Will Lynda ever finish hers?
by Lynda Hall
Thanks to Lonnie, I’ve been inspired to finish the mirror and scope I started about 6 years ago at Cary Chleborad’s telescope workshops. I’m almost finished making my own mirror and am ready to start designing the scope itself.
For those interested in the mirror-making process, I started with a beautiful 12.5” blank given to me by Forrest Lockhart. Cary gave me a 6” steel ring weighing at least 5 lbs and I went to work with the 80 grit aluminum oxide. You really have to put some weight into the rough grinding unless you want to spend a year doing it. It’s the hard part. I got in 3 hours at a workshop and let others have a go at it for about an hour. To measure the amount of glass I needed to grind out to form the required lens curvature for an f5.5, I laid a flat strip of (unbending) metal over the center of the glass and measured the gap between it (which represents the top of the glass when I started) and the new ground out bottom of the center of the mirror. This distance is known as sagitta and the sagitta I required was about 1/7”, or more conveniently– a nickel and a penny stacked together.
I still needed a lot more grinding. So, the next day I worked at home for 8 hours until I could get the nickel and the penny under the “flat”. Ow. I didn’t work on the mirror again for a week. One more hour of grinding with silicon carbide and the rough grinding was done.
The fine grinding requires much less force and went much more smoothly through the #120 grit (3 hrs); 40 micron (2 hrs); 15 micron (1 hr) and finally 3 micron (1 hr) sessions. At last, the mirror was ready for polishing with a cerium oxide slurry and three hours later, I was ready for figuring.
Unfortunately, I ended up with a down-turned edge from too long a stroke somewhere in the process, and I am currently working on correcting that.
Meanwhile, back to work on the scope design. Like Lonnie, I found the boxy traditional Dobs a bit, well– ugly and boxy. I’m also looking for something lightweight as I am a somewhat compact person. I chose the f5.5 ratio for the mirror as it provides the brightness and field of view needed to look at nebulae and galaxies without having to worry too much about coma correction. It also provides a nice height for the eyepiece. Lonnie’s design is very similar to what I had in mind and I think I’ll use his as a basis for my own. Stay tuned to see how well I meet Lonnie’s ATM challenge as I build my own light bucket.