Once both images are aligned, some areas will probably still need adjustments before you can blend them together. Here the author has used Photoshop’s Color Range process to select the brightest stars, then applied the Minimum or Maximum filters (inset), depending on whether he needs to shrink the stars or expand their apparent sizes to make them match in both images. Click on image for larger view.

Robert Gendler

Using Photoshop’s Lasso tool, I select regions in each color channel that still need some adjustment, and I blur the edge of the selection using the Feather option. Which layer I choose to edit often depends on the detail I hope to keep; one image usually goes deeper than the other. I then use the Curves, Levels, Color Balance, and Saturation functions to adjust the area in question.

Images made with instruments having a large difference in focal length inevitably show obvious differences in the visual appearance of the stars. One way to change the size of the stars is to select the bottom layer (the wide-field image) and use the Select command, then Color Range. This will open the Color Range window with a preview of the image and a slider bar above it named "Fuzziness." Here I hold the shift key down and click on some of the brighter stars. I experiment with Fuzziness to either increase or decrease the range of my selection until I’m content with how many stars I want to adjust. I then choose the Select command and Feather to further transition the selection into the rest of the image. After finalizing my selection, I usually go to the Filter menu and, from the Other category, select Minimum to shrink the largest stars on the wide-field layer. Occasionally, though, the difference in star size is still apparent, so I select the top layer and repeat the starselection step, but this time replace the Minimum filter with the Maximum filter, to enlarge the stars by a few pixels.

Once the images are completely blended I save the document as a Photoshop (.psd) file to preserve the layered version in case I feel the need to adjust different parts of the image at a later date. I then flatten the image and save a copy as a TIFF file. Any final tweaks are now done to the flattened TIFF file. Once I’m done, I have an image that is capable of being printed in poster format and displayed in an office or gallery.

So when returning to those "old friends" in the night sky with new equipment, remember that your earlier images aren’t necessarily inferior to the new images you take. While the work involved in combining old and new views is labor intensive, hybrid compositing is well worth the effort.

When not working on celestial masterpieces from his driveway in the suburbs of Hartford, Connecticut, Robert Gendler spends his days as a radiologist.