Software recognises the star patterns in a photo and works out, in seconds, the exact coordinates of its centre, its orientation and scale. The mount can then correct its pointing, or you can check your polar alignment. ASTAP and astrometry.net are examples.
ToolsPolar alignment
Related terms
- GoTo
A motorised, computer-controlled mount that slews by itself to objects chosen from its database, then tracks them. After switching on, you align it on two or three bright stars. It saves time, but learning the sky still helps when it gets lost.
- Polar alignment
The adjustment that points the polar axis of an equatorial mount at the celestial pole. Its angle above the horizon must equal your latitude (51.5° in London), and it must aim at true north, not magnetic north. Good polar alignment keeps stars from drifting, visually and in photos.
- Sampling
The angle of sky covered by one pixel, in arcseconds per pixel: 206.265 × pixel size (µm) ÷ focal length (mm). 3.76 µm pixels behind a 500 mm focal length give 1.55″/px. For deep sky, aim for about a third to a half of the seeing (often 1 to 2″/px).
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