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Space Telescope Imaging Spectrograph



DESCRIPTION: Verify slit locations and slit wheel repeatability. Update PDB and onboard table if necessary.

This activity has five functions: verification of the aperture positions, verification of absolute optical position, verification of slit wheel repeatability, verification of lamp functionality, and verification of CCD imaging mode functionality.

Verification of the aperture positions: Determine the location of the slit positions, in pixel coordinates, on the STIS CCD detector for a representative subset of STIS acquisition apertures and slits. This will be accomplished by acquiring internal exposures using the tungsten lamp for illumination of the selected apertures. An image will be taken at each of eight nominal slit or aperture positions on the slit wheel. These apertures (52X0.05, 52X0.2, 52X0.1, 52X0.5, 52X2, 0.2X0.06, 0.2X0.09, and 0.2X0.2) were chosen to represent frequently used apertures and span the nominal range of selected aperture positions. All images will be taken with the acquisition mirror. Images of the 0.2X0.2 slit will be interspersed with these measurements to provide a reference position. The measured slit locations will be compared with the previous side-2 measurements of the slit positions.

A change in relative slit positions between side-1 and side-2 is not expected, although different thermal conditions and any resulting flexure of the STIS bench may cause an overall shift of how the slit images project onto the detector, and bench flexure during the course of the observations may cause these offsets to drift slightly during the course of these observations. Such thermal offsets will be measured and removed from both the new and the old data by monitoring the interspersed images of the 0.2X0.2 reference aperture.

Verification of absolute optical position: The first "used on orbit" slit above the avoidance zone (330X050 at encoder position 33699) and the last "used on orbit" slit above the avoidance zone (F28X50LP at encoder position 3824324) are imaged to check the absolute optical positions of these slits.

Verification of slit wheel repeatability: In addition to acquiring images at the nominal slit position, multiple images of three slits (52X0.05, 52X.1 and 52x0.2) will be taken after having moved the slit wheel to check for positioning repeatability.

Verification of lamp functionality: Spectra will be taken with each of the Pt/Cr-Ne emission line lamps (HITM1, HITM2, and LINE) at the two commonly used current settings (3.8mA and 10mA) to check lamp functionality for routine science operations. The lamps are used in imaging mode with the 0.2X0.2 slit and dispersed for comparison of individual emission lines (G230LB with 52X.2). The lamp flux and location of the spectra on the detector will be compared to previous side-2 values.

Verification of CCD imaging mode functionality: This is accomplished when the slit is imaged on the CCD detector using the tungsten and HITM lamps.


DEPENDENCIES: Execute after the initial CCD functional test (STIS-06). This assumes that the CCD anneal (STIS-05) has also been performed.

While this activity may result in FSW and aperture table updates, it is not expected to be necessary. Consequently, subsequent STIS activities do not need to wait for the results of this activity before execution.

DURATION: 8000 s


ANALYSES & EXPECTED RESULTS: The slit/aperture centers relative to the reference target acquisition aperture(s) will be determined from the slit images. Comparison of the slit center positions taken before and after slit wheel motion will be used to measure the repeatability of the slit wheel. If there are changes in the relative slit/aperture positions then these changes will be more fully characterized by a measurement of each slit/aperture position.

Comparison of lamp fluxes at different currents will provide current setting and exposure time information for future use of the HITM2 lamp. This will enable redundancy for the 3.8mA current setting of the HITM1 lamp.

Both the on-board table of slit/aperture centers used for target acquisition and the aperture file in the PDB may be modified as a result of the analysis of the data. Changes may be made to the nominal slit encoder positions. However, it is not expected that any changes will be necessary, and SMOV planning should assume that no such updates will be needed as the result of this program.


AUTHOR/telephone/email: Mary Elizabeth Kaiser/401-516-5088/

DATE: September 25, 2007