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March 30, 2008 - James Mulherin

In June of 07 we started a project to build a 48" Dobsonian for Jimi Lowrey in Ft. Davis TX. Just this week the scope passed some important performance tests at our shop in Iowa City.

The views from the shop aren't much to brag about. The optics performed very well through essentially horrible seeing conditions and worse light pollution. Fainter stars focused to nice pin points but brighter stars boiled quite a bit. Of course the primary had very little time to cool so mirror currents were a significant factor. On the night we tested we had only a short window of clear sky at the beginning of the night so there wasn't much time for the optics to equalize.

Pointing and tracking were very good. Even on large slews, for example from Polaris to Rigel, the scope nailed it. We let the scope track on M42 (the Orion nebula) for 20 minutes and detected only a small drift. The e and f components in the Trapezium were very easy at 400X. Pointing and tracking will be even better after careful set up in Texas. On the whole, the telescope performed very well and I'm looking forward to setting it up under the excellent skies of southern Texas.

We have a bit more cosmetic work to do to complete the project (some paint and a light shroud) but this will be wrapped up in the next couple of weeks. We'll install the scope in April. We will present an article in the June issue of Astronomy Technology Today detailing the project from start to finish. In the mean time, the following is a brief pictorial history of the design and construction of the scope but first, a short list of the telescopes specifications and features.

The primary mirror was supplied by Yuri Petrunin at Telescope Engineering Company. It was made from Astro-Sitall by Lomo in Russia. The Physical diameter is 48.875" and it is 5" thick. The focal ratio is F/4 and it weighs a little over 700 pounds. The secondary mirror an 8" minor axis flat made of fused quartz by Howard Johnson Optical Labs.

Pointing and tracking are provided by Gary Meyers at RXDesign. We used components from his standard ServoCat system with some custom made components to interface the ServoCat drives to our friction drive system. Digital telescope control is provided by Wild Card Innovations Argo Navis. Many large Dob owners would feel right at home using the 48" scope.

• http://www.stellarcat.com
• http://www.wildcard-innovations.com.au
• http://www.telescopengineering.com

The mechanical parts of the scope are constructed from steel and aluminum. The whole thing weighs about 3000 pounds. On to the photos. . .

This preliminary line drawing of the scope shows the scale of the thing next to a 6 foot tall person.

These rendered drawings show the completed design right before hand off from engineering to production.

Two rendered drawings showing some of the detail in the primary mirror cell. Notice that the primary mirror has holes cored in the back to lighten the mirror. Not shown in the drawing are aluminum pucks that are bonded to the back of the mirror. The pucks will be visible in one of the photos that follow.

Test assembling the secondary cage. Dave Pasley is holding the head-ring of a 16" scope for comparison.

Primary mirror cell parts on the CNC mill and ready for the anodizing shop

Producing an 80" diameter side bearing on the CNN milling machine. Standing on the machine is Toney Mulherin who runs the machine shop.

Toney Mulherin making the steel parts for the main support structure of the primary mirror cell

Making the round ground plat for the bottom of the mount. The Azimuth drive roller will turn against the edge of this plate to drive the scope.

Making the bottom plate of the rocker box. The annular ring on the bottom of the plate is a stainless steel track for three out-rigger bearings. Most of the scopes weight is carried on a tapered roller bearing in the center of this plate. The three out-riggers that run on the stainless track are for stability.

James Mulherin standing between the assembled primary cell support structure and side bearings. Dave Pasley rocks the bearings back so you can see the internal structure of the primary cell support. The bottom of this structure will be closed off for added strength.

The rocker box welded and ready for paint.

The bottom of the rocker box with the stainless bearing track installed.

The ground plate with center load bearing tapered roller, electrical contact rings and out-rigger bearings, ready to receive the rocker box.

The rocker box installed on top of the ground plate and the view from between the plates showing the center and out-rigger bearings.

The Alt rollers and drive shaft installed

Stainless steel bands applied to Alt bearing surfaces. Note the stop blocks to prevent Alt motion beyond zenith and below the horizon.

The primary cell and Alt bearings installed in the rocker box.

The primary mirror supports

The back of the primary mirror with aluminum pads bonded in place. Each of the primary mirror support points rests beneath one of these pads.

Installing the primary mirror

The primary mirror snug in its cell.

The Az drive and encoder

The Alt drive and encoder

The business end of the scope. The barbell weights will be removed after we add a 4" refractor and light shroud to the OTA.

Out in the parking lot ready for testing.