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Dalriada n:
Kingdom of the Scots,
home of the residents

Deep Sky Stacker

By coincidence Colin Robb and I had both photographed M42, the Great Nebula in Orion, on the same night (Wednesday) before the ASG Kindrogan week-end. Not such a coincidence really because we were both making use of the first decent star gazing night in South Glasgow for some time.

At Kindrogan we both repeated the exercise, Colin in a much more professional way than me. He has posted his result at http://www.colinrobbphotography.com/m42-the-great-nebula-in you really must check it out, and the rest of his fantastic gallery.

Subsequently I asked his advice on astrophotography post-processing and auto-guiding, a technique to keep a telescope pointed precisely at an object being photographed during long exposures. His helpful reply included:

Stacking multiple images of the same short exposures will definitely reduce the noise seen in a single image, (increases the signal to noise ratio by averaging each pixel’s value), and will also slightly increase the dynamic range over a single image…

…So I have no doubt that for bright objects, (such as M42) good images can be created without guiding. Indeed if I had adopted this approach in the first place, I could have created a reasonable M42 as a starting point quite a long time ago….

So for non-guided work, my advice is:

  1. Find the limit of time before star distortion is obvious for your mount
  2. Experiment with stacks of up to fifteen images at various ISOs
  3. Take Dark, Biased and Flat frames, ( especially Darks if you are pushed for time)
  4. Stack each group, (including Darks etc. using DeepSky Stacker, (free download if you don’t already have it)
  5. See which stack gives the best image
  6. Keep the other stacks for combining at a later time , or combine right away if you know how – Photoshop etc

I went back to my Kindrogan images and had a go with Deep Sky Stacker on 4 of my images (2x 30s, 69s, 140s – all at ISO 3200) and 2 30s dark frames from the previous Wednesday in Cathcart! The result below seems a bit less noisy but next time I will try to be a bit more systematic, maybe taking multiple images with a remote control intervalometer, which I already have.
m42stacked