Editing the FITS Header

The header of a FITS Format Image contains a lot of image documentation. Most Mira commands update and change it as necessary to keep track of what is done to the image. However, there will be times when you need to change something in the header by yourself. This topic describes how to edit the FITS Header.

NOTE

Lines in the header table that are tinted light red cannot be changed. For some of these keywords, there are other ways to change them. For example, NAXIS1 defines the number of image columns. This value can be changed by the Crop Image command but changing NAXIS1 here could render the image unreadable.

How to Undo Changes

Mira preserves 1 level of Undo each time you change a cell in the table. To recover the previous cell content, click the button on the lower left corner of the window or use the Undo command in the Context Menu (see below).

Typing New Keyword Data

To change the contents of a particular keyword line, the simplest procedure is to directly edit the value or comment cell in the table. Editing a cell is straightforward. Read through the following procedure once before following it step by step so that you understand the available options.

  1. Bring the cell of interest into view using the arrow keys, scrollbars, or by resizing the window.

  2. Double click the mouse on the cell to enter edit mode. This highlights the entire line and leaves the insertion cursor at the end. Do not type anything yet!

  3. If the entire line is highlighted, typing any character deletes the entire content of the cell. This is equivalent to hitting the [Delete] key.

  4. To position the insertion cursor to a specific position, click the mouse pointer on the location. continue typing from there.

  5. When you are finished typing, exit the cell using the keyboard Up and Down arrow keys or click the mouse in a different cell.

Using the Context Menu for More Complex Tasks

To do more than simple editing, Mira provides extra commands in the FITS Header Context Menu shown below. This menu opens by right clicking anywhere inside the window.

Most of these commands are self explanatory. Their use is described in the table below.

FITS Header Context Menu Commands

Undo (optional)

The Undo command appears only after a change is made. Click it to undo your changes to the cell.

Insert Line

Inserts a new, blank line at the point of the cursor and activates the editor in the Keyword cell.

Delete Line

Deletes the line at the cursor.

Rename Keyword

Opens a list of keywords that Mira knows.

Procedure: Position the cursor on the target keyword and right click to open the Context Menu. Choose Rename Keyword to open the list box of known keywords. Select a new keyword. Mira replaces the cell with the new keyword name.

Insert Keyword

Inserts a keyword from those that Mira knows.

Procedure: Position the cursor where you want to insert a new keyword line and right click to open the Context Menu. Choose Insert Keyword to open the list box of known keywords. Select a new keyword. Mira inserts the keyword name into the Keyword cell and moves the cursor to its Value cell. To edit the value cell

Insert Blank

Inserts a blank line with no keyword, value, or comment. Use this as a spacer to make the header more readable.

Insert History

Inserts a HISTORY keyword line at the point of the cursor. Your text may span both the VALUE and COMMENT fields of the header.

Insert Comment

Inserts a COMMENT keyword line at the point of the cursor. Your text may span both the VALUE and COMMENT fields of the header.

Save your changes

Changes to the FITS header(s) do not take effect until you have accepted them by clicking a button. These changes apply only to the displayed image.

NOTE

Header changes affect only the displayed image. To make the changes permanent in the FITS file, you must save the image using the File > Save command or an equivalent.

Related Topics

FITS Header Dialog, History Keywords, Comment Keywords, WCS Keywords