rmjarvis / pixmappy

Python interface to gbdes pixel map (astrometry) solutions

pixmappy

Python interface to gbdes pixel map (astrometry) solutions

PixelMapCollection class can read the YAML format astrometry solutions produced by Gary's WcsFit program (in gbdes repo). This class can then issue a PixelMap instance, which is a map from one 2d coordinate system ("pixel") to another ("world") 2d system. A PixelMap instance can be used as a function mapping one (or many) coordinate pairs. An inverse method does reverse mapping, and the local jacobian of the map is available also.

A WCS is a PixelMap that additionally specifies a (de)projection from the world 2d system onto the celestial sphere. It uses astropy.coordinates.SkyCoord objects to represent celestial positions, and has toSky and toPix methods to map between the pixel system and SkyCoords.

The YAML deserialization is the slow part, using PyYAML. PixelMapCollection will first try to use the C-based YAML loader, which requires libyaml to be available. If this fails it falls back to the 20x slower all-Python loader.

The type of mapping that can be expressed is very flexible, and PixelMaps can be compounded into chains of tranformations. See the gbdes source files for more documentation on the types of transformations available and their YAML encodings. One type of map is a TemplateMap, which uses a lookup table stored in some other YAML file. Two such template files used for DECam solutions are part of this repository and installed by setup.py. The setup area will be searched automatically for requested template files. Additional paths to search can be specified by the CAL_PATH environment variable, using the usual colon-separated list format.