Imagine we have a photographic plate that clearly shows a few stars, but we do not know where in the sky these stars are located.

We can digitize the location of stars on this plate and now have a set of (x, y) values, but of unknown scale. Perhaps we can also assign a crude estimate of the relative magnitude of each star too.

What is the easiest, practical way of generating a ranked list of candidates for where these stars may be located in the sky? Is there a library available, or a database, that allows such a query? An answer that requires programming is fine.

Moderators, please re-word this question if I have asked it in a way that doesn't make sense! I am an outsider in this community. Thank you for your help!

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    $\begingroup$ ASTAP is another option, you can run it locally (easily), so it ends up being a lot faster than Astronmetry.net, but fails a little more frequently. hnsky.org/astap.htm $\endgroup$ Commented Jul 19, 2022 at 4:22
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    $\begingroup$ different but related: How (the heck) does Astrometry.net work? $\endgroup$
    – uhoh
    Commented Jul 19, 2022 at 11:03
  • $\begingroup$ Doesn't "(x, y) values, but of unknown scale" miss the point? There's a photo within which, Star S has co-ordinates X,Y… Where does scale come in? $\endgroup$ Commented Jul 19, 2022 at 18:19
  • $\begingroup$ Apologies if using incorrect terminology (as an outsider), but I was using "scale" to mean "unknown amount of arcseconds between any two points", e.g. it may be a very zoomed-in image of a small section of sky showing relatively lower magnitude stars, or it could be a zoomed-out image of a large section of sky showing higher magnitude stars. $\endgroup$
    – Matthew
    Commented Aug 29, 2022 at 17:35

1 Answer 1


Upload your photo to nova.astrometry.net and in a few minutes you will get a result page that tells you the plate scale in arcseconds/pixel, the celestial coordinates of the center of the frame, the angular size of the photo, etc. These data will also be stored in a fits image file, along with coefficients that enable identifying celestial coordinates of stars and other objects at any x, y location in the frame.

You can read more info about how this is done on the astrometry.net home page.

This technique is usually called plate solving. It has also been incorporated in many astronomical programs used by professional and amateur astronomers and astrophotographers for polar alignment, telescope pointing and creating models that enable more accurate tracking by measuring characteristics of the telescope and mount.

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    $\begingroup$ Thank you @amateurAstro! “Plate solving” is exactly the phrase I was missing in my googling. $\endgroup$
    – Matthew
    Commented Jul 19, 2022 at 1:41

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