[daip] Can someone perhaps help with this?

[Eric Greisen]

Johan van der Walt writes:
 > 
 > Johan
 > 
 > Can AIPS convert an image that is in galactic coordinates with the CAR
 > projection to an image in J2000 with TAN projection.  IRAF imcctran does
 > not recognise the CAR projection.  I've trying to get the NIR and MSX
 > images into the same coordinate system for direct comparision.

         The -CAR projection is the pure linear "projection".  Per se,
AIPS does not know about this.  BUT, if you use PUTHEAD, to change the
axes to simple 'RA' and 'DEC' it will take them as linear.  Now,
unfortunately comes a problem.  The conversion from galactic to
celestial is a simple rotation IFF the projection is a true projection
(such as the -TAN or -SIN).  So use verb CELGAL to convert the TAN
into galactic, then HGEOM on the linear system to make a -TAN
galactic.  Finally, CELGAL will take both -TANs back to celestial.
You may want to unrotate them (with HGEOM or LGEOM).

Actually OHGEO and OGEOM may be better for you than HGEOM and LGEOM.
They handle blanked pixels better.

 > 
 > I can convert the NIR images to galactic but they have a TAN projection
 > and I don't know how much distortion there is between the two systems.  

See Kerr's Radio Astronomy book appendices - there can be little in
some areas and a lot in others.

Eric Greisen