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16 inch Dob project under way

Daniel-K

By Daniel-K
in DIY Astronomer

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FraserClarke

FraserClarke

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any one know's if we can use the glass tool for the cerium oxide stage or do we have to make a pitch tool?

You'll need to make a pitch tool for the polishing. This can be poured on the existing glass tool if you want.

I'd also think you probably need to spend longer on each of the grit stages to make sure you've got rid of all the pits from the previous stage. What usually happens is that the first 20 min makes it *look* like you've got all the bigger pits by covering them with smaller pits -- but then when you go to even finer stages, the big pits appear again. Very frustrating...

I'd expect to spend at least an hour on each grit stage, if not longer.

Looks like you're making rapid progress though :)

Edit: Oh, and make sure that you keep a bevel on the edge of the mirror (and tool) to reduce the risk of chipping. It looks from your pictures that it's all but gone.... Best to regenerate a good bevel now than have the risk of having to do it nearer polishing...

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Daniel-K

Daniel-K

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ok so after ringing around i have found pitch is like frigging gold dust! beacon hill no long sells kits and orion optics just said no. the i rang a place called Washington mills in manchester didnt have a clue. finaly abit of luck with glavo optics just waiting for a phone call been trying to source pitch now for 3 months!!!

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Daniel-K

Daniel-K

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Have you done any thing else besides work on the mirrors????? do you have a stile in mid?

well barry wants to try and make it into a 16" imaging scope!!! but if that fails we will make into a truss dob. im just eager to get the mirror done and got it coated and then the fun begins putting it all together!

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Glasspusher

Glasspusher

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Nice to hear that you are making progress Danny, I was wondering what had happened. Pitch is difficult to get hold of these days and the price of cerium oxide is, wait for it, astronomical! What grade pitch did you get? Important that it is not too hard or else scratches might result. How are you planning to test the mirror? probably best to give testing some serious thought before it is time to start doing it.

John

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Daniel-K

Daniel-K

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WOW it's been awhile since i posted anything here, thats down to the fact of not having my " tool room" garage free of any space but the project is now back to life first thing we need to do is go back down 2 grits as the mirror has been stored it has got some fine scratch's on the surface nothing to worry about we have the pitch (2LB of medium burgundy ) and cerium oxide and the ronci and foucolt tester waiting to go for the figuring so hopefully in the next few weeks we mite be getting somewhere again. baz will probaly make the base up for the bottom of the scope while im at work, but its all very exciting again! me and baz are having a crack at it tonight will keep it updated

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Daniel-K

Daniel-K

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after a bad first attempt and nearly destroying the mirror i have managed to come up with this pitch lap today, it needs a warm press on the mirror to get the shape but im fairly pleased at this, on to the polishing next

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Daniel-K

Daniel-K

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so after 2 hours of polishing we could really tell the difference of the surface of the mirror it's starting to go clear. were using cerium oxide with a ratio 8:1 with water. the lap has help up well better than what i thought anyway. its tough going i read that it tales 10 hours to fully polish out a 16inch mirror by hand.

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Astrobits

Astrobits

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I assume that you are using powdered Cerium Oxide mixed into water as required. When you come to figuring, especially near the end, you will be doing much shorter wets and it is then that "Sleeks" start to show their presence. These look like short scratches and usually lead to panic as it is believed that contamination has crept in. I think ( but haven't got any proof) that these are actually caused by the cerium oxide powder. When the cerium oxide slurry ( it's ground up in a water base ) is dried it tends to form agglomerates ( a very common phenomenon ) which are sufficiently strongly bound together to cause the short sleeks before being ground down by the polishing action. It has been suggested that the polishing mix is made up and left for a day or so before being used as then most, if not all, of the agglomerates will have been broken down before use.

When I was doing most of my mirror making I purchased the slurry direct before it had been dried and I never had sleeks when using that. Unfortunately the firm that supplied me with the relatively small quantities that I needed has gone out of business and I do not know of any other place to get the slurry.

Nigel

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