M31 and PANSTARRS

When the Cumbrian skies cleared on 1 April we had the opportunity for several nights of catching PANSTARRS in glorious juxtaposition with the Andromeda Galaxy M31.

First up, some test frames on 1 April which turned into a nice capture that I’d had in mind for some time.

50mm f/5.6, ISO1600, 2min
4 x 30-sec frames stacked in DSS

With a standard 50mm lens and some judicious cropping, this letterbox format shows Mirach (one of the guide stars used when finding M31: “from Mirach, hop right one star, then again, then down to the fuzzy blob”) with M33 faintly visible on the left, M31 on the right and PANSTARRS making its way in from the bottom.

Quite a windy evening, so I cranked up the ISO to 1600 and took just four good frames at 30 seconds.

On to the following night…

200mm f/5.6, ISO 1600, 15 min.
30 x 30 sec. frames stacked in DSS.

Quite breezy again, so I kept the exposures down at 30 seconds, but on the 200mm lens.  30 frames stacked for a 15 minute total exposure.

PANSTARRS is only about 7 degrees above the horizon here.  That presents a whole new set of problems.  At this angle, the line of sight goes through about ten times as much atmosphere as at zenith, multiplying the effect of water vapour on the incoming light.  From this location, Helsington Church, the view North-West also passes over the lights of Windermere and Ambleside, giving a street light skyglow to the bottom of the frame.  One of the beautiful features of PANSTARRS is its fan tail, glowing by reflected sunlight, so any attempt to process out the skyglow tends to eliminate the tail too.

300mm f/5.6, ISO 400, 40 min.
20 x 120 sec. frames stacked in DSS.

Another shot I had framed in my mind, estimating there would be just enough room in the 300mm frame to put both M31 and PANSTARRS.  As the wind had dropped, I could get exposure up to 2 minutes, and grabbed 23 frames of which 20 were acceptable.

Finally on 3 April…

200mm f/6.3, ISO 200, 60 min
12 x 5 min. frames stacked in DSS

The birthday fairy brought me a polarscope this year, which dramatically reduces the time taken to polar-align the EQ3-2 equatorial mount.  In  a couple of minutes I can align more accurately than I used to get from 30 to 40 minutes of drift alignment using the camera.  Certainly it is good enough for 5 minute exposures at up to 300mm.  I must find time to test the alignment with longer focal length.  Anyway, this stack of 12 frames at 5 minutes each has cropped nicely.  M31 is not as clear as I would have liked (see M31: up to 200mm) but it was too low in the sky for that.

There was another very pleasing alignment on 5 April but the Cumbrian clouds had closed in.

M31 Andromeda first attempt: fixed tripod

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.

50mm f/1.8, ISO 3200 5 sec.

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“.

50mm f/1.8, ISO 3200 5-sec
40 frames stacked in DSS

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.