Instrument Science Report ACS 2005-05
ACS Coronagraph
Performance in Two-Gyro
Mode
Colin Cox and John Biretta
June 28, 2005
ABSTRACT
In February 2005, as part of a study of HST performance using two gyroscopes instead of
three, the ACS coronagraph was exercised in two-gyro mode and its quality compared with
that in the normal three-gyro operational mode. This is a sensitive test since an inaccurate
location or a small pointing instability when a target was behind the coronagraphic spot
would make a significant difference to the image. No degradation of performance was
detected either by inspection of the images or detailed analysis.
Introduction
In February 2005, a suite of tests (F2G Test) was performed to assess the impact on
HST of operating with two gyroscopes instead of three with the aim of extending the useful life of the gyroscopes. The possible degradation of pointing performance would be
expected to broaden the point spread function with possible effects on resolution and sensitivity. The ACS coronagraph (Krist, 2000, Pavlovsky et al. 2004) is quite sensitive to
pointing variations. A star placed behind the 1.8 arcsecond spot still transmits some light
around the spot edges. A small shift in the spot position could noticeably change the intensity and distribution of this unblocked light, weakening the suppression effect and
complicating later analysis. So a test was devised to compare the coronagraph performance in three-gyro and two-gyro modes. We present the results of these tests performed
in HST program 10445.
Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration
Instrument Science Report ACS 2005-05
Method
There have been a number of measurements made to characterize the coronagraph and the
aim was to repeat one of these and do a direct performance comparison. Necessarily, we
had to choose a target that had been previously observed and that would be visible at the
scheduled time of the two-gyro test. From several plausible targets, we selected the one
which would be available even if the test slipped by a week. The star chosen was a brown
dwarf HD-130948A which had been observed in program 9668 in 2002 by Krist et al. We
copied the filters used in the earlier observations.
We currently use an improved acquisition strategy to allow for the small but rather unpredictable variation of the coronagraphic spot position. This occurs on a time period of a few
days. The first feature of the new strategy is that we perform the target acquisition with the
coronagraph in place (Krist, 2002). A second improvement introduced in June 2003 is that
as part of a weekly flat field monitoring program we include an HRC flat field with the
coronagraph in place, and before any coronagraphic observation, using the most recent
series of spot measurements, we apply an offset to correct as far as we can for the spot
wandering.
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Figure 1: Movement of the 1.8 arcsecond coronagraphic spot. The spot measurement immediately preceding the coronagraphic observation was used to refine the coronagraph alignment.
We were concerned that the positional variation could degrade this particular measurement and so we requested that the preceding flat field be taken as close as possible to the
test, but allowing time for downloading the data, analysis and installing the offset. This
takes a total of two days. We then included flat field measurements as part of the coronagraph test so that we could detect any motion around the time of the two-gyro test.
Figure 1 shows the motion of the spot in a two month period surrounding the two-gyro
test. This was a relatively quiescent time; the spot can move by about three pixels from
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week to week. From the plot it is clear that the position was known to within a tenth of a
pixel for this test. One pixel is about 0.025 arcseconds on a side.
Observations
The two-gyro observations were made in program 10445 and normal coronagraphic
observational procedures were used. In visit 1, the target was acquired in the target acquisition aperture which is displaced from the coronagrapic spot by about 3 arcseconds. Once
the target is acquired the exact position is calculated and a telescope slew occurs to place
the target centrally behind the 1.8 arcsecond diameter spot. This slew includes a small correction to allow for the coronagraphic spot movement. Then a 300 second exposure was
taken with each of four filters, F475W, F625W, F775W, and F850LP. These filters were all
used in the original calibration proposal 9668. Visit 2 used the same sequence to test that
we can return to the same position. Visit 3 was similar but at a roll angle differing by 25
degrees. This permits subtracting the rotated images to show features close to the occulted
star. Visits 4 and 5 and 6 were earth flats which, in combination with the earlier flats, gave
us spot measurements straddling the coronagraphic alignments.
Results
All target acquisitions and visits were successful. Comparison of the intensity distributions from the original and current measurements showed almost identical structure.
Figure 2 shows the coronagraphic images while figures 3 to 6 show comparisons of the
radial profiles centered at the spot center for the original three-gyro operational mode and
the two-gyro mode used in this test. The vertical line indicates the edge of the spot at a
radius of 0.9 arcseconds. During detailed analysis of the data, it was noted that the threegyro images had slightly higher background levels than the two-gyro images. The cause
of this is unclear -- it may be related to differences in the circumstances of the observation
(sky background, scattered light, etc.) or small differences in the calibration reference files
made several years apart. It seems unlikely that the difference is directly attributable to
two-gyro mode. Prior to plotting figures 3-6 the background levels were determined in a
region 16 arcseconds from the star, and then a uniform background was subtracted from
the three-gyro data, so as to equalize the background levels.
The method of subtracting rotated images is illustrated in Figure 7 which shows the
two coronagraph images with a 25° rotation between them and the subtracted image
clearly showing the nearby star as a positive (white) spot and a negative (black) spot. This
star HD130948B is 2.6 arcseconds away from the main target HD130948A, and although
it is not a stringent test of the method, does indicate that it works as expected.
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Three-gyro images
Two-gyro images
Figure 2: Images of the star behind the 1.8 arcsecond coronagraphic spot through each of four filters shown
with a logarithmic stretch. Those on the left are from 2002 while those on the right are from the February
2005 two-gyro test. The same detailed structure can be seen in both sets of images. The images on the right
were exposed for 300 seconds while those on the left had a 30 second exposure. The images are approximately 10 arseconds square.
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Figure 3: Profile comparisons for filter F475W. The values plotted are raw count rates. No normalization
has been performed to force the two sets of data to match.
Figure 4: Radial profile comparisons for filter F625W
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Instrument Science Report ACS 2005-05
Figure 5: Radial profile comparison fro filter F775W
Figure 6: Radial profile comparison for filter F850LP
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Instrument Science Report ACS 2005-05
Figure 7: Subtraction of rotated images. The nearby star shows up as white and black points separated by
the roll difference of 25 degrees.
Acknowledgements
We are grateful for the assistance of George Chapman and Alison Sherwood during the
preparation of the proposal for this study.
References
Krist, J., The predicted performance of the ACS coronagraph, ACS-ISR 2000-04
Krist, J., ACS Coronagraph Update for Cycle 12 Proposers, ACS-ISR 2002-11
Pavlovsky et al. 2004, ACS Instrument Handbook, Version 5.0, section 5.2
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