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FOC Instrument History and Advisories

FOC Advisories

Advisories posted here provided information about changes which affected FOC data and its calibration throughout its operation. These are not new items, rather references for understanding the current status of FOC data, image header information and instrument operation.

Sensitivity Drop in the f/96 Relay [7 Apr 1997]
A drop in sensitivity of approximately 10% has been seen in the f/96 relay. This is attributed to a change in the sensitivity of the FOC itself, not in COSTAR or the OTA. Since we do not monitor the sensitivity of the FOC very regularly, we do not have a very good history of this sensitivity change, but the evidence we have indicates that there has been a steady decline over the past 1-2 years, with little or no dependence on wavelength.

Routine FOC Operations [7 Apr 1997]
Both the f/96 and f/48 cameras on the FOC have operated successfully since the start of 1996. Although the focus was farther from nominal for a short period of time, the f/96 has performed as it always has, producing such results as the map of Pluto and the image of the surface of Betelgeuse. Not to be forgotten, the f/48 has steadily been tested with complete success. In fact, the behavior of the background in f/48 images has begun to prove its stability, with periods of near-nominal levels. In all, the FOC has has been operating well.

FOC f/48 Long-Slit Spectroscopy [7 Apr 1997]
Recent observations taken with the f/48 slit have resulted in a determination of the position of the slit to within 0.2". More details of this determination, along with the behavior of the background in the latest test, were reported in the 20 July 1995 TIPS meeting. The f/48 long-slit spectroscopic facility is being made available for Cycle 7 observations.

FOC User Support Changes [3 Jan 1996]
Each proposal approved for execution in Cycle 6 were assigned two people: a Program Coordinator and a Contact Scientist. The program coordinator will be primarily responsible for the scheduling and execution of the program and will work in concert with the Contact Scientist to insure that technical issues are properly handled. The Contact Scientist, however, will be responsible for the overall success of the program, from proposal writing all the way through the data analysis phase.
FOC programs will be assigned a Contact Scientist from the STScI FOC Group to work with each PI. The CS will be available to answer nearly any question related to the proposal, and can be contacted directly or through the STScI Help Desk.

FOC Filter Shifts [3 Jan 1996]
Recent analysis of calibration data has determined that several f/96 filters produce significant shifts of 0.1" and more in the images. A full listing of the filter shifts is now available.

Image Orientation Calculator [3 Jan 1996]
A new form has been created to assist in the determination of image orientations for both proposal preparation and data analysis. This form not only provides numerical results, but a PostScript plot of the image format with the North and U3 vectors included.

FOC Geometric Correction File [3 Jan 1996]
A new set of geometric correction files were implemented on 9 Mar 1995 for the f/96 imaging formats: 512z X 1024, 512 X 512, 256 X 1024, 256 X 256, 128 X 128. These new files were derived using the methods described in ISR FOC-086 [3202 kb PostScript]. NOTE: Only those interested in highly accurate astrometry should consider re-processing with these new files if the data was taken prior to 9 Mar 1995.

New Near-UV Objective Prism Dispersion Curve [3 Jan 1996]
The latest calibrations of the near-UV objective prism using post-COSTAR observations have been completed resulting in a new dispersion curve. The new dispersion curve will calibrate near-UV objective prism data with much higher accuracy than before over the wavelength range of 1600 to 6000 Angstroms.

FOC Cycle 5 Calibration Plan [3 Jan 1996]
The Cycle 5 calibration plan for the FOC provides a summary of the strategy behind this cycle's calibration efforts, and individual descriptions of each of the calibration proposals.

FOC Reference Point [4 Jan 1995]
The reference position in FOC images was moved from the image center to a position of (556, 536) in the full format. This change was made to position targets away from the reseaux mark near the image center.

Form for Running FOCSIM [4 Jan 1995]
A new form for running FOCSIM has been created. This form allows a person to calculate count rates for a simulated FOC image in real-time. A more complete description of FOCSIM provides explanations about the results of the simulation and how they were calculated.

Image Artifacts Caused by F320W Filter [21 Nov 1994]
Images taken with FOC+COSTAR and the F320W filter suffer from severe elongation of the PSF core, in addition to the 85 pixel offset noticed in pre-COSTAR images. The FOC team has decided this filter is not suitable for science observations. Therefore, the F320W filter will NOT be available in Cycle 5. Any Cycle 4 observations taken with this filter should be compared to the observed PSF obtained by the FOC team to properly identify the artifacts produced by the filter.

A new FOC DQE file has been delivered to CDBS for use with the 'pipeline' and SYNPHOT. This new DQE file corresponds to the DQE file given in the FOC Instrument Handbook Version 5.0 and can be used with the encircled energy curves there.

Aperture Location
The aperture center for the FOC is accurate to within 0.1 arcseconds according to the latest calibration report (Abstract). The full report (PostScript [644 kb]) describes the observations used to verify the aperture location and the errors associated with it.

Photometry Keywords in FOC+COSTAR Images
Images taken with FOC+COSTAR between 1 Jan 1994 and 19 April 1994 suffered from a problem with the calculation of the photometry keywords. During this period, the standard 'pipeline' calibration of FOC+COSTAR images did not recognize the addition of the COSTAR reflectivity when calculating the PHOTFLAM keyword. This resulted in errors of 20% or more in the value of the keyword. The proper value can be calculated using the SYNPHOT package under STSDAS.

Archival Instrument Notices

The following notices were sent out to FOC users to inform them of changes in the status or in the usage of the FOC. They are listed in reverse chronological order with their title and date of release.