NAME · GENERATING_DETECTIVE_EFFICIENCY_FILES
focphot -- Producing photometric 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.
- The median task, also in images, may be used to
remove reseau marks and blemishes.
This will also reduce Poisson noise.
- 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
- 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
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.
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This file last updated on 24 Feb 2011