a 11" F4.3 or a 12" F4.7

[garethmob]

hello guys

just a quick question which would be better a scope at 11" with F4.3 or a 12" F4.7 scope, both dobs

also im guessing the F4.7 would be better for cheaper eyepeices, but how much better? with F0.4?

[estwing]

anything below F5 is screaming for a decent EP,mine's a F4.8 12.5" is boardering on Parracorr's being used(i stand to be corrected as i'm not certain of this) and collamation is going to be spot on here aswell...

[garethmob]

is there a cheaper alternative to the paracorr? lol its more expensive than the scope i have in mind (and any single peice of equiptment i own :'( ) lol

[John]

Baader do a coma corrector which costs less than the Paracorr, as do Skywatcher:

http://www.firstlightoptics.com/coma-correctors.html

I'm in the process of moving from 10" F/4.8 to 12" F/5.3 though as I don't want to use a CC.

Things get demanding sub F/5.

[garethmob]

id like an honest answer then guys

is there any point in me getting said 11" scope at F4.3 (at £ 250 if id have to spend an extra £100 to get any real use out of it?)

[Naemeth]

Hmm.. how much is the 12"?

[garethmob]

twice the price....

[Naemeth]

Well, the F/4.7 12" is 1432.56mm focal length, the F/4.3 11" is 1201.42mm focal length, which is almost identical to the Skywatcher Dobs (6", 8", 10") focal length which come in more manageable focal ratios. They'll also show less coma. It's up to you, but if it's good optical quality, the F/4.3 may be good, if you can get the right eyepieces and coma corrector for it.

[Moonshane]

Just took the PC out of my 16" f4 to see what it was like with and without. using my 15mm TV plossl there was some coma at the very edge of field but very little. there was a lot more in the 32mm TV Plossl. putting the PC back in there was very little coma at the edge of my 13mm Ethos. so the difference is very apparent with a corrector. I think you'd get away with a Skywatcher or Baader CC on your widest field and then nothing on your higher powers but it depends on your ability to ignore coma.

in terms of which is best it's impossible to say - what's the mirror quality?, focuser quality? general build quality? of the two scopes? assuming similar answers to these questions, I'd personally go for the 12" but that's to do with aperture as it's a better jump from your 8".

[garethmob]

well the 3 which id compair

a 10" dark star

10" brand new SW/12" TS special

or this mystery 11" im unable to see it before i pay (ebay) but the seller has said its optically very good, and was going to be used as a travel dob (its a funky truss but not as nice as yours shane)

would that http://www.firstlightoptics.com/coma-correctors/baader-mpcc.html work with it? im looking at eyepeice sizes at 30.. 25 14 and 10 with the possiabilty of 6 (in a perfect world)

[John]

Making an F/4.3 mirror is a difficult task - I'd want to know more about the mirror quality / pedigree before buying. Dark Star scopes were decent in their day but that was quite a long time ago. I seem to recall their optics varying depending on the supplier of the optics used. The TS scopes use GSO optics I think so the same as Revelations and Meade Lightbridges. Of the 3 I'd say the latter was the safest bet if buying "blind".

If you get a mediocre mirror then it won't be a low cost scope any more and a coma corrector won't help.

I paid £250 for my used Orion Optics 300mm F/5.3 optical tube but that had a Zygo report showing good optics and I was able to trace the scopes history.

[Moonshane]

I have a feeling this is a SGL member's scope. it looks quite well made and solid. personally, I'd consider it at that price as it seems OK to me subject to a test. if I were you I'd make arrangements to go along with a view to buying subject to using the scope for an hour or so one night - cannot see why the seller would mind/not agree.

as John says you can get better bargains if lucky, quick and did I mention lucky?? assuming the figure is relatively decent this seems a fair price.

to make it better you could rebuild the dob structure and add a new focuser but this would probably double the cost in due course. in that case you'd possibly be better going for a standard Skywatcher 12" dob which occasionally go for £350-400 but would have a better focuser etc already.

either way you either pay more or take a chance (or both) I suppose it depends whether you want a ready to use with no mods scope (more expensive) a starter scope to do a fair bit of work on scope (medium price with medium future spend/work) or a donor for a new project scope (minimum spend with more future spend/work).

[ollypenrice]

I don't use a Parracor in our F4.1 Dob. I do use Naglers, though, and the difference is huge.

Olly

[Peter Drew]

I don't think an 11" mirror is a commercial size so is likely to have been amateur made. This is not to say that it couldn't be high quality but a try before you buy could be good advice