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Significant inaccuracy likely to occur...
06-10-2009, 15:24
Post: #1
Significant inaccuracy likely to occur...
Hi,

I'm calculating my astrometric solution with scamp, and then stack the data with swarp. Occasionally, when the astrometric solution is incorrect (but scamp does not notice), swarp throws a "Significant inaccuracy likely to occur in projection" warning at me. And in almost all cases this warning is justified.
Based on what parameters does swarp recognise this faulty astrometric solution? And why does scamp not? I know I could also post this in the scamp forum, but I didn't want to complain about scamp, I'm more curious what kinda checking swarp does.

Thx

mischa
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10-18-2009, 21:26 (This post was last modified: 10-18-2009 21:37 by Emmanuel Bertin.)
Post: #2
RE: Significant inaccuracy likely to occur...
Hi Misha,

Hi Misha,

The reason is that SWarp needs to invert the astrometric solution, which otherwise is given in the FITS header as (alpha,delta) = f (X,Y). The inversion is required because, contrary to SCAMP, SWarp works predominantly on distorted astrometric solutions using a reverse mapping approach. In the presence of distortion terms, the inversion process involves fitting a polynomial on a 2D-grid, and checking that the direct mapping followed by the obtained reverse mapping gives back the same coordinates within a small tolerance all over the grid. The message "Significant inaccuracy likely to occur in projection" occurs whenever this condition is not met, which generally means that the solution is not invertible, and therefore is unlikely to be correct.
- Emmanuel.
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