After the STIS failure on 2004-Aug-03, the STIS team put together a close-out plan for STIS re-calibration support.
The majority of the calibrations that were detailed in this plan and which affect the standard pipeline calibration of STIS data have been completed, and a re-calibration of all existing STIS data is well underway. When finished, this re-calibration will allow uniformly calibrated STIS data to be obtained from the HST archive without the need to re-create and re-calibrate the data files via the HST archive's OTFR (On-The-Fly-Recalibration) procedure each time they are requested. This should dramatically improve delivery times and render STIS data readily usable by the Virtual Observatory and related projects. As of 2007-Jan-30, 76,450 STIS datasets, or about 60% of the total, have been re-processed and re-archived.
While these re-processed raw and calibrated data files are not yet directly accessible to the user, apart from a few exceptions detailed below, the calibration enhancements that have been done are already available and are automatically applied whenever data is requested via OTFR through the current archive interface.
Currently available pipeline calibration enhancements include:
- Updated echelle flux calibration and blaze shift corrections (ISR 2007-01 and 2005 HST Calibration Workshop paper)
- Updated flux calibration for first-order medium-resolution spectra (ISR 2006-04)
- Improved flux calibration for first-order spectra taken at E1 aperture positions or with narrow apertures (2005 HST Calibration Workshop paper)
- Improved algorithm to correct for charge transfer inefficiency (CTI) when extracting fluxes for first-order spectra: (ISR 2006-01; also Goudfrooij et al. 2006, PASP, 118, 1455)
- Improved spectral traces for the first-order modes, including date-dependent rotation parameters for the most commonly used modes (October/November 2006 STAN article and 2005 HST Calibration Workshop paper)
- Recommended fringe flat exposures are now delivered with most G750L and G750M data, and the name of this recommended fringe flat is put into the FRNGFLAT keyword in the data file header.
Pending enhancements include:
- Association of GO-specified wavecals with spectroscopic observations that lack auto-wavecals will be implemented in the next few months. Currently OTFR cannot produce wavelength or flux calibrated spectra for visits that had the automatic wavecals turned off, even if the observer included appropriate line-lamp exposures in place of the auto-wavecals. We plan to re-define the dataset associations to allow these GO-specified wavecals to be treated in the same way as auto-wavecals, allowing these spectra to be fully calibrated. We expect this change will be implemented during the spring of 2007.
- We also anticipate delivery of significantly improved geometric distortion coefficients for FUV-MAMA imaging modes sometime in the first half of 2007. Note that improved distortion coefficients for NUV-MAMA imaging modes are already available. See ISR 2004-01.
In addition to these improvements to the raw and calibrated data files, a few new post-analysis tasks are now available in the STIS package of the STSDAS distribution.
- ctestis: This task can be used to estimate charge transfer inefficiency (CTI) corrections for photometric measurements of STIS CCD imaging data (Goudfrooij et al. 2006, PASP, 118, 1455).
- mktrace: It has been realized that the trace tilt of the first order modes has slowly rotated over the years. This PyRAF task will find the rotation angle that best aligns the pipeline traces with an observed spectrum and produce a reference file with the rotated traces.
- wx2d: This task improves on the standard bi-linear interpolation used in the OTFR pipeline to rectify the STIS spatially under-sampled 2D spectral data. WX2D instead uses wavelet techniques to resample the spectral image in the cross-dispersion direction. See 2005 HST Calibration Workshop papers on wavelet interpolation and spectral rectification.
Remaining close-out items:
Work on the documentation describing many of the above items is ongoing, and several new instrument science reports (ISRs) should appear over the next few months. Many of the archive enhancements described in the original close-out plans, such as better previews and PSF libraries, will also be worked on over the next year.
The schedule for a few of the originally planned enhancements is less certain and is dependant on internal resource assignments. We still hope to make available a spectroscopic version of MultiDrizzle, which would enhance the combination of spectra that have been dithered in the cross-dispersion direction.
In the future, we also hope to take advantage of two improvements in STIS wavelength calibration that are currently being developed by the Space Telescope-European Coordinating Facility (ST-ECF):
- The ST-ECF has made new line-lamp measurements that have significantly improved the accuracy of the wavelengths for the lines of Platinum, Chromium, and Neon used in the STIS lamps. The far UV part of this line list (< 1700 A; Sansonetti et al., 2004, ApJS, 153, 555) has been published, and we have verified that the wavelength calibration of the first-order far-UV modes as currently implemented in the CALSTIS pipeline is not affected by the new line wavelengths to within the uncertainties quoted in the STIS Instrument Handbook. Once the near-UV part of the new Pt/Cr-Ne line list has been made available, we plan to evaluate the need for a revised wavelength calibration for the near-UV first-order modes.
- The ST-ECF is also developing a physical model of the wavelength solution for the high-resolution echelle modes that should be more accurate than the currently advertised dispersion solution uncertainties. When ready, we plan to make their software compatible within the CALSTIS structure and turn it into a post-observation task within STSDAS.
It should also be noted that the current plan for SM4, the next
servicing mission to HST, includes time for an attempt to repair STIS.
Some details of NASA's plan for a possible STIS repair can be found at
Completion of a final summary document, giving an overview of the impact of STIS, may be deferred until it is clear whether or not STIS will indeed be revived.