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Legacy Member
You said in thread 109 that it was better to use a SET of lenses as opposed to using separate lenses from separate sources - or telescopes. I have done small repairs to several No32 telescopes. Why is it better to use a set of lenses as they don't all interchange due to the variance in tube sizes and diameters?'
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06-26-2017 12:31 PM
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Good question Skippy. And the reason is this. On the surface - if you'll excuse the pun in a minute - all the lenses should be the same. After all, they were all made to the same dimensions and from the same glass. But then reality kicks in....... In the REAL world it's not quite like that. Because different glass mixes will give differing refractive indexes (the RI value) and slightly different curvatures just compound the problems with varying focal lengths (the FL's) as so it goes on........... And if you are making up a telescope using all new lenses or all part used or a mix of both, then it can go on and on and on and ........anyway.
You certainly CAN assemble one up but you'll find that certain spec ocular lenses, some double convex and others plano (plain or flat) and convex simply will not accommodate similarly configured double convex with plano convex lenses within the erector cell. Then while you might get the graticle perfectly focussed, the image will never focus in a zillion years......... So it's back to the drawing board. This is fine if you are sat at a bench in a clean-room at an instrument workshop with lots to select from. But definitely NOT a bundle of laughs elsewhere especially if you haven't got a light and calibration screen ahead of you. The good news is that if you use a complete set of lenses from a stripped No42 or 53 telescope then someone else has already done the hard work for you while sat in a factory workshop. Simple isn't it?
It might mean that you have to skim the erector cell a tad or sharpen/clean-up the ocular housing thread or fine tune the image focus but these things are simplicity itself compared with starting from scratch. So take my advice. If you need to replace a lens in a No32*, best start with a full set.
*It's usually the OG lens that gets scratched or damaged due to the insistence of those who know best - or refuse to learn - to use a piece of cloth out of their shooting bag to wipe the lens clean. DO NOT, EVER, never, never, ever do this. Buy a roll of the cheapest paper kitchen roll and remember this. This kitchen roll does not contain additives. ONE WIPE, ONE SHEET and throw it away and the same again until it's clean. If it's just the OG lens you are half way home and dry and these can be replaced. EASY to change but NOT easy to set up correctly. But alas, the same criteria applies. Wrong glass mix used in either of the double convex lenses or the wrong curvature and.......... bring on the nightmares.
As a good example of different glass giving different RI values, look at some REL No32's and 42's and notice how deeply into the tube some ocular lenses are set. This due to the slightly inferior quality of some of the glass mix. When you have lenses re-manufactured you should give the lens make a sample of BOTH parts of the lens (a broken lens will do) in order that the physics lab will give him the exact spec in order to get the RI and FL's correct
Good question Skippy. Sorry about the long winded answer
Last edited by Peter Laidler; 06-26-2017 at 01:34 PM.
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The Following 3 Members Say Thank You to Peter Laidler For This Useful Post:
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Legacy Member
Many thanks Peter very helpful answer...
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I'm currently considering purchasing an original Mk 3 scope, but it has some scratches on the front lens (the outer lens on the front of the scope, the one which you can touch with your finger, not any of the inner lenses) although these scratches do not appear in the sight picture. Is this typically a severe enough defect that one should avoid the scope and keep waiting for one which has no scratches at all?
I sadly do not have any photos of these scratches to share.
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I would say that if you can really screw the price down due to them, then do so. You say that you cannot see the scratches or that they don't appear on the screen....... That's not what counts. What counts is the internal light scatter that these scratches cause
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Ok, thanks for that.
Another question: is it true that No 32 scopes are not atmospherically sealed at all, meaning that dust can get inside via the seams thus causing dirt to appear on the crosshairs over time?
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Regarding the scratches on the front lens, I've just received some photos of them. This is my first time seeing them, but they appear to be quite...bad, at least from the perspective of a novice to No32 scopes.
Would very much appreciate the opinions of anyone more well-versed on the issue. Looking at the amount of scratches here, I have a hard time believing that these don't impact the sight picture, or devalue the scope for use in live fire training. But perhaps scratches like these are quite common on 70+ year old scopes?
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Advisory Panel
Love the Ithaca deerslayer in the backdrop...
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That OG lens is beyond salvage. Some might tell you that you can polish the scratches out. Well, you can if it was a very fine scar but when you polish a scratch out you are degrading the curvature of the lens and therefore the focal point - and doubly so if the lens is a doublet! If I were you I would deduct the price of a No42 or 53 telescope from the price. You've then got a spare OG lens to use in your No32. Not exactly the same I hasten to add (oh no they're not.....) but as a starting point you could then have the telescope optically re set-up and be left with some valuable spares to boot.
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Legacy Member
With regards to using lense sets out of a no53 in a mk3. Were any/all of the no53s bloomed?
My no32 has 3 fractures around the edge of the occular lense, they are only around the outer edge but as it will eventually be restored and i just picked up a no53 it might worth while replacing the lenses at the same time.
The scope was converted to a no71 for tank use and painted silver but i have found traces of red and blue markings under the paint so i think it was bloomed and waterproofed.
Would fitting non bloomed lenses make a noticable difference to the quality of the sight picture?
Thanks.
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