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Space Telescope Imaging Spectrograph




DESCRIPTION: The maximum thermal motion of the MAMA detectors occurs in the first portion of those orbits that immediately follow a large angle maneuver, as this produces the maximum external changes on the portion of axial bay closest to the STIS instrument. During the second orbit on the same target, the shifts are smaller, but the largest shift is still near the beginning of the orbit. We will follow these changes for two orbits with each MAMA using internal lamp images done with the medium dispersion echelle formats in order to obtain a two-dimensional series of reference points on the 2-dimensional detector format. Exposures will be executed using one of the small echelle apertures with the medium resolution echelle gratings, and will have exposure times of 120 seconds for deep, sharp spectral line images.

Each orbit will start with a sequence of six of these dispersed lamp images. For the remainder of each orbit, dark exposures of 600 to 800 s in length will be interspersed between the lamp spectra. This provides frequent sampling in the portion of the orbit where thermal flexure is largest, while avoiding excessive lamp use when shifts are expected to be slower. The dark frames will also provide a useful addition to the calibration of the MAMA detector dark current.


DEPENDENCIES: Each of the two orbit internal executions must be done after large angle maneuvers that are predicted by thermal models to have a large effect on the internal echelle optics. This activity should be sequenced after STIS-25 STIS MAMA Spectroscopic Throughputs, but there is no need to wait for downlink or analysis of that program’s data. If possible, this can be done in parallel with similar thermal stability tests for other instruments.

DURATION: Two visits, two orbits each. Total 4 full internal orbits (11,500 s).

DATA REQUIREMENTS: 5.5 Gbits for each of the two visits.

ANALYSES & EXPECTED RESULTS: All spectra are internal wavecals with echelle gratings. Cross-correlations will be performed with respect to the initial exposure and to an averaged exposure. STIS IDT IDL tools will be used with the assistance of Don Lindler.

COMMENTS: These deep exposures will also be of use to confirm the updated wavecals reference wavelengths provided by ESA and NIST. The dark frames will provide information on the dark count rate. Lamp on time will be about 2900 seconds per two orbit visit. This totals to about 16.1 mA-hours. Total LINE lamp usage to mid-2004 was about 2200 mA-hours out of an estimated 15000 mA-hour lifetime, so this test will use only about 0.1% of expected lamp lifetime.

NOTE: make sure that mechanisms are not moved between wavecal and dark exposures. Recommend that this internal motion measurement be for two full orbits as major maneuver is at beginning.

AUTHOR/telephone/email: Ted Gull/301-286-6184/Theodore.R.Gull@NASA.GSFC

DATE: September 27, 2007