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There are several different ways for CCD images to be stored. Each software package has its own format for saving the data. The ST-4 can save data when used as an imaging camera. The format of the saved files is not discussed in the ST-4 manual.
The Lynxx saves data in a format that best uses its memory allocation. Since the Lynxx data is stored in 12 bits, the format rearranges the data to send 2 pixels of information in 3 bytes. This is really just a redistribution of data, so that it is sent faster and stored more efficiently. Two pixels can be represented as 3 bytes of information. The first byte contains the 8 least significant bits of pixel 1. The second byte carries two nibbles. The first nibble has the 4 most significant bits of pixel 1, and the second contains the 4 most significant bits of pixel 2. Finally the last byte contains the 8 least significant bits of pixel 2. The Lynxx manual suggests that data stored in this format be saved with a ccd extension.
The ST-6 offers its own format in compressed and
uncompressed versions, along with FITS (Flexible Image Transport
System) and TIFF(Tagged Interchange File Format) formats. The
ST-6 format saves data in 16 bit words for each pixel value. In
the uncompressed version, the data is read down with each pixel
being described by a 16 bit word. The compressed version saves
the data in a slightly different manner. The first
pixel in a row is described by a 16 bit word. The rest of the
pixels are described by an 8 bit word, provided that the value
varies from the first pixel by 
127 counts. If the difference
is more than 
127 counts, that pixel's value will be
described by a 16 bit word. Clearly, the compressed
format will save the data in less space than the uncompressed
format.
The other formats are FITS, a common astronomical format, and TIFF, a publication format. Under the FITS format, the user has the option of saving data in either a 16 bit or 8 bit mode. To save the original image, the data should be saved with 16 bits per pixel. When 8 bit is selected, the 16 bit image data will be reduced to 8 bits and will be photometrically useless. The ST-6 also allows the observer to record information about the image collection and comments about the collection. A word of caution for those using the FITS format, the ST-6 software will record the time of the FITS creation -- not the time of the file creation. This information can be found in the comments.
The TIFF format under the ST-6 can only write in 8 bits while the ST-6 images are in a 16 bit format. This can be a disadvantage since this compresses the data and slightly alters it. This format is not suggested for images with astronomical data. This program also allows the observer to record collection conditions.