Update of FOC DQE Curve (18 Oct 1994)

Measurements of the FOC f/96 detector quantum efficiency (DQE) have been made from observations of spectrophotometric standard stars using the COSTAR-corrected FOC. The fluxes are now normalized such that the total flux is measured within an aperture radius of 1.0 arcseconds, and the background is also determined at this radius. This is in contrast to the aberrated case, where an aperture of at least 2.5 arcseconds radius was needed to include all of the light from the PSF. Since there is some flux outside the 1.0 arcsecond aperture that contributed to the DQE calibration of sensitivity in the pre-COSTAR case, but is not included now, the net result is that the new DQE appears to be lower than the prediction using the pre-existing FOC DQE curve and the transmissions of the COSTAR FOC mirrors.

Other contributors to differences between the new and old FOC DQE curves are the adoption of the G191B2B absolute calibration scale and a small drop in the FOC+OTA ultraviolet sensitivity since the last time the FOC DQE was measured.

The net result is that the new FOC DQE is linearly related to the old DQE by the following formula:

(NEW FOC DQE)/(OLD FOC DQE) = 1.00 + 0.0732*(lambda - 4675 Angstroms)/1000.0

Users should be aware that the encircled energy curves published in Version 5 of the FOC Instrument Handbook are also normalized to a total flux of 100% within an aperture of 1.0 arcseconds, while up until now the SYNPHOT DQE curve (foc_96_dqe_003) was normalised to a larger aperture size. This means that users who have calculated total fluxes using synphot or using header parameters in Cycle 4 FOC data and also used the encircled energy curves in the Version 5 Handbook will obtain answers that are incorrect by up to 25% at 1200 Angstroms, or more typically 10% at 3400 Angstroms. FOCSIM has been using the correct dqe curve since the publication of the Version 5 Handbook. Also the DQE curve published in the Version 5 Handbook is consistent with the 1.0 arcsecond normalization.

The new FOC DQE curve (foc_96_dqe_004) was installed in the HST pipeline on 18 Oct 1994. Users can check whether their data used the new DQE file by checking the HISTORY records in the .c1h file. Using the IRAF command:

cl>match dqe filename.c1h

should give a result like this:

HISTORY crfoccomp$foc_96_n256_001.tab, crfoccomp$foc_96_dqe_003.tab

where it can be seen that the OLD DQE curve was in use.

Users can calculate the correct PHOTFLAM for their data using SYNPHOT with the correct FOC DQE curve (foc_96_dqe_004) by using the following command in the synphot package:

sy> calcphot obsmode=band('PHOTMODE') spectrum='unit(flam,1.0)'

where you should substitute the value of the PHOTMODE keyword in the .c1h file. The PHOTFLAM inverse sensitivity is then 1.0/(calcphot.result).

Users can determine the FOC DQE file in use by their SYNPHOT installation by doing:

sy>grafpath "foc,f/96,costar" | match dqe

which should give the result:

crfoccomp$foc_96_dqe_004.tab

when the new DQE file is in place.