Subject: Re: IMPORTANT: FITS data from INT/JKT

From: mjc at rlsaxp.bnsc.rl.ac.uk (Malcolm Currie)
Date: 26 Jul 1994 16:12:32 GMT

In article <313ag8$p2c at lyra.csx.cam.ac.uk>, gtr at mail.ast.cam.ac.uk (Guy Rixon) writes:
From: gtr at mail.ast.cam.ac.uk (Guy Rixon)
Subject: Re: IMPORTANT: FITS data from INT/JKT
Organization: Royal Greenwich Observatory

In article <3135fa$lu3 at lyra.csx.cam.ac.uk>, nrt at mail.ast.cam.ac.uk (Nial Tanvir) writes:
|>
|> [There] is that a spurious [FITS descriptor] in the [files of the FITS tapes
!> produced at the INT and JKT]  which is telling 
|> the fits reader that all occurrances of 32768 are bad pixel
|> values.

> Not quite true.  BLANK = 0 is saying that any pixel-values of zero are non-physical
> (which is true for any CCD that's working correctly).  The FITS reader is assuming
> that the test against BLANK can be made before adjusting the pixels according to BZERO and 
> BSCALE.  The FITS writer is making the opposite assumption.  

You took the words right out of my mouth.

> The FITS standard (NOST 100-1.0)
> doesn't make it clear which assumption is legitimate.

The NOST FITS User's Guide does.  "The value of BLANK is the one that appears
in the actual data array, without scaling." 

Malcolm Currie
Starlink Project