Mobile Phone Star Explorer

Matches star candidate right ascension, declination, and magnitude to Yale Bright Star and Henry Draper catalogs.

While mobile phone star gazing apps provide information about stars that you MIGHT locate and see in the sky, astrometry can provide information about stars in actual images.

The Yale Bright Star Catalog (YBS) lists 9110 objects with magnitude <6.5, most of the stars visible to the naked eye and contains the names of most conventional named stars. The Henry Draper catalog list 273k stars and extends to magnitude 9, perhaps the limit for a mobile phone wide-field long-exposure night-sky image but lacks conventional names. Ref [1] provides the catalogs as tables. This Live Script illustrates the use of a function to search either table for matches to candidate stars based on candidate right ascension (RA), declination (DEC), and visual magnitude (MAG) values and their errors. For each candidate, the function provides a table of matches consistent with errors and with an overall chi-square.

The function can be used to identify stars found in a plate-solved image [2], study their known properties, annotate a plot of the stars in RA and DEC, and ultimately to annotate the image by projecting pixel coordinates of star candidates to celestial coordinates, matching these to the catalogs, and projecting back to pixel coordinates. [3].

In a wide-field mobile-phone night-sky image, the candidate star magnitude may not be well measured due to atmospheric effects and overexposure of bright stars. The illustrations ignore the candidate magnitude by setting the corresponding error range to encompass the relevant catalog magnitudes. Using phone gravity and magnetic field sensors alone, one can determine the RA and DEC of candidates to of order 1 degree, the accuracy of anomalous magnetic bearing. As YBS stars have separations of several degrees, the brightest stars are well separated and are mostly unambiguously matched using RA and DEC alone.

Plate solving an image with astrometry-net, which ignores magnitude and includes a nonlinear distortion correction for atmospheric and instrumental effects, can align an image relative to its Tycho 2.5 million star catalog with a precision of order a pixel in the center of the field, a pixel corresponding to 60 degree/3000=0.02 degree for a 60 degree width image taken without optical zoom magnification (3x-10x now available). As HD stars have separations of order 1 degree, for a plate solved image, HD stars may be unambiguously matched without leveraging magnitude information. In good seeing conditions, the relative magnitude of stars can be determined to about a half magntiude from simply the peak pixel value so magnitude could be a useful selection criterion with special attention to overexposed stars.

This Live Script provides a playground for exploring the matching of stars to catalogs and the use of table data structures. It may interest students and educators in physics and astronomy as well as amateur astrophotographers. Some "Try this" suggestions are included for further exploration.