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FITS stands for Flexible Image Transport System, and this format is what professional astronomers use for image data. FITS images consist of a plain text header and the binary image. The header contains at least information about coordinates of the image, but may also contain a slew of other things about, for example, the target of the image, the telescope which took it, the astronomer(s) who observed it, when it was observed, the wavelength used, and any data reduction steps that may have been done to the image. The binary image can be one plane or many planes of images. It can also be a table of data.
Astronomers use FITS images because they are at least 16 or even 32 bits deep -- that is, there are at least 2 to the power of 16 () or 65,536 and possibly (=) 4.3 billion possible discrete levels of data for each pixel. Things like jpegs or gifs are only 8 bits deep, meaning that there are only (=) 256 discrete levels of information for each pixel (per color plane). This is why if you make a nice image in some packages with lots of extra detail, that detail gets lost when you save it as a jpeg. The computer is compressing 65,000 levels into 256.
JPGs (and for that matter GIFs or most PNGs) are "lossy compressed" files, which means that images in those formats actually LOSE INFORMATION, particularly in comparison to the FITS file. JPGs are just fine for images you take with digital cameras - you rarely ever see evidence of the loss of information. (As an aside - you might see evidence of it if you take a picture of something with high contrast, or a sharp edge somewhere in the image. If you look at the jpeg up close, you will see 'ringing' of the sharp edge, which looks kind of like blurring. The Wikipedia page on lossy compression has an example of loss of information with JPGs.)
So, what this means is: any time you are doing science, whether that is using your eye to see small details in the image, or measuring distances, or doing photometry, you always want to be using the FITS file, never a JPG, PNG, or GIF.
Other important CoolWiki pages
Other sources of interest
- More information on FITS format from GSFC
- These issues also affect anyone who creates graphics on computers, not just astronomers. More information on bit depth and file format in the context of graphic design, and graphics for the web
- Wikipedia page on lossy compression
- FITS liberator for Photoshop