Stitching Stars

Every once in a while, I have to try something a little “left field”.

What if, I thought, instead of stitching a panorama or vertorama of a landscape, I tried stitching a 180 degree vertorama of stars.  I have stitched a landscape shot with a star shot before as shown in the image “Above and Below”. For “Above and Below” I combined a stack of star trails with a shot from just before dark of the landscape. Unfortunately after stacking the star trail for the top, I found none of my image tools could be convinced to stitch the portions of the image together satisfactorily and the two halves of the whole lay dormant in the bit bucket bin.

Above and Below

The star shot – that is the top half – is very similar to this shot on Flickr. I opted to use a stack with fewer frames to keep the sky dark and let the smear of the Milky Way stand out. I then very crudely combined the two shots using the “Collage” tool in Picasa3. I followed the crude paste up job with the “touch up” tool to blend the seam lines. The touch up tool is a crude approximation of the Photoshop Healing Tool and Clone tool combined into one. Sometimes it is very effective, sometimes not so much.

But one evening I found myself staring at the dark skies at 9,000 feet above the town of Bishop, California. The Milky Way stretched from horizon to horizon and was nearly directly overhead. So I thought I’d try something that I expected to be difficult… but proved surprisingly easy. I thought I would try to stitch together the stars in the night sky in much the same way that one can stitch a panorama together.

But Would Stitching Stars Work?

To answer the question, I took the following 8 shots at high ISO, 30 seconds each.

I then dropped them into Photoshop CS5’s PhotoMerge tool and got this peculiar and clearly poor result:

Photoshop’s confusion was partially understandable.  When I took the eight shots for the image I shot 4 shots from the horizon up to the zenith. I then turned the camera 180 degrees and shot the remaining 4 shots descending from the zenith to the horizon.  The top of the first shot must be stitched with the bottom of the second shot, the top of the second with the bottom of the third, the top of the third with the bottom of the fourth … and then the great discontinuity: the top of fourth shot needed to be stitched with the TOP of the fifth shot.  Realizing that the problem might be the topsy turvy issue, I rotated shots 5 through 8 to preserve the “top to bottom” alignment. Unfortunately using the default mode Photoshop could not figure out what to do with the topsy turvy or the consistently aligned images.  So I tried another approach. I changed the Photomerge projection mode to “cylindrical” and obtained the following:

There is a peculiar and inexplicable bulge in the result, but it certainly looks quite a  bit more realistic than the first try.  It took about 5 minutes to do the stitching.

To compare, I fired up the Microsoft ICE tool (an Image Composite Editor – free from Microsoft for PC users) and dragged the 8 original un-rotated exposures and got this:

in about 3 minutes. The ICE result is a lot closer to what I was expecting and the tool did not seem to object to having either the topsy-turvy or properly aligned images thrown at it.

After cropping the ICE result and a little image clean up I arrived at the following visual conundrum. Which way is up? Well, the middle of the picture is up! The top and bottom are the East and West horizons.

Path of the Milky Way West-to-East [C_009575-82stitch]

What do you think about this?