All options may be abbreviated to their shortest unique prefix.
This program is part of Netpbm.
fitstopnm reads a FITS file as input and produces a PPM image if the FITS file consists of 3 image planes (NAXIS = 3 and NAXIS3 = 3), or a PGM image if the FITS file consists of 2 image planes (NAXIS = 2), or if you specify the -image option. The results may need to be flipped top for bottom; if so, just pipe the output through pamflip -tb.
The -image option is for FITS files with three axes. The assumption is that the third axis is for multiple images, and this option lets you select which one you want.
You can use options -min and -max to override the min and max values as read from the FITS header or the image data if no DATAMIN and DATAMAX keywords are found.
You can use the -scanmax option to force the program to scan the data even when DATAMIN and DATAMAX are found in the header. If you specify -printmax, the program will just print the min and max values and quit.
The program tells you what kind of PNM image it is writing.
FITS stands for Flexible Image Transport System. A full description can be found in Astronomy & Astrophysics Supplement Series 44 (1981), page 363.