Instrument Science Report ACS 2006-08
SBC FLATS: PRISM P-FLATS
and IMAGING L-FLATS
______________________
R. C. Bohlin & J. Mack
December 2006
ABSTRACT
The internal deuterium lamp was used to illuminate the SBC detector through the
PR110L and PR130L prisms for 12.2 hours each to produce a total of ~12,000
counts/pixel. This illumination does not simulate the OTA optics and, therefore, is not
suitable for the production of a low frequency L-flat. However, the pixel-to-pixel P-flat
is an improvement over the laboratory SBC P-flat currently used in the ACS pipeline for
the two dispersing modes.
In addition, short exposure internal lamp flats were obtained in the standard imaging
filters. These flats have sufficient signal to define the low frequency L-flat field for five
filters relative to the high signal F125LP flat, assuming that the relative lamp
illumination does not vary with wavelength. These five ratio L-flats are smoother than
the ratios of the current pipeline L-flats; but there is evidence for variation of the
internal lamp illumination with wavelength. Thus, the current SBC L-flats may have
some errors of a few percent due to local inappropriate lumpiness; but the alternative
flats defined by the internal illumination may also have errors.
1. Introduction
Because uniform flat field illumination in the 1200-1700Å region is available neither in
the lab nor in space, the low-frequency L-flat field correction for the SBC filters on
ACS is problematic. However, pipeline processing does not require an L-flat correction
for prism data. Instead, Larsen (2006) derives sensitivities that depend on location on
the detector. The internal deuterium lamp does produce a smoothly varying illumination
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pattern that can be used to define a P-flat. Section 2 of this ISR addresses the creation of
a new P-flat for the SBC prism modes, while section 3 discusses the relative L-flat
correction for five imaging filters.
2. Prism P-flat
The prism P-flat differs from the imaging filters, because the illumination has a different
angle of incidence Bohlin, Hartig, and Meurer (1999, BHM). Because the P-flat for the
prisms has probably changed by an amount that is comparable to the change found by
Bohlin & Mack (2005) from ground to space for the imaging filters , an additional set of
internal lamp exposures are analyzed to define a P-flat appropriate for on-orbit use for
the prism modes.
Figure 1 - Relative count rate vs. total time of illumination of the SBC detector by
the internal deuterium flat field lamp. The 32 points after ~21 hours are 16
PR130L and 16 PR110L points, which are normalized to the earlier F125LP data.
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The internal lamp exposure times are all 2750s for the 16 PR110L and 16 PR130L
images that were obtained in program 10739 over the time period of 2005Oct4 to
2005Nov5. The total exposure time is 24.4 hours, which resulted in ~12,000 total
counts/pixel near the center of the SBC MAMA detector. Figure 1 shows the
degradation of the lamp output vs. exposure time. Based on pre-launch testing, the
declining lamp output is probably caused by polymerization of contaminants on the
lamp window. In order to verify the claim by BHM that the SBC flats are independent
of wavelength, the 16 individual exposures for each prism are co-added and fit
following the procedures summarized in Bohlin et al. (1999) The IDL routine flatall
along with its main subroutines flats and fitflat are used to produce the P-flat by fitting
the overall vignetting pattern with low order splines. This internal vignetting of the lamp
illumination is about 10% from image center to the edges, while the P-flat is equal to the
original co-added image divided by the fitted pattern.
The data quality flags in the third extension of the flat field file are all zero, except for
values of 512 around the edges, in the un-illuminated region beyond column 850, in
rows 599:604 with dead MAMA diodes, and for 295 pixels listed in the bad pixel table
lch1502jj_bpx.fits.
The new PR110L and new PR130L P-flats are typically identical to 0.2-0.4% of excess
residual scatter over the differences expected from the Poisson counting statistics. With
an expected Poisson uncertainty per pixel of 0.9% in the combined 32 prism exposures,
the intrinsic differences between PR110L and PR130L are small enough so that the best
P-flat is made from the co-addition of all 32 internal prism exposures. The ratio of this
new prism flat to the original lab P-flat is illustrated in Figure 2.
In the dashed box of Figure 2, the expected Poisson scatter of 1.13% is comprised of
0.7% from the original lab flat and 0.9% from the new in-flight P-flat. The error in using
the original flat is 1.08% from the combined (in quadrature) of its 0.70% Poisson
statistics and the 0.82% systematic error. Assuming 0.3% systematic error caused by
averaging the PR110L and PR130L images and combining with the 0.9% statistical
uncertainty, the new P-flat uncertainty of 0.95% per pixel is slightly better than the
1.08% error expected in the old P-flat. The intrinsic differences at the lower right in
Figure 2 increase from 0.82% to over 1% due to a slight shift in the Moire interference
pattern between the micro-channel plate and the anode array of the SBC MAMA
detector. Thus, the advantage of the new prism P-flat is greater in the lower right corner
of the SBC.
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Figure 2 - Ratio of the on-orbit SBC P-flat to the laboratory P-flat for the prisms.
The name of the lab flat, psbcex99marsm20prism, is the sum 20 images with
external illumination obtained in March of 1999 and is also known as
jref$m5t12222j_pfl in the ACS pipeline processing system. The grey scale
calibration is indicated on the reference bar at the top right. The line of text "P-flat
rms(%)=…" indicates one-sigma values in the 101x101 pixel standard region at
452:552,462:562 (dashed box) for the total pixel-to-pixel scatter, the Poisson
counting statistic, and the intrinsic rms variation of the ratio. The horizontal grey
strip is the region of bad anodes, where the flats are set to unity. The vertical
feature near column 578 is caused by the slightly different illumination in the
region of the repeller wire. The uniform grey region beyond column 850 at right
hand side is outside the field-of-view of the prisms.
3. Imaging L-flats
Even though the P-flat is independent of wavelength, the L-flat is not. Mack, et al.
(2005, MGVB) used observations of the NGC6681 star cluster to determine SBC L-flats
on a lightly smoothed 16x16 grid. The S/N of the hot blue HB stars in NGC6681 is the
highest at the shorter wavelengths. The formal uncertainties in the L-flat are the lowest
for F125LP, which also has the most observations. An alternative method for defining
the remaining five SBC imaging L-flats uses short exposure internal deuterium lamp
flats. These flats have sufficient signal to define the low frequency L-flat field for five
filters relative to the high signal stellar F125LP L-flat, assuming that the relative lamp
illumination does not vary with wavelength. These five ratios of internal lamp images
are smoother than the corresponding ratios of the current MGVB pipeline L-flats. For
example in the worst case with the lowest S/N for F165LP, the ratio F165LP/F125LP is
shown in Figure 3.
If the deuterium lamp illumination of the detector-filter combination is the same for all
six filters, the internal flat field lamp can be used to find differences in the L-flat
between filters. Single exposures of 2500s obtained in program 10739 in each of
F115LP, F122M, F140LP, F150LP, and F165LP are sufficient to define the delta L-flats
relative to the F125LP L-flat from MGVB. The ratio of the MGVB stellar L-flats in
panel c) of Figure 3 is lumpier than the corresponding ratio of the L-flat structure in the
internal lamp images illustrated in panel d) by up to 7% in a hot spot (white) in the
lower left quadrant of panel c). MGVB estimate a systematic uncertainty of ~4% for
F165LP in their table 3. Alternative L-flats, L', for the five filters in Table 1 are formed
by multiplying the F125LP stellar L-flat by the appropriate internal lamp ratio, eg.
multiplying panel b) by the F165LP/F125LP ratio of panel d) from the internal lamp.
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Figure 3 - Top panels: L-flats from MGVB a) F165LP and b) F125LP scaled to
unity in the central 101x101 pixel box. Lower panels: c) Ratio of panel a) to panel
b) for the stellar L-flats from MGVB. d) Corresponding ratio of L-flat response to
the internal deuterium lamp for F165LP/F125LP.
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Table 1 summarizes the large-scale and small-scale differences between the five L-flats
derived from the deuterium lamp data and the MGVB pipeline flats. The two "64 Bin"
columns are the large-scale minimum and maximum values of the MGVB/internal L/L'
ratio averaged over 8x8=64 boxes of 128x128 pixels per box, while the last two
columns are the local, small-scale minimum and maximum of the L-flat ratios within
the central 512x512 pixels of the 1024x1024 pixel SBC image format. The total range
of 64-bin differences exceeds 10% only for F165LP. Both the min and max of the range
of these large-scale differences are usually at an edge of image, where any change in the
internal lamp illumination pattern with wavelength would contribute the most. The min
and max of the local, smaller-scale lumpiness in the central 512x512 pixel region
minimize any effect of changing illumination with wavelength and demonstrate
agreement between L and L' to 4%, except for F165LP.
TABLE 1. Ratio of Stellar to Internal-lamp L-flats
Filter
F115LP
F122M
F140LP
F150LP
F165LP
Large scale Smaller scale
64 Bin
Central 512 px
min max
min max
1.00 1.05 0.99 1.04
0.99 1.07 0.98 1.04
0.95 1.02 0.97 1.04
0.96 1.02 0.97 1.01
0.93 1.06 0.94 1.07
Complete reduction, including time dependent corrections, is done for all the NGC6681
F165LP data; and stellar photometry is derived using the MGVB pipeline L-flats. This
standard photometry, L, can be corrected to L' photometry for the alternative internal
lamp L'-flat. If the lamp L'-flat is superior to the stellar L-flat, then the rms scatter in the
dithered observations of NGC6681 should be lower in the L' photometry.
The biggest differences between L and L' photometry should be for stars lying nearest to
the hotspot, where L/L' reaches a maximum of 1.07 in panel c) of Figure 3. There are
seven stars where at least one observation lies in box centered on the hotspot at
(360,320) with x-size of 64 pixels and y-size of 74 pixels. Each of these seven stars has
33 observations, so that the seven rms measures are robust. Five stars show an improved
rms scatter with the L' lamp flat, where the biggest improvement is from 0.041 to 0.032
mag. However, the rms scatter for two of the seven stars increases; and most of the
outliers for the L' lamp photometry tend to lie near the top of the image. Thus, the L'-flat
may be good locally; but there may be wavelength dependent errors that slowly vary
across the full field.
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To test for local improvements, the only photometry considered is still for the same
seven stars; but every point is restricted to lie within a box of 200x200 pixels, centered
on the hotspot. For this restricted case, the number of repeat observations is 7-8 for three
stars and 19-20 for four stars; and the rms scatter is always better for the L' photometry
by as much as a factor of two.
In summary, the relative L'-flats from the deuterium internal lamp suggest that the flats
derived from the stellar observations by MGVB are somewhat too lumpy; and the final
L-flats should be restricted to have stiffer fits to the stellar data. Unfortunately, the
relative L'-flats from the internal lamp data may have significant low order errors across
the full field that could be attributed to a gradient in the relative lamp illumination that
varies with wavelength. Ongoing SBC observations of NGC6681 will be combined with
existing observations; and revised L-flats will be made that should have ratios similar to
the internal lamp ratios, eg. Figure 3d.
REFERENCES
Bohlin, R., Hartig, G., Lindler, D., Meurer, G., & Cox, C. 1999, Instrument Science
Report, ACS 99-01, (Baltimore:STScI)
Bohlin, R., Hartig, G., & Meurer, G. 1999, Instrument Science Report, ACS 99-02,
(Baltimore:STScI), BHM
Bohlin, R. C., & Mack, J. 2005, Instrument Science Report, ACS 2005-04,
(Baltimore:STScI)
Larsen, S. 2006, Instrument Science Report, ACS 06-02, (Baltimore:STScI)
Mack, J., Gilliland, R., van der Marel, R., & Bohlin, R. 2005, Instrument Science
Report, ACS 2005-13, (Baltimore:STScI), MGVB
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