Subject: BZERO/BSCALE question

From: "Marcelo Magallon Gh." <MMAGALLO%UCRVM2.BITNET at uga.cc.uga.edu>
Date: Mon, 06 Dec 93 10:29:06 UCR

I'm working with some FITS that are pretty funny.

Firts of all, they have one extension, but no EXTEND keyword. The extension
name is IMAGE-IUE

They are have NAXIS=1.

The BITPIX keyword is 16 for both PDA and the extension, and they use
BZERO and BSCALE in order to output IEEE values.

They use CDELTn and CRVALn, too.

My problem is that BZERO and BSCALE keywords are present in the primary
header, but not in the extension. Do I have to assume that they are
the same for the extension?

Marcelo Magallon Gh.                   Have a nice day!
Astrophysics Group, University
of Costa Rica.

MMAGALLO at UCRVM2.UCR.AC.CR              Research is what you're doing when
MMAGALLO at HUETAR.CI.UCR.AC.CR           you don't know what you are doing.
MMAGALLO at UCRVM2.bitnet

---

From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 8 Dec 1993 09:33 EDT

In article <9312061641.AA08715 at cv3.cv.nrao.edu>, "Marcelo Magallon Gh." <MMAGALLO%UCRVM2.BITNET at uga.cc.uga.edu> writes...
>Firts of all, they have one extension, but no EXTEND keyword. The extension

If the FITS files has extensions, it must have EXTEND = T in the 
primary header.  If it does not, it is out of conformance.  Remember, 
however, that EXTEND = T in the primary header does not require that 
extensions be present.

The name 'IMAGE-IUE' is not registered with the IAU FITS Working 
Group.  There is an extension 'IUEIMAGE'.  There is also a question as 
to whether names of more than 8 characters are permissible for an
extension.  The NOST Definition codifies the rule as "Reading the data 
values in a FITS file should not require decoding any more than the 
first eight characters of a character string value of a keyword."  
This language would appear to imply that, since the type of an 
extension must be identified before the data can be read, than that 
name would have to be unique in the first 8 characters.

>They are have NAXIS=1.
Conformant.  The array is one dimensional.

>BZERO and BSCALE in order to output IEEE values.

Before the adoption of IEEE floating point, this kind of scaling was
the only way to us FITS to transfer floating point physical values.

>the same for the extension?
No.  The BZERO and BSCALE keywords apply only to the data immediately 
following the header, in the same HDU.  BZERO and BSCALE must be 
defined independently for the primary data array and each extension.

				Barry Schlesinger
				NSSDC/NOST FITS Support Office