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:
- Find the limit of time before star distortion is obvious for your mount
- Experiment with stacks of up to fifteen images at various ISOs
- Take Dark, Biased and Flat frames, ( especially Darks if you are pushed for time)
- Stack each group, (including Darks etc. using DeepSky Stacker, (free download if you don’t already have it)
- See which stack gives the best image
- 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.