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Showing content from https://astrometrynet.readthedocs.io/en/latest/code.html below:

Astrometry.net code structure — Astrometry.net master documentation

This is meant to be an introduction to what parts of the codebase run during a solve.

• parses command-line args
• chooses output filenames
• downloads URL inputs
• decides whether inputs are FITS x,y lists or images
• calls augment_xylist() to produce *.axy file
• runs astrometry-engine to actually do the solve
• produces plots (*-objs.png, *-ngc.png, etc)

A field to solve is encapsulated in an “axy” file, which is a FITS binary table containing X,Y star positions, and well as FITS header cards that describe the command-line arguments and other information we have about the image. “axy” is short for “augmented x,y list”, and we often abbreviate “x,y list” to “xylist”. augment-xylist.c creates these “axy” files.

• run image2pnm.py to uncompress and convert images to PNM.
• for non-FITS images, run ppmtopgm and an-pnmtofits to produce FITS
• run image2xy (or SourceExtractor) to generate list of (x,y) star coordinates (xylist)
• for FITS files, run fits2fits.py to clean file
• run removelines.py to remove lines of sources from the xylist
• run resort_xylist() to sort by a combination of brightness and background
• run uniformize.py to select a spatially uniform subset of stars
• add headers to xylist to create axy file

This is the astrometry-engine executable.

• reads astrometry.cfg file
• finds index files
• reads axy file
• runs engine_run_job() to actually do the solve

engine_run_job()

• parses axy file
• based on range of image scales, selects index files to use
• calls blind_run()

blind_run()

Runs a set of fields with a set of index files.

• reads xylist
• runs any WCS headers to verify (solve-field –verify)
• depending on whether running with inparallel or not, loads one or all index files and calls solve_fields()
• records good matches that are found (writes WCS, rdls, match, corr files)

solve_fields()

• calls solver_preprocess_field()
• calls solver_run()

Runs a single field with a set of index files.

solver_run()

• load index files
• compute scale ranges of field and index files
• looks at pairs of stars A,B forming the “backbone” of the quadrangle, precomputing geometry and deciding which stars can be C,D
• adds one star at a time, forming all quadrangles where that star is A,B or C,D, and for each index, calls add_stars()

add_stars()

• select stars that will form the quadrangle (or triangle or pentagon)
• calls TRY_ALL_CODES() = try_all_codes()

try_all_codes()

• tests permutations of the C,D stars that are valid (satisfy Cx<Dx constraints), with different parities
• calls try_all_codes_2

try_all_codes_2()

• tries different permutations of A,B stars
• calls try_permutations()

try_permutations()

• recursive
• tries different permutations of C,D stars, checking for cx <= dx constraint
• searches code KD-tree for matches, calls resolve_matches() if found

resolve_matches()

• given a code match between a field quadrangle and the index,
• looks up the index star numbers forming that quadrangle (in the quadfile)
• retrieves the index star RA,Dec positions for these stars (in the star KD-tree)
• fits a TAN projection to the matched quadrangle
• calls solver_handle_hit()

solver_handle_hit()

• calls verify_hit() to confirm the match
• if matched, calls solver_tweak2() to compute SIP coefficients

verify_hit()

• searches for stars within the field in the star KD-tree
• calls real_verify_star_lists() to do the model comparison between true match and false match.

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