FITS File Header Definitions

The native file format for MaxIm DL images is FITS (Flexible Image Transport System), a standard widely used in the astronomical community. This is an excellent choice for all types of astronomical images since the format supports 16-bit and floating-point data, and it includes a flexible and highly extensible header. Various standard and widely used non-standard keywords can transfer information about the image between applications. In addition, public domain ”C” source code for reading and writing FITS files is available on the web.

One shortcoming of the FITS standard is that the 16-bit number format is signed, producing a range of -32768 to +32767. The range of data from many cameras is 0 to 65535. To overcome this problem, MaxIm DL subtracts 32768 from each pixel prior to saving. The FITS standard requires that the BZERO and BSCALE keywords be used when the value in the array is not the physical value; MaxIm DL sets BZERO to 32768 and BSCALE to 1. Other applications that interpret these keywords correctly should automatically add 32768 to the image pixels, resulting in a correct image display. This default behavior can be overridden using the File menu Settings command.

The FITS compression method used by MaxIm DL is proprietary and is not compatible with other software packages. If you need to transfer images to another package, you must save the images as uncompressed. If you need to convert a large number of images, use the File menu Batch Save and Convert command.

MaxIm DL is compliant with the SBIG proposal for FITS keyword extensions. This means that various optional and non-standard keywords are written in a fashion that is compatible with a number of other astronomical imaging packages. See for more information.


Mandatory FITS keywords are as follows:

Optional keywords defined by the FITS standard and used in MaxIm DL:

Extension keywords that may be added or read by MaxIm DL, depending on the current equipment and software configuration:

In addition, when PinPoint Astrometry is used, World Coordinate System (WCS) information will be added to the FITS header.

Some of the above parameters can be viewed in the File Open dialog. All are visible in the FITS Header Panel.