Plate Solving 1 -What am I looking At?

If you are familiar with the constellations then you will quickly recognize the distinctive W-pattern in the image below and identify it as an image of the constellation Cassiopeia.

Image Credit John Sanford –
https://fineartamerica.com/featured/3-cassiopeia-constellation-john-sanford.html

Now how about the image below?

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This one is harder to identify but some amazing software software has become more accessible to amateur astronomers in the last few years that makes identifying images a breeze. Plate Solving software uses pattern matching techniques to match an image of the sky with star catalogs to determine the stars and other objects that appear in the image.

The popular web app plate solver, Astrometry.net (
http://nova.astrometry.net/), can be used without installing any software – just upload an image and after a few minutes Astrometry.net displays results like the following.

It is a bit of a jumble of overlapping star labels but near the center you can see that the Messier cluster M103 has been identified.

Astrometry.net also displays a wider view of the area around the image and the deep-key objects that it found in the image. In this case, we can see that the image is an area near Casseopeia that contains M103.

The term “Plate Solving” is historical and refers to the large photographic glass plates that were used prior to the development of photographic film. Plates continued to be used in astronomy until the 1990s because they were superior to film for research-quality imaging – they were extremely stable and less likely to bend or distort.

Plate solving did not originate with the advent of computers, it was done manually by humans and often by women. The Harvard Computers were a team of women whose work included classifying stars by comparing the photographs to known catalogs. Many significant scientific contributions were made by these women’s subsequent analysis, classification, and processing of the astronomical data in the plates:

  • Annie Jump Cannon developed a stellar classification system using the strength of absorption lines and categorized stars into the now-familiar spectral classes O, B, A, F, G, K, M,
  • Antonia Maury discerned in the spectra a way to assess the relative sizes of stars, and
  • Henrietta Leavitt showed how the cyclic changes of certain variable stars could serve as distance markers in space.

Plate solving software is available in other package in addition to Astrometry.net and a follow-up article will cover other packages and the uses of plate solving by amateur astronomers.