Yes, my first mirror. I started off with a simple 6" f/8, which is the "easiest" to do, since there is no parabolizing, I figured this would be a great choice to cut my teeth with, if I found it enjoyable I'd move on to bigger mirrors, but this way I wouldn't be out an arm and a leg for an 18" blank and accompanying consumables if I failed. They say that even if you want to make your very own large mirror, say 18"+, that you will be able to do it much quicker by actually grinding first a 6", then a 12", then the 18" as opposed to starting out of the gate with an 18". Although I am not complete yet with the 6", I most whole heartedly agree with that sentiment :-)
Well, I'm through polishing and am at the point of testing for spherical, which is all that is needed for an f/8. Well, by gum, when I turned my back (must have been when I let Billy push a few "wets") I went and overshot the f/8 part, by more than just a little. I realized it at about 600 grit or so, so from then on I mainly stroked tool on top (TOT) to try to flatten the focal length out. Well, being very pleased with the new luster the surface was taking on I found it too irresistible to back up a few grades of grit to correct the curvature, so I kept going. I mean what's really the big deal? So it will be less optimized for planetary and lunar and more toward wide-field, a nice balance I think at f/6.91. So, I'm ok with that, I couldn't decide whether I wanted an f/8 or an f/4 anyway! Besides, it's much more sexy to tell someone that you have an f/6.91 focal length mirror rather than the bland "f/8", or "f/6". They would most certainly think "by God that guy knows his #$%, he even has it down to the hundredths!" It's truly amazing how quickly you can knock 6 1/2" off the focal length in a matter of a few mind-numbing hours of pushing glass. I'd just like to have it done so I can finish the rest of the scope for my son while he's still living at home. I still have a few years, but they are flying by let me tell you!
Right now I've been told that the surface is that of an oblate spheroid. Ouch. That sounds downright painful! "Hey Bob how are you doing?"... "Well Frank, last Friday the Doctor told me I've got an Oblate Spheroid in my rectum, how do you think I feel?! This thing HURTS!" (ouch, sorry I asked).
So, it could be worse, but not by much. The good news is that it's only a 6" mirror and fairly slow one at that, so the parabolizing that will need done is relatively minor. That and corrections will by very quick, only a few minutes to notice a real difference on such a small mirror (remember the talk about starting with a small mirror? You were paying attention weren't you?)
Anyway, I will post updates here regularly (hopefully), and am eagerly awaiting the day I can ship it off to be coated.
-Bill Mitchell
October 1, 2009
Bill,
ReplyDeleteAt this stage of grinding, how do you measure the f-ness?
How do you know it is shaped correctly?
How do you know that it is going to see well enough to spend the money on coating?
Then, where are you going to send this for coating?
tz
Hi Tony,
ReplyDeleteFirst off let me declare that I am only a beginner in this, I am learning as I go.
1) "f-ness" is easily measured in the foucault tester with a ronchi grating. I am using 133 lpi, when you're at the Radius of Curvature (ROC) there will only be one line (solid black) reflected off the mirror. That measurement gets you close enough at this point. See this link to the Stellafane site which has plenty of beginner advice, and is where I learned a great deal.
http://stellafane.org/tm/atm/index.html
There is a wealth of knowledge there.
2) Correct shape: Well, the repetitious act of stroking the glass with the tool, randomly rotating both the tool and the blank as you go will automatically create a spherical shape, provided you overhang properly and stroke correctly. From there the slower mirrors need no figuring so it's complete when you've polished it out enough. If faster then you figure using couder mask and aforementioned foucault tester and machinists micrometer. Carefully measuring where specific zones of the mirror come to focus and then data crunching, you can get a very accurate picture of your surface, and can make corrections to fix the errant zones to your liking/tolerance. You shoot for 1/100 wave but first timer here would be happy with 1/16 on this one. Next one will be bigger and much more accurate.
3) Spending money on coating: Well there are tests that you run to #1) make sure you're polished completely out, and 2) have the most accurate shape you are striving for. Yes, I have read that it is common for the excited beginner to send off a mirror that looks very polished to the naked eye, much like mine, but gets rejected by the coater saying it isn't worth the money to coat because of the rough surface. A very humbling scenario no doubt.
4) I haven't decided yet, I have used Spectrum Coatings in FL several times with no complaints. At the time I last used them they were the least expensive. I will cross that bridge when I get to it :-)
Thanks for the awesome questions!
-Bill