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  • eteq · 1 ✖

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  • Idea: functionally-derived non-dimensional coordinates · 1 ✖

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id html_url issue_url node_id user created_at updated_at ▲ author_association body reactions performed_via_github_app issue
565638027 https://github.com/pydata/xarray/issues/3620#issuecomment-565638027 https://api.github.com/repos/pydata/xarray/issues/3620 MDEyOklzc3VlQ29tbWVudDU2NTYzODAyNw== eteq 346587 2019-12-13T22:48:21Z 2019-12-13T22:48:21Z NONE

Thanks for the quick responses @rabernat and @dcherian!

When you say “arbitrary complex mathematical functions”: what are the arguments / inputs to these functions?

The short answer is: pixels. Which in astro is sometimes the same as "array index", but usually off-by-0.5 (i.e., the center of the pixel), and occasionally off-by-1 (for files generated by e.g. FORTRAN or other 1-based indexing schemes...).

The longer answer is: it depends on which direction you mean. Most WCS' are by design invertible, so you can go world-to-pixel or pixel-to-world. In the former case that means there's a bunch of possibly inputs - examples include wavelength (for a spectrum), energy (for a photon-counting experiment), or altitude and azimuth (celestial coordinates that are similar to lat/lon but on the celestial sphere instead of a reference geoid).

Additionally, the WCSs have parameters, which are usually not thought of as "inputs" to the WCS, but rather internal state or metadata (usually they are stored in file headers and interpreted by software).

If you want to see some concrete examples, you might check out Astropy's docs for our WCS interface, or the design document for the most recent iteration of that interface. But those might be impenetrable for a non-astronomer, so I'm open to more follow-ups!

It would also be good to hear about "sub-pixel metadata"

Sorry I wasn't clear there - I meant there are cases where the sub-pixel structure of the coordinates matter - e.g. telescopes often have pretty intense optical distortion, and we sometimes interpolate in such a way that the sub-pixel information in the WCS is critical to getting the interpolation right. And by "metadata" I meant both abstract parameters that encode that transformation (polynominal coefficients, information about which sky-projection is used, etc), and more physical metadata like "the temperature of the observatory, which slightly changes the shape of the distortion".

which we basically support through dask

Oh, tell me more about that - maybe this is the key for our needs?

 
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   Idea: functionally-derived non-dimensional coordinates 537772490

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