Images representing different hobbies

I found an old 50mm x 350mm refractor with half decent coated lenses and a RACI prism, which seemed ideal for using as a finderscope, but it had no eyepiece. Searching the web,  it seems that long focal length (about 40mm) 0.965" eyepieces are as rare as hen's teeth, so what to do?

I made a 40mm eyepiece out of two 25mm doublet binocular objectives with a focal length of about 100mm. This is how.

The lenses were a close fit inside some 1 1/4" aluminium tube with a ~1" bore. I cut a length off the end of an old 1 1/16" bicycle seat pillar and turned one end down to 0.965" for about an inch, this fits in the prism tube. I turned the rest to a good fit in the 1 1/4" tube, then parted off two washers, one ~2/16" thick and one about 1/16" thick. The thicker washer was used as a spacer between the lenses with their most convex sides facing each other. Projecting an image of a distant subject on a wall showed the focal length to be more or less 40mm (I was going to use one lens until Wikipedia revealed to me how plossl lenses are made).

I then shortened the main tube so with the lenses at the top the focal point would be about 15mm in from the end. I checked it all worked with the scope.  I now made a neat housing for the lenses from the larger tube and superglued the thin washer at one end. I could now load it with lens, spacer and another lens and do a quick check by sliding this onto the main tube. All OK,  so make sure lenses are spotless and glue the lens holder onto the main tube.

I turned down the end of the holder so it had two steps to suit the binocular rubber cup, but the hole was far to big for the exit pupil. I drilled a hole in some acetal rod, recessed one end for the curve in the lens and made it a close fit in the thin retaining washer. it popped in snugly and the bino cup holds it in place.

Home made plossl eyepiece

The home-made eyepiece

To finish, I turned another bit of acetal about 1/2" long with a 1/2" hole in it, a groove around the outside and four number 60 holes (~1mm) equally spaced around the groove in two slightly offset pairs for cross-hairs. First try with some fine wire was no good - they looked like steel bars! I extracted some fine wire from some very thin flex, flattened it by rolling between two flat surfaces and use this to make the cross hairs. The ends of each wire were twisted together and superglued into the groove.

I focused on a distant object and by trial and error moved the crosshair holder up the tube to the right position. Run a drop of superglue around the holder and  - bingo!

Home made cross hairs

The cross hairs, made from fine wire.

I don't claim this is a good eyepiece, although as a 'spotting scope' the result is pretty impressive - it seems to give a crisper, brighter image than my existing finder scopes - but its field of view is rather poor for a finderscope. It seems it will be a handy bird spotting scope.

The main thing is, I've learnt a lot about eyepieces very quickly and it will be fun to use one I've made myself.