4. Examining Extensions and Image Data

4.1 fh_extensions()

If no EXTEND keyword is found, or if it is not set to T(rue) then 0 is returned. Otherwise the value of the NEXTEND keyword is returned. Use this to test for a mutli-extension FITS file.

4.2 fh_image_bytes()

This returns the expected unpadded image size in bytes. It should be called only after valid BITPIX and NAXIS* values have been set or read from a file. Here is the formula used to obtain the image size in bytes:

$NAXIS1 \times NAXIS2 \times . . . \times BytesPerPixel$

Where $BytesPerPixel$ is 1 for BITPIX=8, 2 for BITPIX=16, 4 for BITPIX=+/-32, and 8 for BITPIX=-64.

4.3 fh_image_blocks()

This returns the following:

$( fh\_image\_bytes() + 2880 - 1 ) / 2880$

4.4 fh_header_blocks()

This returns the number of 2880-byte blocks fh_write() or fh_rewrite() will need for the header.

4.5 fh_ehu()

Use this only if you are making a program which loops through all extensions (see the example fhextname.c). In all other cases, refer to extensions by IMAGEID or EXTNAME using one of the following two functions.

Do not fh_destroy() the HeaderUnit returned by the fh_ehu() functions. It is destroyed automatically when the corresponding primary header is destroyed.

4.6 fh_ehu_by_imageid()

Returns a HeaderUnit which contains a matching IMAGEID keyword and seeks the file to the start of the data for that extension.

If no matching IMAGEID is found, 0 is returned.

Do not fh_destroy() the HeaderUnit returned by the fh_ehu() functions. It is destroyed automatically when the corresponding primary header is destroyed.

4.7 fh_ehu_by_extname()

Returns a HeaderUnit which contains a matching EXTNAME keyword and seeks the file to the start of the data for that extension.

If no matching EXTNAME is found, 0 is returned.

Do not fh_destroy() the HeaderUnit returned by the fh_ehu() functions. It is destroyed automatically when the corresponding primary header is destroyed.