[daip] BSCALE BZERO

[Eric Greisen]

R. Niruj Mohan writes:
 > Am I right in assuming that ...
 > 
 > 1. AIPS understands BSCALE and BZERO and the flux I get from IMVAL or 
 > IMSTAT is correct. But if I access these values through GETHEAD then aips 
 > doesnt understand them.

      Like cfitsio, when AIPS reads an image in it applies BSCALE and
BZERO from the FITS header.  We do not keep any scaling internally -
allowing tasks to read images without concern for scaling with all
sorts of possible complications from magic-value blanks etc.

 > 
 > 2. EXPLAIN GETHEAD mentioned ISCALE and IZERO instead, and if I 
 > change the keywords in the FITS files from B to I then imheader includes 
 > that info. *But* the flux doesnt change, even after RESCALE, which means 
 > that ISCALE and IZERO have no effect.

ISCALE AND IZERO are kept as keywords in the header to tell you of any
rescaling applied to the image inside aips (by the RESCALE verb).
They are not active values in that they tell of scaling already
applied to the image - not scaling that needs to be applied.

 > 
 > 3. When I use cfitsio to read a file which has BSCALE and BZERO in them 
 > then the image read in automatically has these corrections done.
 > 
 > Atleast thats what I figure from experimenting. Would be helpful if you 
 > tell me if thats indeed so.

I suspect that you have things about right.

Eric Greisen