NAME

focphot -- Producing photometric files.

GENERATING DETECTIVE EFFICIENCY FILES

NORMAL IMAGING MODE

For reasons of convenience, the calibration is done in two stages: relative DE and an absolute sensitivity. The absolute sensitivity step does not change data values themselves---but rather computes a factor that is added to the image header.

RELATIVE DE

The relative DE (UNI) files are images normalized to unity which describe the variation of the detector response over the imaging area when illuminated uniformly with near monochromatic light. In principle, the non-uniformity of the source should be corrected so that we have the response to a uniform illumination. We start with a flat-field image and perform the following steps:

Subtract dark count

The imarith task in the images package may be used to multiply the dark count image by the exposure time and subtract it from the observed flat field.

Smooth

The median task, also in images, may be used to remove reseau marks and blemishes. This will also reduce Poisson noise.

Dezoom

The blkrep task in images should be used to double the length of the first axis (scan direction) so the image will be 1024 x 1024 pixels in size and cover the full photocathode faceplate.

Normalize to get Relative DE

The dqenormx task is then used to create a normalized image. Note that the area used to define the normalization may be chosen by the user, but it should be the same area as was used to calculate the absolute DE if possible.

Note also that RSDP requires the reciprocal of the flat field; the task rsdpunix performs the required inversion and reformatting.

ABSOLUTE SENSITIVITY

During RSDP calibration by calfoc (in the focutility package), a factor is computed which may be used to convert from image counts to flux density input to the OTA. This factor is written into the header of the calibrated image using the keyword PHOTFLAM. The synphot package contains tasks, such as evalband, for computing this factor based on the instrument configuration, and calfoc calls the same subroutine used by synphot to obtain this factor. The factor needs to be checked using in-flight data. This can be done by comparing the counts within actual stellar images with the counts predicted by the FOC simulator FOCSIM.

Three contributing factors

ST optics throughput

Filter transmissions

These are assumed known from ground based measurements on the filters.

Detector (photocathode) Sensitivity

To be measured in flight by imaging stars using various filters to obtain sufficiently narrow band data. Note that f/96 mode has narrow band filters but f/48 does not. The detector in the latter chain may be calibrated using spectrograph mode. However, if you decide to use the broad passbands for f/48 detector sensitivity measurements, then both chains may be processed in essentially the same way.

SPECTROGRAPH MODE

Spectrograph DE files are generated by taking mean values in a small area at as many points as possible within a geometrically corrected spectrograph image. These points are assumed to have known incident flux. Division of measured count rate by expected flux gives a grid of points of sensitivity.