Instrument Science Report ACS 00-10
FLATS: PRELIMINARY WFC
DATA AND PLANS FOR
FLIGHT FLATS
R. C. Bohlin, G. Hartig, and Z. Tsvetanov
2000 October
ABSTRACT
The ACS WFC pixel-to-pixel P-flats with the build-3 detector repeat over a three
month time span to a precision of better than 0.1% rms. The shape of the low frequency Lflat field varies with wavelength, where the ratio of the F435W/F814W L-flats differs from
unity by –10% in two patches near opposite corners of the field of view. At ~0.5%, the high
frequency pixel-to-pixel structure in the WFC P-flats is about half that of the HRC build-2
and does not increase as much toward shorter wavelengths as the HRC build-2 with the
Lesser coating. The amplitude of the fringes for continuum illumination is less than 1%,
even in the narrow band ramp filters, while the fringing in monochromatic light is –10% at
9300Å.
The goals of the laboratory WFC and HRC flat field program are to obtain the set of
external flats that fully populates the pipeline database and to archive a baseline of high
S/N internal flats for tracking post-launch changes.
1. INTRODUCTION
The WFC was externally illuminated by light from the Refractive Aberrated Simulator/Hubble Opto-Mechanical Simulator (RAS/HOMS), which accurately simulates the
full field OTA illumination at 6328Å. The refractive aberrations at other wavelengths
should not affect the fidelity of flat fields. Several flat field exposures were obtained with
the RAS/HOMS and the non-flight detector assembly B3 in 2000 June, as tabulated in
Table 1. A translucent mylar diffuser screen was added to the RAS/HOMS at the pupil
Instrument Science Report ACS 00-10
plane. This source of external illumination should produce the same flat field as the sky
through the OTA+ACS, except for the dominant OTA geometric distortion over the field,
because only the post-pupil plane RAS optics are used in this configuration. This geometric distortion factor, G (Sparks et al. 2000) or M (Bohlin, et al. 1999, i.e. Paper I), must be
incorporated before delivery of Mylar flats to the reference file database.
A complete flat field for use in pipeline data reduction is the product of the low frequency variations, L, times the high frequency, pixel-to-pixel variations, P, where the
geometric factor G is also included implicitly. Complete LP-flat fields are obtained merely
by dividing the images of the flat field illumination by their average over some region of
the detector, such as the central portion of the useful region. In the case of the preliminary
HRC data in Paper I, flat field illumination was NOT obtained because of spurious reflections from the shiny edges of the non-flight detector mask; and only P-flats could be
derived. For filters that fully illuminate both WFC chips, the lab illumination is proper;
and the normalization for the full field flats is defined by the average of rows 1843:2039
near the top of the lower CCD chip 1, adjacent to the bottom of chip 2. This normalization
process preserves any overall sensitivity differences between the two CCD chips across
the ~40 pixel gap between the two independent pieces of the WFC detector. The relative
response across the gap is wavelength dependent. Chip 2 is ~1.5% more sensitive in
F435W, while chip 2 is ~8% less sensitive at 9300Å. For display of the Figures for this
presentation only, the chip 2 LP flats are renormalized to unity at the bottom of chip 2,
rather than to chip 1.
2. FULL FIELD WFC L-FLATS
Flat field exposures have been obtained for the WFC during ground testing at Ball
Aerospace and at GSFC. The ACS IDT has stored these images in a database and has
assigned a unique entry number to each image. In general, pairs of images are co-added
with cosmic ray rejection using acs_cr.pro, which also produces statistical uncertainty and
data quality arrays. The results presented here are preliminary, because the WFC B3 detector assembly that recorded the images in Table 1 is scheduled to be replaced before flight.
Figure 1 is an F435W LP-flat-field image, as illuminated by the RAS/HOMS with a
reflective Spectralon diffusing screen near the RAS source plane and a continuum light
source, while Figure 2 is the corresponding flat, as illuminated by the continuum internal
lamp. The external RAS/HOMS flats with entry numbers before 16791 use this Spectralon
screen and must be corrected for geometric distortion of the pixel size, as quantified in
Figure 8 of Paper I, in order to compare with the other undistorted illumination modes. In
addition, the Spectralon flats are corrected for a left-to-right non-uniform illumination of
~14%. The mylar diffuser at the RAS/HOMS pupil plane replaced the Spectralon setup for
the external observation after entry 16791. Neither the mylar illumination nor the internal
ACS lamp have the Spectralon geometric distortions that reach ~17% from corner-to-corner over the 3.4x3.4 arcmin field of the WFC. After both corrections to the Spectralon
2
Instrument Science Report ACS 00-10
flats, the response to external illumination with F435W declines by more than 10% from
the lower-left to upper-right corner in Figure 1, while this corner-to-corner response for
the internal lamp and F435W in Figure 2 is more uniform. In F814W, the internal flats
become more than 10% brighter at the upper-right vs. the lower-left corner, while the
external Spectralon L-flat is more uniform.
Even though the WFC L-flats are a strong function of wavelength, the same doughnut
pattern of change with wavelength is manifest for the internal and two different external
illumination techniques. For example, Figures 3-4 show similar large-scale patterns that
are contiguous across the two chips for the F814W/F625W ratios for internal and external
illumination, respectively. Furthermore, the doughnut reappears in Figure 5, which is the
smoothed ratio of monochromatic images spanning a similar wavelength interval of 8500
to 7000Å. The monochromatic illumination is produced with the mylar diffuser in the
RAS/HOMS, while the boxcar smoothing is done twice with a size of ~4% of the field
width to remove the fringing pattern. In conclusion, the cause of the wavelength dependence of the shape of the L-flats must be assigned to the CCD detector assembly with the
corollary that the external light sources are probably diffuse with no intrinsic pattern on a
scale smaller than the size of images.
3. COMPARISON OF WFC AND HRC P-FLATS
In order to compare the WFC LP-flats with the build-2 HRC P-flats of Paper I, a
1024x1024 subimage is extracted from the center of the WFC chip2 where the WFC LPflats are most uniform. The center 100x100 pixels of this subimage show no systematic
gradient, so that the rms dispersion can be compared with the counting statistics and with
the statistical results for the HRC P-flats. Figure 6 displays the external-to-internal ratio
for the 1024x1024 WFC subimages in F435W with a 0.98-1.02 stretch for comparison
with the same HRC Figure 3 of paper I. Figure 7 is an individual external LP-subimage,
while Figure 8 is the cross wavelength ratio F625W/F435W as in Figure 4 of Paper I.
There are six salient differences between the WFC and the HRC:
•
There is a residual flocculent structure at the <1% level in the WFC ratio of Figure 6,
which is present neither in the external/internal HRC ratio nor in the WFC external/
internal ratios at longer wavelengths. This artifact might possibly be attributed to the
wavelength dependent focal length of the refractive optics in the RAS/HOMS, which
could partially focus any variations in the surface of the Spectralon screen. For illumination with the mylar diffuser at the RAS/HOMS pupil, this artifact should disappear.
However, testing this hypothesis should be a high priority during the first attempt to
obtain LP-flats with the flight detectors. If any such residual structure persists, the
mylar should be rotated to see if the cause can be attributed to the diffuser itself.
3
Instrument Science Report ACS 00-10
•
At f/25, the shadows from the small dust grains on the detector window are weaker
than for the f/68 HRC in Paper I. There are only two weak dust mote signatures near
the bottom of Figure 6. As in the HRC case, the wider angle illumination with the
internal lamp reduces the contrast in the shadows of the particulate contaminants. The
external subimage for the F814W LP-flat Figure 7 shows these dust motes more
clearly.
•
Vertical lines caused by the step and repeat pattern from the CCD manufacturing process appear in Figure 7 at the fractional percent level every ~70 pixels in the x direction. No such artifacts are visible at the same 0.98-1.02 stretch of HRC images.
•
The HRC ‘freckles’, i.e. small spots with sizes of a few pixels and wavelength dependent depths of a few percent, are caused by irregularities in the HRC CCD backside
treatment but are largely absent in the WFC with its different treatment. Compare Figure 7 with Figures 1-2 of Paper I. After correction for the counting statistics, the intrinsic rms structure of the HRC P-flats increases from 0.8% in F814W to 1.6% in F435W,
while the corresponding WFC rms is in the range 0.5-0.7%. The ratio F625W/F435W
in Figure 8 has an rms residual of 0.4% and lacks the snow storm of white dots that
appear in the corresponding HRC Figure 4 of Paper I, where the residual is 1.0%.
•
Despite the lack of freckles, the WFC F625W/F435W ratio shows a moderate profusion of middle size white spots that are more prolific in Figure 8 than in the corresponding Paper I Figure 4. These white spots have diameters up to ~15 pixels and
correspond to black spots in Figure 7. Since these blemishes cancel in the external/
internal F435W ratio of Figure 6, these spots have the same characteristics as the HRC
freckles, except for their larger size and lower abundance. If an HRC without a Lesser
backside treatment is flown, then the HRC small scale artifacts should be more like the
WFC.
•
In the WFC, particulate contaminants on the filters are not observed. The filters have
been cleaned in the interim, and the faster WFC optics reduces the contrast of these
shadows.
Like the HRC, the short term WFC repeatability is excellent, e.g. over three months,
the pixel-to-pixel repeatability of the WFC flats is better than 0.1%. The ratios July/September of 2000 for internal lamp exposures with F555W, F625W, and F814W have a rms
pixel-to-pixel scatter of ~0.5%, which is the same as the counting statistics to an accuracy
that limits other noise sources to <~0.05%. However, the internal F555W L-flat changed
by as much as 0.5%, while the F814W L-flat changed by <0.2%. These changes cannot be
attributed to the bias, which contributes <0.05% of the signal. Perhaps, the change in CCD
temperature from -77C in July to -67C in September effects the short wavelength L-flats.
Ramp L-flats are problematic, even for the region of monochromatic transmission at
the observed wavelength. Consider uniform streaks of emission parallel to the length of
the ramp filters along the direction where the central wavelength changes with position.
For monochromatic emission the counts along the line rise and fall with the shape of the
monochromatic transmission summarized in Bohlin & Tsvetanov (2000), while the signal
for continuum emission changes monotonically across the entire field of view with the
slope of the overall change in instrumental sensitivity with wavelength. In order to enable
4
Instrument Science Report ACS 00-10
continuum subtraction, the pipeline flatfields for ramps are the LP-flats derived from full
field continuum exposures in broadband filters (Sparks et al. 2000). Additional study is
required to quantify uncertainties in this L-flat correction scheme for the ramp filters.
A question raised in Paper I is whether the broad band flats are appropriate for ramp
filters with monochromatic illumination at wavelengths below 7000Å, where there is no
fringing. Monochromatic 6330Å and continuum flats were obtained with the WFC
FR656N and are divided by the F625W flat from the external Spectralon illumination.
After correcting for counting statistics, residuals are <0.2% for both monochromatic and
continuum light in the central 100x100 pixels at the 6330Å position on FR656N. Any
large scale artifacts in these corrected FR656N images are <<1%, so that broad band flats
are appropriate for the WFC ramp P-flats below 7000Å. For HRC with its wavelength
dependent fine structure and slower optics, the broadband flats will leave larger residuals
for monochromatic light and maybe even for continuum illumination.
4. FRINGING
In continuum light, fringing appears only in F892N, FR931N, and FR1016N at <~1%
rms. F892N is used at only one filter wheel setting and should have a fixed fringe pattern
in its flat. The worse ramp is FR931N with a coherent fringe amplitude of –1%
(rms=0.7%). After correction with a worst case F814W flat, the pixel-to-pixel scatter is
reduced to rms=0.4%; but the fringe amplitude remains at 1%. In the baseline pipeline
data reduction, there is only one flat field per filter, so that fringing in FR931N cannot be
removed because of the multitude of allowed central wavelength settings. In order to
reduce the uncertainty in relative point source photometry to <1% over the field of F931N,
the results of the fringe modeling program must be applied. The baseline pipeline flat for a
ramp filter will be the appropriate piece of the broadband flat at the nearest central wavelength. The three pieces of the flat for each of the five ramps will be assembled into one
flat field image before delivery of the reference file.
In monochromatic light, the fringing pattern seen around the edges of the ratio image
in Figure 5 is ubiquitous. Figure 9 shows the worst case data for a 1024x1024 subimage
region of chip 2, illuminated by 9300Å light with a bandpass of 20Å. Fringes with ~–10%
amplitude are visible everywhere in the field, since the broadband F814W filter is in place.
For the same 20Å bandpass, the rms fringe amplitude ranges from 1% at 7500Å to 6% at
9300Å. Since the fringing is coherent over the size of the PSF, uncorrected photometry of
monochromatic point sources would exhibit the full fringe amplitude range of –10% variation across the field. No correction for fringing is planned for the pipeline data reduction.
Figure 10 illustrates the fringe pattern for the 9300Å light at the FR914M filter wheel
setting, where FR914M is in the middle part of the field. The upper right-hand set of
fringes are through the outer FR931N ramp filter, while the inner FR853N ramp segment
at the lower left does not transmit at 9300Å. Figure 11 demonstrates that the fringe pattern
depends on the bandwidth of the illumination but is nearly independent of which filter is in
5
Instrument Science Report ACS 00-10
the beam. The heavy solid line is a cut through the center of chip 2 in the FR931N region
of Figure 10. The light solid line is the same 20Å bandpass light through the F814W filter
and matches to better than 1%, although the fringe amplitude may be reduced systematically by a fraction of a percent. The dotted line is for a 40Å bandpass, which does
systematically reduce the fringe amplitude by ~1% with respect to the fringes for the 20Å
band width. Therefore, the fringe pattern depends only on the CCD and the spectral distribution of the illumination at the CCD.
Since the fringe amplitude changes rapidly with wavelength, obtaining a dense enough
grid of monochromatic flats to correct monochromatic images at any wavelength is
impractical. For example, dividing a monochromatic image at 8520Å by an 8500Å image
does not diminish the fringe pattern. The only practical way of correcting the fringing is
through modeling. The ECF is committed to modeling the fringing pattern in order to correct the HRC and WFC grism spectra. The results of that effort should be applicable to
fringe removal in the imaging modes.
5. RECOMMENDED LAB PROGRAM
This section updates and expands the requirements in Paper I for lab CCD flats. See
Paper I for a discussion of on-orbit calibration plans. While internal illumination provides
pixel-to-pixel corrections, complete flat fields must be defined by uniform diffuse illumination with a proper f/24 OTA beam. Uniform illumination is difficult to achieve on orbit
(Cox et al. 1987), although narrow band WFPC2 flats are made by labor intensive processing of streak flat observations of the sunlit earth (Biretta 1995). A full set of on-orbit sky
flats from normal science exposures may require years to accumulate sufficient counting
statistics; but checks of the more popular filters should be available near the end of the first
year of science operations. Therefore, our basic philosophy is to produce a complete a set
of flat fields on the ground, while using the internal lamps to monitor changes on-orbit.
The RAS/HOMS provides a convenient flat field source longward of its refractive optics
cutoff at ~3500Å.
For the four HRC short wavelength filters below 3500Å and for a check on the longer
wavelength RAS/HOMS flats, a second ground calibration setup that accurately simulates
the full-field OTA beam into ACS is desirable. Alternatively, the assumption that the
change with wavelength of the flat field is adequately measured by the internal lamps
could be invoked to manufacture these four short wavelength continuum flats from the
ratio of high S/N internal flats. Approximate LP-flats will be obtained at GSFC with a deuterium lamp illuminating a diffusing screen in front of the ACS.
The G800L grism on the WFC covers the 5500-10,000Å region with a peak sensitivity
at 6500-7000Å. P-flats for the broadband filters F625W or F814W are adequate for flat
fielding grism images. Shortward of the onset of fringing at ~7000Å, the rms scatter of
~0.7% should be reduced below ~0.2% by application of an F625W flat, because the worst
case residual should be typical of the 0.14% residual rms scatter in the F555W/F625W
6
Instrument Science Report ACS 00-10
ratio of external flats. Longward of ~7500Å, uncertainties in the fringing correction will
likely dominate uncertainties in grism spectra. Because of the shorter wavelength coverage, the HRC PR200L may require a data cube of P-flats. Likewise, the wavelength
dependence of the L-flat on WFC greatly complicates the low spatial frequency L-flat correction for G800L. If the wavelength dependence of the L-flat is the same for the grism as
for the other filters, then the typical L-flat errors at 5500Å and 8140Å are <5% and <10%,
respectively, when an F625W LP-flat is used as the G800L flat. To test this hypothesis and
verify the uncertainties and to test the possibility of a data cube L-flat correction scheme
for the grism, a monochromatic flat at 6250Å should be obtained on G800L, in addition to
the same illumination specified below for F625W. All fringe flats, which require monochromatic illumination, must be obtained in the lab before launch.
Since the intrinsic rms pixel-to-pixel scatter is as low as 0.5% in the WFC, flat fields
must have at least 40,000 electrons/px to avoid degrading the intrinsic S/N. To make flat
fielding a worthwhile enterprise by significantly improving the noise to say 0.25% rms in a
WFC image with no Poisson noise, the flat field exposures must have at least 160,000
counts/px for each filter. If the preliminary reproducibility result of ~0.1% is maintained,
then exposures with a million counts are useful; and external CCD flats with a million
counts should be obtained, as time permits. HRC images and WFC subimages can be
obtained in less than the two hours for 12 full-field WFC readouts required at the full well
of 80,000 electrons. For polarization measurements of small objects at ~1% precision,
0.1% stability of the flats would be especially useful.
5.1 Broadband Filters
There are 25 HRC and 20 WFC broadband filters, including six polarizers on both
cameras, a grism on the WFC, and a grism and a prism on the HRC. In a few supported
combinations, the polarizers will be crossed with filters in another wheel.
Since the internal lamps will be used to monitor changes in the flats, a high S/N CCD
baseline at the million count level should be obtained on the ground. A set of three baseline flats that cover the wavelength range of each CCD at the best guess orbital
temperature setpoint is suggested. This data set should be obtained at a few epochs before
launch to monitor stability and to measure any decline in brightness of the internal lamps
as a function of wavelength. If significant wavelength dependent changes occur, then a
finer grid in wavelength would be required to transfer the measured changes in the internal
flats to the full set of external flats.
5.2 Ramp Filters
5.2.1 Continuum Light and Short Wavelengths without Fringing
The 12 narrow and 3 medium bandpass ramp filters provide a filter set with bandpasses
of 2% or 9% and cover the wavelength range from 3710-10710Å. Three separate ramp fil-
7
Instrument Science Report ACS 00-10
ter segments share the WFC field of view at each of five filter wheel positions. Only the
five middle segments may be used on the HRC. The central wavelengths of the ramp filters
change continuously with position along each segment. The filters have already been characterized in terms of transmission in each bandpass and to the 10^-5 level out of band
(Bohlin & Tsvetanov 2000). The ramp P-flats for monochromatic light below 7000Å and
for continuum light are sufficiently well defined by continuum light through the broader
band filters that fall closest in wavelength. This broadband set is comprised of F330W,
F435W, F475W, F555W, F606W, F625W, F775W, F814W, and F850LP. The ramp filter
FR388N at 3796A is the farthest (1.13x) from the bracketing central wavelengths of 3354
(F330W) and 4297Å (F435W). Since the flat fields change more rapidly with wavelength
at the shorter wavelengths, the rms residuals will probably be the largest at 3796Å. Bandpass and sensitivity information is available in the STScI Exposure Time Calculator
package for ramp filters (Van Orsow et al. 2000).
To quantify the errors in the ramp flats, high S/N monochromatic and continuum flats
are needed at a few test wavelengths. The acid-test wavelength should be 3796Å. As discussed in Section 3 above, WFC monochromatic 6330Å flats adequately match F625W
flats; but residuals in the HRC could be worse due to the wavelength dependence of the Pflats and to dust particles. An ideal set of monochromatic test flats would consist of the five
wavelengths at the geometric midpoints between the central wavelengths of the broadband
filters F330W, F435W, F475W, F555W, F606W, and F625W, where fringing is not
present. With exposures through the appropriate ramp and adjacent broadband filter, this
test would quantify the differences in the dust motes and in LP-flats for monochromatic
and broadband illumination. In addition, central wavelength monochromatic flats at 3300,
4350, 4750, 5550, 6060, and 6250Å are required to measure the pixel-to-pixel structure
and to fully quantify possible gains from interpolation schemes that might be required to
remove any wavelength dependent fine structure.
5.2.2 Long Wavelengths with Fringing in Monochromatic Light
At the longer wavelengths, the fringing pattern from interference of monochromatic
light in the silicon layer of the CCDs is the dominant flat field effect. The best removal
technique should accrue as a ‘fringe’ benefit from the ECF modeling of the fringe patterns
in grism spectra. Wolfram Freudling has provided the following wish list of 20Å bandpass
monochromatic images to be taken through the nearest broadband filter for both WFC and
HRC: 7000, 7500, 7820, 7860, 7900, 7910, 7920, 7930, 7940, 7980, 8250, 8520, 8560,
8600, 8610, 8620, 8630, 8640, 8680, 9000, 9150, 9320, 9360, 9400, 9440, 9480, and
10000Å.
8
Table 1. ACS Prelaunch Data Catalog of Full WFC Data
FILENAME
OBSTYPE
DATEOBS
EXPTIME
(s)
DETECTOR
FILTER1
FW1
OFF
FILTER2
FW2
OFF
CCD
TEMP
(C)
16066
16067
16068
16069
16070
16071
16072
16073
16074
16075
16076
16077
16079
16080
16081
16082
16083
16084
16085
16086
16087
16088
16089
16090
16092
16093
16094
16095
16096
16097
16098
16099
16100
16101
CSIJ00167185419_1
CSIJ00167190437_1
CSIJ00167191455_1
CSIJ00167192513_1
CSIJ00167193530_1
CSIJ00167194547_1
CSIJ00167195607_1
CSIJ00167200624_1
CSIJ00167201650_1
CSIJ00167202706_1
CSIJ00167203736_1
CSIJ00167204816_1
CSIJ00167212031_1
CSIJ00167213101_1
CSIJ00167214120_1
CSIJ00167215136_1
CSIJ00167220228_1
CSIJ00167221248_1
CSIJ00167222515_1
CSIJ00167223724_1
CSIJ00167224827_1
CSIJ00167225927_1
CSIJ00167231209_1
CSIJ00167232451_1
CSIJ00167235707_1
CSIJ00168000913_1
CSIJ00168002119_*
CSIJ00168003325_*
CSIJ00168004531_*
CSIJ00168005737_*
CSIJ00168010830_1
CSIJ00168011912_1
CSIJ00168012954_*
CSIJ00168014036_*
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
15/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
3.4
3.4
3.3
3.3
2.4
2.4
3.0
3.0
1.5
1.5
4.4
4.4
15.4
15.4
2.0
2.0
5.6
5.6
115.0
115.0
46.0
46.0
127.0
127.0
112.0
112.0
112.0
112.0
112.0
112.0
27.4
27.4
27.4
27.4
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
F555W
F555W
F775W
F775W
F625W
F625W
F850LP
F850LP
F606W
F606W
F475W
F475W
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
F550M
F550M
F502N
F502N
F658N
F658N
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
0
3
0
3
0
3
0
3
0
3
0
3
0
0
0
0
0
3
0
3
0
3
0
0
0
0
0
0
0
0
0
0
0
0
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
F435W
F435W
F814W
F814W
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
F660N
F660N
FR388N
FR388N
FR388N
FR388N
FR388N
FR388N
FR505N
FR505N
FR505N
FR505N
0
0
0
0
0
0
0
0
0
0
0
0
0
3
0
3
0
0
0
0
0
0
0
3
0
3
62
65
-65
-62
0
3
62
65
-76.8
-76.8
-76.7
-76.7
-76.7
-76.8
-76.9
-76.9
-76.7
-76.7
-76.8
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.7
-76.7
-76.7
-76.9
-76.9
-76.9
-76.7
-76.8
-76.8
-76.8
-76.8
-76.8
-76.8
-76.8
-76.9
-76.8
-76.8
Instrument Science Report ACS 00-10
9
ENTRY
FILENAME
OBSTYPE
DATEOBS
EXPTIME
(s)
DETECTOR
FILTER1
FW1
OFF
FILTER2
FW2
OFF
CCD
TEMP
(C)
16102
16103
16105
16106
16107
16108
16109
16110
16111
16112
16113
16114
16115
16116
16123
16124
16125
16126
16127
16128
16129
16130
16469
16470
16491
16492
16521
16522
16523
16524
16525
16526
16527
16528
16530
16531
CSIJ00168015118_1
CSIJ00168020221_1
CSIJ00168024145_1
CSIJ00168025229_1
CSIJ00168030313_*
CSIJ00168031357_*
CSIJ00168032441_1
CSIJ00168033525_1
CSIJ00168034605_1
CSIJ00168035627_1
CSIJ00168040649_1
CSIJ00168041711_1
CSIJ00168042733_1
CSIJ00168043816_1
CSIJ00168161723_1
CSIJ00168162745_1
CSIJ00168163807_1
CSIJ00168164829_1
CSIJ00168165851_1
CSIJ00168170913_1
CSIJ00168171957_1
CSIJ00168173012_1
CSIJ00171192950_1
CSIJ00171193359_1
CSIJ00171232649_1
CSIJ00171233810_1
CSIJ00172160841_1
CSIJ00172162621_1
CSIJ00172164100_1
CSIJ00172165501_1
CSIJ00172170840_1
CSIJ00172172300_1
CSIJ00172173620_1
CSIJ00172175340_1
CSIJ00172182621_1
CSIJ00172184049_1
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
16/06/00
19/06/00
19/06/00
19/06/00
19/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
20/06/00
27.4
27.4
30.0
30.0
30.0
30.0
30.0
30.0
7.6
7.6
7.6
7.6
7.6
7.6
7.2
7.2
7.2
7.2
7.2
7.2
0.5
0.5
35.0
35.0
1.0
1.0
4.0
4.0
8.0
8.0
40.0
40.0
30.0
30.0
40.0
40.0
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
G800L
G800L
F892N
F892N
F625W
F625W
CLEAR1L
CLEAR1L
F555W
F555W
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
-61
-64
0
3
0
0
0
3
0
0
0
0
0
0
FR505N
FR505N
FR656N
FR656N
FR656N
FR656N
FR656N
FR656N
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
FR459M
FR459M
FR459M
FR459M
FR459M
FR459M
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
F814W
F814W
CLEAR2L
CLEAR2L
F435W
F435W
FR914M
FR914M
FR656N
FR656N
-65
-62
0
3
62
65
-65
-62
0
3
62
65
-65
-62
0
3
62
65
-62
-65
0
0
0
0
0
0
0
3
0
0
0
3
0
3
62
65
-76.7
-76.7
-76.9
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.7
-76.9
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.9
-76.7
-76.8
Instrument Science Report ACS 00-10
10
ENTRY
FILENAME
OBSTYPE
DATEOBS
EXPTIME
(s)
DETECTOR
FILTER1
FW1
OFF
FILTER2
FW2
OFF
CCD
TEMP
(C)
17039
17040
17041
17042
17044
17045
17046
17047
17058
17060
17061
17063
17064
17065
17067
17068
17070
17071
17073
17074
17076
17077
17078
17079
17091
17092
17093
17094
17095
19114
19115
19116
19117
19118
19119
19120
CSIJ00175032030_1
CSIJ00175034541_1
CSIJ00175040251_1
CSIJ00175041920_1
CSIJ00175051600_1
CSIJ00175053049_1
CSIJ00175054540_1
CSIJ00175060029_1
CSIJ00175154059_1
CSIJ00175163249_1
CSIJ00175170229_1
CSIJ00175174819_1
CSIJ00175175909_1
CSIJ00175181300_1
CSIJ00175190418_1
CSIJ00175193910_1
CSIJ00175210040_1
CSIJ00175215000_1
CSIJ00175225230_1
CSIJ00175232010_1
CSIJ00176000200_1
CSIJ00176001829_1
CSIJ00176003510_1
CSIJ00176010459_1
CSIJ00176035139_1
CSIJ00176041409_1
CSIJ00176045240_1
CSIJ00176052900_1
CSIJ00176060400_1
CSIJ00256194959_1
CSIJ00256195950_1
CSIJ00256201309_1
CSIJ00256202309_1
CSIJ00256203610_1
CSIJ00256204609_1
CSIJ00256205949_1
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
EXTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
BIAS
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
23/06/00
24/06/00
24/06/00
24/06/00
24/06/00
24/06/00
24/06/00
24/06/00
24/06/00
12/09/00
12/09/00
12/09/00
12/09/00
12/09/00
12/09/00
12/09/00
400.0
400.0
400.0
400.0
300.0
300.0
300.0
300.0
100.0
1000.0
1000.0
200.0
50.0
50.0
1500.0
1500.0
2000.0
2000.0
1000.0
1000.0
400.0
400.0
400.0
1000.0
600.0
600.0
1500.0
1500.0
1500.0
3.4
3.4
2.4
2.4
2.0
2.0
0.0
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
WFC
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
CLEAR1L
F555W
F555W
F625W
F625W
CLEAR1L
CLEAR1L
CLEAR1L
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
FR656N
FR656N
FR656N
FR656N
FR656N
FR656N
FR656N
FR656N
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
FR914M
F814W
F814W
F814W
F814W
F814W
F814W
F814W
F814W
F814W
F814W
F814W
CLEAR2L
CLEAR2L
CLEAR2L
CLEAR2L
F814W
F814W
F814W
30
30
30
30
30
30
30
30
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
-76.8
-76.7
-76.8
-76.7
-76.8
-76.8
-76.7
-76.9
-76.8
-76.8
-76.8
-76.8
-76.8
-76.8
-76.7
-76.9
-76.8
-76.8
-76.8
-76.8
-76.8
-76.7
-76.8
-76.8
-76.8
-76.8
-76.8
-76.8
-76.7
-66.7
-66.6
-66.6
-66.7
-66.6
-66.6
-66.6
* Contaminated by straylight from internal lamp.
Instrument Science Report ACS 00-10
11
ENTRY
Instrument Science Report ACS 00-10
REFERENCES
Biretta, J. 1995, in “Calibrating HST: Post Servicing Mission,” ed. A. Koratkar & C.
Leitherer, (Baltimore:STScI), p. 257.
Bohlin, R. C., Hartig, G., Lindler, D. J., Meurer, G., & Cox, C. 1999, Instrument Science Report, ACS 99-01, (Baltimore:STScI), Paper I.
Bohlin, R.C., & Tsvetanov, Z. 2000, Instrument Science Report, ACS 00-05,
(Baltimore:STScI).
Cox, C., Bohlin, R. C., Griffiths, R. E., & Kelsall, T. 1987, “Standard Astronomical
Sources for HST: 6. Spatially Flat Fields,” (Baltimore:STScI).
Sparks, W. B., Jedrzejewski, R., Clampin, M., & Bohlin, R.C. 2000, Instrument Science Report, ACS 00-03, (Baltimore:STScI).
Van Orsow, D., Boffi, F. R., Bohlin, R. C., & Shaw, R. A., 2000, Instrument Science
Report, ACS 00-07, (Baltimore:STScI).
12
Instrument Science Report ACS 00-10
ACS WFC LP Flat
0.90
1.10
CLEAR1L F435W
rms(%)= 0.97 0.62 0.75
lpwsptrln-00172sm02f435w
Figure 1: WFC LP-flat for the F435W filter derived from external illumination by a Spectralon diffusing screen. The image stretch is from 0.9 to 1.1, as indicated by the reference
gray scale at the top. The image is the sum of two exposures which were recorded on day
172 of 2000 (00Jun20). The three rms(%) values are for the total, counting, and intrinsic
scatter of the flat field in a 100x100 pixel box centered at (2000,1000) in chip 2. There are
two 4096x2048 CCD segments with chip 1 at the bottom and chip 2 at the top, where the
gap in sky coverage between the segments is about 40 pixel wide. The orientation of the
flat is in the prelaunch coordinate frame. The GO science data will be flipped about a horizontal axis so that chip 2 will be at the bottom. The intensity is corrected for the geometric distortion of effective pixel area per Fig. 8 of Paper I.
13
Instrument Science Report ACS 00-10
ACS WFC LP Flat
0.90
1.1
CLEAR1L F435W
rms(%)= 0.83 0.36 0.74
lpwint00167sm02f435w-2
BOHLIN: prtimg 23-Aug-2000 15:29
Figure 2: As for Figure 1, except for internal illumination obtained on 167/2000
(00Jun15).
14
Instrument Science Report ACS 00-10
0.95
ACS WFC RATIO
1.05
CLEAR1L F814W / F625W CLEAR2L
lpwint00167sm02f814w-2 / lpwint00167sm02f625w-2
BOHLIN: prtimg 5-Sep-2000 13:49
Figure 3: Ratio of F814W/F625W LP-flats from the internal lamps with a grey scale
stretch from 0.95 to 1.05.
15
Instrument Science Report ACS 00-10
0.95
ACS WFC RATIO
1.05
CLEAR1L F814W / F625W CLEAR2L
lpwsptrln-00172sm02f814w / lpwsptrln-00171sm02f625w
BOHLIN: prtimg 11-Sep-2000 11:21
Figure 4: As for Figure 3, except for external illumination from Spectralon. The large
scale doughnut pattern is the same as in Figure 3 to 2-3%; and this difference may be due
to non-uniformities in the Spectralon illumination.
16
Instrument Science Report ACS 00-10
0.95
ACS WFC RATIO
1.05
CLEAR1L F814W / CLEAR1L F814W
lpw850-00176sm02f814w / lpw700-00176sm02f814w
BOHLIN: prtimg 11-Sep-2000 12:16
Figure 5: Ratio of monochromatic images at 8500 and 7000Å with external illumination
from the RAS/HOMS and mylar diffuser. The ratio is boxcar smoothed to remove the
fringing, except for 2% of the width around the edges of each CCD chip. Again, the
doughnut of Figures 3-4 appears for light of a similar difference in effective wavelength.
This L-flat pattern matches Figure 3 to ~1%.
17
Instrument Science Report ACS 00-10
0.98
ACS WFC RATIO
1.02
CLEAR1L F435W
rms(%)= 0.76 0.74 0.18
lpwsptrln-00172sm02f435w-2/lpwint00167sm02f435w-2 1024x1024 center: 2000x1000
BOHLIN: wbloup 28-Aug-2000 19:52
Figure 6: Ratio of the F435W external/internal subimages. The major difference in the
two flats is that the external illumination has a spurious flocculent aspect that is not present
in the external/internal ratios at longer wavelengths. The two circular features near the bottom center are caused by dust on the detector window and are ~100px in diameter vs. only
~30px for the HRC.
18
Instrument Science Report ACS 00-10
0.98
ACS WFC LP Flat
1.02
CLEAR1L F814W
rms(%)= 0.73 0.46 0.57
lpwsptrln-00172sm02f814w-2 central 1024x1024
BOHLIN: wbloup 23-Aug-2000 16:14
Figure 7: Subimage F814W flat showing prominent dust motes and the orthogonal stepand-repeat pattern from the CCD manufacturing process.
19
Instrument Science Report ACS 00-10
0.98
ACS WFC RATIO
1.02
F625W CLEAR2L / CLEAR1L F435W
rms(%)= 0.81 0.72 0.36
lpwsptrln-00171sm02f625w-2 / lpwsptrln-00172sm02f435w-2 1024x1024 center: 2000,1000
BOHLIN: wbloup 5-Sep-2000 14:01
Figure 8: Ratio of subimages F625W/F435W, both with continuum external illumination
by the RAS/HOMS and spectralon. The most important difference is that the dust motes
are not as deep at the longer wavelength. The freckles that appear as tiny white dots in the
corresponding Figure 4 of Paper I are largely absent in this WFC ratio, which has a different backside CCD treatment than the build-2 HRC with its Lesser coating.
20
Instrument Science Report ACS 00-10
0.90
ACS WFC LP Flat
CLEAR1L F814W
rms(%)= 5.30 0.84 5.23
lpw930-00175sm02f814w-2 central 1024x1024
BOHLIN: wbloup 23-Aug-2000 20:44
Figure 9: Subimage for 20Å bandpass monochromatic light at 9300Å. The rms fringe
amplitude is 6.4% for the whole frame, while the 5.3% listed on the Figure is for the central 100x100 pixels.
21
Instrument Science Report ACS 00-10
0.90
ACS WFC LP Flat
1.10
CLEAR1L FR914M
rms(%)= 5.66 0.67 5.62
lpw930-20A-sm02fr914m-2
BOHLIN: prtimg 24-Aug-2000 18:17
Figure 10: FR914M filter with monochromatic illumination at 9300Å. The fringe pattern
of the CCD is evident at the two field positions that have high throughput at 9300Å. There
are three ramp segments oriented at 45deg at the FR914M setting: the outer FR931N at the
top right, FR914M in the middle, and the inner FR853N at the lower left. FR853N cuts off
beyond ~8910Å and does not transmit 9300Å light. The FR914M has a double peaked
sensitivity function at 9300Å.
22
Instrument Science Report ACS 00-10
23
Figure 11: Heavy solid line - average of rows 1050:1060 through chip 2 of Figure 10 with the 20Å bandpass illumination. Dotted line same as heavy solid line, except for an image with a 40Å bandpass. Light solid line - 20Å bandpass through the F814W filter instead of
FR931N.