Wednesday, 6 March 2013


Last month I mentioned that I had purchased a small tracking mount at Astrofest-
I had hoped to try it out when we had the recent asteroid DA14 flypast, but we were completely clouded out. As the Moon was high in the sky I then had to wait for a dark period, and last night looked promising. My intention was not to produce pretty images but rather test the various parts of the system, from the mount to camera and lenses. Since I had not actually done any long exposure imaging since 1997 (Hale Bopp) when we were still using slide film and had only undertaken a few quick exposures from the garden with my digital slrs I needed to understand the capabilities of the gear.
We used to go out to a spot near the village of Cromer, and i had found a spot on the map which looked good, but when I got out there at around 1930 I felt it was still a bit too close to Stevenage and drove further east to a spot near Haultwick where i found a nice farm track well away from any lights. Unfortunately due to the mild weather, it was a bit hazy with large areas of patchy cloud drifting slowly over, and I was still close enough to town to see that the sky was still a bit orange. However it would do for the tests.
Setting up was pretty quick, and the IPhone app enabled rapid and accurate polar alignment. I had taken the Zeiss 100mm lens, and based on the garden tests stopped it down to f2.8 to reduce vignetting and blue halos around bright stars. The camera was set to iso 1600 and manual exposures of 30 and 60 seconds. The noise reduction was switched off as I wanted to see what the sensor would deliver, and in order to keep things simple I made no attempt to produce dark frames or flat fields for the same reason.
The only clear area of sky was Perseus low in the west, so I set up to get the Double Cluster and the area around alpha in the same field. A 30 second exposure looked ok suggesting 60 seconds would work, but the stars were not sharp-the tripod had shifted slightly on the soft ground. After getting the polar axis set up again I tried 60 seconds which looked much better.
Cloud was building up so I then concentrated on the Orion region which was now a bit low down. All subsequent exposures were marred by cloud and contrails but I was able to get the Orion Nebula and belt regions in between the gaps, but the haze produced brighter skies and slightly bloated stars.

Subsequent processing has been kept to a minimum. Apart from trying to get levels adjusted to bring out the fainter details all I have done is get the background reasonably neutral ,and there has been no attempt to enhance the images up by sharpening or noise reduction.
All three images are crops. The double cluster is going reasonably deep but faint stars are difficult to distinguish from the noise. The belt crop is interesting as it shows faintly the emission nebula NGC 2024 next to the left hand belt star, and a faint patch to the north is the reflection nebula M78.

So a couple of things have been learnt. The location I found would be pretty good on a clearer night provided the western sky towards Stevenage is avoided. Otherwise I would have to travel a considerable distance further east and north. The mount is more than capable of tracking for many minutes with the 100mm lens and would probably handle substantially larger lenses-the 300mm f2.8 will be tested. The 100mm lens certainly works well at f2.8. Due to noise and dynamic range I would not want to set the camera to more than iso 1000 in future particularly if exposures of several minutes are attempted and it would certainly be beneficial to produce multiple images and stack them, as this will enable me to go a lot deeper. Dark frames and flat fields will also help.
 The drive home was a bit of a let down as I hoped to pick up one or two Barn Owls.

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