Pixel Scale of Observations

[ajamsellem]

I have been using the astroquery.mast functions Observations.query_criteria and Observations.download_products to download HLA FITS files, but I've come to a point where I need to know the degree-to-pixel scale of the observation in order to know if I need to download the files. The pixel scale is available in the HLA querying web browser, but I have not been able to find the parallel functionality in astroquery as the tables downloaded after the calling of either Observations.query_criteria and Observations.download_products do not contain any pixel scale information.

Is there another function I can call to get the pixel scale of an observation or product before downloading any image data?

[keflavich]

@jaymedina @ceb8 Do either of you know whether pixel scale is available through MAST?

[jaymedina]

Hi @ajamsellem - Unfortunately there is no function under Observations that would give you the pixel scale of an observation before download. The metadata in the output table is all the data that would be available.

Do you know if your products have a header keyword that denotes the pixel scale? For example, STIS products have the header keyword PLATESC which denotes the plate scale of an observation in arcseconds/pixel (source: https://hst-docs.stsci.edu/stisdhb/chapter-2-stis-data-structure/2-4-headers-keywords-and-relationship-to-phase-ii). If so, you can use the Mast class (from astroquery.mast import Mast) to do a science instrument keyword search, and specify for products with a pixel scale of a certain range. Here is a Jupyter Notebook of a query made using the Mast class to make a science instrument keyword search for NIRISS observations: https://github.com/spacetelescope/jwebbinar_prep/blob/main/mast_session/Exoplanet_Spectra/Exoplanet_Spectra.ipynb

It's not as straight-forward as working with Observations, but allows you to search with more criteria.

[ajamsellem]

Hi @jaymedina – Thanks for the response. I am pretty sure there are four values (labeled CDX_Y) in the headers correspond to the pixel scale, so using the Mast class sounds like a good option. I'll give it a try and see if it works. Thanks!