20 June 2004

Design Discussions

I have enlisted the help of my good friend, Kevin Parker. Kevin is a gifted electronics engineer and has run his own handyman business in the past. He has built his own canoe from scratch and helped me rebuild part of my basement. Kevin has a lot of nice tools; tools I no longer own. So, we are going to build two identical telescopes based on my 1985 splitring design. We will use 10" f5.6 optics.

Kevin and I have taken an extended trip to visit my daughter (Kevin helped with the driving). While we were traveling, we discussed almost all the design criteria and worked out formulas for construction variables. These things have been integrated into two additional web pages listed below.

We will begin construction as I described 20 years ago: from the eyepiece holder outward. The first thing we need to do is properly mount the eyepiece holder. We chose a standard helical folder because it is simple and stable. We use Televue eyepieces that all have the same focal plane, so refocusing will be minimal.

We have decided to use 9-ply cabinet plywood because of its light weight and horizontal strength. The upper tube assembly will be a single plywood ring. The secondary mirror spider, eyepiece holder and truss tubes will all attach to this ring. By using a single ring, we hope to reduce the amount of weight at the upper-end of the optical assembly..

We will once again be building our own spider instead of buying a commercial one. I have had a lot of experience with these commercial spiders. They are generally designed around a generic concept that fits as many telescopes and secondaries to be commercially viable. As such, they aren't necessarily the most accurate or easy to use. The secondary mirror must be adjustable in 4 directions: along the optical axis (up and down); across the optical axis (side to side); Angular (tilt); and Rotation (left to right).

Commercial spiders put these four adjustments in the center where tiny screws must be tweeked. This tweeking plays one adjustment against the other in order to lock in the final setting. In practice, this process is difficult to perform and the settings aren't necessarily locked in; requiring additional adjustments. Because of our design (where the upper tube assembly is detachable), we will need to have adjustments that can be easily performed and locked. Our attempt at design will move all adjustments to the ends of the spider except rotation. I will cover this in detail as we create and assemble the spider.

The tube assembly will rotate in the mount, facilitating comfortable eyepiece placement. We will also have Surrier truss mounts above and below the balance point. The primary mirror will be mounted on a floating cell and glued with silicon adhesive. We have found a local surplus store that carries low RPM clock motors. Our final design and motor choice will depend on a convergence of the construction variables.

Both Kevin and I have already spent a considerable amount of time looking for local sources of worm gears without success. I finally found a great source of worm gears on line at Stock Drive Products / Sterling Instruments. I recommend visiting this site and ordering what you need rather than searching for days or weeks.

So tune in again soon. We are now building the upper ring assembly. Pictures, descriptions and discussions will follow.

Clear Skies!

Reconstruction Home Page
Construction Calculations
Original 1985 Telescope