Subject: Std. date/time/RA/Dec FITS keywords?

From: timmyd at netcom.com (Tim DeBenedictis)
Date: 7 May 1996 14:20:41 GMT

Greetings all-

I am writing some astrometry software which needs to run with a variety 
of different FITS-format images.  It would be nice to use the R.A., Dec., 
date, and time information stored in the images' FITS headers, but 
unfortunately there does not seem to be any standard FITS keywords which 
provide this information.

I read the defining FITS paper, which describes the DATE-OBS keyword; 
however, there is no corresponding TIME-OBS keyword, and one-day time 
resolution is not sufficient.

I also read about the CRVAL, CRPIX, CRDEL, and CROT keywords, which sem 
to specify the coordinates, rotation, and image scale.  However, as far 
as I can tell, no one actually seems to use these in practice!  I've 
worked with a variety of images from institutions like Kitt Peak, Lowell 
Observatory, as well as amateur images from SBIG cameras.  The only thing 
I've seen is keywords like RA and DEC which then list an R.A. and Dec. in 
hours/minutes/seconds as an ASCII string.

Is there any standard way of handling these things?  What is the 
"correct" way to do it?  Please send e-mail, as I am not able to check 
responses to all the groups I've posted this to.

-Tim DeBenedictis
timmyd at netcom.com

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From: Peter Bunclark <psb at ast.cam.ac.uk>
Date: 9 May 1996 16:20:44 GMT

Tim DeBenedictis wrote:
> provide this information...[deletia - mod.]

Read :
<A HREF="http://fits.cv.nrao.edu/documents/documents.html" >FITS
Documents</A>

[moral: check the 'net before pressing "post" - mod.]

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From: sla at umbra.ucolick.org (Steve Allen)
Date: 13 May 1996 14:36:56 GMT

In article <4mt60p$1pn at pecos.msfc.nasa.gov>,

><A HREF="http://fits.cv.nrao.edu/documents/documents.html" >FITS

This reply may give the impression that there is a definitive answer
to the question.  There is not.

There are no adopted standards specifying the keyword name or format
of RA and Dec in a FITS header.  There exists decimal degrees, decimal
hours, and hexadecimal strings in hours and degrees.  The given RA and
Dec may be expressed in a catalog reference frame or as notably
inaccurate telescope encoder readings.

Similarly there is no adopted standard specifying the keyword name or
format of observation time in a FITS header.  Again, existing practice
varies rather widely as do the accuracies of the clocks used to obtain
the time.

There is a draft document at
http://fits.cv.nrao.edu/documents/wcs/wcs.html
whose adoption should solve this problem, but its recommendations
are not generally found in any existing FITS headers.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 408 459 3046     FAX: +1 408 454 9863
WWW: http://www.ucolick.org/~sla                PGP public keys:  see WWW

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From: Peter Bunclark <psb at ast.cam.ac.uk>
Date: Tue, 14 May 1996 08:46:58 +0100

> are not generally found in any existing FITS headers.

I agree that there hasn't been much standardisation up to present; but
anyone writing new software should study the available documents on
wcs, including the section in NASA's ``A Users's Guide for the Flexible
Image Transport System (FITS)'',  and apply the recomendations to his
instrument/system.
	The disagreements seem to be in how exactly to represent the
image skew, rotation and scale.  The actual convention for representing
the coordinates of the reference pixel are as laid down in the
mystical PAPER I , ie CRVALn in degrees for ra and dec.  Meanwhile,
most observatories have actually used RA and DEC keywords in more or 
less human-readable format, but these shouldn't in fact be part of
a wcs.
	Since widely-distributed software (eg SAOTNG) is becoming available
to exploit images with wcs, there is much more motivation than
previously to exploit the potential.

Peter.

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From: Robert Mutel <rlm at astro.physics.uiowa.edu>
Date: Fri, 17 May 1996 00:02:48 GMT

Readers of sci.astro.research may be interested in a collection of programs for FITS images called ATFtools.
It includes a tool for inserting world coordinates into a FITS image header by pattern matching to the 
Guide Star Catalog. The program (wcs) uses the nominal ra, dec, keywords (along with keyword crpix for
plate scale) to roughly locate the image center and either an online or CD-ROM GSC database to do the pattern
matching. The programs have been ported to several UNIX systems (Linux, Solaris, Unixware) and Windows-95. 
There are also tools for batch photometry, FITS  header editing, and multiple image alignment. and cropping.

The Windows-95 port is available at
<a href="http://www-astro.physics.uiowa.edu/atf_atftools.html>ATFtools</a>

The UNIX port will be available for ftp access in early June.

Robert Mutel
Department of Physics and Astronomy
University of Iowa, Iowa City IA 52242
Telephone: 319-335-1950      FAX: 319-335-1753
Email: rlm at astro.physics.uiowa.edu