Tony Hallas’ Cosmic Imaging: Create a mosaic

April 2011: Multiply your camera’s normal field of view.

Contributed by Tony Hallas

Published: February 21, 2011

There will come a time in your astroimaging career when you will want to stitch (not stack, but combine) two images into one large one. The word mosaic comes from the ancient method of creating a piece of artwork from many small pieces of stone, glass, or other materials. In this case, you will use pieces of the sky you have photographed.

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THOMAS EELLS from OHIO said:

Tony,

My sincere gratitude for your incomparable advice! RegiStar looks phenomenal, but may I humbly (very humbly!) suggest to your readers that similar results may be achieved by Mac users and / or anyone else who doesn't have that particular software; and also w/o using a wide-field reference shot.

The method would of course involve first creating the individual frames to be used in the final mosaic, making sure to have at least a 20% overlap just as you advise. These frames (or 'tiles', if you will) can then be combined in Photoshop in one of several ways.

The first method would be to use the 'Photomerge' command in Photoshop's 'Automate' submenu (in the 'File' menu). There are a couple options here. You can use the 'auto' option and Photoshop will work whatever magic it can to get everything into alignment; in most cases it will probably do a little 'morphing', much like RegiStar. Or you can choose the 'reposition only' option; in this case Photoshop will get the tiles as closely aligned as it can, but will not morph or otherwise deform the original tiles.

Either way, the photomerge method will create a new file with all of the original tiles left intact as layers. You can then use masking, curves, and etc. to make the transitions seamless. It should go w/o saying that Photoshop's success in this process will depend heavily on the quality of the tile frames being used. The process can also be greatly affected by the actual content of those frames. And although it usually works astonishingly well, occasionally it just won't be able to get the result.

My preferred technique for creating mosaics, then, is usually to do it all manually. In this case you need do nothing more than create a new empty document in Photoshop, ensuring that it's big enough to encompass all of the pixels from the mosaic's constituent tiles; then copy and paste in all the tiles (they will automatically end up on their own layers); align them manually as accurately as possible; and finally, complete the mosaic by again applying masks, curves and other corrections as needed.

The manual method is my favorite because it's straightforward, simple and appeals to the 'control freak' in me; and because any 'morphing' of an image necessarily degrades image quality to some degree. I'd rather allow some minor misalignments in small areas of the overlaps and then mask out one of the layers to eliminate them.

I know that none of these techniques will be even remotely new to you-- no doubt you were using them when I was still learning what an 'aperture' is; but I meekly submit them in the hope that they may be of help to some of your readers who are not similarly equipped.

Thanks again, and as always, your awed and devoted fan; your astrophotography is a monumental inspiration!

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