Combining Files

The best way to reduce noise and increase detail in an image is by combining multiple exposures of an object.  MaxIm DL allows you several methods of combining images including Sum, Average, and Median combining.

Below is an excellent example of a file that has quite a bit of noise in it.  Because the Cone Nebula is such a faint object, stretching the picture to bring out the dim nebulosity increases the noise considerably.

Above:  Original single exposure of the Cone Nebula.

By combining five separate exposures of the same object, the noise is significantly reduced.  In this case five images were combined using the Median method, which tends to be one of the best for reducing noise.

Above:  Median combine of five individual exposures

Combining in especially important for very faint objects.  The amount of image processing necessary to reveal these dim targets can cause a significant increase in noise -- the more images you combine, the less noise you will have to begin with and the more processing you can get away with.

Combining

Select File > Combine Files.  Select the appropriately directory and highlight the files you wish to combine (use Ctrl-Click to select multiple files).  Alternatively, you can combine images which are already open by selecting Process > Combine.  In this example three previously opened files have been chosen.  This is the usual procedure for combining after having calibrated a new set of images.

All open files appear in the left column.  Select the ones you wish to combine, or in this case since we want to combine all three open images, you can click Add All.  Click OK.  The Align window will open.  It is necessary to register the individual images with each other.  In most cases selecting Auto -star matching under Align Mode is sufficient.

Choose the combining method you would like to use under Output:

• Sum - Adds the pixel values of each image, enhancing the object but also increasing noise.  Any artifacts (hot pixels, etc.) in each image will be retained in the final image.

• Average - Averages the pixel values in the images producing a less noisy image.  Some artifacts are retained.

• Median - Usually the best method for reducing noise, this takes the median value of each pixel in an image.  Artifacts are effectively removed from the final image, so this method produces the "cleanest" files.  Requires 3 or more files to be used.

Note:  Median combining requires three or more files to be used.  Averaging works with two or more files.

Click OK and the images will be combined and displayed as a new image.

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