How to use the command 'fitstopnm' (with examples)

The fitstopnm command is a useful tool for astronomers and other scientific users who routinely work with FITS files. FITS, or Flexible Image Transport System, is a file format commonly used for storing astronomical data and images. The fitstopnm command serves as a converter, transforming these FITS files into PNM (Portable Any Map) images, which are more widely supported by various image processing software. This conversion allows users to easily visualize and further manipulate the data contained within the FITS files using standard image editing tools.

Use case 1: Convert a FITS file to a PNM image

Code:

fitstopnm path/to/file.fits > path/to/output.pnm

Motivation:

The primary motivation for converting a FITS file into a PNM image is to facilitate easier viewing and processing of the data using general-purpose image editing software. FITS files, while optimal for astronomical data due to their ability to store a lot of metadata and multiple datasets in a single file, are not always compatible with conventional image software. By converting the file to a PNM format, users can view the data as an image and use conventional image processing tools for analysis and presentations, broadening the usability of the data across different applications and platforms.

Explanation:

• fitstopnm: This is the command that initiates the conversion from FITS to PNM.
• path/to/file.fits: This positional argument specifies the file path to the source FITS file that needs to be converted.
• >: The redirection operator is used here to direct the output of the fitstopnm command to a file.
• path/to/output.pnm: This specifies the file path where the converted PNM image will be saved.

Example Output:

Upon executing the above command, a PNM image will be created at the specified output path. The image would appear as a grayscale or color raster image, depending on the complexity of the original FITS file. This PNM file can then be easily opened using various image viewers and editors, allowing for further manipulation or simply for visual inspection.

Use case 2: Convert the image on the specified position of the third axis in the FITS file

Code:

fitstopnm -image z_position path/to/file.fits > path/to/output.pnm

Motivation:

In many FITS files, especially those that are multi-dimensional, there is a need to access and visualize specific slices of data. These multi-dimensional files might, for instance, store a sequence of time-ordered images or spectral data across various wavelengths. The -image option allows users to specify which particular slice along the third axis of the FITS file they wish to convert, providing flexibility in accessing and analyzing specific parts of data without converting entire datasets. This is particularly beneficial in time-sensitive situations where quick visualization of specific slices is needed without processing the entire file.

Explanation:

• fitstopnm: Engages the command to initiate the conversion process.
• -image z_position: The -image flag specifies the exact image slice along the third axis of the FITS file. Here, z_position is a placeholder for an integer that represents the desired position of the slice one wishes to convert into a PNM.
• path/to/file.fits: The path points to the source FITS file. The file contains multiple axes from which a specific slice can be extracted.
• >: The redirection operator, which directs the output to a specified location.
• path/to/output.pnm: The path specifies the destination file where the extracted image slice will be saved as a PNM image.

Example Output:

Executing this command results in a PNM file at the given output path, representing the selected slice from the source FITS file. This output would visually correspond to the image slice extracted from the designated position on the third axis of the original FITS file, allowing for discrete analysis or display.

Conclusion:

The fitstopnm command is an essential utility for those working within scientific fields requiring the analysis of FITS files. By converting these files into the more universally compatible PNM format, users can leverage a broader range of software tools to further inspect, analyze, and present their data. The flexibility of fitstopnm extends beyond simple conversions by enabling selective access to specific datasets within multi-dimensional files, thereby optimizing both utility and efficiency in data handling.