Technical data

Chapter 2 File 9
After the image is loaded in 16 bit format, a histogram is constructed that shows the distribution of the data contained
in the image. Normally the data is distributed in a rather narrow range of values which lends itself to scaling to the 8
bit format. Once the histogram is constructed, the Threshold values of the data is calculated and are used to set the
scaling range. This gives a reasonable starting point for scaling most images. The threshold values may be reset at
any time by pressing the Threshold button.
Since the data is not evenly distributed, some fine-tuning of the limits is usually needed. This is accomplished by using
one of the predefined scaling options or by moving the red markers located under the main histogram display.
Generally, the best images are obtained with the markers set just at the high and low edges of the important data.
Values that are above the upper marker result in pure white pixels, while those below result in pure black.
If the Std. Dev. button is depressed the Mean and Standard Deviation of the image are calculated. The upper limit is
set to the Mean plus 5 Standard Deviations, while the lower limit is set to the Mean minus 1 Standard Deviation.
The Min Max button sets the pointers to the extreme minimum and maximum values contained in the image, while
Full Scale sets the values to 0 and 65535 which are the minimum and maximum values possible in a 16 bit image.
If you are loading images from an RGB composite set of files, you may find it useful to use the same scaling values for
each of the individual images. This is easily accomplished by using the Use Previous Scale Values button located at
the bottom the the dialog box. Since many astromonical images contain a large amount of red light, you may want to
load the red image first, scale it properly, then use the same scaling values for both the green and blue images.
Close
Selecting Close will close the current file and remove its display window and any support windows, such as the
Histogram.
Save
Saves the current file using the current filename and format type. This function is only available if the current format
type is TIFF, FITS or BMP.
Save As
Displays the Save As dialog box and allows saving the current image in any of 3 different formats, TIFF, FITS or
BMP.
To save a file:
- Select the desired format.
- Select the directory and filename from the list boxes
OR
- Enter the desired filename in the Edit box.
If the filename is specified without an extension, a default extension will be automatically added to the filename to
reflect the current format. If the extension is provided, it will over-ride the current selection
NOTE: All images are saved in the 8 bit versions of their respective format type.
Print
The Print command sends the current starmap to the print spooler for the current default printer. The starmap is scaled
so that it completely fills the printing area of the paper in the horizontal direction. The vertical scale is then chosen to
preserve the original aspect ratio of the star map. Additional information pertaining to the starmap is also printed at the
bottom of the page, below the image.