[fitsbits] FITS for DSN spectral line observations

[Tom Kuiper]

Dear colleagues,

I've been studying the FITS standards and now have some idea of the choices
that we need to make regarding data file formats for spectral line work
in the DSN.

As far as I am aware, the software that we currently use -- Velu's data
conversion program, COMB, and CLASS -- understand only simple FITS files.
- Am I correct in that?
- Are there any other software packages we should be aware of and compatible
  with?

Beyond simple FITS there are two extensions that appear attractive.

The IMAGE extension basically allows additional data to be appended to the
basic header and data unit (HDU).  IMAGE extension HDUs look almost exactly
like basic FITS, so modifying existing "dumb" programs would be easy. The extra
HDUs could be additional scans, for example.  Another use would be, if one had
two or more backends, the data for the second and additional backends could be
put in IMAGE extensions. If one took the approach that subsequent header
variables, if specified, override previous ones then one could construct
files that follow somewhat the COMB approach to data storage, with "main",
"scan", and "array" headers.  An additional advantage is that a dumb software
package would at least be able to read the primary HDU.

The proposed Single Dish Format (SDF) is based on the BINTABLE extension.
One might think of this extension as a spreadsheet, with the additional
feature that a column can be a data array.  A typical structure consists
of a series of rows, and the fields in each row contain some of the variable
names and values that would appear in a simple FITS header, and the last
"field" would contain the data array.  This type of structure is more
efficient (for example, the blank space in standard headers is avoided), and
many scans can be included in a file.  There would have to be some trade-off
between parameters specified in the main header (those that seldom change)
and those that are specified in the table (like scan number and time).  Any
time you change a main header parameter, you'd have to start a new file.

The drawback to the SDF format is that I doubt that any package except
AIPS++ or UNIPOPS could process SDF files.  It's also worth pointing out that
the standard was drafted in 1995 and is still a draft, so I don't think that
there is a real ground swell here.  On the other hand, if the GBT adopts this
(the proposal was developed at Green Bank), then we may have to conform
sooner or later.  I don't know if I'm right about this, so I'm copying this
message to the FITS exploder in the hopes that other radio spectral line folks
can comment.

Tomorrow, I will try (a busy day before I go on vacation) to put the
information I've gathered on FITS standards on our website in case you want to
dig deeper.

Best regards

Tom
--
Internet:       kuiper at DSNra.JPL.NASA.gov (137.79.89.31)
SnailMail:      Jet Propulsion Lab 169-506, Pasadena, CA 91109
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