Brass spacers in Petzvals' rear cells

[Steven Tribe]

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.