Dobsonian TelescopeProject Goal: I am making a 6" f/8 Dobsonian telescope.
Pictures: Available in the Gallery
July 1, 2010I have purchased a 6" mirror kit (complete) from ATMoB. I am planning to build a 6" f/8 equatorial reflector as per Richard Berry's plans. I figure that it will be best for my first scope, where I have no skills and know that I don't know what I'm doing, to start with someone else's known-good plans. I have started to put a bevel on one oface of the tool. Three more sides to go.
July 2, 2010I have ground the bevel on both the obverse and reverse sides of the tool.
July 9, 2010I have put the bevel on both sides of the mirror now. My left thumb has been a little numb for the last several days. I think I was holding the glass at an odd angle initially, and applying enough force while chamfering to make holding it that way a strain.
July 15, 2010I have started to grind. I'm using #80 grit, full strokes. Counter to what I had thought, grinding in this manner makes both the mirror and tool concave initially, then flat. Once the surfaces are good and flat, then you shape one to be concave and the other convex.
July 22, 2010I learned that I was told to start with full strokes because my mirror is small enough for that to work. My mirror continued to develop a concave shape, while the tool is now becoming convex. Tonight's final measurements revealed a sagitta of -0.00665 at the center of the mirror, and -0.00465 toward the edge, while the tool was at +0.00225 at the center and +0.00375 at the edge. To achieve f/8, I'll need a sagitta of -0.01775 in the mirror, so I'm 1/3 done.
Grinding Schedule for a 6" Mirror
August 18, 2011
After 52 wets with #320 grit, I finally got rid of all my #220 pits. The master optician overseeing mirror grinding examined my mirror and told me that, now that all those #220 pits were gone, the leftover #80 pits were really plainly visible. This meant that I would need to start over from #120.
The above table does not accurately reflect the process we use in the club, at least from what I've heard. Our process goes: #80, #120, #220, #320, 30µ, 12µ, and if we're feeling really daring, 5µ. The risk at 12µ - and even more so at 5µ - is that the two pieces of glass will seize up. If this happens, they are to be placed in the freezer until frozen. Then you get out a wooden mallet and a pillow, and give them a nice, firm whack on the side. Hopefully one of them falls off, onto the pillow, and neither one is damaged. It is best to avoid needing to do this at all, so some people proceed straight from 12µ (or 30µ) to polishing, and just polish a while longer. Also, the times and wets are more targeted towards people who know what they're doing.
Progress since 8/18/2011
March 27, 2013I am advised that it's taking me so long not just because I haven't had all that much time to put toward mirror grinding, but also because I'm doing it wrong. What I thought was a 1/3 stroke is instead a 1/6 stroke. Thus, regardless of how hard I'm leaning on the glass, the grit is not really moving enough to take out the pits.
December 18, 2014
Back in May of this year, I was given the go-ahead to move on to 30µ grit - not because all the #220 pits were gone, but because there were still a bunch of them all over the place, and we couldn't figure out why. It felt like a hollow sort of progress, to be sure.
Today, we figured out what was going on: I had been putting my whole weight into grinding, even with the finer grits. The correct approach is to decrease the amount of pressure you apply as you move to finer grits. When you get to polishing, the weight of the glass by itself is sufficient to do the job.
I was given approval tonight to move on to 12µ grit despite only putting in 14 wets with 30µ (versus 62 with #320 grit), because all my #220 pits are now gone. It's very exciting to feel that progress is being made.
March 5, 2015
After only 10 wets with 12µ, I am told that I am definitely ready to move on to 5µ!
May 28, 2015
After only 10 wets with 5µ, I am told that I am definitely ready to move on to polishing!
June 18, 2015
Poured a pitch lap! I'm told that cerium oxide is rather expensive at the moment and has availability issues, so I'll be using barnesite instead.
October 29, 2015
I didn't actually polish tonight, but I did spend some time working on my mirror. I'm told it's very important to make sure that the pitch does not overhang the edge of the mirror. If it does, I'll get a turned-down edge, which I'm told takes forever and a day to fix. I spent about 40 minutes trimming the pitch, and need to spend a little more time.
We also popped my mirror into the test chamber for the first time. My focal length is ~103 inches, so my tube will need to be at least 51.5 inches long - we'll fine-tune that number as I get closer to done. Ed said that he could see zones all over the place and a turned-up edge (which isn't as bad as a turned-down edge). Phil said that all of this was simply because it's too early to be testing with this mirror, and further polishing (I've done ~1.25 hours so far) will smooth it out.
January 24, 2016
I think I've done about 3 hours or so of polishing so far. I haven't been keeping track very well, on the theory that I'm not done until I'm done.