Improvised Eyepiece

[Stub Mandrel]

I found an old 50mm x 350mm refractor with half decent coated lenses and a RACI prism, which seemed ideal for a better finderscope, but it had no eyepiece.

it seems that long focal length 0.965" eyepieces are as rare as hen's teeth and even an eBay special will set you back £17, so what to do?

To avoid repeating what I've posted elsewhere, 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 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 traffic about 50 metres away 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 ploslls 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, space and another lens and do a quick check by sliding this onto the main tube. All OK, 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.

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 4 number 60 holes (~1mm) equally spaced around the groove in two slightly offset pairs. First try with some fine wire was no good - they looked like steel bars! I extracted some very 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.

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

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 - so it should work well as an 8x50 finder scope - certainly better than the little 30mm thing I have now.

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.

[ed_turco]

I really like this idea.  It is not generally known that amateur DIY astronomers can make their own eyepieces from scratch, as illustrated here. 

All one needs to do is stick with a simple type, like a Ramsden.  One needs to get the appropriate tubing first, measure the inside diameter and then search out plano convex lenses that will fit in the tube.  I've built quite a few 40mm ramsdens and I've kept a few for my own use.

While such an eyepiece isn't a Nagler, there is a special feeling one gets when looking through an eyepiece of one's own construction.    

Great job here!!

ed

[Stub Mandrel]

Thanks,

Unfortunately the eyepiece  greatly exceeded the capacity of the scope - the field of view wasn't wide enough to be a useful finderscope, I could see the same view through a 10mm eyepiece but four times bigger! Now I know why you need short focal length objectives for finderscopes!

Yes I find it really rewarding to make things that work, especially from 'found' items, it's also a great way to learn.