I am doing an astronomy project. I want to have the information about our images stored in a spatially enabled database. This, I would think, should be a very easy special case for GIS functions because the sky can be treated as perfectly spherical, and does not require an elliptical treatment like the surface of the earth. Unfortunately, I have not found a way to do this yet and I have been dodging mines with spatial functions that use an elliptical earth. (Pretty much any function that returns meters instead of degrees may be using an elliptical calculation. Luckily, many of the PostGIS functions I've needed appear to have incomplete implementations where the documentation explicitly states that the returned results are for a sphere and not for the ellipsoid. But that may change with future versions, which is a cause for concern.)
Background: I am presently using PostgreSQL with PostGIS and WGS 84 coordinates (SRID=4326). This works fairly well. I am creating a closed POLYGON from the right ascension and declination of the four corners of the image. I have a lot of images (10k or more), covering a large area of the sky. Each images is about 1 degree square. From the set of these images, I am making mosaics from small subsets of 15 to 30 images. Each mosaic is about 1.5 degree square.
Presently, I am storing the geography of the mosaics as a MULTIPOLYGON that consists of all the POLYGONS corresponding to each image that went into the mosaic. [A better solution would be to create a single POLYGON that describes the perimeter of the union of all the individual polygons. I don't know if this can be done in spherical coordinates (i.e., that the geography type). This would also be an interesting answer for me too.] The date line and celestial poles may be included in an image in the dataset so I have been avoiding projecting to planar coordinates to the extent possible.
My specific question is:
What coordinate system ought I use for celestial coordinates with PostGIS functions? I have looked at http://spatialreference.org/ but have not found anything so far. Google has turned up little. I am stumped. Basically, I want to ensure that if a function returns meters as a distance, it is meters along a great circle on a sphere.
More generally, some advice for using celestial coordinates in a spatial database would also be appreciated. Have I erred by choosing PostGIS? Are there far superior commercial choices? FOSS choices?
EDIT: I am using PostGIS 1.5.2. I have not yet tried PostGIS 2.0. I am curious if the ST_CoveredBy function works with a POLYGON and a MULTIPOLYGON of type geography. If anybody is running 2.0, could you tell me if you get the same error as this:
mydb=# select ST_CoveredBy(ST_GeographyFromText('MULTIPOLYGON(( (10.37795 -69.57926,8.9498 -69.54875,9.0178 -69.21643,10.4242 -69.24648,10.37795 -69.57926),(10.42436 -69.24618,9.01774 -69.2162, 9.08363 -68.88389,10.46914 -68.91344,10.42436 -69.24618)))'),ST_GeographyFromText('POLYGON((10.46915 -68.91315,9.08371 -68.88364,9.14755 -68.5513,10.5125 -68.58038,10.46915 -68.91315))')); ERROR: geography_covers: only POLYGON and POINT types are currently supported CONTEXT: SQL function "st_coveredby" statement 1
EDIT 2: I have tried PostGIS 2.0. This function still only works on points and polygons, not more general shapes. :-(