Week 10 — Extracting waveunit from FITS files
Motivation
The query method of the class Database already works with Wave(100, 200, 'angstrom'), but not as the user might expect it. Currently, querying for the wave instance shown above will only search for database entries with the saved wavelength unit angstrom and then check if the stored wavelength is between 100 and 200 (inclusive). The goal is to check every single database entry and convert units, if necessary.
Getting the waveunit from a FITS file
To get the wavelength unit from a FITS file, the first step that has to be performed is to get a list of FITS headers using the function sunpy.io.fits.get_header. Do not get confused by the name; the function returns a list of headers, not just one header. My new function extract_waveunit receives one header and attempts several ways to find the value of the waveunit. One goal of this function is to return strings which can be used easily to make astropy.unit instances. Here is an example of how to use it:
>>> from sunpy.io import fits
>>> from sunpy.data.test.waveunit import MEDN_IMAGE
>>> from astropy import units
>>> header = fits.get_header(MEDN_IMAGE)[0]
>>> waveunit = fits.extract_waveunit(header)
>>> waveunit
'nm'
>>> getattr(units, waveunit)
Unit("nm")