Once your images are calibrated you can begin the basic image processing steps to enhance your pictures.  Basic image processing involves stretching the image for the best display and using processing filters to enhance the image.

Stretching

There are two types of image stretching.  The first -- display stretching -- simply determines how the image is displayed.  The second, called permanent stretching, actually affects the data within the image.

Display Stretching

Display stretching is important because most CCD cameras are 16-bit systems, which means an image taken by the camera contains over 65,000 levels of brightness.  Even 12-bit cameras contain over 4000 brightness levels.  A computer monitor is an 8-bit system, displaying only 256 levels of brightness.  In order to bring more of the brightness levels from the CCD image into the range of the monitor, you stretch the image.

Above:  An unprocessed image of the Whirlpool Galaxy.

The first thing you probably notice is that the background is grey rather than black, as it usually appears in astronomical images.  What also can be seen is that the core of both the main galaxy (on the right) and that of its companion on the left are overexposed, as are some portions of the inner spiral arms.  In some images the overexposure is unavoidable because of the brightness range of the object itself.  But very often it is possible to stretch the display to show more detail.  In the next section we also see how to permanently stretch the image to bring out even more detail.

Note:  There are advanced procedures which are available using programs like MaxIm DL which can bring out even more detail in bright regions of objects like galaxies and bright nebulas.

In CCDOPS, display stretching is done using the Contrast window.  The Contrast window for the image above is shown below.

The important numbers are the Back and Range values.  Back determines the brightness at which the background is displayed.  It is the value of a black pixel.  In other words, every pixel in the image with a value of less than 164 (in the above example) will be displayed as pure black.  The range determines how many levels of brightness are compressed into the remaining 255 levels available for display (remember there are 256 levels in an 8-bit system like a monitor).  In the above example the Range is 440, meaning that pixels with values between 127 (the Back value) and 604 (the Back plus the Range, 164 + 440) will be visible as shade of grey.  Everything with a value higher than 604 will be pure white.

So, to get the background darker, you need to raise the Back value to place more of the low-value pixels under the black threshold.

Raising the Back value to 188 results in the darker background below:

Now, to bring more of the bright core into view you need to increase the Range value to bring more pixels within the visible range.  Raising the Range to 669 brings everything from 188 to 857 into view.

The result is shown below:

Permanent Stretching

While display stretching affects only the way the image is seen on the monitor, permanent stretching changes the actual pixel values.  Permanent scaling is usually called stretching.  To stretch an image in CCDOPS, select Utility > Pixel Utilities > Scale from the main menu.

The scale command allows you to select a Background value, Range value, and scaling type.  The values are scaled according to the type of scaling selected.  The choices are linear, log, and auto-log.  The most common choice is to use the auto-log feature.

Log scaling is used to enhance faint areas of an image and to allow both bright and dim regions to be displayed simultaneously.

Log scaling is most easily selected by choosing Auto-log in the Log pull-down menu.  This will adjust the range of values between Background and Background + Range to a logarithmic scale rather than a linear one.  This has the effect of enhancing faint detail.  For some objects, such as galaxies, the noise produced is significant.  A program like MaxIm DL is better suited to scaling most images.  However, Auto-Log works well on objects such as nebulae and globular clusters.

Select Auto-Log from the Log pull-down menu.  In this mode there is no need to input the other values as they are selected automatically.

Above:  A 15-second image of Omega Centauri, the largest and brightest globular cluster in the sky.

Above:  The Scale window set for Auto-Log.  The Background and Range values are the same as those from the display Contrast window but are ignored by the Auto-log feature.

Above:  The same image after log scaling.  The bright and faint stars are compressed into a narrower brightness range, allowing more stars to be seen.

Next, Color Image Processing....

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