The Baader Hyperion eyepieces accept fine-tuning rings which increase the magnification. One unscrews the lens group at the bottom, screws in one or more fine-tuning rings, and screws the lens group onto that. One can buy the fine-tuning rings, of course, for a modest price. But cheap as I am, I decided to save. Instead, I purchased two bunches of 48mm camera filters on ebay. The latest bunch was a lot of ten Niko skylight filters for $13 shipped. One then removes the glass from the filters, and screws the filter housings into a tube. The tube is very rigid. (Removing the glass destructively is easy. Put filter in some thick bag, hit with hammer or screwdriver or handle of butter knife through the bag. If the glass breaks, it is easily removed from the housing, though of course one should wear some kind of safety gloves and be careful not to let the glass out of the bag. Eye protection and a breathing mask are a good idea. Surprisingly, a number of the Nikos had the glass survive the procedure.)
The ten Nikos add up to 43mm total, and the three Gemkos I have from an earlier purchase give me 14mm. I could control the length of the fine-tuning tube in increments of about 4.5mm by adding and subtracting one filter housing at a time.
For a total extra price of about $20, my single 13mm Hyperion varies in increments of under a millimeter of focal length between 13mm and about 7.5mm. The views are really good, though the focus has to be close in with all 57mm of rings attached. The apparent field of view increases very significantly, which is nice (at full extension, I need to press the eye into the eyeguard to see the whole field of view). I guess this is because the field stop doesn't increase. The eyepiece becomes absurdly long, and maybe some focusers won't be happy with the added torque. I suppose for added fun, I could put in a Barlow, but then the magnification would be more than I can really use.
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