Primary Mirror grinding

vincentm

So one of my friends from my Astronomy club has given me a 6in mirror blank at our club mtg last week. I'd like to try my hand at making my own mirror. I didn't have time to stay and ask questions ( dropped in to return an astronomy lecture vid to our library). How do i go about doing so? I'd like to eventually use it in a dobsonian, for photography purposes. turbo-1 i'm looking at you bud, been awhile since i posted here, how've you been?

turbo-1

I've been around! You should start Googling on "ATM" "Amateur Telescope Making" for detailed information on how to hog out and figure that blank. Your biggest concern will be cleanliness and prevention of cross-contamination during figuring, since as you move to finer and finer grits, you have to do a LOT of work to remove marks made by the coarser grits. If you get onto some good ATM sites, you'll find tips on how to mount and support your blank, how to test the blank during the process, etc.

If you want to use a Dob for photography, you'll have to figure out how to tilt and drive your alt-az mount to track properly. You might want to haunt the astronomy club for a while and talk about your project. Chances are that there are some members who have upgraded their mounts and will have their old ones available for sale. A 6" Newtonian OTA is not all that heavy, so you might be able to find a suitable mount for reasonable $$$. Remember that as you figure your mirror, you'll be shooting for a focal-length appropriate to your goals. A long focal length will be easier to figure and polish accurately, but the OTA will be longer and will require a beefier mount. A short focal length is tougher to finish accurately, but will result in a more compact tube that can be mounted more easily.

Chronos

6" is a good starter size, big enough to be worth the effort and small enough to be manageable. I prefer aluminum oxide for rough grind. It takes twice as long as carborundum to hog out, but, you shouldn't have nearly the pitting problems - which will spare you many hours later on. Nowadays I wouldnt even bother with a flat blank, premade sphericals are cheap enough to be attractive. The Ingalls series on ATM is still the gold standard in my book. Turbo is right about mounts. Short f mirrors are more difficult to figure, but a smaller mount is the difference between closet and viewing time. My first scope was a 6" f10. You needed a pack animal to set up and it quivered like a frightened butterfly in a faint breeze. My next was an f4, which I could carry around in one hand and was rock solid.