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Posts Made By: David Tarshis

April 25, 2004 07:51 AM Forum: Equipment Talk

Mirror grinding question

Posted By David Tarshis

I'm in the rough grinding process of my 12.5 (60 grit). I am one inch from the edge. When I check the f ratio I get about a f6.3-6.9. I am using 2 methods. Sun check I get 67 inches, which calculates to f6.3, when I do a sagitta check, I get about f6.9 [F = r²/(4D)], D=. 095 r=5.25 (I’m only calculating the part of the mirror that has been ground, approx 10.5) which comes out to f6.9. My question is, when I go to parabolize, does the f ratio change? I want about f7-f8. I was told to go the depth of a dime and a penny (.110). Do I keep grinding with the 60 with the MOT until I get the .110? If I do, I’m afraid the f ratio will be to fast. My next class is Tuesday night, I was wanting to keep up with the 6-10 inch mirrors that are done with the rough grinding. I elected to do some grinding outside the class. I don’t want to have to go back and correct any errors so I’m asking the experts here. I am also using a 10 inch tool. Don't know if that is going to make any difference. Thanks, DaveT

NOTE: I posted this yesterday in the telescope making forum, no responses yet, but I think I will just wait until I go back to class on Tuesday to see what the instructor has to say. Besides, I used the last of the 60 grit I had. But with all the mirror talk here lately, I would like to get an opinion from some of the experts here. Ed? Thanks again, DaveT

April 25, 2004 11:16 AM Forum: Equipment Talk


Posted By David Tarshis

Mark, just FYI, caps usually implies yelling. Plus it hurts the eyes to read. I have a 9.25, I'm happy with it. Clear skies, DaveT

April 30, 2004 12:42 PM Forum: Telescope Making

Need help, grinding 12.5

Posted By David Tarshis

My 12.5 mirror is ground to about 3/8 from the edge. That gives me about a 11.75 inch convex. The sun test measures about 67 inches which gives me a f/5.7. I want a f/7 or f/8. I am using a 10 inch tool. I have been grinding TOT and the convex is reaching the edge, but the f ratio is staying the same. What can I do to flatten the convex or lengthen the f ratio? Stroke? long, short? how far over the edge? Thanks, DaveT

May 3, 2004 02:55 PM Forum: Telescope Making

12.5 mirror grinding dilema

Posted By David Tarshis

Once again I have a question about my mirror. I was able to push it out to the edge. Thanks to those who answered, it helped. But now that I'm out to the edge, I'ts still only a f/6, I was hoping for a f/7, f/8, f/7.5 would be ideal. So here's my question: is it worth the extra grinding time for the f/7.5? or is the f/6 good enough? what would I gain having a f/7.5 vs f/6? (smaller secondary?), f/6 vs f/7.5 more portability?). Thanks, DaveT

One more note: I am up to grit 180, I found out that the f ratio doesnt change much at this grit. If the longer focal length is worth it, I dont mind going back a couple of grits. I also have a couple of pits I missed when I changed grits that I missed during inspection. Wouldn't mind getting rid of those also, probably being too picky at this point concerning the pits. Thanks again

May 8, 2004 09:31 AM Forum: Telescope Making

12.5 mirror cell vender list

Posted By David Tarshis

I have looked at UO, Parks, Starsplitter, Novak, and a couple of others. I would like some opions on these makers. It seems like UO is not popular. Let me know if I'm wrong. Please add to the list, thanks. Oh, and this is for a tube.

May 17, 2004 11:17 AM Forum: Telescope Making

Truss dob for my 12.5 f/6

Posted By David Tarshis

I have a tube for my mirror. Now I want a truss. I would like to be able to store the scope where the 2 and 6 year old can't get to it. A 6 1/2 foot tube doesnt store very well in a garage. I like the teeters obsession thing going on. Does anyone know where I can get plans to build something like that? I havent yet tapped into my local club's ATM shop yet, that will be next. Would like some pointers from this group. Thanks, DaveT

May 18, 2004 11:20 AM Forum: Telescope Making

Parts I'm considering for my 12.5 f/6

Posted By David Tarshis

Protostar seconday (2.14)

Novak 9 pt cell (I would like to have a cell that doesnt have fingers, any thoughts?)

Moonlite CR1 focuser

Any other suggestions?

May 18, 2004 12:20 PM Forum: Telescope Making

Maple veneered plywood?

Posted By David Tarshis

I picked up 3 sheets of 3/4 Maple veneered plywood at Homedepot the other day. I had too, it was only 4.00 a sheet (4x8). It fell of a pallet and has one dented corner. Will this be good wood for a dob? I will more than likely coat it with marine varnish. What, if any, problems will I have? Thanks

May 22, 2004 07:05 AM Forum: Telescope Making

Truss poll recomendations

Posted By David Tarshis

Looking for truss poles and connectors for my 12.5 project. I have seen a couple already, Moonlite and Obession's. I would like to know what you have used and what you have made. If you have bought from a ATM supply place like the previously mentioned, what you would have done different if anything. Thanks.

May 24, 2004 11:48 AM Forum: Telescope Making

How long?

Posted By David Tarshis

Can someone tell me about how long my truss poles would be for my 12.5 f/6.4 would be? Would 44 inches be too short or would I be able to make those work? Having never built one, I have no idea. Thanks