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dstndstn / astrometry.net

Astrometry.net -- automatic recognition of astronomical images
http://astrometry.net
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Link to legacysurvey.org - how it's calculated #222

Closed lkosz closed 3 years ago

lkosz commented 3 years ago

Hello

eg. link: https://www.legacysurvey.org/viewer/?ra=286.7403&dec=-12.4767&layer=unwise-neo6&poly=286.5333,-13.0876,287.3981,-12.4980,286.9463,-11.8656,286.0825,-12.4537,286.5333,-13.0876

On each recognition page there is "Legacy Surveys sky browser" link as above. Ra and dec are center of picture got from wcs file, but how is this "poly" array generated? It's a simple rotation of ra_min, ra_max, dec_min, dec_max around ra/dec center?

dstndstn commented 3 years ago

Those corners are generated using the WCS file, computing the sky locations of the four corners of the image.

lkosz commented 3 years ago

Can I find anywhere formula for it? Or maybe there is an application which makes it?

dstndstn commented 3 years ago

Yes, but it's not a trivial formula. https://github.com/dstndstn/astrometry.net/blob/main/util/sip.c#L115

lkosz commented 3 years ago

Yes, it's quite complicated, but found other method:

#!/usr/bin/env python3

import numpy as np
from astropy import wcs
from astropy.io import fits

imagew=1280-1
imageh=960-1

wcsfile = fits.open('wcs.fits')
w = wcs.WCS(wcsfile[0].header)
corners = np.array([[0, 0], [imagew,0], [imagew,imageh], [0, imageh], [0, 0]], dtype=np.float32)
world = w.wcs_pix2world(corners, 0)
poly = ','.join(map(str, world.flatten()))
print(poly)