September 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - September 1999 Issue

The Telescope Nut
by Jeff Baldwin

The 40 Inch f/3.6 Project: Part I

Here is the first installment of the 40" project.

At this time, the mirror cell, spider and secondary holder have been manufactured by Glen Youman. Let's start with the secondary holder.

The most difficult part of making the secondary holder on this scope is that the secondary mirror is huge: 7" wide by 10" long by 1" thick. The holder not only has to grip the mirror without any optical distortion, it has to carry a lot of weight.

Glen whipped up an aluminum holder that operates like a mirror cell, spring loaded pull bolts with wing nut adjustment.

The plate that is rigid to the bolt that holds it onto the spider is opposed with springs that are placed against another plate. This plate has a cylinder welded to it, and welded to that cylinder is an oval flat plate to which the mirror will be RTV'd. (RTV is Room Temperature Vulcanization, otherwise known as silicon.) There will also be a safety wire holding onto it in case of RTV failure. This protects the secondary mirror, the primary mirror and the observer in case of failure. All components are made of aluminum. The whole assembly was then powder-coated black.

The spider is huge. It spans 41.5" of optical light path and is very wide for rigidity. Each arm has two female thread sections to accept the bolts that connect it to the focusing cage poles. The pipe in the center has grooves machined in it where the spider arms were stuck prior to welding. Again, the whole assembly was powder-coated.

The mirror cell, also known as the tailgate, is enormous. It is a framework of flat plate and square tubing that will hold the weight of the mirror and distribute it into the mirror box. It is a fairly typical Kriege style framework, three collimating bolts, 27 point flotation with well-measured triangles, three fans for cooling the glass and light path, bolts for the strap that hangs the mirror, and three locator cylinders made of Delran.

We also finally decided on our altitude bearing parameters. They will be 60" in diameter, half-moon shaped, spoked design, and will be layers of aluminum and steel. No ebony star on these, they will have trucks or roller bearings on three points and a stepper motor on the fourth point.

I am fortunate to have Glen doing this work for me. I can't cut a straight line or a right angle, and I swear that each part of this is exceptionally beautiful and of professional quality. I hope the optics will meet the same specifications. He'll kill me if I sluff them up after making the telescope to such high precision.

The glass is not as flat as I thought it was when I received it. I measured its thickness and diameter and was a little surprised at how round the back was. It is about 1/4" convex, but only in one direction, otherwise known as cylinder, which means that I will have to remove about 100 cubic inches of glass just to make the back flat. The front is very flat, and most folks would have just said to make that side the back. The edges of the bow side has chips in it, so I chose the flat side to be the front. The front will have to have 436 cubic inches of glass removed. Wow, between 500 and 600 cubic inches of glass being removed from a mirror blank. That shouldn't take me more than about a million years.

Anybody just starting on their first mirror might get afraid reading this string of articles. I have a feeling that it will get fairly gruesome before it gets better. However, with patience, hope, forward progress, and enjoyment of the project, this telescope will eventually become somewhat of a landmark. At this time there are only a handful of 1 meter telescopes in the hands of amateurs. By the time it is finished there may be dozens, who knows.

For now it is very exciting, and seeing it begin to develop makes me realize that it really is happening. By the summer or fall of 2001, we'll be looking through a light bucket that can easily grasp 18^th magnitude. That will make Pluto appear as a 2^nd magnitude dot. We might be able to see the jet in M87. We should easily split the lensed quasar in Ursa major. Imagine M13 nearly 3 times brighter than in Black. Moons of Uranus. Moons of Mars . . .

OK, I'm getting ahead of myself.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.