2/9/2023 0 Comments Fields of astrometryWe have found that the optical catalogs of stars, as for instance the USNO, are in general not good to calibrate near-infrared images. To have an accurate astrometric calibration of a frame it is advisable to find the astrometric solution for that field or, if there are not enough stars, on a nearby field. This mode it is also affected by chromatism with a change in the scale of about 0.02 arcsec/pix from J to Ks. Of all the imaging modes available in SOFI the SO is the one more affected by distorsions, it is required a polynomial fit with order higher than 2 to bring the rms below 0.1 arcsec. The coefficients c and e give the X and Y scale of the field. The coeficients a and d represent a linear shift that depends on the position and on the pointing model used. We obtained the coeficients and the rms of the fit in arcsecond given in the following table: As an example for a linear transformation of the form: In most cases a linear astrometric transformation will be able to provide a quite accurate astrometric calibration of the field. All modes have a very limited amount of distorsion over the entire field of view. SOFI has five imaging modes, mainly Large Field (LF), Small Field (SF), Large Field + Focal Elongator (FE), High Resolution (HR) and Spectroscopic Objective (SO). So the estimations of the distortions that you can find here are not up to date. !!!WARNING : The image quality of the large field mode had been modified at the end of august 2003.
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