Olympus 200mm Lens for Astrophotography

If you'd like to do some wide-field astronomy with a SLR lens and don't have a DSLR, it's possible to mount the lens on your CCD. The particulars of mounting vary from camera to camera. Some of the adapters can be relatively expensive, but adequate lenses are well within the reach of the low-budget imager.

In my case I have an SBIG ST-8300M camera with a filter wheel. SBIG sells a Canon lens adapter for about $300 that mounts on the filter wheel (a spacer is needed if you don't have the wheel). This opens up the possibility of imaging with any other lens that can be used with canon EOS cameras.One such lens is the Olympus OM lens type. An adapter is needed to mate an OM lens with the SBIG canon adapter. (Yes, two adapters are needed between the lens and filter wheel.)

I've already imaged with an old 135mm Tamron lens that lived its previous life on my OM-1 SLR. Given the good images that lens produced I shopped around for a fixed 200mm lens.

What I found was a used Olympus AUTO-T E-Zuiko f/4 200mm lens at a local camera shop. These sell on eBay for about $50.

Olympus E-Zuiko AUTO-T 200mm f/4.0 with lens shade retracted. Image by Jone Quinn

Technical information about this lens can be found here. Why 200mm? It plugs a focal length hole between my 135mm lens and AT65 (422mm). Yes, I'm actually buying telescopes and lenses partly on the basis of their fit to a trend line. See how neatly the 200mm fits in in the chart below?

Everyone uses a semi-log chart for choosing imaging telescopes, right?

This lens is semi-automatic; it is wide open until the shutter opens and the camera tells it to stop down. There's no way to handle this with the CCD, so I inserted a plastic wedge into the lens base to keep the iris stopped down to the dial setting.

Last night I took a few images just to see how it well it would serve for astrophotography. My concerns were if it could focus infinity properly (which isn't a given thanks to those adapters) and how far itwould need to be stopped down to get reasonable stars across the field.

It reaches focus if you put the "3" of the 30m mark over the left index line for f/16. In other words, infinite focus is at about 35m.

And field flatness? It's very good for a $50 price point. Here's a full-frame bias-calibrated and stretched luminance images from last night, first at f/5.6:

f 5.6

and next at f/4 (same exposure and processing):

f/4

Vignetting can be handled with flat frames, and the stars are decent at f/5.6, a little less so at f/4 (although some of this is probably star bloat from processing). This shouldn't be a surprise as this is a fixed focal length lens designed to illuminate 35mm format cameras. The KAF-8300 sensor size is about half that, so it avoids the worst of the off-axis distortions.

For ALBN imaging f/4 is perfectly adequate. For "pretty pictures" I'll probably used f/5.6.

When used with the ST-8300, the 200mm lens gives a 3.9 x 5.1 degree FOV, a nice size for some remaining ALBN objects, either singly or in combination. (My last post lists my planned targets for this lens.)