Sunday, 21 December 2014

Astronomical Imaging

I have been dabbling in astro-photography again. Last year I picked up a small tracking platform, and the early tests out in the countryside were pretty successful. The only real problem, apart from the hassle of packing everything up in the car and driving to a dark site was finding a suitable surface to put the tripod-soft ground is not good for accurate polar alignment.
For some time I have been discussing light pollution filters with some of the LDAS members and I finally purchased a 77mm diameter one made by Hutech. The idea was to enable me do do some sort of imaging from my badly light polluted Stevenage garden. Back in the 80's I used to do quite a bit with lenses ranging from 35mm to 135mm, generally using Fujichrome slide film. Even from the garden I could run to several minutes exposures, but as Stevenage expanded to the north and east it became more and more difficult. My last images on slide were of comet Hale-Bopp in 1997. When I went digital in 2003 I did a few trials with the D1x and later D2x but rapidly came to the conclusion that anything more than a few seconds would produce bright orange-brown backgrounds, limiting the amount of information recorded.
The filter arrived last week and I have had two brief sessions in the garden. With the first, the tracker was not polar aligned as I was on the patio and Polaris is hidden. I just set it up so the axis was pointing in more or less the right direction. The camera was set between iso 4000 and 10000 and using the Zeiss 100mm at f2 I was able to get decent images using exposures of up to 8 seconds. I also tried my Sigma 100-3000mm F4 lens. This like many Sigma lenses was a bit fragile and after a lens motor repair, followed a few years later by the focussing lens assembly falling to bits no longer autofocuses, but being an ED lens is pretty good optically. The image of M42 the Orion Nebula is a stack of two ten second images at iso 4000 and 8000 with the lens at 300mm f4. Some Photoshop work brought out a lot of the fainter nebulosity.
The second session used the 300mm again. This time I was properly polar aligned, the iso was reduced to 2000 and exposures ran up to 30 seconds. The iOptron tracker has come in for some criticism regarding tracking accuracy and load carrying. Supposedly anything more than a 200mm lens is asking for trouble, and the pro body D3s and 100-300f4 is very heavy. Seems to work for me though as the images of M31 the Andromeda Galaxy and the Pleiades M45 show.

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