Quote:
Originally Posted by astro744
You'll find the Naglers are highly corrected and show no astigmatism. In fact they show beautiful coma that can be corrected by a Paracorr. I say beautiful coma since only a highly corretced eyepiece will show it properly as poorer corrected eyepieces have astigmatism added.
Any astigmatism you may see using the Nagler would be either the eye or mirrors (pri or sec). Often the eye when straining to see the edge of field will show considerable defects from the eye.
The Nagler has a much flatter field than cheaper 80deg clones also giving sharper stars at the edge.
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I'm glad you mentioned coma & the paracorr in one breath, astro. You answered a question I've had about coma correctors and their part in correcting for astigmatism- they don't! I had suspected this when trying out a Baader MPCC, and the astigmatism was still there, but no coma
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Quote:
Originally Posted by Steffen
I'd prefer if eyepiece makers kept them at moderate viewing angles if that meant they'd be cheaper/more lightweight/more user-friendly and had good sharpness and contrast.
I've never quite shared the obsession with super-wide, ultra-wide and even wider FOV eyepieces. I'd agree that a wider field of view is handy for finding things and keeping them in view when using non-tracking telescopes. However, the eye isn't able to take in big panoramic views. Our angle of sharp vision is tiny (probably 5º or less) so we're scanning our vistas all the time. I found scanning around some of the very wide FOV eyepieces to be hard work. I don't care how sharp stars are at the very edge that I can only see if I roll my eyeball all the way to the stop.
I'm more comfortable with 52 to 60 degree eyepieces. I recently picked up an Edmund RKE 28mm on astromart and despite its "measly" 45º FOV this thing is something else! No hint of barrel vision, on the contrary - the eyepiece seems to project a floating image right in front of your face. It's also cheap and hardly weighs anything. Why can't we have more eyepieces like this?
Cheers
Steffen.
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I agree, Steffen!! I advocate the max. FOV as being 68º (how did you get your degree symbol? I copied and pasted yours here) for the reasons I've outlined. Only two of my EPs go to 68º, everything else is less, and I'm ok with that. However, it is good to have a 60º+ high power EP for a dob as the objects stay in the field of view for longer as they drift. Really good in a push-pull dob!
I LOVE my RKE 28mm
. It is by far the best performer in my fast scopes. Its narrower field of view is really not an issue here as its actual field size is great for low power. I've wondered why there isn't a 2" version of it too! I guess that most manufacturers try to convince us that wider is better, and it is not seen as being sexy enough to only have three elements- its got to have seven, or more, be made of exotic glass and designs, and cost us a fortune, not $60. Steffen, next time we're up at Katoomba, try your RKE in my dobbie.
Quote:
Originally Posted by mbo
Hey Alex, as I seem to have a case of early onset aperture fever, I'm considering an F5 12" GSO, so I'm curious if you have tried the 40mm superview? Would an increase in fov and a reduction in F value exacerbate the problems some people seem to have with these ep's?
Also, what celestial objects suit really wide FOV's (not AFOV's), in other words, are there things that wont fit inside the 1.4 degree field of the 30mm that would be better in the 1.8 field of a 40mm?
BTW, I'm going to buy a scope on Friday so I'll be free of the pain of procrastination soon :-)
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MBO, you need to be aware that reflectors have a limiting factor to the maximum focal length eyepiece that can be used with them. Exceed this limit, and you will begin to see the shadow of the secondary mirror through the eyepiece, and there is no way around it.
Wavytone gave me this formula to find out this upper limit:
max focal length = f/ratio of scope X 6
So, for the 12" scope you are looking at, with its f/5 ratio, the longest focal length EP you can use on it is 30mm. Some say you can push this to 7 X the f/ratio, making it 35mm. The thing is, the 30mm limit is safe, the 35mm is the max limit.
As an example, my 17.5" f/4.5 scope's safe limit is 28mm. Its theoretical limit is 32mm. When I look for a low power EP, its focal length needs to sit between 28 & 32mm. I've got an expensive 35mm EP, I just can't use it with this beastie! The 30mm SuperView I have sits within this upper limit range for this scope, so it works.
The 40mm superview may just be too long for the 12" f/5 scope. I'd suggest getting the 30mm first. You know it is safe for your scope, but 40 pushes things a little far if you don't have another low power EP. Once you are up and running, you can borrow someone's 40mm EP and see how it goes in your scope.
Object wise, there are only few DOS's that exceed the 1.4º AFOV. It is something I wouldn't worry about. You'll soon learn how to nudge the scope to see any areas that fall outside this. Your eyes can only see so much at once too. Eta Carina nebula is one, the Andromeda galaxy is another. Maybe the Orion Nebula, and a few other open clusters would fall into this category. Oh, and the Large and Small Magellanic Clouds too. I wouldn't concern yourself about it though.
Whilst you are at it, look at getting a pair of binos if you don't have a pair already. These are fantastic as rich field telescopes! 7X50's are the classic astro size, and double up nicely for daytime viewing too. They don't need to be expensive too, that they are aligned properly is more important.