Fits can print or manipulate the FITS file HDUs (extensions), meta-data keywords in a given HDU. The executable name is astfits with the following general template
$ astfits [OPTION...] ASTRdata
One line examples:
## View general information about every extension: $ astfits image.fits ## Print the header keywords in the second HDU (counting from 0): $ astfits image.fits -h1 ## Only print header keywords that contain `NAXIS': $ astfits image.fits -h1 | grep NAXIS ## Only print the WCS standard PC matrix elements $ astfits image.fits -h1 | grep 'PC._.' ## Copy a HDU from input.fits to out.fits: $ astfits input.fits --copy=hdu-name --output=out.fits ## Update the OLDKEY keyword value to 153.034: $ astfits --update=OLDKEY,153.034,"Old keyword comment" ## Delete one COMMENT keyword and add a new one: $ astfits --delete=COMMENT --comment="Anything you like ;-)." ## Write two new keywords with different values and comments: $ astfits --write=MYKEY1,20.00,"An example keyword" --write=MYKEY2,fd ## Inspect individual pixel area taken based on its WCS (in degree^2). ## Then convert the area to arcsec^2 with the Arithmetic program. $ astfits input.fits --pixelareaonwcs -o pixarea.fits $ astarithmetic pixarea.fits 3600 3600 x x -o pixarea_arcsec2.fits
When no action is requested (and only a file name is given), Fits will print a list of information about the extension(s) in the file.
This information includes the HDU number, HDU name (EXTNAME keyword), type of data (see Numeric data types, and the number of data elements it contains (size along each dimension for images and table rows and columns).
Optionally, a comment column is printed for special situations (like a 2D HEALPix grid that is usually stored as a 1D dataset/table).
You can use this to get a general idea of the contents of the FITS file and what HDU to use for further processing, either with the Fits program or any other Gnuastro program.
Here is one example of information about a FITS file with four extensions: the first extension has no data, it is a purely meta-data HDU (commonly used to keep meta-data about the whole file, or grouping of extensions, see Fits).
The second extension is an image with name IMAGE and single precision floating point type (float32, see Numeric data types), it has 4287 pixels along its first (horizontal) axis and 4286 pixels along its second (vertical) axis.
The third extension is also an image with name MASK.
It is in 2-byte integer format (int16) which is commonly used to keep information about pixels (for example, to identify which ones were saturated, or which ones had cosmic rays and so on), note how it has the same size as the IMAGE extension.
The third extension is a binary table called CATALOG which has 12371 rows and 5 columns (it probably contains information about the sources in the image).
GNU Astronomy Utilities X.X Run on Day Month DD HH:MM:SS YYYY ----- HDU (extension) information: `image.fits'. Column 1: Index (counting from 0). Column 2: Name (`EXTNAME' in FITS standard). Column 3: Image data type or `table' format (ASCII or binary). Column 4: Size of data in HDU. ----- 0 n/a uint8 0 1 IMAGE float32 4287x4286 2 MASK int16 4287x4286 3 CATALOG table_binary 12371x5
If a specific HDU is identified on the command-line with the --hdu (or -h option) and no operation requested, then the full list of header keywords in that HDU will be printed (as if the --printallkeys was called, see below). It is important to remember that this only occurs when --hdu is given on the command-line. The --hdu value given in a configuration file will only be used when a specific operation on keywords requested. Therefore as described in the paragraphs above, when no explicit call to the --hdu option is made on the command-line and no operation is requested (on the command-line or configuration files), the basic information of each HDU/extension is printed.
The operating mode and input/output options to Fits are similar to the other programs and fully described in Common options. The options particular to Fits can be divided into three groups: 1) those related to modifying HDUs or extensions (see HDU information and manipulation), and 2) those related to viewing/modifying meta-data keywords (see Keyword inspection and manipulation). 3) those related to creating meta-images where each pixel shows values for a specific property of the image (see Pixel information images). These three classes of options cannot be called together in one run: you can either work on the extensions, meta-data keywords in any instance of Fits, or create meta-images where each pixel shows a particular information about the image itself.
GNU Astronomy Utilities 0.22 manual, February 2024.