How to compute distortions for calculating Ra/Dec from pixel coordinates in a fits image ?

[Paul Evangelopoulos]

Hello,

I need to compute ra/dec from pixel position in astrometry.net solved image for use in an app I am building that makes use of the Astrometry.Net api.

By using these formulas that I found in an internet discussion

delta_RA = CD1_1 * (x - CRPIX1) + CD1_2 * (y - CRPIX2)
delta_Dec = CD2_1 * (x - CRPIX1) + CD2_2 * (y - CRPIX2)

RA = CRVAL1 + delta_RA
Dec = CRVAL2 + delta_Dec

I managed to compute coordinates but I noticed that in some cases i.e high declination, the results are incorrect due to calculation inaccuracies.

I need the additional formulas to make these corrections for distortions but I do not know how to proceed.

Also, I tried to include astrometry.net code that makes these corrections in my own program but without success. My app is written in C#.

Do you know how to add the needed corrections (formulas or code) ?

Thank you

[Dustin Lang]

That formula is not correct.  The delta-RA,Dec part is correct, those are called "intermediate world coordinates", but then you have to do a projection operation around the CRVAL point.

In the astrometry.net code, we go from pixels to IWC, then IWC to x,y,z unit-sphere coordinates (xyz), then to RA,Dec.

https://github.com/dstndstn/astrometry.net/blob/main/util/sip.c#L232

iwc2xyzarr is the projection;

https://github.com/dstndstn/astrometry.net/blob/main/util/sip.c#L167

cheers,

dustin

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[Paul Evangelopoulos]

Thank you Dustin for your reply!

My understanding is that I must use the following functions to get Ra/Dec from pixel values

void tan_pixelxy2iwc(const tan_t* tan, double px, double py, double *iwcx, double* iwcy)

void tan_iwc2radec(const tan_t* tan, double x, double y, double *p_ra, double *p_dec)

Can you please clarify what is the value of  const tan_t* tan argument ?

Paul

[Dustin Lang]

tan_t is the TAN (tangent-plane) WCS header, defined here.

https://github.com/dstndstn/astrometry.net/blob/main/include/astrometry/sip.h#L15

For a typical TAN WCS header, the "anbool sin" would be false - TAN projection rather than SIN.

cheers,

dustin

To view this discussion on the web visit https://groups.google.com/d/msgid/astrometry/78d37979-47d2-4d1b-aabd-4de3aee5efafn%40googlegroups.com.

[Paul Evangelopoulos]

Hi Dustin,

I finally managed to port the astrometry code to c# and from some tests that I made so far it works without problem. 

cheers

Paul