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4 Output

 4.1 Log output
 4.2 FITS WCS output

The report file (named by the report= parameter, and defaulting to astrom.lis) is a human-readable report of ASTROM’s actions and its results. The summary file, which normally appears on the terminal, contains observations and warnings about ASTROM’s progress, and should be monitored. Neither of these files, however, is intended to be easily machine-parseable, and so if ASTROM is used within a larger system, the other output files will be useful. The log file is a report on ASTROM’s actions and results which is easy to parse, and easy to extract information from. The WCS files contain ASTROM’s results in the form of a FITS header, containing FITS WCS keywords. Both these files are described in the subsections below.

4.1 Log output

The log file is a report on ASTROM’s actions and results which is easy to parse, and thus easy for another program, acting as a harness for ASTROM, to find out about ASTROM’s progress.

The log file consists of a number of statements describing the fits ASTROM has performed, and its success. The statements describing each fit are bracketed in a pair of statements ‘FIT n’ and ‘ENDFIT’, for some fit number n. The statements thus bracketed are in table 1.




Result RESULTkeyword value
Status STATUS[OK|BAD]
Residuals RESIDUALsource-number dx dy dr
Information INFOcode details…
Warnings WARNINGcode comment
Errors ERRORcode comment



Table 1: Log file statements

The INFO, WARNING and ERROR statements share a common set of codes, and are described in Appendix D.

The RESIDUAL message reports the size of the residuals for the given source number in the input file. The residuals are reported in the x and y directions, plus δr=δx2+δy2.

The STATUS message is OK if the fit succeeded, and BAD otherwise.

The RESULT statements give feedback about the results of the fit. Much of the information is also in the FITS WCS headers, if they are generated. The keywords are listed in table 2.




keyword meaning


nstars number of ref stars
xrms RMS errors in fitted x, arcsec
yrms …in y
rrms …in r
plate (mean) plate scale, arcsec
prms rrms in pixels
nterms number of terms in fit
rarad projection pole RA, radians
decrad projection pole Dec, radians
rasex projection pole RA, sexagesimal
decsex projection pole Dec, sexagesimal
wcs name of FITS WCS header file



Table 2: Information returned in log-file RESULT statement

4.2 FITS WCS output

If requested (by the presence of the fits= specifier on the command line), ASTROM will write out the plate solution in a series of FITS files, containing headers conforming (largely) to the FITS WCS standards (known as ‘Paper I’ and ‘Paper II’) [12]. ASTROM will generally attempt more than one fit. The file names will start with the string given in the fits= parameter.

There is more than one way to encode the required WCS information, and which way is used depends on the value of the parameter wcsstyle= on the command line. The allowed values of this are qtan and xtan, and these are discussed now.

There is a standard for specifying world coordinate systems in FITS files [2], and ASTROM conforms to this. At present (May 2003) there is only an early draft standard for representing distortions, Representation of distortions in FITS world coordinate systems, Calabretta et al. (also known as ‘Paper IV’), available at Mark Calabretta’s web pages [3]. Part of ASTROM’s function is to determine and report such distortions, but since there is not yet any standardised way to do this, we have something of a problem.

This program does not attempt to produce output using the distortion model described in Paper IV; that seems premature. Instead, it describes distortions using the model described as ‘distorted gnomonic’ (TAN) in the late draft versions of Paper II. If you specify wcsstyle=xtan (not recommended), then ASTROM emits FITS headers which fully conform to these drafts; if you specify wcsstyle=qtan, the FITS headers are essentially the same, but with the draft paper’s PVj_m headers replaced by non-standard QVj_m. In this latter case, the headers are conformant with the final Paper II, but the distortion information is available to software which knows how to use it. You are strongly advised not to produce new FITS files using option wcsstyle=xtan, unless you are obliged to by old versions of software.

The ‘distorted gnomonic’ TAN projection is not documented here (deliberately), and the drafts describing it are no longer readily available on the web. However, should you need to, you would probably be able to find a copy through Mark Calabretta’s WCS web pages [3].

This is an interim solution. It’s anyone’s guess how long it will take for the FITS community to agree on a final version of Paper IV. Once that is finalised, however, it’s quite possible that ASTROM’s support for the above header styles will be removed.