[daip] coordinates in AIPS

[Eric Greisen]

Anna Bartkiewicz wrote:
> Dear Eric,
> 
>   Thank you for your willing help. In the meantime I just tried:

> CTYPE1  = 'RA---TAN'           /the coordinate type for the first axis
> CTYPE2  = 'DEC--TAN'           /the coordinate type for the second axis
> CD1_1   = -6.089454412084e-006 /partial of first axis coord w.r.t. x
> CD1_2   =  3.802123089418e-008 /partial of first axis coord w.r.t. y
> CD2_1   =  2.450182223808e-008 /partial of second axis coord w.r.t. x
> CD2_2   =  6.054210442537e-006 /partial of second axis coord w.r.t. y
> FRAME   = 'FK5     '           /Target coordinate system
> CRPIX1  =              121.130 /Ref pix of axis 1
> CRPIX2  =              345.380 /Ref pix of axis 2
> CRVAL1  =         278.31204800 /RA at Ref pix in decimal degrees
> CRVAL2  =          -8.40106300 /DEC at Ref pix in decimal degrees
> FLUXSCAL= 'mJy/pixel'          /Flux scale of image

> ring2 > FITLD1: Task FITLD  (release of 31DEC10) begins
> ring2 > FITLD1: WARNING: 64-bit input stored in 32 bits inside AIPS
> ring2 > FITLD1: WARNING: 64-bit input stored in 32 bits inside AIPS

> ring2 > FITLD1: PCHDR: IMAGE APPEARS TO BE SKEWED:  179.77 NOT  179.64
> ring2 > FITLD1: PCHDR: IMAGE SKEWED:  0.02192 -0.02180 NOT  0.01712 -0.02657

> ring2 > FITLD1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
> ring2 > FITLD1: RA---TAN  1305    18 33 14.892     121.13       0.019522    0.00
> ring2 > FITLD1: DEC--TAN  1306   -08 24 03.827     345.38      -0.024182  179.70

I include the relevant lines:  I am glad to see the 64-bit warning since 
AIPS assumes that 64-bit float is not needed for real image data I was 
afraid that I had not fixed the reader to handle it.

Note that the coordinate info you give is quite complicated: CD1_1 and 
CD2_2 are not equal 0.021922 and 0.021795 arcsec.  Furthermore CD1_2 and 
CD2_1 are both positive meaning that a rotation interpretation is 
impossible.  The skew and "rotation" are small but still affect a 
difference for 6 or more pixels at the edges of your image.

I fear that AIPS is not equipped to help you - what you need is a 
program that understands the CD parameters in a general sense and 
regrids the image onto an unskewed grid.  Some of the general FITS 
viewers can handle this issue although I do not know if they have the 
regridding software.

Eric Greisen