Which are the better corrected EPs?

[mental4astro (Alexander)]

Hi all,

My likeing of fast Newtonians has led me to have to deal with issues of astigmatism. While more, much more, expensive eyepieces tend to be better corrected, they can cost more than the scope they are being used in. They also can be really, really heavy and with too wide a field of view, over 68 degrees, only complicating matters.

The only modestly priced eyepiece to be well corrected I have found is the 30 year old Edmund Scientific RKE series, only available in the 1.25" barrel size.

While I love my 28mm RKE, I know I'm cheating myself of precious light due to its small field stop.

What are the better corrected EPs in the 2" format not costing a mint, less than $300? This is the type of info missing from just about all the literature I can find.

While coma in a fast Newtonian is one issue, and can be corrected for with coma-correctors, and tolerable using the eyepiece neat, astigmatism is something else.

Mental.

[casstony]

A Paracorr will allow many eyepieces (cheap & expensive) to perform better in Newtonians - it slows the light cone a little and reduces field curvature in addition to correcting coma.

Titan II ED eyepieces sold by Astronomics reputedly perform very well in Newts with a Paracorr - they're US$160 new but can be had on Astromart for around US$110 + shipping. AFAIK these are the same design as the TMB Paragons. I have the 35mm & 40mm - they're nice eyepieces though the outer field is poor in my C11 due to the aberrations inherent to the SCT.

[mental4astro (Alexander)]

Thanks Tony.

I too had heard of the Astro-Tech Titan II's being the same as the Paragons. I've had one chance to use a Paragon 30mm ($300) through my 17.5" f/4.5 dob- a marked improvement over my GSO SuperView 30mm ($60), with a small amount of distortion at the edge of the FOV.

I've also tried another EP three times to price of my SuperView to get an image that was totally useless in my fast scope. I was surprised that my 'cheap' EP so outperformed a much more lorded EP.

Any other suggestions?

[casstony]

Originally Posted by mental4astro

I've also tried another EP three times to price of my SuperView to get an image that was totally useless in my fast scope. I was surprised that my 'cheap' EP so outperformed a much more lorded EP.
Any other suggestions?

I'm curious what the more lorded eyepieces was?

I'm not surprised at the lack of responses to your question; to quote someone else "cheap, wide, good - pick any two". However one other eyepiece in paticular that I've noticed getting good reviews is the '22mm AF series', a 70 degree eyepiece also sold by Astronomics for US$120.

[mental4astro (Alexander)]

Orion Q70 series. I tried the 30mm one, very disappointing. I remember, if I'm not mistaken, that these were tauted as good in fast scopes!?

Also, the RKE series is not being made any more. Edmund Scientific now selling Plossls instead. A real shame. Probably a commercial decision as Plossls can be made really, really cheaply, and probably more cheaply than the more specialised and a smaller quantity order of RKE.

[Wavytone]

Alex,

"better corrected" usually means aberration free, but doesn't usually equate to "best" in an eyepiece.

In an optical system the aberrations in the image is the sum of the aberrations from the optical components along the way.

An eyepiece that has a curved field and a modest amount of negative coma will give superbly sharp images across the field in a Newtonian, far better than eypeieces with flat fields and no coma.

The point about the RKE's is that Edmund designed them to match the old Astroscan - which was an f/4 Newtonian, hence why they suit your scope (it's almost an exact match), and they do this WITHOUT the need for coma correctors or field flatteners - which actually would make the image WORSE with the RKE's.

There is no other eyepiece that will give you such a good match with just three elements. There is one downside - a fair amount of pincushion distortion.

If you wanted to use "better corrected" eyepieces, you'd also need a coma corrector and field flattener to eliminate the contributions from the primary mirror; but with so many more pieces of glass in the path the image may be degraded in other ways.

Conversely, while the RKE is OK in an SCT or Mak, many eyepieces can give much wider sharp fields because the light cone is so narrow - f/10 - f/15 - that the contribution from the eyepiece won't matter.

Similarly, for refractors, depending on what sort of scope you have, the colour corection of the eyepiece may either make matters worse, or better, depending on the characteristics of the eypeiece and scope.

Conclusion ? The "best" eyepiece depends on the aberrations of the scope you will use it with.

[koputai (Jason)]

Originally Posted by mental4astro

Also, the RKE series is not being made any more. Edmund Scientific now selling Plossls instead.

Are you sure about that? The Edmund site still has the RKE available.

Have you tried a Panoptic 24mm?

Cheers,
Jason.

[mental4astro (Alexander)]

Where, Jason?

All I can see are Plossls listed, and these are made to look like the old RKE. Even the ad for the Astroscan lists Plossl eyepieces, not RKE.

RKE's are a three element design, Plossls are a 7 element.

Wavy, I had thought that I was clear in asking for astigmatism correction in my first post. Sorry if I wasn't that clear. I can only guess for my lack of knowledge on optics, that fast reflectors aggravate astigmatism in an eyepiece. This is what I've noticed between the RKE, TMB/Titan II, SuperView and Q70 in decending order of image quality.

Leaving coma asside, which is the common problem to all fast reflectors, astigmatism in fast reflectors is what I'm looking to reduce in the eyepiece.

[mental4astro (Alexander)]

RKE's are supplied by "Edmund Optics", not Edmund Scientific!

http://www.edmundoptics.com/onlineca...productid=2075

I came across this link from an old post of yours, Wavy.

[ColHut (Colin)]

I do not think that the field stop is limiting the performance of your RKE 28mm. I calculate it would need a field stop of about 22mm to give it full field with an AFOV of 45 degrees, and the RKE website lists it as having 23.3mm (if the same). Only if you go above about 55 degrees AFOV at about that focal length eyepiece will you need a 2" barrel. At that point I cannot see how you can escape an expensive well corrected eyepiece like a Nagler 31mm which will give you twice the field but with slightly larger exit pupils (~6.5mm). Good luck

[mental4astro (Alexander)]

Very good point, Colin. When I designed my 17.5"er, I aimed for a 22mm fully illuminated field. Just the ticket for this EP to be given its full potential.

This does make the RKE a special beasty. In my 17.5" f/4.5 dob, my 28mm RKE is remarkable, and easy to use. Of course, not as wide FOV as a Paragon (68*), but just a bright. In my 8" f/4 scope, coupled with the 28mm, this union outperforms my 10" f/5 dob, and a 13.1" f/4.5 from my home in Sydney. A really unique combination for a light polluted enviroment.

Another unique feature of the 28mm RKE is its extraordinary long eye relief, nearly a full inch. A little tricky to use at first as you just cannot bring your eye in closer or you just cannot see. Makes that you need to be able to hold your eye steady, floating above the EP, and not resting on a rubber eyecup. I like it, some other fellows just can't get used to it.

[mental4astro (Alexander)]

In my trawling the net, I've come across the SkyWatcher Aero Series of eyepieces, aviable only in 30mm, 35mm & 40mm. By all accounts, they are the same as the Astro Tech Titan II's and the TMB Paragons.

Anyone have experience or has heard of them too? Oz supplier as I've only seen them on UK sites?

I've also seen that the Williams Optics UWAN series in the UK is marketed as the SkyWatcher Nirvana.

[AG Hybrid (Adrian)]

Hey alex, I was scouting out vixen eyepieces in my never ending quest for knowledge about all things astro equipment and I came across a shootout of 7-9mm eyepieces, theres about 8 brands there.
http://www.vixenoptics.com/PDF/lvw_review.pdf

Now I know the focal length is a bit short but its an interesting read in price/performance in specific brands.

Also I was looking at Meade SWA the 24,34,40mm eyepieces are going for a steal right now from Optcorp.

[mental4astro (Alexander)]

I've come across the manufacturer of the TMB Paragon, Astro-Tech Titan II, and SkyWatcher Aero eyepieces. They also manufacture the Williams Opitcs SWAN and UWAN eyepieces:

http://www.united-optics.com/Product...cessories.html

Interestingly, in the US, the RRP difference between the Paragons and the Titan IIs is close to $100, for the same EP, . You need to do your home work, don't you!