Stitching!

I now write a regular piece for the Eddington Astronomical Society, setting out what’s going to be on view in the night sky each month. In all honesty, it will probably lean towards astrophotography!  You can see the March 2014 version here.

One of the opportunities identified as coming up in March was the view of Jupiter reaching its highest point in the sky, elevated 59° in the early evening of the 13th. This looked like a challenging photo project, with bright Moon, bright Jupiter, bright and faint stars and the inevitable Morecambe Bay glow. It would also be at its best during twilight.

Sadly the Cumbrian rain was forecast for that evening, but we enjoyed a few days of high pressure just before then, with a couple of clear nights.

On Tuesday, 11th March, I strolled up to the Mushroom on Scout Scar with no more than camera and tripod. The 18-200mm zoom is pretty wide at 18mm, but I wanted a field of view of about 150°, so I set it to 24° and took nearly forty frames in a grid pattern looking south.

Photoshop can stitch shots together to make panoramas, but it needs clear points of reference in the shots and can only run on automatic. So, if it can’t identify the overlapping shots itself, there is no option for user-intervention. It simply doesn’t work on night sky mosaics.

That’s when I found PTGui, a programme that does exactly the same, but with a much wider range of projection options and, most importantly, the facility for the user to identify the overlap points (ie the stars) manually.

Here is the result, the view south from Scout Scar, Tuesday evening13th March. The sky isn’t flat, of course, and stitching frames requires distortion according to various rules depending on the projection settings. Circular projection seems to give the best compromise on distortion, then I cropped the result to a nice tidy rectangle.

All frames ISO 1600, 10 seconds 24mm f/5.6.

Big sky!  Nearly 40 frames stitched in PTGui, each 10 sec ISO 1600, 24mm f/5.6

Big sky! Nearly 40 frames each 10 sec ISO 1600, 24mm f/5.6, stitched in PTGui

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