This is a short description of various eyepiece adaptors for camera lenses I have acquired or made. They work pretty well with cheap second hand manual long slow lenses to provide a reasonable wide field view. Some of the better examples of the Russian MTO f10 1000mm Mak lenses will work very well as a compact portable spotting scope.
A cheap second hand manual or preset 300mm f4 or 400mm f5.6 or similar lens will make a pretty good wide field finder if combined with a suitable eyepiece using one of these adaptors. Any of the now unfashionable slow fixed focal length long lenses will work pretty well as a straight through finder scope. To use as a telescope you don't care if the metering and aperture linkage works properly works as long as it stays fully open.
Unfortunately there is not usually enough back focus to accommodate a normal star diagonal in combination with this simple adaptor so it has to be a straight through telescope. My measurements suggest that it might be possible to place a right angled prism against the back of the adaptor and retain enough back focus but I have never tried it. The Russian telescope converter for the MTO Mak uses a barlow lens in combination with a right angle prism. I guess this arrangement could also be copied. Markus Ludes has some notes about how to modify MTO and Rubinar lenses to increase their back focus - I have never tried to do this myself.
|The first is a Japanese all
plastic moulded push fit carrier for 1" eyepieces. These are widely
available in Japan for a range of camera lens fittings. I have never
seen them outside of Japan.
I regret not buying more of them when I had the chance.
|The second is a fairly simple DIY 1.25" adaptor made out of a spare lens protector which has been carefully drilled out and then fitted with a 1.25" plumbing pipe adaptor. Lined with cardboard tube it makes a decent push fit eyepiece adaptor for lightweight eyepieces. The assembly is glued together with epoxy glue - when testing it for the first time take care not to foul any lens aperture linkage mechanisms on the inside.|
|Finally the most expensive solution is to take an old extension tube and bore out the inside on a lathe to make a perfect eyepiece adaptor with a tapped hole for a locking screw. This can take heavier eyepieces without any danger of them falling out. Some short focal length eyepieces will also fit in this for eyepiece projection. One tip - dismantle the aperture linkage completely before turning the inner diameter on the lathe. It is embarrassing when spare bits fly out unexpectedly.|
Some of the Japanese camera makers also offer terrestrial telescope converters for their series lenses. You very rarely see these available in the West but they sometimes are offered for sale second hand. I also have a Pentax manufactured Monocular Converter for my K series manual lenses. It makes a pretty reasonable spotting scope out of any of the longer focal length lenses. It contains a 13mm erecting eyepiece with it's own helical focuser.