  This paper describes a lossy method for compressing raw images produced by
CCDs or similar devices. The method is very simple: lossy quantization followed
by lossless compression using general-purpose compression tools such as gzip
and bzip2. A key feature of the method is that compressed images can be
converted to FITS files simply by decompressing with gunzip or bunzip2, and
this is a significant advantage for distributing compressed files. The degree
of quantization is chosen to eliminate low-order bits that over-sample the
noise, contain no information, and are difficult or impossible to compress. The
method is lossy but gives guarantees on the maximum absolute difference, the
expected mean difference, and the expected RMS difference between the
compressed and original images; these guarantees make it suitable for use on
raw images. The method consistently compresses images to roughly 1/5 of their
original size with a quantization such that no value changes by more than 1/2
of a standard deviation in the background. This is a dramatic improvement on
lossless compression. It appears that bzip2 compresses the quantized images to
within a few tens of percent of the theoretical limit.
