A simple Unix cmp of the original and decompressed files does not always work because of syntactic differences if the original header did not conform entirely to the FITS standard. This is because the cfitsio library produces conforming FITS format files, correcting any non-conforming header cards from the original file. Relative Performance Tests were run to characterize whether running compfits2 has introduced a time penalty or provides a significant gain over other methods. The results are summarized in Table 1 . Results for gzip and bzip2 are not included because they were consistently 10 times slower.
Comparison tests on single image files.
Compression Number Original Compr. The final tests were to determine performance on large multi-extension FITS image files. The results are summarized in Table 2 . Performance results on large multi-extension FITS files. Compression File Number Original Compr.