Scout Regiment Telescope lens check

[MikB]

One of the things that led me to this site was Peter Laidler's informative article on the Scout Reg telescope. I found this passage in the article:-
(begins)
Originally, OG lenses were a set of the usual flint and crown. But later, a cemented doublet. Now, here’s a little tip for you. One of the doublets consists of a very slight convex (outwards) curve and a concave (inwards dish) while the other is a double convex (outwards curves). But each convex isn’t exactly the same. So which convex do you mate with the concave? I’ll tell you. Clean both sides of each with methylated spirit until perfectly clean and grease free then put a bit of spit into the dish of the concave lens. Now put one side of the double convex lens into the dish and note how far it spreads out the blob of spit. Now do the other. The radiused side that spreads the spit the furthest is the matching side.
(ends)

There's another way to check whether you've mated the right side of the bi-convex without gobbing on the lenses you've just carefully cleaned.

All you need is a lamp.

Mate them the way that looks right, then hold the doublet so you can see reflection from the lamp. There should be 3 images: on from the front face, one from the interface, and one from the back face. This last is larger and more diffuse than the other 2. It's also more 'slippery' - it whips across the face of the lens faster when you move it. With the other images showing close to the centre of the lens, rock it carefully so as to move them closer to the edge.

Does one of the images split in two?

If it does, you have the bi-convex lens back to front, and away from the middle of the lens, the surfaces are no longer touching. This effect has been easily visible on all Scout Regs and Tel Sigs that I've seen.

Now you don't have to re-clean (and possibly re-scramble) your lens before reassembly.

Regards,
MikB

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