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Thread: Brass spacers in Petzvals' rear cells

  1. #1

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    Brass spacers in Petzvals' rear cells

    The rear cells in Petzvals have two different basic designs. The first system is a dream. This is where the cell separates into two - each with its own lens. The distance between lenses is important for optical correctness (to some extent) and this is fixed by careful machining/thread cutting of the two parts. This system is quite necessary for the rear use of softness adjustment of the Dallmeyer patent Petzvals, but is also found in non Dallmeyer type petzvals. The extra refinement of the Dallmeyer rear cell is unfortunately mismatched by his burnishing seal of the front achromat - which is devil to service!

    The problem with most rear cells is that the lenses can be on the wrong position, both in sequence and direction of the assymetric faces. Other problems I have seen have been the presence of two brass spacers. Where these used to give a combined distance between the lenses or was one a distance piece to effectively shorten the distance between the front and rear cells?

    As I recently purchased a small Petzval with a very obvious spacer problem, I have given the spacer problem some thought. Old Petzvals often have lens edge damage at the rear. It is usually the internal face of the double convex outer lens which has, if you are lucky, lots of "flea bites". If you are unlucky, you may a few edge shears which intrude into the light path. The first, I think is due to an untightened cell, whilst the second is more likely a over-tightened cell.

    The brass distance piece seems to work well in average circumstances. When you consider the contact area between the ring and the curving glass surfaces there is a very minute "real contact ring" where pressure will be exerted. The ring is narrow and flat - with no allowance for the curvature of the lenses it touches. And all this takes place at the very edge of the lenses. It is certainly a method which must be unforgiving of any sort of mistake.

    The reason for these "deep" thoughts is this rear cell from a Hermagis! There a couple of shears on one of the rear lenses. Damage due to the very unusual substitute for the missing brass spacer. This is a steel wire shaped to fit, but with no real flatness and cut ends which are definitely oversize in diameter.
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  2. #2

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    Re: Brass spacers in Petzvals' rear cell

    Making a replacement thin brass ring which is exactly flush with the two lens edges is not for someone without a lathe or experience. There is also the problem of what thickness it should have, when the original has been lost and the replacement is likely to have been a makeshift "solution". I have looked at some other small Petzvals and have an idea of the right distance.

    I decided to make a thin flat brass ring - just narrow enough to hide behind the mounting stop. The brass is thin enough to distort a little to even out the pressure from the locking screw ring. To allow for some distance adjustment - spread the load on the more fragile outer double convex lens - I glued a rubber (synthetic) ring with more than double the brass ring thickness on this ring. A loose rubber ring would not be reliable in assembly. The X-section is shown below. I think this will :-

    - make the pressure on the inner concave surfaces less than the usual mounting with plain brass tube spacers.

    -provide a degree of distance adjustment to find the optically best distance (+/- 0.5mm).

    I have used faily hard rubber, but I can see that a more foamy material, with elasticity, could provide a wider range pf adjustment. I think it would still be necessary to have a "disc" on one side to ensure integrety. Anyone have any other ideas/solutions?
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  3. #3

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    Re: Brass spacers in Petzvals' rear cells

    In my lens mounting experience, brass rings are reliable but yes, do require an amount of precision that is not always available to the photographer/collector.

    I'm involved in a lot of complicated lens mounts in my professional life and can attest that brass, aluminum and steel are used for lens setting purposes. Personally, I wouldn't go for rubber as it decomposes rather quickly in the bigger scheme of things but I wouldn't go black Delrin out of the way.

    Determining the optimal distance requires in most cases tools better than the human eye to evaluate the difference. The distance impacts focal length in a minute matter, typically less than 0.5mm in the average small Petzval.

    Key is to distribute the stresses over the full circumference and not have a stress point that can lead to fracture.

    There's a good read online (pdf) with respect to mounting optical elements in tubes and the associated spacer and the stresses etc associated with them.

    https://wp.optics.arizona.edu/optome...f-lenses-1.pdf

    Cheers,

  4. #4

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    Re: Brass spacers in Petzvals' rear cells

    A good and relevant read indeed!

    Lots of information in note form.

    I like the comment about lens mountings have to stand up to from "3 G to 15 G" forces in normal transport and shipping!

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