Friday, December 31, 2010

DSLR digiscoping

I can't believe it is so long since I last posted something on this blog! Anyway, things move on and I am now into digital SLRs. For reasons I won't go into, I have in fact ended up with two: a Canon EOS 550D and also a 60D.

I wanted to try out the DSLRs on my Kowa 823 telescope. There are two ways of taking photos through a telescope:
  1. Replace the eyepiece with an adapter (which usually connect to the camera via a T-mount)
  2. The same way as a compact camera - i.e. photograph the image that the telescope's eyepiece produces by pointing a camera with a normal lens mounted at the eyepiece.
The first option can be expensive. Kowa do an adapter, but it is hard to get hold of and costly (c. £100). It turns the 'scope into the equivalent of an 800mm lens on a 35mm camera. You can readily tell whether the second option will work for you by setting up the scope, placing your camera lens close to the eyepiece and seeing if you can see anything though the viewfinder.

Since I already have the LCE adapter for my Coolpix 4500 (which is essentially a bucket shaped piece of metal which slips over the eyepiece of the scope and has a hole surrounded by a 28mm filter thread to fit to the camera lens - see). So, if I can find a suitable lens for the DSLR which will successfully form an useful image, I just need a way to mount it on the 28mm thread of the LCE adapter. After some Internet searching, it seemed that a 50mm lens was often mentioned as suitable - and I have a Zuiko 50mm f1.8 lens from a defunct Olympus OM1 film camera (whose metering system gave up the ghost in the dim and distant past). I have an adapter to mount Olympus lenses on the Canon DSLR (bought for a few quid off eBay) and it did indeed form and image (without any vignetting!) when held against the scope's eyepiece. It has a 49mm filter thread, so a 49 - 28mm step-down ring solves the mounting problem.

LCE adapter, 49-28mm step-down ring, 50mm lens, Olympus to Canon EOS mounting adapter

Canon EOS 60D mounted on Kowa 823 telescope

So what are the results like? I set up the 'scope in my garden (on a rather dull day) and used my car numberplate as the target (at a range on 12m as accurately as I could manage to set it up).
Car number plate - the red line shows the height of the inside of the letter "L" which is 64mm high
First, here is a 100% crop from an image taken using my Sigma 150-500mm lens, set at 500mm (I don't think I got this quite in focus!).
100% crop - 500mm
Here are 100% crops from images taken with Coolpix 4500 on the scope (eyepiece set to minimum zoom - 1.e. 15x) with the camera at the minimum zoom setting to avoid vignetting and then at its maximum optical zoom. I used the camera's auto focus here.
Coolpix 4500 - min zoom to avoid vignetting
Coolpix 4500 - max optical zoom
And finally here is the image taken using the 60D/50mm lens combination. The camera was set to manual mode (M) and ISO800 - which gave a shutter speed of around 1/4 sec - so a remote release was used. I used live-view mode and focused by zooming in on the LCD screen to adjust the focus as well as I could.

Canon EOS 60D,/Zuiko 50mm mounted on Kowa 823 telescope
Measuring the height of the up-stroke of the "L" in these images viewed at full size (100% zoom): using the Coolpix it was 439 pixels at min zoom and 1110 pixels high at maximum optical zoom. The 500mm lens gave an image in which it was 594 pixels high and using the scope + 50mm lens it was 1454 pixels high.

Just like with the Coolpix, the whole setup is quite cumbersome and it can be quite difficult to find the subject, because the field of view is so small. Focusing is also a problem. The auto focus system won't work of course - since the scope and 50mm lens do not have focus motors. Even focus confirmation is not possible because the Canon need a lens aperture of f5.6 or wider for its auto focus system to work. I don't know what the effective aperture of the scope setup will be - but its a lot smaller than f5.6! The live-view mode and articulating LCD screen of the 60D is a tremendous help here.

I will be interested to try it out for real next time I am in a hide somewhere and have a subject that isn't moving very much!