[daip] coordinates in AIPS

[Eric Greisen]

Anna Bartkiewicz wrote:
> 
> Dear Eric,
> 
>  Please, help me finding what should I change in the FITS file that will 
> enable AIPS to obtain a proper pixel size.
> 
> I got the map (FITS file) of Gemini data with a header:
> [...]
> 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
> [...]
> 
> When I load it to the AIPS the header is:
>> AIPS 1: Image=G23.389+ (MA) Filename=G23.389_KCOR.IMAP . 1
>> AIPS 1: Telescope=Gemini-N Receiver=NIRI
>> AIPS 1: Observer= User #= 101
>> AIPS 1: Observ. date=BAD DATE Map date=25-OCT-2010
>> AIPS 1: Minimum=-1.17929012E-03 Maximum= 1.95409203E+00
>> AIPS 1: ----------------------------------------------------------------
>> AIPS 1: Type Pixels Coord value at Pixel Coord incr Rotat
>> AIPS 1: RA---TAN 1305 18 33 14.892 121.13 0.019522 0.00
>> AIPS 1: DEC--TAN 1306 -08 24 03.827 345.38 -0.024182 179.70
>> AIPS 1: ----------------------------------------------------------------
>> AIPS 1: Coordinate equinox 2000.00
> 
> Is the "coord incr" values the pixels scale for the two axes?
> If so, they should be 0.0219 and -0.0219 arcsec/pixel as I got the 
> informations from my Collaborators.
> 
> I read the page:
> http://tdc-www.harvard.edu/wcstools/wcstools.wcs.html
> but still I do not know how AIPS determines the "coord incr" values? 
> Should I add CRPIX1 and CRPIX2? Please, reply if you find time for my 
> question.
> 
> I will appreciate your help.
> 
> Best regards,
> 
> Anna
I am just back from 24 days vacation - please let me look in more 
detail.  The CD2_1 and CD1_2 indicate that there is some skew but only 
at a small level so I am surprised that aips gets this much difference 
between the 2 axes.  It will be a couple of days until I can get to this.

Eric Greisen