image orientation on the screen

[Thierry Forveille]

mnewberry at axres.com (Michael Newberry) writes
 > You are right about using the signs of CRDELT1 and CRDELT2.  But you
 > are talking about World Coordinate keywords whereas I'm talking about
 > keywords that do not require an world coordinate calibration.  Most
 > CCD users do not WCS calibrate their images---all I want is North
 > almost up and East almost left for *display* purposes.
 > 
If that's all you want, just set the signs of CDELT1 and CDELT2 right,
using either +1 or -1 as is fit for your orientation. That will leave
you with an image displayed with axes labelled in pixel units, and the 
(aproximately) right orientation. 

I'd personally argue for the value of inserting an aproximate value of 
the angular size of a pixel (which even amateur CCD users know to better
(and often much better) than 10%), and get your axes labelled in arc 
seconds. But if you don't want to do that, you don't have to.

Let's not add keywords where they are not needed....


Thierry Forveille
Observatoire de Grenoble
France


PS: Technically CDELTi, CRVALi and CRPIXi are not part of the WCS definitions,
which they predate by many years. They are defined in the original FITS
paper.
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