Author Topic: Trying to figure out Astrometry and WCS keywords  (Read 357 times)

Offline mpelzel

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Trying to figure out Astrometry and WCS keywords
« on: 2019 January 18 14:54:31 »
I guess my main question is where do the WCS keywords come from, is it part of the FITS header?  I open a single image and click the WCSHeader in utility and it show RA and DEC only.  If i check an integrated image, its completely blank.  As a result, it appears the Astrometry feature does not seem to work.

Thanks for any info/explanation.

Mike

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Re: Trying to figure out Astrometry and WCS keywords
« Reply #1 on: 2019 January 18 16:30:59 »
the WCS keywords are inserted in an image after plate solving. the ImageSolver script in PI will add those keywords to the image it is applied to (assuming a successful solve) and you will then be able to see RA/DEC coordinates as a cursor readout, or run AnnotateImage to get an annotated image, etc.

rob

Offline chris.bailey

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Re: Trying to figure out Astrometry and WCS keywords
« Reply #2 on: 2019 January 19 03:28:28 »
Star Alignment involves co-ordinate transformation and interpolation so the RA/Dec numbers in the individual images are no longer valid. An integrated images is therefore lost-in-space unless astrometry is used to find it again. Note that single images RA/Dec co-ordinates are normally the centre of the image. WCS entries in the header are much more feature rich and allow interrogation of the RA/Dec of any point in the image. Note also that any form of cropping and/or rotation renders the WCS entries invalid again.

Offline mpelzel

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Re: Trying to figure out Astrometry and WCS keywords
« Reply #3 on: 2019 January 19 06:45:27 »
Thanks for the info guys.  I tried ImageSolver but no luck on the integrated images. However, when I tried it on a single Ha light image it solved quickly with no problem.  Don't understand why it will solve a single image but not the stacked Ha image.

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Re: Trying to figure out Astrometry and WCS keywords
« Reply #4 on: 2019 January 19 10:18:25 »
it might solve the sub more easily if the sub already has some RA/DEC info in it from the capture program.

how did you tell ImageSolver the approximate location of the center of the integrated image? if you give the coordinates from the solved sub it should work fine. it's also important to make sure the image scale is set properly.

rob

Offline mpelzel

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Re: Trying to figure out Astrometry and WCS keywords
« Reply #5 on: 2019 January 19 15:50:33 »
Thanks again Rob, Going back to try again, I realized I tried to solve on the cropped image.  When I tried with the non cropped image, it worked.

Mike