[daip] fits convention question

[Niruj Mohan Ramanujam]

Hello,

This is not strictly an AIPS question but I thought this is the best 
place to ask it ...

If I have an NCP image, with CRPIX as the north pole (which is what all 
LOFAR images we are getting these days here are), and hence CRVAL as 
(RA=0, dec=90), is there a convention for calculating coordinates of 
other pixels ?
There seems to be a 180 degree offset in RA for pixels between different 
softwares - in particular, I use WCSLIB inside my software and it gives 
a 12h offset as well.
Any help would be great !

regards
Niruj