Focal Pointe Observatory
Astrophotography by Bob Franke

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Using Astrometry.net Plate Solves

 
 

Astrometry.net is a free online service for plate solving astronomical images. With a successful solution, it creates the image "new-image.fits". This image is loaded into the WCS File text box, as shown above. EXcalibrator uses "new-image.fits" to obtain the required World Coordinate System data.

EXcalibrator assumes that the RA and DEC reference point is at the center of the image. This is true with PinPoint, Pixinsight and TheSkyX plate solves. The calculated search radius covers the image from corner to corner with a maximum of 75.0 ArcMin. With its default settings, the Astrometry.net service may put the reference point anywhere in the image. The worst scenario is if the reference point is at one of the image corners. In this case the number of potential stars for calculating the color ratios can be reduced by 75%.

Do not use a previously plate solved image. Tests show that the RA and DEC reference point is placed at 0, 0.

 

Fortunately, Astrometry.net provides an option to force the reference point to the center of the image. On the upload page, select "Advanced Settings".

 

Near the bottom of the page, select the option "CRPIX in center".

 

Also select "use SExtractor". Focal Pointe Observatory tests show that this may greatly increase, but not reduce, the number of usable stars for eXcalibrator. All plate solves are not equal.