Protector scopes

[Mrs Vampire]

I am in the process of making protectorscopes for the Stuart.

Have done the pistol port ones and most of the two Forward door bits.

I have drawings and have cast up the prisms for both.

[Mrs Vampire]

other bits

[BCA]

A small version of the protectoscope (about 5" wide, 3/4" thick x about 3") was used the M5 Stuart, the M8/M20 armored car as well as the WW2 Canadian Lynx scout Car. These were made by the Standard Products Co (Thermo Plastics Division). They had other obscure uses too. They are a combination of a plexiglass core, thin mirrors and a black plastic protective shell outer. ........ Brian

[Mrs Vampire]

Hi Brian

M3A1 Stuart. I have been unable to source bits so I am making them .

[Ian Fawbert]

Hi Gina,

Might be abit late now, but did you see this post from Steve Greenberg on g503.com?

http://g503.com/forums/viewtopic.php?f=82&t=193757

Cheers,
Ian.

[Mike Cecil]

I see Steve G refers to trying to gluing them together, but the resulting optical qualities were not good. Resin glues tend to have a yellow colour, even when applied very thinly.

When I was training to be a geologist many many moons ago, we had to make our own thin-section rock slides for microscopy. The 'glue' was a thing called Canada Balsam which we used to glue the rock to the glass, then the cover slip to the rock and glass after grinding the face to the desired thickness. Optical qualities were not diminished. I wonder if that has an application here? It was what I was going to experiment with to make the glass vision blocks for the Dingo, but of course, relocating to the USA got in the way of that!

There are probably much more advanced mounting media these days, but microscopy could provide a type that is durable and does not diminish the optical properties of the glass.

Just a thought....

Mike

[Mrs Vampire]

I live and I learn.

Casting them costs about eight bucks a piece. The optical quality can be made near perfect if one is prepared to grind them back to a dead flat surface and polish them. I have done one and it has taken around two hours to get it right.

I am using crystal clear Polyester Styrene casting resin and Silicone casting mold.

I have abandoned that technique in favor of getting the local Perspex supplier to make me an acrylic extrusion that is optically perfect . Cost for enough to make six is sixty bucks. I only have to slice them into appropriate lengths with the little rebate at the end.

I opted for acrylic mirrors which I am assured can be bonded perfectly clear using acrylic glue'''the trick is to get the bubbles out as you glue it. I will experiment with that. The alternative is to bond then from the sides using silicone rubber.

The centre prism for the Fwd protector scopes has proven to be time consuming but straight forward. I have sliced a 15mm Acrylic sheer with a 25 deg bevel along one edge and the opposite angle on the other side. I used a Dremel tool routing tip and shaping bench to make the 1/8 x 1/4 rebate along the rear edge. works fine so long as overheating is avoided.

Perfect clarity with acrylic sheet is had by flattening and flame melting the surface....something I am happy to leave to an expert given the cost per unit.
Not counting my stuffing around I estimate around $15 a unit for the pistol port scopes and $60 for all the mirrors and windows front cover leather pads and bearing blocks for the front scopes including the direct vision side window.

[Mrs Vampire]

Have all the bits made now .

[BCA]

I recall from my work with rock thin sections that Canada Balsam was used because the index of refraction was the same as glass. Differences in the index of refraction between different layers of glass or other substances would cause optical problems.
I'm pretty sure that the M3 protectoscopes look the same as the ones for the M5 Stuart and M20/M8 armored cars but I don't have a Stuart M3 book to confirm: Gina: Do you have a part number from your M3 book.
Finally, it may be that much of the periscope effect is from internal reflection within the prism, not from mirrored surfaces.

[Mike Cecil]

Hi Brian,

Interesting, and you are probably dead-right, but I don't think, as a student, I ever really concerned myself with the 'why' because I detested the hours of hand-grinding rock sections with Al powder when I could have been sinking beers or waterskiing!! Especially as the professional slide-maker in the Dept had all this automatic section-making equipment - and here we were 'learning' to make slides the hard way. I think it was my metamorphics lecturer's method of showing us who was in charge .... no wonder I majored in sedimentology and fluvial dynamic systems!!

Mike

[Mrs Vampire]

Hi Brian

I suspect the pistol port protectorscopes in the M5 etc are the same. The drivers and gunners protectorscopes in the M5 etc are quite different to the Stuart Drivers and Bow gunners scopes.

The pistol port scopes I cast are optically perfect if I put in enough work to flatten them of and polish them . The issue is that I have no desire to put in the twelve to fourteen hours necessary to do to the six pistol port scopes when I can pay sixty dollars and get acrylic sheet professionally cut and polished.

The drivers and gunners periscopes on the M5 etc are all standard 346 https://www.marstar.ca/dynamic/produ...roductid=82590

of which the Stuart had One up until the late production M3A1s which had two in the turret .

The pistol port Protectorscopes are quite different and so far as I can find unavailable.

The M3 turret had a direct vision "peep" hole system that consisted of a thick piece of flat glass. The later M3 hybrid and M3A1 turrets had a periscope like device instead.


The driver and Bow Gunners Protectorscopes are made up of two mirrors and two windows all of which are separate components and separately replaceable .
I don't have part numbers for any of the components either for Pistol ports or front hatch scopes.

[BCA]

Gina; In your figure sheet 2 of 4, is item A, "glass". I am pretty sure this is an item that have NOS: mine are plexiglass, 5-7/8" long, 3/4" thick, maximum width 2-1/4" with the polished faces being 1-5/8" x 5-7/8". They are different than the glass pieces that you posted earlier in photographs. PM me for more info or with an email where I can send you a photo.

[Mrs Vampire]

Progress on the Pistol Port Protectorscopes.

I rate the clarity at around 85% . I can see through them but not well enough to read my computer screen. I can certainly recognize someone at a distance of twenty feet though.

the first two are pictures with mirrors attached the third the extrusion I had made . It all looks remarkably clear but viewing acrylic sheet coupled with the slight distortion of the two polished faces means a less than optically pure result.
It is good enough for a view port in non combat situations and the costs of getting an optically perfect scope are not worth it .

So that's my limit on these tricky gadgets.

[Mrs Vampire]

I have now assembled my prototype using super glue...

I was advised against this but I threw caution to the wind.

Clarity is improved to near perfect when the entire face of the mirror is covered and all air bubbles excluded.

The suggestion of Canada Balsum would also be a very good fix...but I am impatient and the only supply is from E-bay

There are also a number of UV activated lens glues available but I have no experience with them and they are expensive when I consider the number of mirrors I have to mount.

The issue it seems is the distortion caused by the gap between the mirror and the prism. I worked it out and light has to penetrate fourteen surfaces from object to eye....lots of opportunities for distortion.

[Mrs Vampire]

Protectorscope perfection

Using Acrilifix glue to hold the acrylic mirrors onto the shaped acrylic sheet I have achieved perfect clarity.

My protectorscopes are absolutely clear and usable as original....very very satisfying. Not much trouble to make sure all the air bubbles are squeezed out and the glue goes right to the edge.

This project is done

[Mrs Vampire]

Steve asked me if

"using the "acrylafix " to bond mirrors onto cast resin prisms would produce the same results as bonding mirrors to the acrylic sheet extrusion I had made?"

The answer is definitely yes!!

I bonded two acrylic mirrors to the best of the cast resin prisms I had made. It instantly improved the optics to usable clarity. The only reason they were not perfectly clear was that I had abandoned the cast prisms before I had ground the surfaces to dead flat.

Had I flattened them I am sure I would have produced the same results I have achieved with the extrusion I purchased.

I am unsure how effective this would be if I used glass mirrors but have a very strong feeling that if the "acrylifix" would bond to glass the results would be the same.

Brian also recommended Canada Balsum . I have a similar feeling about that as a bonding medium . That is if it will bond perfectly to both the mirror and the prism "whatever they are made from, then a perfectly optically clear scope will result. My only reason for not using Canada Balsum was that It would take a while to get it to me from an e-bay supplier.

I think if anyone was undertaking scope building then an experimental cast up and bonding would qualify the mixture of products selected.

What I am able to say is that using acrylic sheet , acrylic mirrors and acrylifix produces very clear scopes and that if clear casting resins are used the same results are obtainable so long as all surfaces are sanded/ground dead flat.

[Mike Cecil]

Good job, Gina! Well done.

I admire your skill and your perseverance in the pursuit of excellence.

Mike

[Mrs Vampire]

Thanks Mike

[Douglas Greville]

Gina

The ongoing problem with the prisms always appears to be the glue
either shrinking or yellowing.

This is something that has plagued US military periscopes and episcopes and
still does. I have seen NOS ones still sealed in their hermetic packets that the glue had started debonding or/and yellowing. Most seem to go no more than 2 years out of the wrapper before they go yellow.

However, I note that I have seen Churchill, Ferret, Saracen and Centurion scopes that were as perfect optically as the day they were made. Most had been in a vehicle or sitting unwrapped on a shelf all that time. So whatever it is that the British are doing and have always done works.
I would like to find out what it is?

The ultimate test of your acrylic glue will be how it withstands normal daylight. This is also the reason why the normal UV setting glues cannot be used for optics. They are fine for home craft work for something that spends its life indoors.

Regards
Doug

Originally Posted by Gina Vampire

What I am able to say is that using acrylic sheet , acrylic mirrors and acrylifix produces very clear scopes and that if clear casting resins are used the same results are obtainable so long as all surfaces are sanded/ground dead flat.