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Feature: FITS Keywords
Date
DATE-OBS is not a reliable keyword due to the PC that talks to the
detector, so for a reliable time of exposure begin use DATE-BEG.
This may affect the WCS coordinates put out by SAO image if DATE-OBS
is much different from DATE-BEG.
In this respect we are hampered from following the FITS
standard by the proprietary software of the IR detector.
In the case of this instrument DATE-OBS can be unreliable.
That could adversely affect the interpretation of the celestial coordinates.
In all but the most extreme cases the pointing uncertainty of the
Shane telescope will be larger than the shift in the celestial
reference frame that will result from an aberrent value of DATE-OBS.
To be entirely safe, the data reduction software should overwrite the
value of DATE-OBS with the value of DATE-BEG.
This is documented in the User's Guide to ShARCS at the following URL:
http://mthamilton.ucolick.org/techdocs/instruments/sharcs/header/
WCS
L. Hirsch (2017-02-10): There seem to be several pixel scale keywords, and
I am not sure which to use:
CD1_1 = -1.31865E-07 / [deg/FITSpix] CTM i_j from FITS j to WCS i
CD1_2 = 9.44352E-06 / [deg/FITSpix] CTM i_j from FITS j to WCS i
CD2_1 = -9.44352E-06 / [deg/FITSpix] CTM i_j from FITS j to WCS i
CD2_2 = -1.31865E-07 / [deg/FITSpix] CTM i_j from FITS j to WCS i
What is the difference here?
I think the 1.318E-07 one corresponds to what I think it should be
(~0.027 arcsec/pix), but I am not sure why it is negative, and why it is
doubled (what is CD1 vs. CD2, and what do _1
and _2 refer to?).
S. Allen (2017-02-10): These keywords do not stand alone. They must be
interpreted with the context provided by all of the rest of the WCS.
CRVAL1
CRVAL2
CRPIX1
CRPIX2
CUNIT1
CUNIT2
EQUINOX
RADECSYS
CNAME1
CNAME2
CTYPE1
CTYPE2
WCSNAME
CRDER1
CRDER2
CSYER1
CSYER2
DATE-OBS
All of this conforms to section 8 of the FITS standard:
https://fits.gsfc.nasa.gov/fits_standard.html
Most modern FITS software knows the meaning of these keywords.
Associated Issues:
Change Log:
- 2017-01-14: First version.
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