Instrument Science Report ACS 99-01
FLATS: PRELIMINARY HRC
DATA AND ON-ORBIT PLANS
R. C. Bohlin, G. Hartig, D. J. Lindler, G. Meurer, & C. Cox
1999 April
ABSTRACT
The Advanced Camera for Surveys (ACS) has three detectors: a large format 4096x4096
CCD Wide Field Channel (WFC), a 1024x1024 CCD High Resolution Channel (HRC),
and a 1024x1024 MAMA Solar Blind Channel (SBC). The flat field baseline goal is to
obtain a complete set of pixel-to-pixel P-flats before launch and to use the onboard lamps
to track changes. A second goal of the ground calibration program is to obtain the low frequency L-flat variation over the field of view. Changes in the L-flats can be determined
from dithered observations of star fields on orbit. An overall precision of better than 1%
per resolution element in the combined fine and coarse flat field structure is required.
1. INTRODUCTION
Figure 1 is an HRC flat field image in the F435W filter, as illuminated by the Refractive Aberrated Simulator/Hubble Opto-Mechanical Simulator (RAS/HOMS), which
provides a full field simulation of the OTA at 6328Å. The refractive aberrations at other
wavelengths should not affect the fidelity of flat fields. Several external flat fields were
obtained with the RAS/HOMS in 1998 Nov., as tabulated in Table 1. However, the HRC
detector package has spurious illumination from shiny edges of a mask in front of the
CCD. These edges will be modified during a rebuild of the flight HRC detector in spring
1999. The problematic illumination pattern is removed to make the Figure 1 P-flat. Figure
2 is the corresponding flat, as illuminated by the internal lamp, while Figure 3 is the ratio
of the internal to external F435W flat. The small black spots with sizes of a few pixels and
depths of a few percent on Figures 1-2 are known as 'freckles' and are caused by irregularities in the CCD backside treatment. The occulting finger is at the lower left.
Whether the ~30 pixel scale regions of reduced sensitivity are on the CCD or on the
detector window can be determined by examination of the flat field ratio in Figure 3. The
cosmetic features are well corrected, except for the dust particles on the window. These
dust motes are more diffuse in the wide angle illumination of the internal lamp and more
sharply defined in the external f/24 beam from the RAS/HOMS. Furthermore, the mote
1
Instrument Science Report ACS 99-01
positions and the occulting finger shift a few pixels because of an angular offset between
the two beams. The faint vertical bright bar to the right of center may be a residual glint
off the edges of the detector mask. In addition, there is a faint, <0.5%, ~200 pixel wide
ring in the lower left, just above the amoeba shaped dust mote. This faint, large feature
may be caused by a dust particle on the filter. Figure 4 is the external F435W flat from Figure 1, as compared to the external F625W flat. This cross wavelength comparison
demonstrates a relatively severe increase in the contrast of the freckles and the intermediate scale CCD blemishes at shorter wavelengths, in comparison to little difference in the
dust motes. These qualitative statements are quantified in section 3. A careful examination
of Figure 4 also reveals five residual sets of diffraction rings at the locations of the five
strong dust motes with bright central Poisson spots in Figure 1.
While internal illumination provides pixel-to-pixel corrections over much of the detector, complete flat fields must be defined by external illumination with a proper f/24 OTA
beam. Such 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 ~1% counting statistics; but checks of the more popular
filters should be available near the end of the first year of science operations. For example,
the signal rate from the average sky is ~0.057 counts sec-1 px-1 in the WFC F555W filter.
For a typical 20 min integration time per orbit, 68 counts/px from the sky are recorded
along with the 20 counts of read noise for CR-split=1. About 200 orbits of observations of
sparse fields are required for a flat with S/N=100 per px. 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 continuum flat field
source longward of its refractive optics cutoff at ~3500Å.
For the wavelength range from 2000-3500Å, for the narrow band ramp filters, and for
a check on the RAS/HOMS flats, a second ground calibration setup that accurately simulates the full-field OTA beam into ACS is required.
For the SBC, laboratory illumination is provided by the STUFF apparatus, which is
not designed as a high fidelity OTA simulator. The resulting illumination pattern is
removed by the procedure in section 2; and starfields must be observed on orbit to determine the low frequency components of the flats. From STIS experience and from
preliminary results from the ACS thermal vacuum calibrations in 99 March, the MAMA
flats should be wavelength independent to better than 1% (Bohlin et al. 1997, Kaiser et al.
1998). One alternative that might produce proper external flat field illumination on-orbit is
the bright earth airglow at Ly-α. Since the bright earth has not been observed with the
STIS FUV-MAMA, bright object concerns are paramount.
2
Instrument Science Report ACS 99-01
2. FLAT METHOD COOKBOOK
To produce full field flats for reduction of flight science images, all the available CCD
flat field data with the same illumination pattern must be co-added with a cosmic ray rejection algorithm to achieve the best S/N. For the SBC, simple addition is sufficient. If
properly baffled f/24 OTA illumination is used, then the final flats are merely the above coadded image normalized to unity, say over the central 200x200 pixels.
If OTA illumination is not available, the following steps are required to produce a Pflat.
a. Mask the edges of the data image and regions that have no valid data like the occulting finger of the HRC, the six SBC rows 599:604 that are affected by the bad anode, and
the SBC corners that are not illuminated.
b. Mask the SBC columns containing the repeller wire and the columns 1000:1023
with severe vignetting. For the CCDs, mask the dust motes and other features that are
smaller scale than ~50px.
c. Median filter (11x11) the CCD flat field images to remove the sharp features.
Median filters are not appropriate for a MAMA detector with residual odd-even electronic
patterns.
d. Fit the unmasked points in each row of the filtered flat field image with a spline
function with a node spacing of ~100 px. Add more nodes as needed, e.g. near the edges
where the smooth illumination pattern may drop precipitously in the row direction for the
HRC.
e. To avoid discontinuities in the column direction and for the SBC (see below) iterate
the fitting procedure of (d) by fitting the row fits with similar splines but in the column
direction to make a final smooth fit image.
f. Divide the original, unmasked image by the smooth fit.
g. Set the masked points with no valid data from step (a) to unity.
h. Retain the statistical RMS uncertainty image, as well as a data quality extension.
If the fitting of the image is done only in the row direction, artificial noise can be introduced in the column direction because of discontinuities between the spline fits to adjacent
rows. To quantify this problem after step (d) above, the differences in the adjacent smooth
row fits for the F435W flat of Figure 1 are computed and shown in Figure 5. The maximum error is ~1% near the edges, where the row fits are not tightly constrained, while the
maximum row difference for a central column is <0.1%. For the SBC, fits along just the
row direction will not produce a proper flat because of the odd-even structure from row to
row. Iterating to fit the original fit, as detailed above in step (e), produces a final proper
smooth fit, where artificial noise along the row direction is less than 0.01%.
3
Instrument Science Report ACS 99-01
3. RESULTS FOR EXISTING HRC DATA
Flat field exposures have been obtained for the HRC during ground testing at Ball
Aerospace and at GSFC. The ACS IDT has stored these images in an on-line database and
has assigned a unique entry number to each image. Data of P-flat quality exist for three
broadband filters in the HRC mode, as summarized in Table 1. The results presented here
are preliminary, because the HRC is scheduled to be disassembled and reworked before
flight. The HRC Flight #2 with Lessor's coating recorded the images in Table 1.
Tables 2-4 characterize the P-flats and quantify their differences for sub-images from
the pixel range (480:680,480:680), which is free of any dust blemishes.
Table 2 compiles the statistics of internal and external flats for three broadband filters.
These data were obtained on Nov. 6-7, 1998 with illumination by the RAS/HOMS or the
internal cal lamp and are the 12 entry numbers in the range 1109-1137 at the top of Table
1. The pairs of images are co-added with cosmic ray rejection and processed to make six
flats, as described above. The Poisson counting statistics, the actual one σ rms scatter, the
intrinsic rms variation, and the minimum and maximum value of the flat are tabulated for
each flat. The Poisson scatter is removed from the 'Actual σ' to calculate the 'σ Flat', which
is just the intrinsic rms variation of the flat itself. The RATIO section of the Table shows
the result of correcting the external flat by the internal denominator flat, where the Poisson
statistics are just the numerator and denominator Poisson statistics combined in quadrature. The 'Actual σ' entries are just the standard deviations within the ratio sub-images. For
all three bandpasses, the actual σ barely exceeds the counting statistics of the total electrons recorded, so that the external and internal flats are the same to better than 0.2% rms
in the (480:680,480:680) sub-image.
Table 3 analyzes the internal F435W and the F625W exposures in Table 1 for stability
over time. The F625W data from Table 1 with two sec exposure times are subdivided by
date and gain setting to make five flats that are characterized in the first five columns, as
labeled by the yyddd (year and day) and by the gain. The first column compares the
gain=1 flats on day 317 of 1998 with the gain=1 on day 338, while column 2 compares a
gain=2 flat obtained on day 338 with an independent gain=2 flat on day 343. Columns 3-5
compare different gains. All of ratios in the last row of columns 1-5 are consistent with a
repeatability of better than 0.1%. Having demonstrated the agreement between gain settings, the 29 F625W internal flats in Table 1 are divided into 2 groups, the first of 16
images and the second of 13 images. The ratio of these two independent data sets with
about 250,000 electrons each, again shows a tiny residual of 0.03% at the bottom of column 6, which suggest that the repeatability may be even better than 0.1%. However, the
final column shows a somewhat larger residual of 0.2% in the comparison of the internal
F435W 98Nov7 data with the 99Jan12 high S/N baseline data set of 17 images and almost
a million electrons. Perhaps, the small change is due to the higher set point of the CCD
temperature of -68C vs. the more normal -80C of the 98Nov7 flat. Another set of data like
4
Instrument Science Report ACS 99-01
the 99Jan12 at the normal operating temperature may demonstrate a better agreement with
the 98Nov7 baseline.
In summary, the repeatability of the ACS HRC channel is excellent with the quantum
statistical noise limit dominating up to a million counts. If the current stability level is
achieved on orbit, photometry to ~0.1% per pixel may be possible. The monitoring of the
stability of F625W and F435W should be continued with the internal lamp, although an
increase of exposure time from 2 to 4 sec for F625W would produce better precision.
In contrast to the good news about repeatability for flat illumination with the same
spectral distribution, the HRC CCD shows considerable variation in flat field structure
with wavelength. Table 4 contains the ratios of the external flats of 98Nov6-7 with mismatched wavelengths. The residuals are worse for greater wavelength differences:
σ(F435W/F814W) is the worst at 0.93%. Since the actual rms of the flats is larger at
shorter wavelengths, the residual of 0.64% for σ(F435W/F625W) is expected to be larger
than the 0.33% for σ(F625W/F814W). Difference in the spectral flux distribution between
astronomical sources and the ground flat field illumination may be the limiting factor in
the precision of flat field corrections for broadband filters on-orbit.
For the STIS CCD, the flight flats change over time with a repeatability that is ~0.3%
between the monthly flat field monitoring exposures at the same wavelength. For a G430L
vs. G230LB flat, the agreement is also 0.3% for a one month time difference so that there
is no significant difference with wavelength in the STIS px-to-px flats (Bohlin 1999, in
preparation.) Thus, STIS flat fielding precision is limited by the change in the flats with
time, while the preliminary evidence is that HRC flats are constant in time, but vary more
strongly with wavelength.
4. RECOMMENDED LAB PROGRAM
Standard Filters
Table 5 contains the list of ACS filters which require standard external flats. There are
six SBC filters, 23 HRC filters, and 20 WFC filters. The exposures should have at least
10,000 counts/px for each filter. If the above preliminary HRC 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 whenever the exposure times are short. For
polarization measurements of small objects at ~1% precision, 0.1% stability of the flats
would be especially useful. Because of the count rate limitation of ~280,000 total counts/s
for the SBC MAMA, each flat with 10,000 counts per 2x2 px resolution element requires
a minimum of 3 hours of integration.
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 base-
5
Instrument Science Report ACS 99-01
line flats that cover the wavelength range of each CCD is suggested. This data set should
be obtained at a few epochs before launch to monitor the P-flat stability and to measure
any decline in brightness of the internal lamps as a function of wavelength. The set of
F435W flats from 12/01/99 in Table 1 is an example for this monitoring program. If significant wavelength dependent changes occur, then a finer grid in wavelength would be
required to transfer the measured changes in the internal P-flats to the full set of external
flats. Because of the shorter lifetime of the internal deuterium lamp on the SBC, one internal P-flat with 10000 counts per 2x2 px resolution element must suffice.
Ramp Filters
The ramp filters in Table 6 provide a narrow band filter set with a bandpass of 2% or
9% for the HRC and WFC and cover the wavelength range from 3710-10710Å. The central wavelengths of the ramp filters change continuously with position on each ramp filter
segment. Only the central segments can be used on the HRC. The filters have already been
characterized in terms of transmission in each bandpass and to the 10-5 level out of band.
The flat fields are expected to be sufficiently well defined by the broader band filters from
section 4.1 that fall closest in wavelength. This broadband set is comprised of F330W,
F435W, F475W, F555W, F606W, F625W, F775W, F814W, and F850LP, while F892N can
be used to extend the wavelength range. Thus, the greatest fractional extrapolation is 1.2x
from 8920Å to the 10,714Å limit of the ramp filters. Otherwise, the ramp filter at 3769Å is
the farthest (1.14x) from the central wavelengths of 3354 (F330W) and 4297Å (F435W).
Since the flat fields change more rapidly with wavelength at the shorter wavelengths, the
interpolated 3769Å ramp flat will probably have the greatest error. Bandpass and sensitivity information is indexed in the STScI Synphot package by ramp filter name from Table 6
and by a wavelength within the range covered by each ramp filter.
To quantify the errors in the ramp flats, high S/N monochromatic P-flats are needed at
a few test wavelengths. The two obvious wavelengths are 3769 and 10,714Å. In addition,
7 more ramp P-flats every 1000Å from 4000 to 10,000Å, should verify the technique and
fully quantify the uncertainties in the flat fields for the ramp filters. To further quantify the
uncertainties in the flats, monochromatic illumination on a subset of the 7 wide filters
would complete the characterization of the P-flats. This test would quantify the differences
in the dust motes for monochromatic and broadband illumination.
One fly in this ramp ointment will be residual dust and inclusions in the coatings of the
filters after the final pre-flight cleanup.
A related calibration issue is the delineation of the sub-areas on the detectors that are
valid for the in-band response of the ramps. The positions of the in-band regions change
slowly enough with wavelength, so that the 9 monochromatic P-flats from 3769 to
10,714Å should adequately define this additional calibration parameter.
6
Instrument Science Report ACS 99-01
Dispersers
Table 7 lists the low resolution spectroscopic modes for ACS. Since all five modes are
objective, i.e. the spectrum of an object anywhere in the field is recorded, there is no oneto-one correlation between position on the detector and wavelength. Therefore, no flat can
be applied during pipeline data reduction, unless the flat is wavelength independent. However, off-line processing might benefit from a series of monochromatic P-flats separated
by say 10% in wavelength over the wavelength range for the two HRC and one WFC
modes. These P-flats might be sufficiently close to the flats for the broadband filters that
cover the same wavelength regions, so that an early test might vitiate the need for this data
set.
5. SUMMARY OF RECOMMENDED ON-ORBIT PROGRAM.
The set of external laboratory flats, along with on-orbit monitoring with the internal
lamps is a fairly robust plan to characterize the ACS flat fields. Perhaps, the greatest risk to
this plan is particulate or other contamination that changes from ground to orbit. The
exceptions are the SBC flats, where no high fidelity OTA illumination is available in the
lab.
On-orbit L-flats
Starfields of the appropriate brightness and stellar spacing can be stepped around the
field to measure the low frequency L-flat variations in sensitivity for the SBC and to verify
the technique and check the CCD flats. Brown (1998) used observations of the globular
cluster NGC6681 to measure the STIS MAMA L-flats with this technique. These data
were collected for the primary purpose of measuring the geometric distortion (Malumuth
1997), who also observed a field in Omega Cen (NGC5139) for the CCD with a 5x5 step
pattern with a step size of about 30% of the field of view. For the MAMA measurements, a
less robust cross pattern was used with only 5 steps of ~20% of the field in orthogonal
directions.
To produce L-flats, the photometry for each measurement of a star is computed relative
to the response to the same star when located in the central 40x40% of the detector. A low
order polynomial fit, for example 2-D or Legendre, determines the relative sensitivity over
the field, i.e. the L-flat. Since this stellar data set comes for free from the geometric distortion calibration, the first step in the determining L-flats should be to analyze these
starfields. If the uncertainties are too large or inconsistencies arise, then more observations
with a finer step pattern or of a diffuse source can be made to refine the first results. No
suitable diffuse source is currently known. The Orion Nebula is a bright enough diffuse
continuum UV source (Bohlin et al. 1982), but there are probably too many stars or even
stars that are too bright for the SBC. Jupiter is a possibility for the CCD modes, but rotates
7
Instrument Science Report ACS 99-01
too fast (~.2 arcsec/min) for the SBC, which is the mode that requires on-orbit L-flat
determinations.
Optical Distortions
Because of geometric distortions that are intrinsic to the optical design, the plate scales
mx and my in arcsec/px are not constant across the fields of view. The area of the sky (mx
my) seen by a pixel is not constant; and therefore, observations of a constant surface
brightness object or of a perfectly uniform calibration lamp source will have count rates
per pixel that vary as M(x,y)=(mx my)/(mx(C) my(C)), even if every pixel in the detector
has the same sensitivity. In order to produce images that appear uniform for uniform illumination, the flat fields make an implicit correction for the geometric distortion across the
field that is equivalent to dividing each pixel by the optical distortion function M(x,y),
which is normalized to unity at the center (C) of the field (cf. the WFPC2 procedure of
Biretta 1995).
A consequence of the above flat field procedure is that two stars of equal brightness do
not have the same total counts after the flat fielding step. For example, the optical geometric distortion does not affect the total count rate of point sources, although the extraction
aperture in pixels varies as 1/M in order to sample a constant area of the PSF in arcsec2.
Thus, point source photometry extracted from a flat fielded image must be multiplied by
M(x,y). This point source correction can be accomplished by reading M from the contours
in Figures 6-8 for the SBC, HRC, and WFC, respectively, or by multiplying by the actual
image displayed in Figures 6-8 before extracting stellar photometry. The distortion corrections displayed in Figures 6-8 are approximations from a ray-trace analysis, while the
actual geometric distortions will be measured on-orbit from the same starfield data that
determine L-flats (Malumuth 1997). The values for M are within 2% of unity for the small
SBC and HRC fields, while this distortion correction for photometry ranges from 0.91 to
1.07 along the diagonal of the WFC that lies along the tilted focal plane.
The above discussion is for the case of the ACS pipeline, which produces no geometrically corrected images. For offline geometric corrections, the use of the M corrections of
Figures 6-8 depends on whether “charge is conserved” in the geometrically correct pixel
coordinates for point sources or for diffuse sources.
8
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Table 1. ACS prelaunch data catalog
04/12/98
04/12/98
13/11/98
13/11/98
7/11/98
7/11/98
7/11/98
7/11/98
7/11/98
7/11/98
6/11/98
6/11/98
6/11/98
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50.0
50.0
4.0
4.0
1.2
1.2
4.0
4.0
1.0
1.0
3.0
3.0
EXP
TIME
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
DETECTOR
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
CCD
AMP
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
CCD
GAIN
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CCD
OFF
F625W
F625W
F625W
F625W
CLEAR1S
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CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
F625W
F625W
F625W
F625W
FILTER1
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
F435W
F435W
F435W
F435W
F814W
F814W
F814W
F814W
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
FILTER2
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
NAXIS1
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
NAXIS2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
MEB
ID
-74.8
-74.8
-78.0
-78.0
-80.0
-80.1
-80.1
-80.1
-80.0
-80.0
-80.1
-80.0
-80.1
-80.1
-80.1
-80.1
CCD
TEMP(C)
Instrument Science Report ACS 99-01
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10/12/98
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DATEOBS
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2.0
2.0
1.0
1.0
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2.0
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1.0
2.0
2.0
EXP
TIME
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
DETECTOR
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
CCD
AMP
8
8
8
8
2
2
2
2
2
2
2
2
2
2
4
4
4
4
2
2
2
2
1
1
CCD
GAIN
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CCD
OFF
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
F625W
FILTER1
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
CLEAR2S
FILTER2
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
NAXIS1
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
NAXIS2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
MEB
ID
-76.7
-76.7
-76.8
-76.8
-78.1
-78.0
-78.0
-78.0
-78.0
-77.9
-78.0
-78.0
-78.0
-78.0
-74.7
-74.7
-74.8
-74.8
-74.8
-74.8
-74.7
-74.7
-74.8
-74.8
CCD
TEMP(C)
Instrument Science Report ACS 99-01
CSIJ99012221205_16
CSIJ99012221205_10
2795
2801
CSIJ99012221205_9
2794
CSIJ99012221205_15
CSIJ99012221205_8
2793
2800
CSIJ99012221205_7
2792
CSIJ99012221205_14
CSIJ99012221205_6
2791
2799
CSIJ99012221205_5
2790
CSIJ99012221205_13
CSIJ99012221205_4
2789
2798
CSIJ99012221205_3
2788
CSIJ99012221205_12
CSIJ99012221205_2
2787
2797
CSIJ99012221205_1
2786
CSIJ99012221205_11
CSIJ99012205814_1
2785
2796
FILENAME
ENTRY
11
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
JABCDEFG
ROOT
NAME
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
INTERNAL
OBSTYPE
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
12/01/99
DATEOBS
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
80.0
EXP
TIME
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
HRC
DETECTOR
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
CCD
AMP
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
CCD
GAIN
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CCD
OFF
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
CLEAR1S
FILTER1
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
F435W
FILTER2
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
1062
NAXIS1
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
1044
NAXIS2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
MEB
ID
-67.9
-67.9
-67.9
-67.9
-67.8
-67.9
-67.9
-67.9
-67.9
-67.9
-67.8
-67.8
-67.9
-67.9
-67.9
-67.8
-69.9
CCD
TEMP(C)
Instrument Science Report ACS 99-01
Instrument Science Report ACS 99-01
Table 2. STATISTICS OF THE EXTERNAL/INTERNAL FLAT FIELD IMAGES
F435W
F625W
F814W
NUMERATOR
Poisson(%)
0.37
0.34
0.38
Actual σ(%)
1.59
1.09
0.91
σ Flat(%)
1.55
1.03
0.82
Minimum
0.53
0.70
0.73
Maximum
1.03
1.02
1.03
F435W
F625W
F814W
DENOMINATOR
Poisson(%)
0.40
0.64
0.33
Actual σ(%)
1.60
1.21
0.88
σ Flat(%)
1.55
1.03
0.81
Minimum
0.53
0.71
0.76
Maximum
1.03
1.03
1.06
RATIO
Poisson(%)
0.54
0.72
0.50
Actual σ(%)
0.55
0.73
0.53
Resid. σ(%)
0.10
0.09
0.17
12
Instrument Science Report ACS 99-01
Table 3. REPEATABILITY OF THE FLAT FIELD OVER TIME
98317G1
98338G2
98341G4
98317G1
98341G4
1st16
98NOV7
NUMERATOR
Poisson(%)
0.45
0.44
0.44
0.45
0.44
0.18
0.40
Actual σ(%)
1.11
1.10
1.10
1.11
1.10
1.03
1.60
σ Flat(%)
1.01
1.01
1.01
1.01
1.01
1.01
1.55
Minimum
0.71
0.72
0.72
0.71
0.72
0.72
0.53
Maximum
1.03
1.03
1.03
1.03
1.03
1.02
1.03
98338G1
98343G2
98344G8
98343G2
98343G2
2nd13
99JAN12
DENOMINATOR
Poisson(%)
0.45
0.31
0.44
0.31
0.31
0.20
0.11
Actual σ(%)
1.10
1.08
1.11
1.08
1.08
1.05
1.44
σ Flat(%)
1.01
1.03
1.03
1.03
1.03
1.03
1.43
Minimum
0.72
0.70
0.71
0.70
0.70
0.70
0.57
Maximum
1.08
1.03
1.03
1.03
1.03
1.02
1.03
RATIO
Poisson(%)
0.64
0.54
0.62
0.55
0.54
0.27
0.42
Actual σ(%)
0.63
0.54
0.63
0.55
0.54
0.27
0.45
Resid. σ(%)
0.00
0.00
0.10
0.00
0.08
0.03
0.18
13
Instrument Science Report ACS 99-01
Table 4. CROSS WAVELENGTH FLAT FIELD COMPARISON
F435W
F625W
F435W
NUMERATOR
Poisson(%)
0.37
0.34
0.37
Actual α(%)
1.59
1.09
1.59
σ Flat(%)
1.55
1.03
1.55
Minimum
0.53
0.70
0.53
Maximum
1.03
1.02
1.03
F625W
F814W
F814W
DENOMINATOR
Poisson(%)
0.34
0.38
0.38
Actual σ(%)
1.09
0.91
0.91
σ Flat(%)
1.03
0.82
0.82
Minimum
0.70
0.73
0.73
Maximum
1.02
1.03
1.03
RATIO
Poisson(%)
0.50
0.51
0.53
Actual σ(%)
0.81
0.60
1.07
Resid. σ(%)
0.64
0.33
0.93
14
Instrument Science Report ACS 99-01
Table 5. STANDARD FILTERS
F165LP
Suprasil
SBC
F150LP
Crystal Quartz
SBC
F140LP
BaF2
SBC
F125LP
CaF2
SBC
F122M
Ly-α
SBC
F115LP
MgF2
SBC
F250W
Near-UV filter
HRC
F344N
NeV
HRC
F220W
Near-UV filter
HRC
F660N
NII
WFC,HRC
F814W
Broad I
WFC,HRC
F435W
Johnson B
WFC,HRC
F330W
HRC u
WFC,HRC
POL0V
Vis Polarizer 0 Deg
WFC,HRC
POL60V
Vis Polarizer 60 Deg
WFC,HRC
POL120V
Vis Polarizer 120 Deg
WFC,HRC
F555W
Johnson V
WFC,HRC
F775W
SDSS i
WFC,HRC
F625W
SDSS r
WFC,HRC
F658N
H-α (1%)
WFC,HRC
F850LP
SDSS z
WFC,HRC
F892N
Methane
WFC,HRC
F606W
Broad V
WFC,HRC
F502N
OIII (1%)
WFC,HRC
F550M
Narrow V
WFC,HRC
F475W
SDSS g
WFC,HRC
POL0UV
UV Polarizer 0 Deg
WFC,HRC
POL60UV
UV Polarizer 60 Deg
WFC,HRC
POL120UV
UV Polarizer 120 Deg
WFC,HRC
15
Instrument Science Report ACS 99-01
Table 6. RAMP FILTERS
FR388N
OII Ramp (middle)
WFC,HRC
FR423N
OII Ramp (inner)
WFC
FR462N
OII Ramp (outer)
WFC
FR656N
H-α Ramp (middle)
WFC,HRC
FR716N
H-α Ramp (inner)
WFC
FR782N
H-α Ramp (outer)
WFC
FR853N
IR Ramp (inner)
WFC
FR931N
IR Ramp (outer)
WFC
FR1016N
IR Ramp (outer)
WFC
FR459M
Broad Ramp (middle)
WFC,HRC
FR647M
Broad Ramp (inner)
WFC
FR914M
Broad Ramp (middle)
WFC,HRC
FR505N
OIII Ramp (middle)
WFC,HRC
FR551N
OIII Ramp (inner)
WFC,HRC
FR601N
OIII Ramp (outer)
WFC,HRC
Table 7. DISPERSERS.
G800L
GRISM
PR200L
HRC PRISM
HRC
PR130L
CaF2 PRISM
SBC
PR110L
LiF PRISM
SBC
16
WFC,HRC
Instrument Science Report ACS 99-01
6. REFERENCES
Biretta, J. 1995, in “Calibrating HST: Post Servicing Mission,” ed. A. Koratkar & C.
Leitherer, (Baltimore:STScI), p. 257.
Bohlin, R. C., Hill, J. K., Stecher, T. P., & Witt, A. N. 1982, ApJ, 255, 87.
R. C. Bohlin, D. J. Lindler, & M. E. Kaiser 1997, Instrument Science Report, STIS 9707, (Baltimore:STScI).
Brown, T. M. 1998, “Low Frequency Flat Fields for the STIS MAMAs,” STIS IDT
Report.
Cox, C., Bohlin, R. C., Griffiths, R. E., & Kelsall, T. 1987, “Standard Astronomical
Sources for HST: 6. Spatially Flat Fields,” (Baltimore:STScI).
Kaiser, M.E., Bohlin, R. C., Lindler, D. J., Gilliland, R. L., & Argabright, V. S. 1998,
PASP, 110, 978.
Malumuth, E. 1997, “STIS FUV-MAMA Geometric Distortion” and “STIS CCD Geometric Distortion,” STIS IDT Reports.
17
Instrument Science Report ACS 99-01
0.98
1.02
ACS HRC P Flat
PHRCEXT98311SM02F435W
BOHLIN: prtimg 1-Apr-1999 11:09
Fig. 1
Figure 1: HRC P-flat derived from external illumination through the F435W filter. The
image stretch is from 0.98 to 1.02, as indicated by the reference gray scale at the top. The
occulting finger is the unity mid-gray region at the bottom left, while the various cosmetic
features are described in the text. The orientation of the flat is in the prelaunch coordinate
frame. The GO science data will be flipped about a vertical axis.
18
Instrument Science Report ACS 99-01
0.98
1.02
ACS HRC P Flat
PHRCINT98311SM02F435W
BOHLIN: prtimg 1-Apr-1999 11:09
Figure 2: As for Figure 1, except for internal illumination.
19
Fig. 2
Instrument Science Report ACS 99-01
0.98
1.02
ACS HRC P Flat RATIO
PHRCINT98311SM02F435W/PHRCEXT98311SM02F435W
Fig. 3
Bohlin: prtimg.pro 1-Apr-1999 11:10
Figure 3: Ratio of the images in Figures 1-2. The major difference in the two flats is that
the dust motes are weaker in the wider angle internal illumination. The bright vertical
stripe to the right of center may represent a difference in the contaminating glint off the
edges of the non-flight detector mask.
20
Instrument Science Report ACS 99-01
0.98
1.02
ACS HRC P Flat RATIO
PHRCEXT98310SM02F625W/PHRCEXT98311SM02F435W
Fig. 4
Bohlin: prtimg.pro 1-Apr-1999 11:22
Figure 4: Ratio of a flat through the F625W filter to the F435W flat from Figure 1, both
with external illumination by the RAS/HOMS. The most important difference is that the
freckles are not as deep at the longer wavelength.
21
Instrument Science Report ACS 99-01
Flat Field Fit for F435W
3
Artificial Noise (%)
2
22
1
0
0
200
400
600
800
1000
Column Pixel
Fig. 5
BOHLIN: flatdiff 1-Apr-1999 12:45
Figure 5: A measure of the errors after the fitting step (d) of Section 2 for the external F435W HRC flat. The fits to the original data
image are in the row direction, while the residual differences in percent along columns 10 (top), 511 (middle), and 1013 (bottom) are
displayed with an offset of one percent, progressively from bottom to top. Errors approach 1%, rarely, at the edges, while the algorithm
introduces errors of < 0.05% in the middle of the flat. An iteration of the fits in the column direction with procedure step (e) reduces the
corresponding residual errors in the row direction to <0.01%.
Instrument Science Report ACS 99-01
Variation of SBC effective pixel area
1024
0
02
1.
10
1.0
5
01
1.
768
05
1.0
00
1.0
lse
xi
p
Y
512
95
0.9
90
0.9
256
5
98
0.
0
0
256
512
768
1024
X pixels
Figure 6: M(x,y) function, i.e. relative area of sky seen by the HRC pixels. The variation
is caused by a small amount of geometric distortion and is removed in the flat fielding process to make regions of uniform sky intensity appear uniform in a flat fielded image. To
perform photometry of point sources, divide by M(x,y). The coordinates are the flight GO
science orientation.
23
Instrument Science Report ACS 99-01
Variation of HRC effective pixel area
1024
1.010
5
01
1.
1.005
768
1.000
sl
ex
ip 512
Y
0.995
256
0
0
99
0.
0.985
0
256
512
X pixels
Figure 7: As in Figure 6 for HRC.
24
768
1024
Instrument Science Report ACS 99-01
Variation of WFC effective pixel area
4096
2
.9
0
4
0.9
6
0.9
8
0.9
0
.0
1
3072
02
1.
)s
le
xi 2048
p(
Y
4
.0
1
1024
0
06
1.
0
1024
2048
3072
4096
X (pixels)
Figure 8: As in Figure 6 for WFC. Not shown is a gap of ~45 px at y=2048, which is the
physical separation of the two 4096x2048 CCD chips.
25