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Data Formats

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.



next up previous contents
Next: Data Storage Media Up: Data Storage Previous: Data Storage



Scott J. Wolk
Mon Nov 25 15:41:03 EST 1996