FITS Format Overview

FITS is a digital data format developed for storing and transporting astronomical data in form of images and tables. The acronym FITS stands for "Flexible Image Transport System", which was developed in the 1970's at Kitt Peak National Observatory and was standardized in 1981. It has been maintained by NASA for more than 2 decades. A FITS format file consists of one of more Header-Data Units, with each "HDU" consisting of an ASCII text "header" containing metadata, followed by a multi-dimensional array that may contain pixel data for an image, or table data in the form of ASCII text or binary data. The FITS header and data array adhere to a rigid structure defined in the standard. It is critical to note that FITS is a data format, not an image format, as it was originally designed to hold one HDU of image data. It was later extended to hold multiple HDUs which may also hold table data.

The components of the Header-Data Unit ("HDU") depend on the type of data it contains. For images, the data array contains the image pixels. For an image, the header contains image metadata that describe the array, such as the number of rows and columns, and descriptive information such as the exposure time, date and time the image was acquired, user comments, etc. The HDU length is an integral multiple of 2880 bytes, and the header and data array must each be padded to a total length that is an integral multiple of 2880 bytes. Within the header, each 2880 byte "record" consists of 36 "card images" of 80 bytes each (originally based on 80-column Hollerith punch cards when FITS was developed). Each card image contains one item of metadata as an 8-character keyword, like NAXIS1, followed by a 72 character field that may contain a value, such as "512", and an optional comment, such as "number of image columns". The final keyword of the header is the END keyword, which is blank padded to the end of the 2880-byte record.

All characters in the FITS header must be in the range of ASCII codes 32 through 126 (hex 0x20 through 0x7e). Mira automatically scans the header and replaces characters outside this range.

Header Contents

The FITS standard refers to each 80-character line of the header as a card image. In this Use's Guide, a card image is termed a keyword line. Each header line consists of the following components:

• A uppercase keyword up to 8 characters long, starting in column 1, that identifies the information contained in the card image.

• A value, sometimes called the keyword value, beginning in column 10.

• An optional slash in column 32 that separates the value from an optional comment.

• An optional comment, in columns 33 though 80, that describes the keyword.

There are other details relating to issues such as the range of columns used for specific types of keyword values, but these will not be addressed here. A typical keyword line looks similar to the following one:

NAXIS1  =                 512 / number of image columns

Beyond the last non-blank character, the remainder of the 80-byte line is padded with blanks (ASCII code 32, or hex 20). The format of the NAXIS keyword is typical of all keyword lines.

The keyword name, including spaces, is exactly 8 bytes long and is left justified with the 80-character keyword line. Except for HISTORY, COMMENT, and <blank> keywords, an equals sign, =, follows in column 9. An optional slash may be placed in column 32. Between columns 10 and 32 is placed the value of the keyword, in this example, the number 512. Notice that the value 512 is right justified with one blank space after. Right justification to column 30 is required for integer, real, and character values, although this detail is sometimes missed by some software authors. Keywords with a character string value, must be left justified and enclosed within single quotation marks (') beginning in column 10. Character value keywords such as SIMPLE do not use quotation marks, but instead use a single character as T (true) or F (false).

Examples of Proper Format

The Basic FITS Header

The 4 mandatory keywords at the beginning of every FITS file are, in order, SIMPLE,BITPIX, NAXIS, and END. The first four keywords define a basic image and the END keyword signifies the end of the header. Note that a FITS file may contain more than one HDU and that the second and higher HDUs use theXTENSION keyword in place of the SIMPLE keyword.

A classic FITS file has one HDU and contains a 2-dimensional image. It has a header like the following:

The NAXIS1 keyword used in the example is an essential FITS keyword because it specifies the image dimension in the direction of the most rapidly varying subscript. The term most rapidly varying subscript is intended for those who write computer programs to read and write FITS format. For a 2-D image, the value of NAXIS1 corresponds to the number of columns in the image, which is 512 columns in the example. The number of image dimensions is specified using the NAXIS keyword, and this would have a value of either 1 or 2 for classic FITS files. If the image has 2 dimensions, an NAXIS2 keyword must exist in the header. Note that FITS images may have an NAXIS value between 0 and 999.

FITS File Types

Classic FITS Files

The classic FITS file contains a 2-dimensional image array. Such a file contains the keywordSIMPLE = T at its beginning and has NAXIS = 2. This type of file contains one Header-Data Unit, (HDU) consisting of a FITS header followed by a two-dimensional pixel array.

FITS Extension Files

Mira Pro x64 can also open 2-dimensional image arrays from images containing more than one Header-Data Unit (HDU). In this type of FITS file, the data portion of the HDU is optional, so the HDU may haveNAXIS = 0. A FITS file may have a PRIMARY HDU that contains only a header but no data array; this is identified by NAXIS= 0.

The first HDU of a FITS file always has SIMPLE = T. The additional keyword EXTEND may also be present in the first HDU to indicate that the file contains additional HDU's. Additional HDU's must use the keyword XTENSION to describe the type of information the HDU contains, with one of the values 'IMAGE','TABLE', or 'BINTABLE'. Mira Pro x64 version 8 supports only'IMAGE' type extensions. See the EXTEND and XTENSION keywords in the FITS Keywords topic. Mira handles FITS files and FITS images in the following ways:

• Classic FITS images (images with NAXIS = 2 and no extensions) and File Lists containing them may be opened using the standard Open command.

• FITS Extension files, classic FITS files, and File Lists containing them may be opened using the Open FITS File command.

• Files with a PRIMARY HDU (NAXIS= 0), IMAGE type extensions, and NAXIS> 2 are opened into an Image Window by separating the components into an Image Set containing images with NAXIS = 2.

• A dimensionless PRIMARY header havingNAXIS = 0 is opened into the FITS Header Viewer.

• After a FITS Extension image is opened into an Image Set using Open FITS file, you may save individual HDUs using the Save,Save As, and Save Image Set commands.

• An Image Set may be saved to a FITS Extension file using the Save FITS Extension File command.

• If the second or higher HDU includes the keyword INHERIT = T, then the header of the extension integrates the header of the first, orPRIMARY HDU having NAXIS = 0.

FITS Headers for XTENSION HDUs

A FITS file with more than one HDU is known as an Extension file. The first HDU is identified by the SIMPLE keyword in the first card image. Each additional HDU is a FITS XTENSION HDU and has some differences with the first HDU. Whereas the first (or only) HDU begins with theSIMPLE keyword, the second and higher HDUs begin with the XTENSION keyword. There may also be other keywords present that describe the extension, such asEXTNAME and EXTTYPE, but these are not required. TheEXTNAME keyword is used for the Name column entry in theFITS File Summary window shown below.

An XTENSION Header looks like the following:

Selecting Multiple HDU's and Image Planes

When a FITS Extension file is selected for opening, Mira opens a secondary window listing a summary of extension details. This window also opens when any HDU has NAXIS > 2 to permit 2-axis planes to be selected for opening. For details, see the FITS File Summary topic.

Related Topics

Contents

Image Windows

Working with FITS Format

FITS File Summary

Open FITS File

View and Search FITS Header

Save and Save as Commands

FITS Keywords

Pixel Scale Sensitive FITS Keywords

FITS Header Editor

FITS Header Pane

Modify Keywords

Add Metadata

Mira Pro x64 8.83 User's Guide, Copyright Ⓒ 2026 Mirametrics, Inc. All Rights Reserved.