[fitsbits] EXPOSURE or OBSTIME?

[Arnold Rots]

I sent a response to this message earlier but neglected to copy it to
fitsbits (or myself, for that matter).

The problem with the one-pixel time axis solution, though elegant, is
that it is confusing, or at least not useful, in cases where the
integration is not a single contiguous time range.  Does the pixel
increment then refer to integration time or covered time range?

The difference is crucial for instruments where sampling is not
uninterrupted (such as package drop outs and dead time), but becomes
unmanageable when one takes, for instance, a radio image derived from
an observation with interspersed calibrations or, worse, an image
obtained by combining observations made months apart.

  - Arnold

Eric Greisen wrote:
> 
> Tom Kuiper writes:
>  > I'm working on a radio spectral line data acquisition package and need to
>  > decide
>  > what keyword to use for integration time. An example FITS file that I have from
>  > the SEST telescope at La Silla uses OBSTIME. The OGIP Spectral File Format
>  > specifies EXPOSURE.  Are there any other keywords in use for this parameter?
>  > Is there any hint of a convergence on a standard?
>  > 
> 
>    Perhaps this is a case for me to recommend the use of a 1-pixel
> axis to specify time, with the axis increment being the integration
> time. 
> 
>    The VLBA uses INTTIM as a random parameter in a groups UV data set
> and would use the same as a keyword or column label in a UV data
> table.
> 
> 
>     Eric Greisen
> 

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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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