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Space Telescope Imaging Spectrograph




DESCRIPTION: This activity is the first in a series of measurements that will be made to characterize the dependence of the CCD Charge Transfer Inefficiency (CTI) as a function of signal level, source position, and time. The remaining measurements will be conducted as part of the Cycle 17 calibration program to fully characterize the CTI. This activity, executed during the SMOV4 period, will enable measurement of the magnitude of the CTI for a limited parameter set and will provide a comparison of the current CTI performance with the CTI performance prior to the Side-2 failure.

For CCD detectors, the transfer of charge from one pixel to the next is not perfect. CTI measurements quantify the fraction of charge left behind after this charge transfer. The further the charge needs to be shifted before the read-out, the more charge it will lose.

These CTI measurements are made using an "internal sparse field test", along the parallel axis. This test utilizes the ability of the STIS CCD and its associated electronics to read out the image with any amplifier, i.e., by clocking the accumulated charge in either direction along both parallel and serial registers (Goudfrooij, et al. 2006). A sequence of nominally identical exposures is taken alternating the readout between amplifiers on opposite sides of the CCD. Amplifier D (the default) and amplifier B are used to check the parallel CTI at the default gain=1 setting. Comparison of the charge readout using the D and B amplifiers yields a measure of the CTI. If there were no CTI, then the ratio of the signals read out using the two different amplifiers for a source at the same position would be identically 1.

This activity consists of a series of bias images taken with amplifier D and amplifier B. This is followed by a series of 5 tungsten lamp exposures with the 0.05X31NDA slit in mode G430M at the 5471 central wavelength setting. The image of the long-slit, 0.05X31NDA, lies along the dispersion direction and has a narrow (2 pixel FWHM) profile. Measurement of the CTI using a slit oriented in the dispersion direction presents a “worst case” since there is no background source (“sky”) to fill the charge traps on the CCD. This test is executed for a single signal level and a single slit image position on the detector. Subsequent executions of the internal sparse field CTI test will move the mode select mechanism over a range of MSM positions to vary the location of the slit image (and hence the number of parallel shifts) on the detector. The complete test, which spans a yearly calibration program, will also evaluate the CTI as a function of signal level.


DEPENDENCIES: Should be executed after STIS-08, the lamp and aperture functional test.

DURATION: 1 Internal Orbit


ANALYSES & EXPECTED RESULTS: The “internal sparse field method” quantifies two key aspects of Charge Transfer Inefficiency (CTI) effects on spectroscopic measurements: measurements: (1) the amount of charge lost outside a standard extraction aperture, and (2) the amount of centroid shift experienced by the charge that remains within that extraction aperture (Goudfrooij, et al. 2006).

The SMOV4 STIS-14 CTI activity comprises the first visit in a more extensive Cycle 17 CTI measurement program that will establish a new baseline (post Side-2 failure) for CTE performance. The full Cycle 17 program will characterize the CTE losses as a function of signal level (and CCD gain settings) and signal position, characterize CTE losses for spectroscopic versus imaging modes and monitor CTI as a function of time.

This activity is the first visit in the Cycle 17 calibration program to characterize the STIS CCD CTI.


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

DATE: September 27, 2007