Opening the lens cell on an old brass telescope

[jnb]

I have acquired an old brass telescope; age unknown, manufacturer unknown. It's a 9cm refractor, approximately F12 - F15 (condition is too poor to be certain as yet). The objective is filthy, inside and out so I need to open up the lens cell to extract, clean and reinstall the lens.

A bit of context. This came from someone who had the box and tripod sitting in a garage for years and they know nothing of it's provenance. The box for the scope is rotten and rusted. The scope itself is corroded (not just patina but actual corrosion), the eyepieces are in reasonable condition but the objective is close to opaque. I have cleaned off the front surface of the lens but most of the grime seems to be on the inner surface, confirmed with a checking the scope from the inside with an endoscope. Looking at the inside with the endoscope showed no obvious joints where it should disassemble.

Stripping and cleaning the focus with a wire wheel on a dremel and some patience has the mechanics moving again but the lens cell remains the problem.

A couple of images to show what I have. The lens cell as is the way of these things has multiple grooves and ridges some of which may be joints most of which are probably decorative.

There is a knurled ridge around the front of the scope at the front of the very first ring which suggests it may unscrew but if that has been sitting there for decades it may be difficult to remove and as the first ring is only a few mm deep there's not a lot to grab hold of. I have cleaned it a little (and yes I know about people's opinions on whether you should or shouldn't clean old telescopes but I don't know how old this is and without doing a little cleaning I can't see where I might get into this.)

BTW that image is the cleaned condition. Yes that is the "clean" lens!

 

So how might I open that lens cell? Suprisingly enough with a lens in there I don't want to put it in a vice and twist 😉

 

Edited June 16, 2019 by jnb
fixed typos

[Gina]

I can only guess as I'm not an optical expert - just know a bit of engineering.  Penetrating oil is one possibility and/or a bit of gentle heating.  Think I'll leave it to someone who knows a bit more about lenses than me though...  Can you see/tell how many elements the objective has?

[Bizibilder]

Yes - try some WD40 or similar to try and get into the joints.  Then try using something like this rubber jam-jar opener (less than £2 on the bay of fleas):

You may be able to get things moving.  If not then try a little heat from a hair dryer (or hot air paint stripper gun but be careful!)

When all else fails brute force may be required  

[Ships and Stars]

If it's threaded on, try wrapping an old leather belt for trousers around it by simply running the belt through the buckle until it's drawn down tight around the tube, then wrap the remainder around your fist and unscrew it. This is an old trick to remove stubborn car oil filters when a strap wrench isn't available. It does work. PS look up strap wrench online and basically you are doing the same thing with a belt. Good luck! Cool looking old scope.

[Gina]

I've often used a strap wrench to undo tight things.  Very useful tool.

[Carbon Brush]

I have successfully used an ultrasonic cleaner, intened for jewellery and other small items to free stubborn parts.
In this case you would either hold the tube in the tank, or arrange to suspend it.
As the cleaning fluid is water, and may alcohol or soap, if you choose to add this, there is little risk to the item.

I'm not saying it would unscrew itself (Dr Who sonic screwdriver style). But it does sometimes make things less tight.
It might allow you to tell the difference between decorative grooves and joints by getting rid of grime.

Good luck, David.
 

[Peter Drew]

I would expect the knurled front ring to be the lens retaining item. As already posted, a penetrating medium such as Plus-gas or WD40 should help to free it. I would be wary of applying much in the way of heat, old glass tends to be a bit fragile. Rubber domestic gloves provide a good grip and worth trying before employing a more drastic approach. The objective is most likely to be a doublet considering its vintage.  Once you are able to remove the objective be sure to mark the radial orientation of the optical components as well as the original order of assembly. 😀

Edited June 16, 2019 by Peter Drew
typo

[Gina]

Including which way round the lenses are - this is not always obvious.

[philj]

The ring is the objective retaining ring. I have restored many of these scopes and never had a seized one yet despite the ages. Just put the palm of your hand across the front of the cell and press then unscrew whilst holding the tube with the other hand. If it doesnt unscrew due to age or muck use a little release oil as Peter suggested.

 

Dont use heat, thats the best way to crack your objective.

Some better manufacturers put orientation marks on the element either pencil or knicks ground in to the element edges but dont bank on it.

I have never yet come across a lens set in these old scopes that hasnt been messed with by someone in its history so be prepared to do some testing and adjusting when you put it back together.

 

Edited June 17, 2019 by philj