Author Topic: ES 82 30mm vs ES 68 40mm  (Read 267 times)

Chris Jiles

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Re: ES 82 30mm vs ES 68 40mm
« Reply #15 on: January 31, 2018, 01:28:47 PM »
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Let’s not forget that the baffle of a C8 is only 38mm.

Mike R.

Mike:

Apparently the baffle is far enough from the focal plane that an eyepiece with a 46mm field stop does significantly vignette.

A couple of thoughts to consider.

- A standard 8 inch SCT has significant field curvature. Because of the greater magnification of the 30mm that maybe be more visible than in the 40mm.

- A 4mm exit pupil is still small compared to a 7mm pupil, it's about 1/3rd as bright, a 3mm exit pupil, is less than 1/5th as bright. For dark skies with filters, the 4mm exit pupil definitely has advantages. A 56mm Plossl would fit as well.

- At F/10, there's a number of long focal length, wide field eyepieces that are reasonable performers. If there's the possibility of a faster refractor or a Newtonian up ahead, investing now in eyepieces that are well corrected at F/5 might make sense. At F/10, there will be some difference and one would be set for the future.

Jon

ransgesislu

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Re: ES 82 30mm vs ES 68 40mm
« Reply #16 on: February 02, 2018, 03:39:20 PM »
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Let’s not forget that the baffle of a C8 is only 38mm.
Mike R.

True. Still, visually, the field curvature and residual coma is way more troublesome than the barely visible vignetting. The field curvature is very apparent with a 82 deg apparent field eyepiece of 30 or 31mm. So much that I’m planing to upgrade for an EdgeHD

adectisun

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Re: ES 82 30mm vs ES 68 40mm
« Reply #17 on: February 03, 2018, 05:53:11 AM »
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I’m considering gettingES 82 30mm or ES 68 40mm for my Celestron Evo 8.
It looks like they provide similar TFOV (different magnification though), have similar size/weight/price.
Which one would you choose and why?

Unless the EVO is an Edge, I would consider the .63 reducer/corrector. It reduces field curvature and will give you a better corrected field. If your Evo is an Edge, then the eyepiece advice given about the brightness of the sky is spot on.

The reducer/corrector will give you a 38mm equivalent when using a 24mm. A 19mm will give you the equivalent to a 30mm.

exjeraca

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Re: ES 82 30mm vs ES 68 40mm
« Reply #18 on: February 09, 2018, 08:32:56 AM »
The 38mm baffle has never been an issue with an f10 light cone. The vignetting is not visually detectable.
Just avoid SCT threaded diagonals, some of those cause vignetting from the thread on nose in SCTs, but they didn't cause an issue with my orion 127 since the baffle of the scope was smaller.
Looked great with 30mm 1rpd and 42mm superview I was using back then. Narrow f12 light cone, and not obstructing the baffle like an SCT diagonal can do on an sct, is probably the key. It might be possible that faint stars might not have come through as well, but there was no visible vignetting or light falloff in the field itself.
Now using the 2"SCT threaded diagonals and focusers on my c11 or a standard Meade 10" lx200 was another story. But that is a case of obstructing the baffle. An eye opener type focuser/visual back adapter or visual back that threads direct to the 3" threads is necessary. The 3"-SCT thread plate on the c11 needs to be removed, simply threading an SCT diagonal in place of the original 1.25" visual back, was vignetting city.
This has been argued on paper in theory on these forums ad nauseum, that I remember from taking part in them in the past.
30 ultrawides, 40 superwides and 50-56mm max field 2" eyepieces work fine on 8" SCTs as long as an SCT nose diagonal isn't used, a 2" visual back or micro focuser and 2" refractor style diagonal needs to be used.