Magnification

[perrin6]

My brain hurts !

[Ajohn]

Easiest thing to do is download the book. Skip the telescope making bits unless interested and read the interesting bits. The telescope making info is better than most if anyone wants to try that but there are other ways of testing a mirrors profile. I tried several. His, Dall's and the wire rather than a knife method. The last one is probably best for making sure it really is a quality mirror. Ronchi in my view is a load of Rubbish.

The reason I posted really is that given any scope it gives people an idea where to start and what to expect with a warning at the higher end to initially shoot on the low side and also that things get more difficult as the scope size goes up which in real terms means shooting even more to the low side. As to what Lick and Polamar might use I doubt if he had that sort of thing in mind for amateurs. He would probably see 12in as a max and I suspect too big really. Wish I could get my hands on the paper on just how accurately the 2ndry has to be held in place. That would give people a head ache and show why he might think 10in is the limit.

Thin mirrors and supports are sore point. "Unfortunately" Texereau gives figures that indicate just what is needed if you happen to have a 1/10 wave mirror and want it to remain like that in the scope with very simple mounting. How far things can be taken with say a simple 9 point cell is open to debate but the full thickness mirror sizes do give an idea how strong the cell needs to be. There used to be a lot of info on this subject about on the web including some of the arrangements the really big scopes use - weights and leavers etc. The problem is that 1/10 wave is a mere 0.00005mm or 2 millionths of an inch. If the outer parts of the mirror move that much it's bad news as that's where most of the light comes from. Personally I think his thicknesses are a little over the top. When proper blanks were available 10in for instance might well be under 2in. I'd guess about 1 3/4.. A 10in F4.5 meade I have uses a 1in thick blank. That I feel is too thin really but the mirror is on 3 cork annular pads about 1in in from the outer diameter and it's a fairly hefty cell. How much difference the mounting points make? I don't know.

John

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[Josh Wilson]

Josh using 125x on an 8in scope is also us using a 1.6mm exit pupil on small feint fuzzies.

Sorry, I forgot to mention that this was not with my 8 inch scope I have now. It was with a 90mm Mak I owned for a little while.

Josh

[Ajohn]

Interesting. I recently bought a sw 127 mac but from terrestrial views I wouldn't have thought it would cope with that. No clear skies since though, Also a bit misty when I tried it and it may need collimating. That's crucial on macs.

Interesting number though as it comes out at 0.7mm just out of the "see the diffraction rings" size along with GOOD small scopes should take higher mags. I am surprised by that figure though.

John

[alan potts]

John

Another very good point made about looking over things. I know this is a bit of an extreme but I used to use the 12 inch at the town apartment. I know this is not bright. Near by is another block you should have seen the view when looking over the top of that, it was like a mirage. You are of course correct anything like concrete that take ages to cool will cause a simlar problem, or buildings that are giving off heat. I don't have problems like this now I have finished the house.

Alan.