Technical data

Autostar IP

-12-

Although the display shows only 256 levels, all of the image data are retained and processed by the

program. Whenever you perform a processing step, you are given an opportunity to rescale the display

data. The scale command, also allows you to rescale the image any time you desire.

When prescaling an image, 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 maximum and minimum values in the 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 dialog box. Since many astronomical 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.

Selecting Close will close the current file and remove its display window and any support windows, such

as the Histogram.