Re: [Astrometry] Digest for astrometry@googlegroups.com - 7 updates in 2 topics

[Paul Leyland]

I have been bitten, several times, by "errors" which turned out to be caused by stars with unusually high proper motion.

Pays to be careful. The term "fixed stars" is at best an approximation.

On 01/12/2021 10:34, astro...@googlegroups.com wrote:

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• Correct SIP order (tweak oreder) - 5 Updates
• Difference between code-tolerance and pixel-error - 2 Updates

Correct SIP order (tweak oreder)

"Toni Šarić" <tonis...@gmail.com>: Nov 30 02:46AM -0800 Thanks you very much Dustin for your thorough answer and clear explanation. I will use tweak_order=2 from now on. I noticed in .corr file fairly large separation between index and field star of 180 arcseconds for one particular star while mean separation is around 11' with std of 22''. Would it be smart to remove that star from solving? Will I get better accuracy if this star is not used? I can exclude it, and other similar(bad) matches that with parameter "pixel-error", right?     BR Toni

Dustin Lang <dstn...@gmail.com>: Nov 30 07:53AM -0500 The tweak algorithm downweights matches at large radius -- there corr file should have a column named WEIGHT or something like that.

"Toni Šarić" <tonis...@gmail.com>: Nov 30 07:12AM -0800 Yes it does. It gives weight of 0.5 for that particular star. Just for clarification, 180 arcseconds in my image is roughly 10 pixels difference.   If I put "pixel-error" = 0.2, that star is excluded from the solving. But still there are other matched stars with separation ~ 14 arcseconds which is more than 0.2 pixels, but it shouldn't be, if I understood that parameter correctly.   My apologies for so many questions, if you have some user guide with more detailed explanation for parameters, it would be really helpful.   BR Toni

Dustin Lang <dstn...@gmail.com>: Nov 30 10:17AM -0500 pixel-error isn't a maximum, it's a Gaussian standard deviation

"Toni Šarić" <tonis...@gmail.com>: Nov 30 07:20AM -0800 That makes sense, thanks for clarification.

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Difference between code-tolerance and pixel-error

"Toni Šarić" <tonis...@gmail.com>: Nov 30 04:45AM -0800 Raising from the dead. I am also curious about those two parameters.

Dustin Lang <dstn...@gmail.com>: Nov 30 07:49AM -0500 --code-tolerance is about quadrangle matching and determines which possible matches will be evaluated.   --pixel-error is used when evaluating potential matches, and also during tweak. It is supposed to be the uncertainty on detected star positions.   So yes, both would affect which matches will be accepted.

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