Re: Coding of floating values in MIDAS BDF files in VAX/VMS and AXP/VMS

["Dr William Tobin, in Vannes, France." <tobin@club-internet.fr>]

Bernard Debray wrote:
> 

> 
> Warning: data is VAX Float instead of Gfloat...
> frame: WD_MRK691_1_202  (data = R4)
> NAXIS:                    1
> NPIX:                   571
> START:               0.26897961173593E+32
> STEP:                 2048.0000000000
> IDENT:           WD_MRK691_1_202
> CUNIT:           none given
> LHCUTS:            -7.046526       162.8535      -7.046526     162.8535
> 
> so the values in START and STEP are totally meaningless.
> 

The comment I am about to make may appear a bit like the one librarians
fear - the reader who comes into the library asking for help to find a
book that they've seen before, but they can't remember the title or the
author --- but it is green.

As you say, START and STEP are double precision descriptors---or should
be.  I did some years ago come across some MIDAS procedure somewhere
---and this is where I am like the clueless reader in the library, but
it may have been in the echelle spectroscopy package---where these
descriptors were incorrectly treated as single precision.  (I do
believe/hope I reported this fault.)  Your user wrote
the images in 1992:  you should perhaps keep in mind the possibility
that the descriptors were written wrongly (and presumably within the
version of MIDAS/package used at the time also read wrongly but
consistently) and that there is nothing wrong with the decoding under
AXP/VMS.

There might be some way you could test this hypothesis by copying
START and STEP into single precision descriptors and seeing if they are
then correct...

Best wishes,
William Tobin.
-- 
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* Dr William TOBIN, Department of Physics & Astronomy, University of *
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