Former astronomer here! You can download a lot of data from open access websites such as data.nasa.gov or any of the big digital surveys.

I don't know about today's data format, but it used to be all on the FITS format, with lots of libraries in Python/Matlab/etc., and also the excellent SAOImage DS9, all available for free.

Here are some links:

• SAOImage DS9: https://sites.google.com/cfa.harvard.edu/saoimageds9/download

• https://en.wikipedia.org/wiki/FITS

• Space Telescope Science Institute, managing Hubble and JWST data: https://www.stsci.edu/scientific-community/stsci-library-and-institutional-archive/our-collections

• Sloan digital sky survey also has a lot of open data: https://www.sdss.org/dr19/data_access/