From fa080de7afc95aa1c19a6e6fc0e0708ced2eadc4 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 8 Jul 2015 20:46:52 -0400 Subject: Initial commit --- noao/twodspec/longslit/doc/extinction.hlp | 87 +++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+) create mode 100644 noao/twodspec/longslit/doc/extinction.hlp (limited to 'noao/twodspec/longslit/doc/extinction.hlp') diff --git a/noao/twodspec/longslit/doc/extinction.hlp b/noao/twodspec/longslit/doc/extinction.hlp new file mode 100644 index 00000000..39579a07 --- /dev/null +++ b/noao/twodspec/longslit/doc/extinction.hlp @@ -0,0 +1,87 @@ +.help extinction May87 noao.twodspec.longslit +.ih +NAME +extinction -- Apply atmospheric extinction corrections +.ih +USAGE +extinction images +.ih +PARAMETERS +.ls input +List of input images to be extinction corrected. +.le +.ls output +List of output extinction corrected images. Output images may be the +same as the input images. +.le +.ls extinction = "onedstds$kpnoextinct.dat" +Extinction file to be used. The standard extinction files: + +.nf + onedstds$kpnoextinct.dat - KPNO standard extinction + onedstds$ctioextinct.dat - CTIO standard extinction +.fi +.le +.ih +DESCRIPTION +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 \fBhedit\fR. Note that if the image has +been transposed (\fBimtranspose\fR) the dispersion axis should still refer +to the original dispersion axis unless the physical world coordinate system +is first reset (see \fBwcsreset\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. +.ih +EXAMPLES +1. A set of dispersion corrected images is extinction corrected in-place as +follows: + +.nf + cl> extinction img* img* +.fi + +2. To keep the uncorrected image: + +.nf + cl> extinction nite1.004 nite1ext.004 +.fi + +3. If the DISPAXIS keyword is missing and the dispersion is running +vertically (varying with the image lines): + +.nf + cl> hedit *.imh dispaxis 2 add+ +.fi +.endhelp -- cgit