The moment of revelation that turned me on to the possibilities of amateur astrophotography occurred on the evening of 5th March 2012, when Jeremy Hunt (not that one) came from Cockermouth Astronomical Society to present some examples of his work in Kendal. All his slides included a note in the corner, giving details of the focal length of the lens used. In the Q&A, I commented that there must be something amiss – if only with my understanding of the process. If he had taken that photo with, say, the 300mm lens as noted, then he must have enlarged or cropped it because I knew the object in the sky simply wasn’t that big. Jeremy’s answer turned my understanding on its head: “It is that big, it just isn’t that bright”.
Some of the night sky objects are visually enormous. The reason that we cannot see them is not that they are small, but that they are faint. Next time you look at the full Moon, imagine 12 of them arranged six by two like eggs in a dozen box. That’s the visual size of M31, the “Andromeda” galaxy.
The key to photographing faint objects in the night sky is not magnification, but exposure. You don’t need a long lens, but you do need to hold the shutter open for a long time. Unfortunately for the astrophotographer, the sky moves (the Earth rotates) so long exposures of stars appear as lines rather than dots. Holding the camera stationary for long exposure against the rotating Earth requires technology which I didn’t have for my first attempt at M31. However, using a 50mm lens and 5-second exposures meant I could get satisfactory results from a fixed tripod.
This is a 5-second frame using a standard 50mm manual lens at full aperture. There is nowhere for lens defects to hide in astrophotography, and the distorted shape of the stars towards the edge of the frame is a fine example of coma. Coma is most noticeable at wide apertures, and can be fixed by stopping the lens down. I took 40 frames of 5 seconds and stacked them using the free software “Deep Sky Stacker“.
This is the stacked result, cropped to pick out the galaxy. It’s not brilliant, but it is:
- an object that is almost invisible to the naked eye
- photographed with a standard lens from a fixed tripod
- stacked using free software
From here, as the song says, the only way is up.