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adamsp123

Eyepiece for F/4 10" SNT

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One thing I noticed, the my eyepieces I have (Revelation Plossels) although are fine on my 120ED seem a bit lacking on my Meade 10" F/4 SNT, I believe with this type of scope needs more exotic eyepiece design is required so any advice most welcome, especially if it doesn't break the bank!

Cheers Pete

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Exotic design and not breaking the bank seem to be a bit mutually exclusive Pete !.

F/4 is a fussy focal ratio - even Naglers will show a bit of astigmatism.

You might try a better quality plossl (eg: Vixen NPL or Tele Vue ?) for a better quality 50-ish degree FoV view - the Vixens are just £35 apiece from Orion Optics which seems a steal to me from the reports I've read on them.

For a few more £'s Hyperions seem to hold the mid-price wide-angle crown at the moment.

At F/4 I would steer clear of the budget ultrawides such as the Moonfish and clones.

I'm sure there are other choices as well - see what other come up with :(

John

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F/4 is a fussy focal ratio - even Naglers will show a bit of astigmatism.

You might try a better quality plossl (eg: Vixen NPL or Tele Vue ?) for a better quality 50-ish degree FoV view - the Vixens are just £35 apiece from Orion Optics which seems a steal to me from the reports I've read on them.

For a few more £'s Hyperions seem to hold the mid-price wide-angle crown at the moment.

At F/4 I would steer clear of the budget ultrawides such as the Moonfish and clones.

I'm sure there are other choices as well - see what other come up with :(

John

I thought about Baader Hyperions, anything under £100 would be good, breaking bank was reference to some of the multi £100s EPs

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I use my trusty Ol' TV Plossl's on my 6" f3.5 Cometseeker. Works for me.

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I'm not sure that would be astigmatism John. More likely coma from the mirror. The on axis sweet spot of an F4 mirror is pretty small. Any minute lack of accurate collimation would be indicated also.

A coma corrector might be worth consideration. Also, televue test their optics at F4 so should give good results.

Just my 2p.

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Don't forget this a Schmitt-Newt, the coma shown should be around the same as a f5.5 - f6 conventional Newt because of the corrector plate, so you might not have to spend as much on eyepieces as you would otherwise with an f4 mirror (I'd still stay away from the "budget" widefields though).

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Good point Gaz, I wonder in what way the plossls are lacking. I used to use vixen lvws in my F6 reflector, they were very well corrected with a pretty flat field. Hyperions maybe? If you could break the bank a little then I suppose there's panoptics.

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As I understand it, an eyepiece will have to work just as hard in a f/4 newt as an f/4 SN to get the light rays parallel. This is inherent solely in the focal ratio and the EP's ability to deal with it. It will not correct the coma in either scope, so it will be more evident in the newt, which doesn't have a Schmidt corrector.

However, coma is not as bad as it's made out to be, as I found out when I tried an excellent wide field eyepiece that was up to the job of coping with my 8" f/5 newt. The stars at the edge of the FOV still looked great and the imperfection due to coma was very minor. I realised then that all the mushiness I'd been seeing in inferior eyepieces could never be attributed to coma, but rather to the eyepieces' inferiority.

If you're on a budget and want good correction in such a fast scope, you will be restricted to a FOV of about 50° - decent plossls or Vixen LVs would be good bets.

Andrew

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I'm not sure that would be astigmatism John. More likely coma from the mirror. The on axis sweet spot of an F4 mirror is pretty small. Any minute lack of accurate collimation would be indicated also.

A coma corrector might be worth consideration. Also, televue test their optics at F4 so should give good results.

Just my 2p.

Actually I think you are probably right Ian - they say that the Naglers (and Pentax's) are so well corrected that the coma inherant in newtonians below F/6 becomes obvious.

John

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You could always try to find a Pretoria eyepiece. I had the 28mm for a while on a 18" f4.5 Newt.

Smart piece of work, maybe that's why I still like the Klee Barlows!!

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Getting back into this "hobby" nowdays is like walking into a minefield I thought choosing scopes with the range that is on offer was hard enough and now all these EPs. :shock: :scratch:

I need to listen to all your excellent inputs, read up some more generally and certainly spend a bit of time collimating the beast, first light was straight after moving it by car 50 miles!

But please keep the ideas coming it is most helpful. :(

Pete

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An example of what coma does to reflectors, the diffraction limited field of view of my 8" F6 reflector is about 4.5 minutes of arc, not much is it?. This figure would go down even more as the F ratio goes down and infact go down as the aperture increases due to collimation tolerances becoming tighter and therefore the chance of achieving collimation reducing.

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Pete, it sure is like a minefield. Although as long as your scope is quite well collimated it'll be fine. I use a laser collimator, spending about 2-5 minutes on collimation before use, after the cool down time. Although reflectors aren't necessarily the best for planets, I managed to snag saturn with a webcam with it (my sig caption).

Don't forget to collimate your collimator though. Don't ask (-:

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Pete, it sure is like a minefield. Although as long as your scope is quite well collimated it'll be fine. I use a laser collimator, spending about 2-5 minutes on collimation before use, after the cool down time. Although reflectors aren't necessarily the best for planets, I managed to snag saturn with a webcam with it (my sig caption).

Don't forget to collimate your collimator though. Don't ask (-:

I have my SW120 ED for alot of visual work (I love the clean crisp views) and later AP, I wanted a larger aperture (start of aperture fever maybe) I was going for a 12" Dob but prices went up and then this 10" SNT came up secondhand, it worked fine on my vixen mount so I bought it, I want to use it more for the faint fuzzies and also AP if the mount can take it.

Collimate the collimator........eh - what, now I must lie down :crybaby:

:( Pete

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