STIS Instrument Science Report 97-12
STIS Target Acquisitions During SMOV
Rocio M. Katsanis, Ron Downes, George Hartig, and Steve Kraemer
July 25, 1997
ABSTRACT
We summarize the first results on the analysis of in-flight STIS target acquisition (ACQ and
ACQ/PEAK). These results show that the STIS target acquisition (ACQ) is working very
accurately for point sources (within 0.5 pixels = 0.025 arcseconds), about 4 times better
than specified in the Instrument Handbook. As a result of the accuracy of the ACQ algorithm, we are no longer recommending to perform ACQ/PEAKs for the 0.2 arcsecond wide
slits. For diffuse acquisitions the accuracy varies with target size. Although analysis of
ACQ/PEAK data is hampered by a flight software problem, we anticipate that peakups will
be accurate to roughly 5% of the slit width (instead of the 15% previously advertised). We
are implementing several enhancements to the flight software that will take effect by midAugust to improve the quality of the acquisitions.
1. Introduction
To have a successful STIS science observation in a slit or behind a bar/wedge, the target has to be accurately located in the instrument aperture. Following the initial guide star
acquisition for a field, the target location in the aperture plane will be known to an accuracy of ~1-2 arcseconds. For science observations taken through spectroscopic slits that
are less than 3 arcseconds in either dimension and for imaging observations taken using
one of the coronographic apertures, an on-board STIS target acquisition, and possibly a
peakup acquisition, will be needed to fully center the target.
There are two types of STIS target acquisitions, ACQ and ACQ/PEAK (see STIS ISR
97-03 for details). The ACQ observation uses the CCD camera to take an image of the target’s field in a 100x100 pixel target acquisition subarray. The on-board flight software
then processes the image to position the target at the center of the TA subarray (coarse
locate phase). A second image of the field is then obtained, along with an image of the reference aperture (0.2X0.2). These are used to calculate the fine slew needed to center the
target in the reference aperture. The ACQ exposure should center the target on the slit or
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behind the coronographic bar/wedge to an accuracy of ~0.1 arcseconds (based on prelaunch measures).
The ACQ/PEAK observation should be performed if a narrow slit is used for a science
image. The slit is scanned across the object with a predetermined pattern, and the telescope is then slewed to center the object in the aperture; a confirmation image is obtained
(32x32 grid) as the final step. The accuracy of an ACQ/PEAK was expected to be 0.3 and
0.2 times the dimension of the slit or bar for CCD and MAMA peakups, respectively.
2. Data Analysis and Results
We have collected data from the Servicing Mission Orbital Verification (SMOV) of
STIS to analyze the STIS ACQ and ACQ/PEAK accuracies. The data were selected from
the proposals listed in Table 1.
Table 1. STIS Proposals used for Target Acquisition analysis.
Proposal
Number
Title
7067
STIS CCD Point Source Acquisition
7068
STIS CCD Diffuse Source Acquisition
7071
CCD Target Centering
7148
STIS CCD Target Acquisition Workout
7076
STIS Corrector Alignment, Fine
7147
STIS Fine Corrector Alignment with CCD
The analysis of the ACQ exposures was performed using two methods. In the primary
method, the positions of the reference aperture (in the third science image of the ACQ
exposure) and the object (in a confirmation image taken right after the ACQ exposure)
were measured. Comparison of these positions provides an estimate of the errors in the
acquisition. For those ACQ exposures that did not include a confirmation image, the following ACQ/PEAK exposure was used to estimate the ACQ errors (by looking at the
motion derived from the ACQ/PEAK data).
Table 2 below shows the individual ACQ exposure results for this analysis. The results
are listed by Proposal#, Visit#, target name (and size if extended), type of acquisition
(POINT or DIFFUSE), estimated error (in detector pixel units; 1 pixel = 0.05 arcseconds),
and method employed to estimate the errors. The estimated total error is 0.5 pixels (0.025
arcseconds) for point sources, and varies from 0.2 (for the point-like object NGC 6683) to
2 pixels (for the 1.3 arcsecond nuclear region of M86) for diffuse sources. The error for
M86 is not scientifically significant due to the size of the object itself.
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Because of general problems with the flight software (section 3 below), the individual
ACQ/PEAK results are not being included in this report. Simulations where the flight software problems were corrected on the peakup data show an improvement in the final
position of the target in the aperture, close to the expectation errors quoted above. We
expect to confirm these results on the ACQ/PEAK errors after the flight software has been
updated.
We did, however, analyze a series of 20 ACQ/PEAKs (10 in the X direction and 10 in
the Y direction) in the 0.1X0.09 slit. We performed a flux-weighted centroid on the individual points in the peakup to determine where STIS should have pointed. To determine
where STIS did point, we fit the peakup fluxes with a Gaussian, and derived where in that
Gaussian the flux level seen in the confirmation image fell. The difference between the
two positions gives an estimate of the accuracy of the ACQ/PEAKs. The average offset so
derived was 0.103 pixels (0.005 arcseconds), or about 5% of the slit width.
Table 2. Individual acquisition errors for all the ACQ exposures analyzed.
Prop. ID
Visits
Target
Acq. Type
Error (pixels)
Method
7067
1
GD153
Point
+0.1,-0.1
Confirmation image
7067
2
GD153
Point
+0.6,-0.2
Confirmation image
7067
3
GD153
Point
-1.3,+0.0
Confirmation image
7068
1
NGC1399
(0.8 arcseconds)
Diffuse with Geometric Centroiding
N.A.,-1.6
Confirmation imagea
7068
1
NGC1399
Diffuse with Flux
Centroiding
N.A.,-1.6
Confirmation imagea
7068
1
NGC1399
Point
N.A.,-1.8
Confirmation imagea
7068
2
M86
(1.3 arcseconds)
Diffuse with Flux
Centroiding
-1.7, +0.4
Confirmation image
7068
2
M86
Diffuse with Geometric Centroiding
-1.7, +1.4
Confirmation image
7071
1a
GD153
Point
-1.3,+0.0
Confirmation image
7071
1b
GD153
Point
-1.2,+0.3
Confirmation image
7071
3
GD153
Point
+0.4,+0.1
Confirmation image
7148
1
PKS1255-316
Point
+0.0,+0.0
Confirmation image
7148
2
NGC6624
Point
-0.3,+0.1
Confirmation image
a. Confirmation images were diffuse extended objects along the 52x0.5 slit; therefore, only the
centering of the object along the spatial axis was possible to estimate.
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Table 2. Individual acquisition errors for all the ACQ exposures analyzed (continued).
Prop. ID
Visits
Target
Acq. Type
Error (pixels)
Method
7148
3a
NGC6683
(point like)
Diffuse
+0.2,+0.1
Confirmation image
7148
3b
NGC6683
Diffuse
-0.1,-0.1
Confirmation image
7076
4a
BPM16274
Point
-0.1,+0.0
Peakup
7076
4b
BPM16274
Point
+0.0,-0.7
Peakup
7076
4c
BPM16274
Point
-0.4,-0.5
Peakup
7076
5a
BPM16274
Point
+0.0,+0.1
Peakup
7147
1a
BD+28 4211
Point
+0.0,+0.0
Peakup
7147
1b
BD+28 4211
Point
+0.1,-0.3
Peakup
7147
1c
BD+28 4211
Point
+0.2,-0.3
Peakup
7147
1d
BD+28 4211
Point
-0.3,-0.4
Peakup
7147
1e
BD+28 4211
Point
-0.6,+0.1
Peakup
7147
1f
BD+28 4211
Point
+0.0,+0.0
Peakup
7147
2
BD+28 4211
Point
+0.0,+0.0
Peakup
7147
3
GRW+70 5824
Point
+0.2,-0.5
Peakup
The main result is that the basic STIS target acquisition (ACQ) software is working,
and for point sources to a better accuracy (a factor of 4) than was specified. As a result, it
is no longer necessary to perform an ACQ/PEAK in science slits that are 0.2 arcseconds
wide in either direction, since the ACQ itself is positioning the target within 0.025 arcseconds of the center of the aperture. For smaller slits, an ACQ/PEAK should still be
performed following the ACQ exposure.
3. Problems encountered during the analysis.
Observations during SMOV resulted in the discovery of 3 problems with the target
acquisition flight software. Solutions to these problems have been identified and will be
implemented by mid-August (or are already corrected). The problems are
•
When a CCD sub-array is read out (such as for ACQ observations), the first 5 columns
have elevated counts due to the readout timing pattern construction. If a target is
underexposed, it is possible for the bright columns to be acquired rather than the target. The software will be modified to ignore these “bad” edge columns.
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•
The high background level that biases the centroid toward the center of the ACQ/
PEAK scan when using the flux-weighted centroid method. Increases in the background level can be due to bias level fluctuations, hot columns (such as those
explained above), and hot pixels that are not rejected by the current bad pixel table.
The high background level will be corrected by subtracting the minimum value of the
flux (integrated over the subarray) from each of the dwell point fluxes before the
search method (centroiding) is applied.
•
The HST computer (NSSC-I) that commands the spacecraft motion for acquisitions
was truncating the STIS-generated slew requests, thus leading to improper peakup patterns and consequent miscentering of the target. This truncation error has been corrected, so that the slews are properly rounded to the nearest (~1.54 milli-arcsecond)
step size.
4. Further work on Target Acquisition.
As part of the Cycle 7 calibration program, we will be obtaining more data on target
acquisitions. To verify the flight software updates described above, proposal 7605 (“STIS
CCD Target Acquisition Workout”) will perform point source and peakup acquisitions of
a bright target. We will monitor all STIS target acquisitions in Cycle 7 to determine the
slews needed to center the target in the STIS reference aperture. The target acquisition
slews represent the error in the initial HST pointing, which is a combination of the error in
the Guide Star positions, the target position, and the STIS-to-FGS alignment. We will also
track the pointing offsets over time and as a function of the dominant FGS to search for
trends, which would imply a STIS-to-FGS misalignment, or a degradation in the stability
of an FGS.
For each peakup acquisition (ACQ/PEAK), we will determine the motion of HST
based on the acquisition results. This motion represents the error in the fine position of the
target, and is a combination of the error in the target position (from the ACQ), the slit
wheel repeatability, and the error in the relative aperture locations in the (PDB) database.
We will track the pointing offsets over time to search for trends.
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