In years past, a telescope with a mirror over eight inches or so was quite out of financial reach for many if not most amateur astronomers, including this one. Within the last few years, however, relatively inexpensive glass and Pyrex mirrors, many of excellent quality, have been available from China. Large telescope manufacturers such as Hardin, Meade, Celestron and others were able to offer instruments in the twelve inch range for prices under $1,200. Mirrors became available to Amateur Telescope Makers (ATM’s) at unheard of prices, making large instruments affordable to most if not all of them.
I had always wanted an instrument in the twelve inch range, and a couple of years ago, I found a good buy on a 12.5″ mirror from an outfit I found on EBAY called Hubble Optics. I paid less than $400 for a mirror of good quality, that would have cost almost $2,000 just a few years ago.
The first scope I built with my new mirror was a simple classic Dobsonian, with a solid tube, huge bearings and a large base. It worked sort of OK, but was very heavy, hard to transport, and when we decided to move to Mexico, it was obvious I would never be able to take it with us.
It wasn’t a bad scope, it tracked OK, was fairly well balanced but it was very heavy, and hard for me to move around let alone transport. I chose this design mainly because I already knew how to make it, it was easy to make, and it didn’t take too long to complete.
When Gethyn and I decided to put our house on the market, and try to move south, it was clear that I would have to build another telescope, one that was easier to transport. I thought about some of the new ultra-light designs, using just a couple of struts and some string, like this one, but for several reasons I settled on the well-known “Kriege” design. For those not familiar, Dave Kriege is the owner of Obsession Telescopes, producer of the finest large-aperture ‘scopes anywhere, and the co-author of what has become the “bible” of ATM, The Dobsonian Telescope.
I built the first telescope the way I did because I did not want to deal with the complexities of the Kriege design. In the simple Dobsonian design I built, there are a couple of critical dimensions, but nothing like the multiple truss design of the Obsession. I spent a couple of weeks re-reading Dave’s book, and planning the fabrication of the various components. There are some choices in Dave’s design, such things as cage components, truss connectors, mirror support system and other things, but this isn’t a how-to manual, suffice to say I designed around materials I had or could easily acquire, and tools I had.
Here it is, after three months work! It is, beyond a shadow of a doubt, the best scope I have ever had. It moves like a dream, is easy to use, only takes about ten minutes to set up, and takes down in just a little longer (it’s always dark when I stop, after all). Oh, by the way, that thing the ‘scope is sitting on is a “tracking platform” made by Roundtable Platforms. Dobsonian telescopes don’t follow the stars the way the modern, expensive scopes do, so something like my platform is required so the scope can automatically stay on an object for an hour. Otherwise, using a dobsonian means moving the ‘scope every few moments to keep an object in the field of view.
To the right is a close-up of the top of the ‘scope showing the eyepiece holder, Telrad optical finder and a 7X 50mm finder for dim deep space objects. In this shot you can also see the upper truss connections which are simply wooden nuts I made with 5/16th T-nuts and 5/16th bolts.
To the left is a shot looking down from the front showing the spider which holds the diagonal mirror, and the primary mirror down in the mirror box. If you don’t understand how a newtonian telescope works, take a look at this.
To the right is a close-up of the mirror box and rocker showing the split-blocks that hold the truss poles. They were the hardest part of the telescope to make, and necessated the purchase of an inexpensive drill press. The bearing arcs, and all of the other circles and arcs were cut with a router. One last picture, the bottom of the telescope below, with the fans that cool the mirror after a hot day in the shop.
That does it. It’s 9PM here in the east, and Jupiter will be up in a couple of hours. ‘Till then I have M13 and a whole host of other things to look at. I will be taking some pictures over the next few months and I hope to share those as well.