Zeiss C50/540 project

To my knowledge, this is the smallest Zeiss telescope objective ever made - the C50/540. It is a 50mm f/10.8 Fraunhofer-type achromat doublet lens made by Carl Zeiss in Jena Germany until about 1995. Some were sold in OTA's, but the majority were sold in a small ATM kit which included with the lens - .965" 25mm and 16mm Huygen eyepieces and an M44 .965" telescope eyepiece adapter (pictured below).

Getting started.......The first consideration was the OTA tube material. I had made several refractors in the past, using aluminum tube and this has always worked well. This time though, I wanted to try something different. After a couple of weeks investigating several different materials, I decided upon fiberglass. The fiberglass I chose was a dense fiberglass used in antenna masts. It has good characteristics when it comes to strength, rigidity, and thermal. And it can be machined. I found at the supplier, a large number of sizes and a few color choices - black, white, olive green and gray. I chose gray for the color and 2-1/2" O.D. / 2-1/4" I.D. for the size.

Next I shopped around for a focuser and purchased an all-metal 1.25" rack and pinion focuser that nearly fit the tube.

Nearly, but not quite. I had a small amount of machining to do......... there goes the warrantee!

Now it's a perfect fit.

Next the tube needed to be cut to length and to fit the focuser. For this I used a laser and a small piece of paper; adjusting the fit until the laser beam hit directly in the center of the paper, at the other end of the OTA. Once finished, an ultra-flat black paint was applied to the interior of the OTA

The odd cell housing, with no threads, posed a interesting problem. Normally I like to thread the cell to the OTA, however, in this case I needed to do something different. To insure the lens was set parallel to the OTA, I decided to shim it into the tube. First I machined a stop-ring with the same I.D. as the lens cell and the same O.D. as the tube's I.D. I then pressed it into the tube to where I wanted the lens to seat. I then shimmed the OTA with plastic shim of .040" (about 1mm).

This enabled the lens to fit snuggly into the OTA. It remains in place no matter how the OTA is held, yet removes easily with a pull on the cell.

A section of the tubing was cut for the dew shield. After flocking, the dew shield fit snuggly over the exposed section of the cell.

A satin-black was then applied to the dew shield.

I then drilled and screwed the OTA to the focuser and added some metal focusing wheels.

For mounting, I added a little padding to an old Unitron cradle. Here is a pic of the finished OTA with decal. The little Zeiss, waiting for first-light.

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