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As anyone heard of or done any mirror grinding for a primary mirror on a telescope?
I just got into astronomy and would like to see if I can use my routers to grind mirrors! I would love to make a enormous 24"+ Newtonian reflector!
[url]Http://www.glenspeymillworks.com[/url] *Techno LC4896 - 2.2Kw Water Cooled Spindle | *Moving Table Mill from an Omis 3 CMM, 500Lb granite base, Hitachi router, Mach3
Hi, I build telescopes. There are particular machines for grinding mirrors, most likely your router won't do the job. Many also hand grind their mirrors although that is most definitely a lot of work but you'll get excellent results.Originally Posted by Pplug
I wouldn't try a 24" first unless you've had some practice. Here's some links to get you going...
Mirror grinding
Making a Mirror.
Mirror Grinding Machine
Grinding-Polishing-Figuring-Testing Mirrors
Stellafane ATM Main Page
...and this is by far the best book for making large aperture scopes, you'll definitely want to have it...
The Dobsonian Telescope - Kriege and Berry
Obsession Telescopes / Publications
I would recommend starting with a small mirror to learn the techniques. You can hog out a mirror pretty fact. I would not use a router to hog out the mirror, you WILL need water cooling and that does not get along with your electric router at all. You would also was a respirator to keep that glass out of your lungs when power hogging, or worry about silicosis - nasty stuff. I used to scrape out a few inches of ground glass from my water tub every week, I would not want that in my lungs.
I have hogged out perhaps 400-500 mirrors from 3" to 24", and taught about 150 people to build their own complete Dobsonian telescope. The largest mirror a student built successfully as a first attempt was a 16" mirror, the larger ones, I ended up finishing for them, and that was attending class every week or two. Yes, there have been larger scopes built on a first attempt, but I recommend a 10 or 12" mirror as the sweet spot for a first mirror. Smaller than this and you put in as much work and time, but larger mirrors take exponentially more time and effort. A good time estimate would be 10 hours per inch of mirror diameter - just working on the mirror. Yes experienced workers can do this much faster like the 24" mirror John Dobsom built in 24 hours straight. Mirror making is cave mans work. You work like heck, sweat like heck and sleep like heck. It is amazing that you can take your two hands and create a surface that is so accurate that it would be measured in fractions of a wavelength of light.
To hog out your mirror SAFELY, get a bag of 60 or 80 grit sand blasting silicon carbide, get that book referenced above, and follow their instructions. You would be surprised how fast it goes. Due to selling curve generated mirrors, I designed and built a diamond curve generation machine using a 14" diameter diamond cup wheel. Thatnwas for production work, i was generating as many as 10 mirrors a night. But my first 12" mirror I hogged out in about 6 hours using the techniques in the Kreig and Berry book referenced above. (note, we shut down the business due to health issues, I no longer diamond generate mirrors)
Anyway, I am rambling now. If you have any specific questions, just ask.
John Z
It is a shame that the Hubble mirror makers (Perkin Elmer ?) didn't read the book! Another demonstration that 'precise' and 'accurate' are not the same thing.
I have always wanted to try a mirror and build a nice Palomar-type strut-framed 'scope around it. I will look for the book.
Oh boy! Thanks for all the advice! The reason I want to go big is because I already own a 12" dobsonian. Anything 12" or less would be a waste of time and money. Are there any sources for cheap primary's larger than 12"? Everything I see costs many times more than my whole scope and that's just for the primary!
[url]Http://www.glenspeymillworks.com[/url] *Techno LC4896 - 2.2Kw Water Cooled Spindle | *Moving Table Mill from an Omis 3 CMM, 500Lb granite base, Hitachi router, Mach3
That's just the way it is, the primary IS going to be your biggest cost especially if you want something that's not going to give you issues when viewing. If you get a cheap primary, you're also wasting money if your viewing is crappy, which is exactly what will happen. As well, many companies that make mirrors don't make them that large, usually the largest is 16" or 18" - anything over 18" is usually considered commercial grade.
I get my primaries from Steve Swayze or Carl Zambuto for the very best in quality...
Swayze Optical! - Home Page
Zambuto Optical Company - Mirrors
That being said, there are some very good alternatives, but the prices for large aperture are still high...
Galaxy Optics
Company Seven | AstroSystems, Inc. Products Prices
Another option is to buy used, this is the best site for telescope classified...
Used Telescopes, Binoculars & Cameras | Astronomy Classifieds
Again, if you want to go big on the scope, you're going to have sink some serious cash into a primary mirror.
You may want to consider something a little smaller. Large aperture scopes over 16" will require step ladders or platforms in order to view through the eyepieces. Something as large as a 24" will require a tall ladder that you'll have to go up and down continuously. This all depends on the focal length of the mirror, of course. Shorter focal length will mean a shorter scope.
The zenith for a 25" f/5 will be 10 feet while a f/3 will be 7 feet, for example. Smaller focal lengths are more difficult to keep collimated and are prone to viewing issues like coma.
If you can afford an 18" mirror, that is a very nice size for most purposes, does not require a large ladder and isn't difficult to transport.
Of course, I would never discourage anyone from building a 24" "yard cannon" because that is exactly what I want to do, too. Justifying thousands of dollars for a primary is something your lawyer will take into consideration when the wife files for divorce.
John from Johns Telescopes? If so, nice to hear from you again, sir. You gave me some excellent advice back in 2003 when I was doing my first scope build with a 10" Swayze mirror.
Sorry you had to close down the business. Hope all is well.
There is a significant jump from a 12" to an 18" primary mirror. Also, there is a significant advantage to having great optics. The absolute best viewing I have seen was from an excellent 12.5" f3.8 student built mirror. Awesome views and you could use the scope sitting in a chair which helped enjoy the view. The brightest images were with a 32" primary, but the scope was extremely cumbersome to transport or use. If you have a club nearby, search them out. You cannot beat watching a skilled hand, or have them watch you push the glass to provide needed guidance. It is especially helpful watching someone perform a Ronchi or Foucault test, or even a more advanced interferogram. They may even have the ability to perform a two pass test using an optical flat. Then when you are in the final figuring phase (which WILL make or break your mirror) you will want their extra advice. Finally, plan on spending a year or more learning all of this if you will be starting out with an 18 or 24" mirror as your first mirror. And do not forget that you can put that unfinished and uncoated mirror in a scope to try it out, perform a star test, or view the moon comfortably without a moon filter.
...
Yes, I am that John. It makes me happy to hear that I was able to provide assistance back then. Yes we had to shut down. I was working full time, plus going to school, plus doing the business. I landed in the hospital twice and was headed there a third time, and the outlook did not look good. Something had to go, so we shut down, and I stopped working on a second degree too. Glad I did since my career took off after that since I was able to concentrate at work. Now it is just a hobby.
I have a 24" mirror I am working on. But the Pyrex blank was nearly $2,000. However, if you can FIND one, it would be significantly more expensive now since Pyrex is no longer manufactured. I need to get back to work on that one, especially since I am half way through the final figuring, and it has sat in the optics shop for two years now un-touched.
Hope you finish the mirror, John. Two years is a long time for a mirror to sit idle when it could be bringing you fantastic views.
I've still got a couple of sheets of Ebony Star and read that Wilsonart isn't making the stuff anymore, and also still have a whack of PTFE. I was considering making the bearing assembly for a cnc with these materials but have not yet come up with a design. Any thoughts on whether this would work as well as it does for telescope bearing assemblies?
The teflon on ebony star bearing surface works best when you have something like 30 lbs / square inch of bearing surface. Yes it does have great smooth motion without any "Sticktion" but I would be concerned about the accuracy of using this as a bearing surface on a CNC machine. Supporting a dobsonian style telescope, it is great ... But on a machine ...Hope you finish the mirror, John. Two years is a long time for a mirror to sit idle when it could be bringing you fantastic views.
I've still got a couple of sheets of Ebony Star and read that Wilsonart isn't making the stuff anymore, and also still have a whack of PTFE. I was considering making the bearing assembly for a cnc with these materials but have not yet come up with a design. Any thoughts on whether this would work as well as it does for telescope bearing assemblies?
But hey, try it and let everyone here on the Zone know how it works out for you.
Thanks - John Z
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