January 2002 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - January 2002 Issue

The Telescope Nut
by Jeff Baldwin

Stress Figuring

A new way of figuring telescope mirrors is becoming popular amongst ATMers across the country. Actually, it may not be new, but it seems to be catching on more and more. It's called stress figuring, and it apparently works quite well. I don't know this through personal experience, but it seems logical, and people say it works.

I've seen this used mostly on small, slow mirrors, primarily due to the fact that small, slow mirrors require the least amount of correction from a sphere to a paraboloid.

The idea is to make an extremely perfect spherical mirror, no turned down edge, no astigmatism, super polished, super smooth. Then have it aluminized rather than parabolize it.

When you get it back from the coating company, and before you install it into the telescope, epoxy a hex nut to the center of the mirror's back. After the epoxy is cured and is strong, assemble a ring about the circumference of the back of the mirror. A plate, or a set of arms, can be suspended across the back contacting this ring, and a bolt slid through its center that will feed into the epoxied nut. When the bolt is turned it will apply force onto the mirror in a pulling direction. This will cause the center of the mirror to be pulled back slightly, deforming the sphere by making the center of the mirror more concave than the outer regions of the mirror.

This is exactly what correcting a sphere to a paraboloid does-it shortens the radius of curvature of the center of the mirror as compared to the outer regions of the mirror.

When the bolt is turned, one of three things can happen: the bolt will apply less than enough force to figure the mirror, just the right amount, or too much force. So, how do you figure out the right amount of force? You simply star test the telescope as you turn the bolt. If it is under-corrected, turn more; over-corrected, return it somewhat; and if it is just right, STOP. It's that simple (said the guy who has never actually done this!).

During an observing session, as the night progresses and the temperature changes, you may have to tweak the bolt to regain that perfect figure. This ought to be fairly easy. With a 6" f/8, the spherical mirror is so close to a paraboloid that you may not want to mess with this. However, at very high magnifications, you will be able to determine that the spherical mirror is under-corrected and that turning the bolt will slide you right into figure. If you have an 8" f/6, same focal length, faster optic, the difference ought to be striking. Obviously the thinner the glass the easier this might be to do.

I wonder with how large an aperture you can get away with this? One guy on the ATM Internet list said he was doing this with a 16" f/7, but I'd have to look through the scope to decide if it was a successful project or not. Anyway, if he's happy with it, it was successful, right?

This brings to mind another idea. Some off-axis paraboloids may be deformed to perfection by this method. Also, some off-axis paraboloids use toroidal secondary mirrors, which must be a true joy to make. Now the secondary mirrors can be made by bending the glass. How about the classical cassegrains with the hyperboloidal convex secondary mirrors? Make them spherical, then bend them into a hyperboloid.

It seems that those of us who are afraid to figure a mirror may have a way out. I don't plan on making more than one of these just to satisfy my curiosity. I really think the best method is to make the most accurate mirror you can, meaning that you should parabolize the mirror through figuring, rather than through stress, but it's just one more thing to know how to do. It's all fun, right?

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