[fitsbits] Associating ancillary data with primary HDU

[Randy Thompson]

Hi Bill,

This is actually the application that first prompted the Image extension
to be proposed as a FITS standard. Before the binary table extension was
adopted, the IUE project wanted to store an array of data quality flags
with an associated image. The image was stored as a primary array, the
data quality flags as an image extension.

Randy Thompson




On 5/9/14, 4:24 PM, "William Thompson" <William.T.Thompson at nasa.gov> wrote:

>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
>
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