Old Scope Meets New Mirror

In 2020, my interest in mirror-making was revitalized through classes offered by the Springfield Telescope Makers, hosting the annual Stellafane convention in Vermont. Since then, I’ve worked on two 8-inch mirrors, a 6-inch, another 8, and, most recently, a 12-inch F/5.6.

About 30 years ago, I learned to make mirrors with a group of AAA folks who called themselves the Optical Division. Former member Barry Levin was its leader, and we used Foucault testers and Ronchi gratings to get a 12.5-inch mirror to about a 7th wave. Then we made a truss tube Dobsonian mount to house the mirror, and the scope was always in use at Starfest and other public events. These days, that scope is in storage in Brooklyn.

Fast forward to early 2024, when I completed a 12-inch F/5.6 mirror. Because I had made the mirror with the help of the STMs in Vermont, I planned to enter it in the optical competition that’s part of the convention every year. But I quickly realized I didn’t have time to make a 12-inch telescope that would be ready by this year’s Stellafane convention (Aug. 1-4).

So after speaking with Bart Fried and Peter Lipschutz, I drove down to Brooklyn from my home in Upstate, NY, to sign out of storage the AAA’s 12.5-inch dob, which I had made with the club’s Optical Division more than three decades ago. Now, my task was to raise my mirror higher in the existing optical path so that it would come to focus on the old dob. This achieved, I’d be able to use the scope in the optical competition.

I decided to use two 1” x 1” posts attached to my mirror mount, screwed laterally into the rocker box. This would allow me to reach under the mirror and adjust it for collimation. And I would only make 4 screw holes in the body of the rocker box. I just had to find the right spot to place the mirror.

Doing the math, the original 12.5-inch F/6.3 mirror had a 78.75-inch focal length. My 12-inch F/5.6 is just shy of 65.25 inches in focal length. That means, if I placed my mirror 13.5 inches higher in the optical path, I should come to focus. But I discovered a problem: I didn’t have the original mirror! I had left it safely tucked away in storage, figuring there was no need for me to have it sitting around if it wasn’t going to be used. Without it, I could not tell where the original focal path started. So I had to measure from an assumed focal point, down the optical pathway, to a spot where I thought my mirror should rest. Thus began a series of trial-and-error approaches to get to the desired placement of my new mirror.

The first time I did it, the scope worked great with my 2-inch eyepieces but wouldn’t reach focus with my 1.25-inch eyepieces. The focal length was a little too far outside, demanding I racked the focuser out to its very end to get to focus. So I had to lower the mirror a few inches down the rocker box and try again. I thought 2.5 inches would suffice. Close but no cigar. You see, for optical judging at Stellafane, a star is racked outside of focus, into focus, and then inside of focus the same amount, to compare the star images for visual similarity. This is called star testing the mirror. It is very simple and extremely accurate if done with an eyepiece that yields 25 times magnification per inch of aperture. That means I had to use a 5.5-mm eyepiece. The second iteration of my mirror placement did not allow me to equally rack in the eyepiece. Finally, I found the Goldilocks spot. Just right for all my eyepieces, 2-inch and 1.25, and most importantly, the 5.5-mm would work for star testing, as well.

Next, because the weight of the mirror was shifted up the optical path, the scope was no longer balanced. It was top-heavy. I added two black metal drawer handles to the back of the scope’s rocker box, and from them, I hung diving weights, which I had found in a thrift shop. The scope was now balanced. I tested the scope at home one night before packing it up, and then on Thursday the 1st of August my wife Bonnie and I were on our way to the hill at Stellafane. I set it up the night before judging, and the scope performed like a champ. We had a wonderful sky, and I observed many deep-sky summer objects. Everyone who looked through my scope complimented the view. So with fingers crossed, I wrapped the scope up in a tarp and went to sleep.

The next morning it rained. It was announced that Friday night would be a washout for optical judging, but Saturday night was a possibility. My fingers still crossed; I held out hope for Saturday. But it wasn’t to be. For the first time in many attendees’ collective memory, there was no optical judging at Stellafane in 2024. So now I have an entire year to build a 12-inch F/5.6 telescope and return the former Optical Division’s scope to Brooklyn. I plan to make a low-profile lightweight scope so the observer will not have to use a step stool to reach the eyepiece.

I want to thank the AAA, particularly Peter Lipschutz for taking the time to oversee the storage and lending program. I encourage the membership to take advantage of the scopes we have accumulated over the years and get them out there for the public to look through.

Cheers everyone, and clear skies.

Michael O’Gara