L-mode dust mote images. In this case, the
comparison of the two P-flat shows that the differences are only significant in the masked
area4 . The ratio of the two frames shows that they are comparable as the residuals are
within the expected noise. In particular, we measure residuals of ∼0.0043 and ∼0.0026 over
the whole detector area and the central 100x100 pixels, respectively.
4.1 Testing the P-flats
We downloaded from the archive the spectrophotometric observations of the standard
star agk 81D266 taken in cycle 18, program 12411. In particular we downloaded the three
172 s exposures taken with the g430l grating at the standard position (row y≃530) and
the three 144 s exposures taken with the g430l grating at the E1 position (row y≃913).
We reduced each exposure using the calstis pipeline. However, we reduced the same data
multiple times, each time adopting a different reference P-flat or no flat correction at all. In
particular, we used the L-mode P-flat created from the MR exposures of cycle 17, the L-mode
P-flat created from the LR exposures of the cycle 18 monitoring program and the L-mode Pflat created from the LR exposures of the cycle 19 monitoring program. The idea is to check
whether flat fielding is necessary or not and whether the use of different reference flat makes
a significant difference or not (thus justifying the production of a reference P-flat per cycle).
However, it should be noted that the stddev of the reference P-flats is always in the range
0.009-0.011; while it is in the range 0.007≤stddev<0.009 in the extracted and normalized
star spectra, which have not been corrected for flat field. Hence, the flat field correction is not
expected to improve the SNR of the science and in fact, we are unable to notice consistent
differences between different flat field corrections when single spectra/visits are analyzed.
However, if we combine the spectra taken with the same instrument setup (including the
position on the detector), similarly to Bohlin (1999), then we can notice the effect of the
flat field correction. In particular, (see Table 4), the flat field generally improves the final
SNR with respect to no flat fielding; while the use of the cycle specific reference P-flat tends
4
As mentioned before the masked area is larger in the L-mode p-flat created from the LR flat exposures,
due to dropping the noisy slit exposures.
11
to improve the SNR only slightly with respect to adjacent cycle reference P-flats. The user
should not forget, as already pointed out in Bohlin (1999), that flat fileding in the case of
the STIS CCD is a delicate procedure where both the signal of the science spectrum and the
epoch of the observation play a role in the final SNR.
With the LR and MR flat exposures taken during cycle 19 we created L-mode P-flat
files from LR exposures, M-mode P-flat from MR exposures as well as L-mode P-flat from
MR exposures (pasting the cycle 18 L-mode dust motes into the new M-mode P-flat) and
M-mode P-flat from LR exposures (pasting the cycle 18 M-mode dust motes into the new
L-mode P-flat). Ratioing the M-mode P-flat from LR exposures and the M-mode P-flat from
the MR exposures, the residuals amount to 0.40% over the whole detector area and to 0.26%
in the central 100x100 pixels, as mentioned before. Similarly, ratioing the L-mode P-flat
from LR exposures with the L-mode P-flat from MR exposures, the residuals are 0.49% and
0.37% over the whole detector and the central 100x100 pixels, respectively. These differences
are due to the different spline fitting (along columns and rows, which uses different number
of nodes depending on the grating resolution).
We then downloaded from the archive the spectra of the spectrophotometric standard
star agk 81D266 observed in cycle 19 (program 12772) and reduced the MR observations
(2 setups) with either the M-mode P-flat created from the LR frames and the M-mode Pflat created from the MR frames. As we have only 2 exposures (120 s each) per setup even
after combining them we notice no significant difference between one data reduction and the
other: the SNR of the data is comparable to that of the flats. Repeating the same test on
the LR specphotometric observations reduced either with the L-mode P-flat created from
the LR flat exposures and the L-mode P-flat created from the MR flat exposures, we obtain
comparable residuals. However, the spectra reduced with the P-flat created from MR flat
exposures is possibly slightly better: we measure residuals of 0.0045- 0.0050 in the spectra
reduced using the reference P-flat from LR exposures and residuals of 0.0044-0.0047 in the
data reduced using the reference P-flat created from MR flat exposures.
In summary, continuing with the old strategy is probably better both because of the
slightly reduced noise and because of the wider illuminated area, though it should be noticed
that a flat monitoring program which relies on LR frames could save a significant amount of
telescope time.
12
texptime=172.8 s, y pos=530, opt elem=G430L
ref pflat
residual in pixels range:
[130:320]
[750:1026]
no flat
0.0089
0.0054
cy 17
0.0059
0.0055
cy 18 (default)
0.0049
0.0052
cy 19
0.0050
0.0053
texptime=144 s, y pos=913, opt elem=G430L
ref pflat
residual in pixels range:
[180:270]
[560:650]
[750:930]
no flat
0.0055
0.0049
0.0047
cy 17
0.0045
0.0043
0.0045
cy 18 (default)
0.0042
0.0039
0.0041
cy 19
0.0044
0.0038
0.0043
Table 4: Residuals in the continuum of the spectrophotometric standard agk 81D266 normalized spectra after calstis processing of the frames with different reference P-flats. Each
spectrum is the average of three identical exposures taken at three different epochs.
5. Summary conclusions & recommendations
We have analyzed the archive g430m/50ccd frames to investigate the evolution of the
flat noise. Our analysis suggests that the flat noise has increased abruptly after the cycle 7
monitoring program but has been fairly stable since then. A ∼0.1% increase is noticeable
also after SMOV4. It should be investigated whether this 0.1% is due to the larger number of
warm pixels or instead implies the need of a revision in the cosmic-ray rejection parameters
currently in use. .
The analysis of the dust mote images has shown that they are stable with time and
suffered a significant shift only during cycle 9. This implies that the adopted strategy for
the creation of the L-mode P-flat can be continued. The new LR frames should be taken
with very low cadence (≥4 cycles). In addition LR flat exposures with slit apertures should
be avoided as the g750l/slit flats are affected by fringing, while the g430l/slit flats
have such low counts in the bluer wavelengths that they add noise in the final combined
reference P-flat. This suggests also that it is preferable to continue with the current strategy
of creating L-mode and M-mode P-flats from MR flat exposures rather than from LR flat
ones. In fact, in the first case the use of the slit aperture allows the creation of a P-flat over
a wider range of detector rows (from 4 to 998 included, instead of from 18 to 998). Given the
above, we created a L-mode P-flat from MR flat exposures for cycle 19, despite having taken
LR frames. We have used the new frame lmode dustMotes map.fits as reference image
for the dust motes at the LR magnification and position. This was created by combining
all the LR flat field exposures taken within the cycle 18 and 19 flat monitoring programs.
Only for cycle 18 the delivered L-mode reference P-flat was created from LR flat exposures.
The option of creating L-mode and M-mode P-flats from LR (50ccd) exposures could be
considered when/if it becomes necessary to reduce the number of internal orbits devoted to
the flat monitoring program. Similarly it might be worth exploring the possibility of taking
g750m/50ccd exposures in place of the g340m/50ccd ones for its higher throughput
13
and more uniform illumination and to offset the 50ccd mask rather than the slit when
maximizing the illuminated detector area.
Finally by testing a variety of reference P-flats on high signal to noise observations
of the spectrophotometric standard star agk 81D266, we have verified that the correction
for pixel-to-pixel variations remains as recommended. However, in the case of lower SNR
observations (probably the majority) the use of the flat filed correction should be evaluated
on a case-by-case basis as already suggested by Bohlin (1999).
Acknowledgments
The author is very thankful to Sean Lockwood for having processed a number of frames
through his CTE correction code helping part of my data analysis. I am also thankful to
Rolf Jansen for having helped with his script that removes high frequency (herringbone)
noise from low signal frames, and to Ralph Bohlin for the having looked back into his old
analysis helping comparison and the interpretation of the data.
References
Bohlin, R., 1999, STIS ISR 1999-06
Diaz-Miller, R., Goudfrooij, P., 2001, STIS ISR 2001-04
Dressel, L., Davies, J., 2003, STIS ISR 2004-06
Mason, E., 2013, STIS TIR 2013-02
Niemi, S-M., 2010, STIS TIR 2010-01
Proffitt, C.R., Davies, J., 2003, STIS ISR 2003-02
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