Header and Data Units

As the name implies, each HDU consists of an ASCII formatted header unit followed by a data unit. (Actually, the data unit may have zero length and thus not exist). To view the header unit, click on the `Header' button on the right side of the Summary window to bring up an edit window (Figure 4). As can be seen, the header consist of a series of fixed length 80-character keyword records, where the keyword name (up to 8 characters long) is usually followed by an equals sign, then a value, and then an optional comment. The first few keywords in the header are required and define the type and size of the data that follows the header. The remaining keywords are optional and provide descriptive information about the data or the processing of the file.

The entry box near the top of the window is used to search for any character string in the header. A copy of the selected header record will then appear in the window below the scroll bar where it can be edited. (You must hit the `Enter' key to copy any changes into the header). The upper Edit menu contains additional functions for inserting or deleting keywords, and for undoing the previous edit operation(s). By default, fv will not let you edit the values of any of the mandatory keywords that determine the overall structure of the file. Also, as a safety feature, whenever you edit a file, fv first creates a backup copy which is then modified; the original file is not overwritten until you explicitly tell fv to save the changes. Practice some edit operations on this header (but don't save the changes!), then close the Header window and return to the Summary window.

 

Figure 4: The Header Editor

Next, to view the raw numbers in the data unit in this primary array click on the `Table' button in the right side of the Summary window. This brings up a scrollable spread sheet (Figure 5) of all the rows and columns in the image. From this window you can edit the value of any individual pixel, and you can also plot cross sections along specified rows or columns in the image. For now, however, just close the window when you are finished and return to the Summary window.

 

Figure 5: The image spread sheet