Dark-Count Subtraction
The dark-count reference file multiplied by the exposure time is subtracted from the input science file. The dark-count file is a full-frame image (512 1024), so if the science file is smaller than full frame then only the appropriate section of the dark-count file is used. Use imarith or darkx IRAF tasks. Standard processing does not apply the dark-count subtraction to images at this time.
Format-Dependent Photometric Correction (ITF)
These reference files are called ITF (Intensity Transfer Function) files for historical reasons. There is one such file for each format. The format-dependent correction is applied by multiplying the image from the previous step (i.e. the dark-count subtracted image) by the appropriate ITF file. Use imarith IRAF task. Standard processing does not apply the photometric correction to images at this time.
Correct For Zoom Mode
If the image was taken in zoom mode, the next step is to split the data values along the first image axis (the sample direction). The length of the first axis (NAXIS1) is doubled, and the length of the second axis (NAXIS2) is not changed. Use dezoomx IRAF task.
Compute Absolute Sensitivity
This does not affect the data values. The inverse sensitivity, pivot wavelength and RMS bandwidth of the optical mode selected are computed and stored in the header of the output image. The zero-point magnitude and the observation mode are also saved in the output header. Multiplying the data numbers in the image by the value of the header parameter PHOTFLAM and dividing by the exposure time converts to flux density F in units of ergs per square centimeter per second per angstrom. Use evalband IRAF task.
Geometric Correction
A raw FOC image is distorted by a few percent for two reasons, the optics and the detector. Both distortions are comparable in magnitude. The optical distortion was computed by ray tracing, and the detector distortion is measured by taking flat-field images and observing the positions of reseau marks that are uniformly spaced on the photocathode. A geometric correction reference file includes both optical and detector distortion. It gives the location in the input (distorted) image of each corner of every pixel of the output (corrected) image. The geometric correction is performed for each output pixel by adding up the counts in the corresponding region in the input image. This procedure rigorously preserves flux. Use newgeom IRAF task.
Relative Calibration or Flat Field Correction (normal images only)
This correction is called the uniform detective efficiency (UNI) correction, and removes the nonuniform detective efficiency of the detector. It is applied by multiplying the image by the UNI reference file (format independent), which is the reciprocal of a flat field. The UNI files are full-frame in size, which is 1024 1024 because the image is dezoomed. As with the dark-count correction, if the science image is smaller than full-frame then only a subset of the UNI file is used. Since the sensitivity of the detector depends on wavelength, six different UNI files are provided for different wavelengths. For the F/48 configuration, lambda = 3345 and 4800 Angstroms are provided; for the F/96 configuration, lambda = 1360, 4800, 5600, and 6600 Angstroms are provided. The file to select is determined by comparing wavelengths of the UNI files with the pivot wavelength of the optical mode. Use imarith IRAF task.
Spectrographic Detective Efficiency Correction
The SDE correction is only applied to spectrographic images. It includes both the flat-field correction and a conversion from counts to flux density. It is applied after geometric correction because the absolute sensitivity depends on wavelength, and a major function of the geometric correction for spectrographic images is to align the spectrum with the axes and set the dispersion. The correction is applied by multiplying by a spectrographic detective efficiency reference file. The use of an order-selecting filter can change the location of a given wavelength on the photocathode, so there are several reference files; the appropriate one is selected based on the filters used. These files are full-frame (1024 1024), so only a subset will be used if the science image is smaller than this. Use imarith IRAF task. Standard processing does not apply the SDE correction to images at this time.
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