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Re: Fits Headers Again
On Sat, 10 Mar 2001 22:18:15 -0600, Tom Droege wrote:
>Below is some stuff copied (now directly) out of TN-67. My plan is to make
>two files. One is the fixed stuff and one varies run by run. My question
>is, what is a fits extension? Is it everything after the: EXTEND
>= T/ line below or is it something else?
According to the standard, this simply allows you
to have an extension if you want to. As far as I can see
nobody ever has and this is simply a line using up space
to no purpose. Like much else ...
>
>Tom Droege
>
>
>Recommendations:
>· Switch RA and DEC axis.
Er? I hope somebody is going to shuffle all the data
so this is true, rather than just changing the header?
Or can you rotate the camera the right amount without
shearing off five cables?
>· CTYPE1 and CTYPE2 must read 'RA---TAN' and 'DEC--TAN', respectively.
>· EXPOSURE keyword must be present, with correct exposure time in
>secomds.
>· It's better to use reference pixels in the middle of image: CRPIX1,
>CRPIX2=1024.
Or is that 1016? 1016.5? ... or some other number allowing
for the edge pixels? More on this below.
Is there a standard way of dealing with the edge pixels?
I couldn't find one in the FITS standard.
>· CDELT1 and CDELT2 are missing now, they must contain pixel
>increment at the reference pixel. In simpler terms, pixel size with
>possible negative sign if coordinate is decreasing with increasing pixel
>coordinates. Size is expressed in decimal degrees.
>· Keyword IMAGETYP with possible values 'object', 'dark', 'flat' and
>'zero' should be included if IRAF proceessing of images will be used.
>· Change DATE-OBS to include time as well, not only date.
>· Inclusion of the following keywords may be useful
>· RDNOISE in electrons.
>· GAIN in electrons/ADU.
>· CCD-TEMP or CAMTEMP with temperature in degrees Celsius.
>· AIRMASS
Yes. I once came across a subroutine that claimed to compute
Airmass. I can't find it again - could somebody point me
at it? IIRC, it was actually computing zenith distance. I
forget whether it dealt with altitude (height). It certainly
didn't ask for barometric pressure.
>Example of enhanced header after star matching (image H3R1659-826.fts):
>SIMPLE = T / file does conform to FITS standard
>BITPIX = 16 / number of bits per data pixel
>NAXIS = 2 / number of data axes
>NAXIS1 = 2043 / length of data axis 1
>NAXIS2 = 2037 / length of data axis 2
>EXTEND = T / FITS dataset may contain extensions
>COMMENT FITS (Flexible Image Transport System) format defined in Astronomy an
>COMMENT Astrophysics Supplement Series v44/p363, v44/p371, v73/p359, v73/p365
>COMMENT Contact the NASA Science Office of Standards and Technology for the
>COMMENT FITS Definition document #100 and other FITS information.
>CTYPE1 = 'RA---TAN' / Inserted by fitsblink
>CRVAL1 = 2.263508539420E+02 / Inserted by fitsblink
In spite of its general clumsiness and unfriendliness, the FITS
standard does not make it compulsory to use these ugly unreadable
E-formats!
CRVAL1 = 226.350854 / This is also allowed
>CRPIX1 = 1.021500000000E+03 / Inserted by fitsblink
Now why does fitsblink do that? You told it that the data ran 1..2043
and the midpoint of that is 1022.0 not 1021.5.
Unless, of course you take the other convention where 1.0 is the
corner of the pixel so the pixels run 1.0 to 2.0 .. 2043.0 to 2044.0
and the centre is 1022.5.
fitsblink must be using the corner convention but counting
from zero. Naughty. FITS doesn't believe in zeros.
This sort of niggling only matters if you are really
trying to share data ... that's why I usually work alone.
>CROTA1 = 8.974035779811E+01 / Inserted by fitsblink
>CTYPE2 = 'DEC--TAN' / Inserted by fitsblink
>CRVAL2 = 1.723629532873E+00 / Inserted by fitsblink
>CRPIX2 = 1.018500000000E+03 / Inserted by fitsblink
>CROTA2 = 8.974035779811E+01 / Inserted by fitsblink
>BSCALE = 1.
>BZERO = 32768.
>LATITUDE= 42.
>LONGITUD= -88.
>FILTER = 'V '
>DATE = '2000-04-25'
>DATE-OBS= '2000-04-25'
>UT = '07:49:52.00'
>TIMESYS = 'UT '
>COMMENT This file produced by TASS RawToFITS program Version 1.1
>CDELT1 = 2.150392161597E-03 / Inserted by fitsblink
>CDELT2 = 2.150050490463E-03 / Inserted by fitsblink
>IMAGETYP= 'object ' / insert_keywords
>EXPOSURE= 1.500000E+01 / insert_keywords
>EXPTIME = 1.500000E+01 / insert_keywords
>MATCHED = T / Inserted by fitsblink
>END
>
Andrew Bennett, Avondale Vineyard