No-one is getting out much for astrophotography in Cumbria at the moment, as we are seeing endless cloudy skies. Plenty of time to practise processing.
I made it up to my favourite dark lay-by near Tebay on 23 November and captured an hour of the Andromeda Galaxy M31. 29 frames of two minutes each. If you’re wondering what a single two-minute frame looks like, here is one. This is what you get if you open the shutter on your camera for two minutes. The darkness of the background shows what an excellent location it is on the remote country road. I’ve not done any processing on this frame, just converted it to jpeg for display here.
Stacking the 29 frames and applying the usual processing tricks of adjusting the background, stretching the histogram, and a little colour saturation boost, gives this:
That’s a shot I’ve wanted for a long time. M31 nicely framed through a 300mm lens. What more can we do with this? Well, today’s lesson is all about High Dynamic Range “HDR” transforms: re-stretching the brightest parts of the image to bring out further detail. Pixinsight, my chosen processing software, permits this:
The effect of this adjustment is noticeable at the centre of the galaxy, where previously the detail had been hidden by the excessive brightness.
I’m also on a mission to use every scrap of data I can gather on M31, and Pixinsight also allows me to re-scale last year’s frames (taken through a 200mm lens at 5 minutes per exposure) and amalgamate them with the current set of 300mm frames. Here’s the combined set, processed as normal:
Next, the combined set with the HDR transform pushed hard to enhance the detail in the spiral arms of the galaxy:
I could be happy with that, but it’s quite hard on the eye, so I layer the original and the transformed versions together in Photoshop, which gives us the best of both worlds.
There’s still a long way to go with this. I’d like to take longer exposures without the stars trailing, and so get finer detail in the outer reaches of the galaxy’s arms. Even at two minutes through the 300mm lens, the stars trail by a couple of pixels or so. A couple of pixels doesn’t really matter for stars (they just appear slightly elongated rather than perfectly round) but it matters a great deal for the fine detail in the arms of the galaxy.