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extinction noao.twodspec.longslit



extinction -- Apply atmospheric extinction corrections


extinction images


List of input images to be extinction corrected.
List of output extinction corrected images. Output images may be the same as the input images.
extinction = "onedstds$kpnoextinct.dat"
Extinction file to be used. The standard extinction files:

	onedstds$kpnoextinct.dat - KPNO standard extinction
	onedstds$ctioextinct.dat - CTIO standard extinction


The specified images are corrected for atmospheric extinction according to the formula

correction factor = 10 ** (0.4 * airmass * extinction)

where the extinction is a tabulated function of the wavelength. The extinction file contains lines of wavelength and extinction at that wavelength. The units of the wavelength must be the same as those of the dispersion corrected images; i.e. Angstroms. If the image is dispersion corrected in logarithmic wavelength intervals (DC-FLAG = 1) the task will convert to wavelength and so the extinction file must still be wavelength. The table values are interpolated to the wavelengths of the image pixels and the correction applied to the pixel values. Note that the image pixel values are modifed.

The airmass is sought in the image header under the name AIRMASS. If the airmass is not found then it is computed from the zenith distance (ZD in hours) using the approximation formula from Allen's "Astrophysical Quantities", 1973, page125 and page 133

AIRMASS = sqrt (cos (ZD) ** 2 + 2 * scale + 1)

where the atmospheric scale height is set to be 750. If the parameter ZD is not found then it must be computed from the hour angle (HA in hours), the declination (DEC in degrees), and the observation latitude (LATITUDE in degress). The hour angle may be computed from the right ascension (RA in hours) and siderial time (ST in hours). Computed quantities are recorded in the image header. Flags indicating extinction correction are also set in the image header.

The image header keyword DISPAXIS must be present with a value of 1 for dispersion parallel to the lines (varying with the column coordinate) or 2 for dispersion parallel to the columns (varying with line coordinate). This parameter may be added using hedit . Note that if the image has been transposed (imtranspose ) the dispersion axis should still refer to the original dispersion axis unless the physical world coordinate system is first reset (see wcsreset\R). This is done in order to allow images which have DISPAXIS defined prior to transposing to still work correctly without requiring this keyword to be changed.


1. A set of dispersion corrected images is extinction corrected in-place as follows:

	cl> extinction img* img*

2. To keep the uncorrected image:

	cl> extinction nite1.004 nite1ext.004

3. If the DISPAXIS keyword is missing and the dispersion is running vertically (varying with the image lines):

	cl> hedit *.imh dispaxis 2 add+

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