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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) 197.9592 252.5192 9.4492 114.5193 197.9592 252.5192 9.4492 114.5193 197.9592 252.5192 9.4492 114.5193 197.9592 252.5192 9.4492 114.5192 197.9592 252.5192 9.4492 114.5192 197.9592 252.5192 9.4492 114.5192 197.9592 252.5192 9.4492 114.5192 197.9592 Rotation Direction Relative X position (pixels) 0.0000 0.0000 0.0000 0.0000 -0.2722 0.8734 0.0188 -0.5254 0.1590 0.1433 -0.1819 -0.0656 0.7952 0.3359 -0.1603 -0.1931 0.1266 -0.0851 -0.2174 -0.3341 1.4701 0.1637 -0.0566 -0.3712 0.6550 1.0323 -0.0014 -0.2009 0.2970 Relative Y position (pixels) 0.0000 0.0000 0.0000 0.0000 -0.0757 -0.1033 0.1894 0.2568 0.0099 -0.1041 0.0999 -0.0132 0.0235 0.0305 0.2940 0.1332 0.0738 0.0387 0.2143 0.2331 0.1353 0.0836 0.2972 0.2513 0.2680 0.0800 0.2551 0.2951 0.2302

s16f1p s47f1p s63f1p s20f1p s16f2p s47f2p s63f2p s20f2p s16f3p s47f3p s63f3p s20f3p s16f4p s47f4p s63f4p s20f4p s16f5p s47f5p s63f5p s20f5p s16f1m s47f1m s63f1m s20f1m s16f2m s47f2m s63f2m s20f2m s16f3m

16 47 63 20 16 47 63 20 16 47 63 20 16 47 63 20 16 47 63 20 16 47 63 20 16 47 63 20 16

Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise

Stage Location: x= 5500, y= -1341500, z= -1286500

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Image Name

Slit ID

Slit Wheel Location (degrees) 252.5192 9.4492 114.5192 197.9592 252.5192 9.4492 114.5192 197.9592 252.5192 9.4492 114.5192 252.5192 197.9592 9.4492 252.5192 114.5192 9.4492 197.9592 114.5192 252.5192 197.9592 9.4492 252.5192 114.5192 9.4492 197.9592 114.5192 252.5192 197.9592 9.4492

Rotation Direction

Relative X position (pixels) 0.2707 0.0628 -0.5577 0.3031 0.0055 -0.0221 -0.4213 0.6654 0.9303 0.1234 0.1422 0.5382 -0.1217 -0.3177 -0.2392 0.6673 0.9852 0.0051 -0.7009 0.8146 -0.8373 0.0461 -0.1980 0.5421 0.5935 0.0451 -0.6645 0.4512 -0.3492 -0.1481

Relative Y position (pixels) 0.0703 0.3555 0.3580 0.2557 0.0177 0.4370 0.3638 0.1729 0.0060 0.3152 0.2742 0.1056 0.0078 0.0909 0.0009 -0.0375 -0.1954 0.0286 0.1510 0.0406 0.0231 -0.0163 0.1308 0.0164 -0.0354 0.2349 0.3682 0.0060 -0.0009 -0.0210

s47f3m s63f3m s20f3m s16f4m s47f4m s63f4m s20f4m s16f5m s47f5m s63f5m s20f5m s47f1r s16f1r s63f1r s47f2r s20f1r s63f2r s16f2r s20f2r s47f3r s16f3r s63f3r s47f4r s20f3r s63f4r s16f4r s20f4r s47f5r s16f5r s63f5r

47 63 20 16 47 63 20 16 47 63 20 47 16 63 47 20 63 16 20 47 16 63 47 20 63 16 20 47 16 63

Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise Clockwise

Stage Location: x= 5500, y= -1341500, z= -1286500

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Image Name

Slit ID

Slit Wheel Location (degrees) 252.5192 114.5192 9.4492 197.9592 114.5192

Rotation Direction

Relative X position (pixels) -0.5067 0.2115 0.0835 -0.2506 0.3229

Relative Y position (pixels) 0.1169 -0.0488 -0.1662 0.1026 -0.0791

s47f6r s20f5r s63f6r s16f6r s20f6r

47 20 63 16 20

Counter Clockwise Clockwise Counter Clockwise Clockwise Counter Clockwise

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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