12.5" F/12.5 Open-Tube Classical Cassegrain

If you've looked through my web pages describing the CUAS 16.25" F/12.5 Cassegrain work, you have probably seen it mentioned that I was working on my own 12.5" Cassegrain.  I made its secondary in parallel with the one for the club scope, and I refigured the primary after it was given to me by Mike Conron.

Click  here  to go to the article that I wrote about this telescope for the Cloudy Nights ATM contest.  (I didn't win anything.)

Here are links to pages with construction photos:
Mirror box and cell construction photos
Secondary holder and offset wire spider photos
Optical fabrication photos
Other assembly photos
 The birth of the idea for this scope came when I saw Dick Wessling's 13" Cass, mounted like a conventional dobsonian, and sitting on an equatorial platform.  Taking a cue from his scope, I also decided to divert the cone of light out the top of the mirror box (even though the mirror was perforated) with an elliptical flat mirror so that it would be at a more convenient viewing position.  The cored mirror allowed the diagonal (tertiary) to be supported by an assembly that protruded through the hole - no spider required.  Here's a photo of me (left) and Dick Wessling with our classical Cassegrain telescopes.

Two cassegrains and their owners

This is where our designs diverged.  I decided to use four 1.5" aluminum poles as a tube, and add diagonal bracing only if significant flexure was observed.  The poles have threaded inserts in each end and screw onto bolts that are mounted in the mirror box.  The secondary cage attaches with knobs which screw into the inserts.  The secondary cage is extremely light weight and features an offset wire spider to support the 4" secondary.  The wire is stainless steel, 0.015" thick.  To string the spider, the secondary holder was temporarily attached to a piece of wood that was attached to the wooden cage, holding the secondary holder in the proper position.  Then the wire was strung and cut, and the wood was removed.  The spider is extremely strong, and does not move laterally at all.  Once properly tentioned, it can be made to move toward or away from the primary mirror (along the optic axis), but only with significant pressure.

Side view 1 Here are few photos of the telescope.  Construction is mainly from 1/2" Baltic Birch plywood (half a sheet).  The mirror box and rocker box are joined with biscuits and glue, and no fasteners.  The primary mirror cell frame is made from 3/4" baltic birch, glued and screwed together.  It is a 4-point flotation cell.  The primary is 2" thick, and the secondary is 1" thick.  No ventilation fans are installed yet, but they will be in the future.  More will be added later.

The first picture shows a side view of the scope.  The poles are painted Rust-Oleum "Royal Blue".  I have not found reflections from the poles to be a problem yet.  The tertiary mirror acts as quite a good baffle, since it is not oversized.

Note the minimal rocker box, and the holes drilled out of the secondary cage to reduce weight.  This results in a very low center of gravity.  The focuser is a Moonlite single-speed crayford, anodized blue.

The side bearings are 1" thick, and are wrapped in 1/8" thick by 1" wide aluminum.  The focuser sits on a wooden pedestal that raises it up from the mirror box.

When disassembled, the secondary cage sits safely on top of the mirror box.

The last two photos here show another view of the scope from the top, and also looking "down the barrel", showing the offset wire spider and secondary holder.

Top view Front fiew

So far, the scope has performed well.  Planets show good detail, as do deep sky objects.  There were six stars in the Trapezium the first time I looked through it.  Scattered light due to spider diffraction is present, but in small amounts - less than with a conventional spider.

I'm still in the tweaking process, and I'll be working on a folding platform on which the equatorial platform will sit.  (I guess that's a plaform platform.)  This will raise the scope up to a convenient height for viewing while seated.  I'll add a page on the platform platform when it's done.

Update:  The "platform platform" is done.  It folds up for storage.  Here are a couple of pictures of the scope, set up at sunset on April 9th, 2004.  The clouds dissolved and we enjoyed good seeing in twilight.  The planets looked excellent at 400x and up.


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