[fitsbits] Associating ancillary data with primary HDU

[William Thompson]

To the general FITS community:

I've been asked if there are any specific conventions for associating ancillary 
data with primary data arrays.  The specific application is one where the 
exposure time differs from pixel to pixel (something that can be done with 
Active Pixel Sensors), but which could easily apply to other parameters which 
vary between pixels.

The simplest and most obvious approach would be to store the actual data in the 
primary HDU, and then store the exposure times in an extension with the same 
dimensionality.  For example, if the primary HDU had

SIMPLE  =                    T
BITPIX  =                   16
NAXIS   =                    2
NAXIS1  =                 1024
NAXIS2  =                 1024
EXTEND  =                    T

The extension would have

XTENSION=              'IMAGE'
BITPIX  =                  -32
NAXIS   =                    2
NAXIS1  =                 1024
NAXIS2  =                 1024
EXTNAME =            'XPOSURE'

In essence, this is similar to the Green Bank Convention, but applied to the 
individual pixels in a data array rather than to rows in a binary table.

Is this a commonly used method for associating ancillary data with primary 
images?  Are there any additional conventions that are appropriate to this 
situation?  I tried looking in

http://fits.gsfc.nasa.gov/fits_conventions.html

but couldn't find anything that seemed relevant.

One could also imagine binary tables where the primary data array is in one 
column, and the array of exposure times is in another column.  However, for the 
present application, the use of IMAGE extensions is far simpler, and more likely 
to be actually adopted.

Thank you,

Bill Thompson


-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov