17.4 Cycle 7 Calibration

The STIS Cycle 7 calibration plan included additional characterization of STIS performance, along with periodic monitoring of sensitivity, flat fields, dark current, gain, etc. Because it is not feasible to calibrate the instrument in all possible observing modes, the calibration emphasizes measurements in all the supported modes. The results of all Cycle 7 calibration programs are presented in the close-out report STIS ISR 2000-04.

17.4.1 Calibration Priorities

The task of calibrating STIS involves a number of important trade-offs. HST calibration observations are planned to use a limited number of orbits (no more than about 10% of the total science time allocated in the Cycle), and to stretch out through each Cycle so that the observing schedule is not too heavily front-loaded with calibrations. The allocation of spacecraft and staff resources to the calibration effort for STIS has followed roughly the following set of priorities:

1. Monitor the health and safety of STIS. Carry out the necessary periodic monitoring of STIS to ensure that it is operating correctly. Revise operations as necessary to ensure that it will maintain its scientific performance over its lifetime.
2. Update and Maintain Pipeline Reference files. This includes things such as darks, biases, flats, and sensitivities. Information on newly released files is announced via the Reference Files Update mailing list (stis_reffiles_upd), the Space Telescope Analysis Newsletters (STANs), and is posted to the STIS webpages. To subscribe to the stis_reffiles_upd mailing list, send an email to listserv@stsci.edu with a blank subject line and “subscribe stis_reffiles_upd” in the body of the email. As new reference files are incorporated into the pipeline, the “recommended reference files” are updated as appropriate for each dataset in the HST Archive.
   
3. Basic sensitivity calibration of spectroscopic modes. The majority of STIS science observations use the spectroscopic modes. Sensitivity calibration is important for instrument safety, science optimization, and data analysis. The sensitivity calibration includes basic measurements of on-orbit throughput, and monitoring of time variations either due to contamination or due to gain variations in the detectors.
   
4. Characterization of optical performance. This includes point spread functions (PSFs), line spread functions, aperture throughputs, enclosed energy for different spectral extraction heights, and imaging and spectroscopic geometric distortion calibrations. Detailed characterization of scattering (e.g., in wavelength in the gratings, or in the far wings of the imaging PSF) is included, but is orbit-intensive and therefore will be carried out over a longer time scale.
   
5. Characterization of detector and observation specific peculiarities. This includes detector nonlinearities, charge transfer effects, fringing, long-wavelength scattering within the CCD, grating scatter and extended PSF wings, etc.
   
6. Calibration and testing of future observing mode strategies. The original goals to test cross-dispersed gratings and other operating modes were scaled back through the course of Cycle 7 due to the press of the higher priority calibrations.

Within each of these priority groups, calibration priority is in the following order by observing mode:

1. First order prime L grating modes (G140L, G230L, G430L, G750L).
2. Echelle spectroscopy.
   
3. First- order M-mode gratings.
   
4. CCD imaging (broad band first, then narrow band).
   
5. MAMA imaging (broad band first, then narrow band).
   
6. G230LB and G230MB backup modes, including analysis of scattered red light.
   

In addition, on-axis calibrations have higher priority than off-axis calibrations. That is, we sought first to establish the calibrations at the field/slit center and thereafter, to expand the calibration to two dimensions.

17.4.2 Calibration Status

The overall status of STIS calibration was summarized in the STScI Newsletter in October 1998 and in the Cycle 7 Calibration Close-out Report (STIS ISR 2000-04). Many reference files were updated with new calibrations from on-orbit data. The HISTORY and PEDIGREE fields of the calibration reference files should be consulted if you are in doubt about the origin of the calibration. HISTORY keywords contain commentary on the file’s creation. The PEDIGREE keyword describes the type of data (GROUND, INFLIGHT) and gives the date range over which the calibration data were taken.