Instrument Science Report ACS 2006-005
Relative Astrometry Within
ACS Visits
Richard L. White
August 07, 2006
ABSTRACT
The log files from APSIS, the ACS science team’s image processing pipeline, have been
analyzed to determine the relative astrometric scatter among ACS images observed
within a single visit. For a sample including more than 200 visits, approximately 2% of
the visits are found to have images with alignment shifts greater than half an ACS WFC
pixel (0.025 arcsec); more than 25% of visits include exposures with shifts larger than
0.25 pixels (0.0125 arcsec). Observations taken in 2-gyro mode are slightly more likely to
show 0.25 pixel offsets. The typical offset size drops dramatically if only exposures taken
using the same filter are compared: only 3.5% of visits include exposures with shifts
larger than 0.25 pixels, and 1.1% have shifts larger than 0.5 pixels. Half-pixel or larger
shifts result from the failure to acquire two guide stars, from (apparently) bad guide
stars, and when the ramp filters are used. These shifts should be corrected to produce the
highest-quality combined images, especially if images taken in different filters are to be
accurately aligned.
Introduction
HST observations taken within a visit rely on the same pair of guide stars for pointing
and so are expected to have very accurate relative astrometry. While thermal effects can
lead to pointing drifts of up to 50 milliarcseconds during long observations (Gilliland
2005), most images from the same visit align with an accuracy comparable to the
pointing jitter of less than 10 milliarcseconds. The pipeline processing currently being
used to produce the standard CALACS drizzled images and the CADC ACS associations
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Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and
Space Administration
both rely on the accuracy of the pointing and assume that the relative astrometry within a
visit is stable so that it is not necessary to align images before combining them.
The ACS science team’s image processing pipeline APSIS (Blakeslee et al. 2003)
determines the astrometric alignment of individual exposures. The pipeline does not
assume that exposures within visits are accurately aligned; even individual CR-split
exposures within the same visit have independently determined shifts. Is fine alignment
of images within visits is necessary or not? This report addresses the question through an
analysis of the APSIS pipeline results.
The ACS team has processed hundreds of ACS WFC image associations
incorporating more than a thousand exposures using the APSIS software. Among of the
products of that processing are log files and a processing database that record all the steps
taken by the pipeline along with diagnostic information. These files include detailed
information about the astrometric corrections determined for each image in the group.
I have analyzed the APSIS history files for many different ACS WFC fields to assess the
accuracy of the nominal relative alignment (as specified in the header WCS information)
for exposures acquired within the same visit. The properties of the image astrometry are
presented, and specific cases where the relative alignment is poor are analyzed to
determine the reason for the pointing errors.
The APSIS Image Alignment Algorithm
The APSIS alignment algorithm is described in the Blakeslee et al. (2003) paper.
I summarize the relevant details here. SExtractor (Bertin & Arnout 1996) is run on each
exposure, and the resulting catalogs are filtered using the object size and shape
parameters to remove cosmic rays, CCD defects (hot pixels, etc.), and objects that are too
diffuse to use for alignment. If too few sources remain after filtering, SExtractor is run
again using lower detection thresholds. The x,y pixel coordinates of each source are
corrected for the ACS geometric distortion using the distortion model from the IDCTAB
table. Sources on different WFC CCD chips from the same exposure are placed on a
common rectified frame.
The catalog from each exposure is matched to that from a reference image, by default
the one having the most “good” sources, to derive shifts and rotations with respect to the
reference. Note that all images are aligned regardless of the filters being used; this is a
key feature of the APSIS pipeline that enables the production of multicolor images and
catalogs. The matching is first attempted using a simple algorithm that assumes the
header coordinates are approximately correct; it falls back on a more general but more
time-consuming algorithm if the simple approach fails. Both shifts and rotations with
respect to the reference image are fitted, but if a model with only shifts and no rotation is
found to be consistent with the data, it is preferred. The plate scale of the images is
assumed to be known and is not independently determined during the fits.
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For image mosaics covering wider fields where no single reference image has
sufficient overlap with all the exposures, Rychard Bouwen’s “superalign” algorithm is
used for the alignment (Bouwens, private communication). The algorithm is similar but
is generalized so that it does not rely on the selection of a single reference image.
This automatic, adaptive matching procedure works quite well in practice. Typical
WFC GTO fields produce one-to-several hundred matched sources per exposure, and the
resulting shift uncertainty is typically ~0.02 pixels (0.001 arcsec).
Data and Analysis
List of Observations
The table below lists the targets that were used for analysis of the APSIS astrometric
corrections.
Targets Included in Analysis
ABELL1413
ABELL1689
ABELL1703
ABELL2218
ABELL2390
ABELL665
CL0024+16
CL0152-1357
CL0848+4451
CL0910+5422
CL1056-0337
CL1252-2927
CL1604+4304
CL1604+4321
MS1008-1224
CLJ1226+3332
MS1358+62
CONE-NEBULA
MS1455+2232
CT344
MS2137-2353
ESO208-G21
NGC104
FIRSTJ164311+315618 NGC2768
GAL_CLUS_CL0152 NGC2832
HDF-NORTH
NGC2915-OUTER
HDF-SOUTH
NGC3078
M83-NUC
NGC3226
MRC1138-262
NGC404
MS0016+1654
NGC4676A
MS0451+0305
NGC4789A-OUTERDISK
NGC6166
NGC6338
NGC6503
NGC6543
NGC6618
NGC7720
SDSSJ0813+3508
SDSSJ0836+0054
SDSSJ0913+5919
TN0924-2201
TN1338-1942
UGC10214
XMMUJ2235
Only ACS WFC images were used. (APSIS also constructs combined HRC images, but
because the HRC field is so much smaller than the WFC, there are usually only a few
sources per exposure and the resulting astrometric alignment is less accurate.) The APSIS
pipeline constructs mosaic images from data that span multiple filters, multiple visits and,
in fact, multiple HST proposals. A typical image includes data acquired in several
different visits; the data analyzed include a total of 1160 exposures belonging to 207
obsets. The great majority of the observations were taken in 3-gyro mode (before
September 2005), but 72 of the 1160 exposures (6.2%) were taken in 2-gyro mode.
For each exposure the APSIS processing logs provide alignment information
including the x,y shifts in pixels and roll angle compared with the reference exposure.
The APSIS logs also include the predicted pointing shifts from the reference images
based on the nominal WCS information in the header of each exposure. The expected
orientation differences from the reference image were determined using pointing
information extracted from the acs_chip database in DADS.
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The basic information for each exposure consists of the differences between the
predicted pointing shifts/roll angles and the measured shifts/rolls. I will refer to these
quantities as the pointing and roll offsets. If the nominal header WCS pointing
information were perfect, these offsets would be zero; non-zero values measure errors in
the pointing.
Analysis
The complete list of exposures with pointing offsets were grouped by obset (as
determined from the first six characters of the exposure dataset name.) There were a few
cases where exposures from the same obset had alignments determined by comparison
with different reference images (typically because the obset was part of a large-area ACS
mosaic that was processed in several pieces by APSIS.) In those cases the obsets were
split up so that all exposures in each group relied on the same reference image for the
astrometry.
For each obset group the mean pointing and roll offsets were determined using all
exposures in the group. Note that these mean offsets can be quite large since the reference
image was often observed during a different visit using different guide stars. Shifts
between visits larger than 1 arcsec are not uncommon due to uncertainties in the Guide
Star Catalog-1 astrometry used for the HST pointing.
For every exposure we compute the difference between the pointing offsets for that
exposure and the mean offset for the obset. Guide Star Catalog position errors are
removed by this differential measurement. The only remaining sources of scatter are HST
pointing errors within the visit and the ~0.001 arcsec uncertainty of the position
measurement for each exposure.
Results
Most exposures have pointing positions that are very close to those expected. Figure 1
shows a scatter plot of the x and y offsets (in arcsec) for 1160 ACS WFC exposures in
207 obset groups. The bulk of the exposures have pointing offsets smaller than
0.01 arcsec. The median offset is only 6 milliarcseconds; 62% of the exposures have
offsets smaller than 0.01 arcsec. Clearly this confirms the expected excellent pointing
accuracy for HST observations once the Guide Star Catalog astrometric uncertainties
have been removed.
However, much larger offsets, though rare, are sometimes seen. In addition to the
more widely scattered points seen in Figure 1, there are 19 exposures with offsets so large
that they fall entirely outside the bounds of the plot. The properties of these outliers are
discussed in detail below.
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Figure 1: Astrometric offsets of ACS WFC exposures observed in the same visit. Most
exposures have pointing errors smaller than 0.01”. There are 19 points that fall outside
the bounds of the plot, with offsets ranging up to 0.4 arcsec.
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Figure 2: Histogram of pointing offsets. The median and mean offsets are 6 and 9
milliarcseconds, respectively. Observations taken in 2-gyro mode (after August 2005,
shown as light-shaded histogram) are more likely to have offsets larger than 0.25 pixels.
Figure 3: Cumulative distribution of offsets. The line shows the fraction of exposures
with astrometric offsets larger than the given value. The inset is an expanded view (with a
linear x-axis) showing the distribution for larger offsets.
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Figures 2 and 3 show the histogram and cumulative distribution of pointing offsets
(combining the x and y errors). 2.3% of exposures have offsets larger than half an ACS
WFC pixel (0.025 arcsec), and 20% of exposures show offsets larger than a quarter pixel.
The offsets for 2-gyro visits are slightly larger than those for 3-gyro visits, with 30% of
the exposures showing offsets greater than a quarter pixel.
Pointing Scatter Between Filters
Much of the scatter seen in Figures 1–3 comes from comparison of observations taken in
different filters. The choice of filter has an effect on the astrometric zeropoint of the ACS
WFC. Figures 4–6 show the results if we restrict our offset calculations to observations
taken using the same filter (and still in the same obset). The number of obset/filter
combinations increases from 207 to 285 (because exposures using different filters from
the same obset get analyzed separately.) The total number of exposures included drops
slightly (from 1160 to 1155) because the offsets cannot be determined for obsets that
contain only a single observation through a given filter.
The reduction in scatter from comparing only observations in the same filter is
dramatic – e.g., compare Figure 1 to Figure 4. Figure 6 shows the cumulative distribution
of offsets and includes a dashed line showing the distribution from Figure 3. The number
of exposures with offsets greater than about 10 milliarcseconds is far smaller when the
filter is the same. Note that the filter-dependent offsets (and other smaller distortions)
have been calibrated by Anderson (2006) for a subset of ACS filters. Further analysis of
the APSIS pipeline log files would probably permit a zero-point calibration for many
additional WFC filters.
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Figure 4:Astrometric offsets of ACS WFC exposures observed in the same visit. This
version separates the obsets by filter, so that offsets are computed only between
exposures taken using the same filter. Note that the scatter is much reduced compared
with Figure 1. There are 10 points that fall outside the bounds of the plot, with offsets
ranging up to 0.4 arcsec.
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Figure 5: Histogram of pointing offsets for observations using the same filter. The
median and mean offsets are 3 and 5 milliarcseconds, respectively. As for Fig. 2, the
light-shaded histogram shows 2-gyro observations.
Figure 6: Cumulative distribution of offsets for observations using the same filter. The
dashed line shows the distribution from Figure 3; restricting the comparison to exposures
taken in the same filter sharply reduces the frequency of shifts larger than 5 milliarcsec.
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Sources of Large Astrometric Errors
The distribution of offsets shows a tail of rare large pointing errors that extends up to
several tenths of an arcsecond. An examination of the specific cases where this occurs
sheds some light on the phenomenon.
There are 4 obsets that include exposures with shifts larger than 0.03 arcsec. Two are
observations of the cluster CL0910+5422 (obsets j8pv31 and j8pv32), one is an
observation of the strongly lensing cluster Abell 1689 (j8e955), and the last is an
observation of the planetary nebula NGC6543 (j8ck01).
Case 1: Guide Star Acquisition Problems: Two of these 4 obsets (j8pv31 and
j8e955) had guide star acquisition problems that explain the large pointing drifts. In
both cases only one guide star was successfully acquired, so the telescope orientation was
not well constrained. Figure 7 shows small cutout images from CL0910+5422 around a
region selected to have a moderately bright galaxy for reference. These images come
from the flat-fielded (FLT) images, so most of the objects are cosmic rays (which change
from exposure to exposure). The header WCS astrometry was used to shift the images to
remove the dithering, and a cross is plotted at a reference sky position to make the shifts
more easily visible.
Figure 7: Individual flat-fielded exposures from obset j8pv31. A 51x51 pixel cutout
(2.55x2.55 arcsec) is shown for each exposure. The cross marks the same sky position
using the predicted WCS coordinates from the headers; large shifts are evident,
particularly in the top right panel where the pointing shift is 0.38 arcsec.
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There are obviously large shifts between the images, which were acquired over a period
of 6 orbits. Cases such as these can be recognized automatically using the jitter files or
other sources of information on guide star acquisition problems.
Case 2: Bad Guide Star? The second source of pointing errors is more problematic.
CL0910+5422 was observed again in obset j8pv32, which was scheduled a day after
j8pv31. On this second visit both guide stars were acquired successfully, and the
pointing data indicate normal 2-GS fine lock. However, there are still large pointing
offsets between exposures (Figure 8).
Figure 8: Individual flat-fielded exposures from obset j8pv32. In this case both GSs
were acquired successfully, but large pointing shifts are nonetheless seen.
Interestingly, in this case the dominant guide star being used to point the telescope,
0380500201, is the same object that failed acquisition during visit j8pv31 the previous
day. It seems likely that there is a problem with this guide star – perhaps it is a close
double. Unfortunately it does not appear possible to automatically detect this sort of
problem without doing an exposure-level pointing alignment similar to that used in the
APSIS pipeline.
Case 3: Ramp filters: The third source of large pointing errors is the use of the ACS
ramp filters. Apparently the ramps induce a significant apparent pointing shift when they
are used in the ACS WFC. The observations of planetary nebula NGC6543 included
observations using both the ramp filters and other filters, and the ramp-filter observations
are seen to be systematically shifted (Figure 9). My guess is that the ramp filter optical
characteristics induce this shift; however, I have not attempted to understand this effect in
detail. Presumably this is related to the (much smaller) shifts seen between other filters
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and could be put into the IDCTAB distortion models if desired. Possibly at least part of
the shift can be attributed to the fact that most ramp filter observations require placing the
target at a non-standard detector position in order to observe it at the requested
wavelength. The intervening telescope motion may contribute to the offsets.
Figure 9: Obset j8ck01 exposures of the field of planetary nebular NGC6543. The ramp
filter observations (labeled with frXXX, top row and right two panels of the bottom rows)
are shifted by 0.2 arcsec compared with the non-ramp filters.
Conclusions
Most HST exposures show pointing shifts less than 0.01 arcsec within a visit; the median
shift is 6 milliarcseconds. However, in an analysis using 1160 ACS WFC exposures
belonging to 207 obsets, about 20% of exposures are shifted by more than 0.25 pixels
(0.0125 arcsec), and 2.3% are shifted by more than 0.5 pixels (0.025 arcsec). 27.5% of
the obsets (57 of 207) include at least one exposure that is offset by more than 0.25
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pixels. The largest shifts (bigger than 0.5 pixels) are explainable as the result of usage of
a single guide star (so roll is not corrected), the use of ramp filters (which apparently
induce optical shifts in the positions), and (probably) bad guide stars. There may well be
other similar types of rare problems that were not encountered in this sample of data sets.
When only exposures taken in the same filter are considered, the nominal astrometric
scatter is much smaller. The fraction of visits featuring shifts larger than 0.25 pixels drops
to only 3.5% (10 of 285).
If the construction of image mosaics using associations within or across visits does
not take account of these effects, a few percent of the combined images are likely to
exhibit problems in astrometry, morphology and photometry due to errors in the image
combination. The APSIS image pipeline (Blakeslee et al. 2003) has demonstrated that
alignment of individual exposures before cosmic ray rejection can be practical and
robust. A significant side benefit of performing this alignment is the ability to align
images taken in different filters, thereby facilitating the construction of multicolor
catalogs and true color images. Such cross-filter alignment is especially important in light
of the shifts between exposures taken using different filters, though it might be possible
to calibrate those shifts through further analysis of the APSIS pipeline log files.
Acknowledgements
Thanks to John Blakeslee and David Golimowski for providing both data from the APSIS
data processing and for much useful information about the pipeline and its products.
Thanks to Karen Levay for help in extracting pointing information for all the exposures
from the database and to Dorothy Fraquelli for guidance in extracting jitter and other
guide star information on the observations. Thanks to Ron Gilliland and Ken Sembach
for useful comments on a draft of this report.
References
Anderson, J. 2006, “Empirical PSFs and Distortion in the WFC Camera”, in The 2005
HST Calibration Workshop, eds. A. M. Koekemoer, P. Goudfrooij & L. L. Dressel,
NASA/CP2006-214134, p. 11
Bertin, E., & Arnouts, S. 1996, “SExtractor: Software for source extraction”, A&AS,
117, 393
Blakeslee, J.P., Anderson, K.R., Meurer, G.R., Benitez, N., & Magee, D. 2003, “An
Automatic Image Reduction Pipeline for the Advanced Camera for Surveys”,
ASP Conf. Ser. 295: Astronomical Data Analysis Software and Systems XII, 295,
257
Gilliland, R. L. 2005, “Guiding Errors in 3-Gyro: Experience from WF/PC, WFPC2,
STIS, NICMOS, and ACS”, STScI Instrument Science Report TEL 2005-002
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