Instrument Science Report STIS 95-011
STIS Slit Wheel Repeatability
Ellyne Kinney and George Hartig
July 22, 1996
ABSTRACT
This report outlines the results of the STIS Slit Wheel Repeatability Test. The slit wheel
repeatability was found to be +/- 7.5 milli-arcseconds in the direction of rotation and +/2.5 milli-arcseconds in the radial direction, as projected onto the sky. This is well below
the performance requirement of +/- 20 milli-arcseconds.
1. Introduction
The STIS Slit Wheel Repeatability Test was performed at Ball Aerospace on May 8, 1995.
Three tests were performed with the slit wheel powered using the GSE test set. The first
measured the repeatability of the slit wheel when it was rotated in the clockwise direction.
The second measured the repeatability when rotated in the counter-clockwise direction.
The final test measured the repeatability when the direction of rotation was reversed after
measuring each slit. A series of four slits, one from each quadant of the slit wheel, were
chosen to make the measurements. For each test the slits were illuminated from the back
using an opal glass diffuser and fiber light. Images were taken with the Ball Image Analyzer (BIA) which remained stationary. An Infinity lens system was mounted on the BIA,
providing a magnification of about 4x. Each slit was measured in turn and the sequence
was repeated five times for the first two tests and repeated six times for the final test.
2. Data Analysis
The center of each illuminated slit (s16, s20, s47, s63) was measured using the XCOR.PRO
procedure in IDL which uses cross correlation to find the relative slit position. The first
measurement of each slit from each test series was used as the cross-correlating template
and baseline for the nominal slit position. Subsequent measurements were subtracted from
their corresponding baseline to determine the offsets from nominal position. The procedure XFORM.PRO was used to transform the slit position from the image coordinates to the
BIA stage frame coordinates, in which Y lies in the direction of the wheel center. These
offset were then converted from the BIA coordinate pixels to the angular projection
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through the OTA onto the sky in milli-arcseconds, using a scale factor of 5.990 milli-arcseconds/pixel (BIA). (Assuming 1.67140 microns/pixel and an OTA plate scale of 3.584
arcseconds/ millimeter.)
3. Conclusion
The repeatability is on the order of +/- 7.5 milli-arcseconds peak-to-peak, or 1.9 mas
RMS, in the direction of rotation and +/- 2.5milli-arcseconds peak-to-peak, or 0.7 mas
RMS, in the radial direction as shown in Figure 1. This is well within the repeatability
specification of +/- 20 milli-arcseconds specified in SER STIS-OAT-017A. There is some
indication of dependence of repeatability on wheel position. Figure 5, illustrates the positioning distribution for each of the four measured slits, which were in a different quadrants
of the slit wheel. Each slit has a slightly different distribution and RMS values. Thus, there
may be wheel positions at which the repeatability is somewhat worse then that measured
here. The fact that the scatter in the x direction is greater than the scatter in the y direction
shows that the effect plotted is due to actual variations in the slit wheel position, rather
than to measurement error. At maximum, the measurement error is equal to the scatter in
the y direction, althought the effect of bearing wobble is likely to affect this measurement
as well.
Plots were made of the offset of each slit center versus the order each slit image was taken,
to look for any possible drift of the BIA relative to the slit wheel coordinate frame; no
obvious trends were seen in the data (to the level of 1 BIA pixel). (Figure not shown.)
If the slit wheel continues to perform with this accuracy on orbit, some operational advantages may result. In particular, it maybe possible to move between slits, even the narrower
ones, without incurring the need for repeated peak-ups. It may also be possible to hide the
slit locate phase of the standard target acquisition in the guide star acquisition period. In
this scenario, the science slit is selected prior to GS acquisition and located in the detector
coordinates using the line lamp (not HITM); then the TA aperture is selected and the target
located before the science slit is returned to position.
4. References
Sullivan, J. F., Slit Wheel Performance (530688 Rev A), SER STIS-OAT-017A, Ball Aerospace Systems Group, March 1995.
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Table 1: BIA Images
Image Name
Slit ID
Slit Wheel
Location
(degrees)
Rotation Direction
Relative X
position
(pixels)
Relative Y
position
(pixels)
s16f1p
16
197.9592
Clockwise
0.0000
0.0000
s47f1p
47
252.5192
Clockwise
0.0000
0.0000
s63f1p
63
9.4492
Clockwise
0.0000
0.0000
s20f1p
20
114.5193
Clockwise
0.0000
0.0000
s16f2p
16
197.9592
Clockwise
-0.2722
-0.0757
s47f2p
47
252.5192
Clockwise
0.8734
-0.1033
s63f2p
63
9.4492
Clockwise
0.0188
0.1894
s20f2p
20
114.5193
Clockwise
-0.5254
0.2568
s16f3p
16
197.9592
Clockwise
0.1590
0.0099
s47f3p
47
252.5192
Clockwise
0.1433
-0.1041
s63f3p
63
9.4492
Clockwise
-0.1819
0.0999
s20f3p
20
114.5193
Clockwise
-0.0656
-0.0132
s16f4p
16
197.9592
Clockwise
0.7952
0.0235
s47f4p
47
252.5192
Clockwise
0.3359
0.0305
s63f4p
63
9.4492
Clockwise
-0.1603
0.2940
s20f4p
20
114.5192
Clockwise
-0.1931
0.1332
s16f5p
16
197.9592
Clockwise
0.1266
0.0738
s47f5p
47
252.5192
Clockwise
-0.0851
0.0387
s63f5p
63
9.4492
Clockwise
-0.2174
0.2143
s20f5p
20
114.5192
Clockwise
-0.3341
0.2331
s16f1m
16
197.9592
Counter Clockwise
1.4701
0.1353
s47f1m
47
252.5192
Counter Clockwise
0.1637
0.0836
s63f1m
63
9.4492
Counter Clockwise
-0.0566
0.2972
s20f1m
20
114.5192
Counter Clockwise
-0.3712
0.2513
s16f2m
16
197.9592
Counter Clockwise
0.6550
0.2680
s47f2m
47
252.5192
Counter Clockwise
1.0323
0.0800
s63f2m
63
9.4492
Counter Clockwise
-0.0014
0.2551
s20f2m
20
114.5192
Counter Clockwise
-0.2009
0.2951
s16f3m
16
197.9592
Counter Clockwise
0.2970
0.2302
Stage Location: x= 5500, y= -1341500, z= -1286500
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Image Name
Slit ID
Slit Wheel
Location
(degrees)
Rotation Direction
Relative X
position
(pixels)
Relative Y
position
(pixels)
s47f3m
47
252.5192
Counter Clockwise
0.2707
0.0703
s63f3m
63
9.4492
Counter Clockwise
0.0628
0.3555
s20f3m
20
114.5192
Counter Clockwise
-0.5577
0.3580
s16f4m
16
197.9592
Counter Clockwise
0.3031
0.2557
s47f4m
47
252.5192
Counter Clockwise
0.0055
0.0177
s63f4m
63
9.4492
Counter Clockwise
-0.0221
0.4370
s20f4m
20
114.5192
Counter Clockwise
-0.4213
0.3638
s16f5m
16
197.9592
Counter Clockwise
0.6654
0.1729
s47f5m
47
252.5192
Counter Clockwise
0.9303
0.0060
s63f5m
63
9.4492
Counter Clockwise
0.1234
0.3152
s20f5m
20
114.5192
Counter Clockwise
0.1422
0.2742
s47f1r
47
252.5192
Clockwise
0.5382
0.1056
s16f1r
16
197.9592
Counter Clockwise
-0.1217
0.0078
s63f1r
63
9.4492
Clockwise
-0.3177
0.0909
s47f2r
47
252.5192
Counter Clockwise
-0.2392
0.0009
s20f1r
20
114.5192
Clockwise
0.6673
-0.0375
s63f2r
63
9.4492
Counter Clockwise
0.9852
-0.1954
s16f2r
16
197.9592
Clockwise
0.0051
0.0286
s20f2r
20
114.5192
Counter Clockwise
-0.7009
0.1510
s47f3r
47
252.5192
Clockwise
0.8146
0.0406
s16f3r
16
197.9592
Counter Clockwise
-0.8373
0.0231
s63f3r
63
9.4492
Clockwise
0.0461
-0.0163
s47f4r
47
252.5192
Counter Clockwise
-0.1980
0.1308
s20f3r
20
114.5192
Clockwise
0.5421
0.0164
s63f4r
63
9.4492
Counter Clockwise
0.5935
-0.0354
s16f4r
16
197.9592
Clockwise
0.0451
0.2349
s20f4r
20
114.5192
Counter Clockwise
-0.6645
0.3682
s47f5r
47
252.5192
Clockwise
0.4512
0.0060
s16f5r
16
197.9592
Counter Clockwise
-0.3492
-0.0009
s63f5r
63
9.4492
Clockwise
-0.1481
-0.0210
Stage Location: x= 5500, y= -1341500, z= -1286500
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Image Name
Slit ID
Slit Wheel
Location
(degrees)
Rotation Direction
Relative X
position
(pixels)
Relative Y
position
(pixels)
s47f6r
47
252.5192
Counter Clockwise
-0.5067
0.1169
s20f5r
20
114.5192
Clockwise
0.2115
-0.0488
s63f6r
63
9.4492
Counter Clockwise
0.0835
-0.1662
s16f6r
16
197.9592
Clockwise
-0.2506
0.1026
s20f6r
20
114.5192
Counter Clockwise
0.3229
-0.0791
Stage Location: x= 5500, y= -1341500, z= -1286500
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Figure 1: All tests.
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Figure 2: First Test.
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Figure 3: Second Test.
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Figure 4: Third Test.
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Figure 5: Each Slit.
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