FITS long integer support (was Re: [fitsbits] ADASS FITS BoF on Sunday)

[Clive Page]

On Wed, 20 Oct 2004, Thomas McGlynn wrote:

> I don't think there is any implication that header keywords are limited
> to what is permitted in 4 byte integers, i.e.,

Tom is quite right, and I was mis-reading or mis-remembering the Standard.
So I guess the only question is whether there is interest in getting
current software packages and libraries to be compatible with long files.

I take note of Eric's comments that FITS users should avoid generating
files over 2 GB in size because not all current systems can handle them.
In the long term, however, surely the 2 GB file size limit will be seen in
the same light as the comment ascribed to Bill Gates on the MS-DOS memory
limit: "surely 640k is enough for anyone".  I'm old enough to remember the
painful transition from 16-bit to 32-bit machines, so can't help feeling
that a bit of advanced planning would ease the transition to 64-bit
addressing that is surely inevitable.

> There are also three changes to the FITS standard that would be needed
> to accommodate long integers.
>
>     BITPIX  = 64
>
> would indicate arrays of 8 byte integers in images.

I don't think we need these yet in high-energy astronomy, perhaps
optical/IR astronomers would comment on whether they are needed?

>     TFORMxx = 'K'
>
> would indicate arrays of 8 byte integers in tables.

I think that 8-byte integers are starting to appear, e.g. as pixel-code
numbers for pixelations of the sky with resolution below around 30
arc-seconds, so that seems a desirable feature.

>     TFORMxx = 'Q'
>
> would indicate use of longwords in pointers in variable length columns.

I don't know of any need for this yet, but if files over 2 GB become
common surely the pointers will have to move to more than 4-bytes?


-- 
Clive Page
Dept of Physics & Astronomy,
University of Leicester,
Leicester, LE1 7RH,  U.K.