Wide F.O.V. EP

[michael.h.f.wilkinson]

They are mate, I sold my 25mm BST after I got mine. I rate it that highly.

Granted it's not sharp to the edge but it's much sharper than it should be for the price and it definitely shows more pin sharp area of sky than the 60 degree BST.

I stand by my review and my challenge for anyone to find better for the price.

For that kind of price they are definitely worth a punt.

[Naemeth]

They are mate, I sold my 25mm BST after I got mine. I rate it that highly.

Granted it's not sharp to the edge but it's much sharper than it should be for the price and it definitely shows more pin sharp area of sky than the 60 degree BST.

I stand by my review and my challenge for anyone to find better for the price.

I do wonder how the comparison would be at F/5. Of course, I couldn't do the comparison as I only have a 1.25" focuser.

[bomberbaz]

Wow, £70, don't pay that lol

Skies the Limit have them for £59, they're not on their ebay site but email Alan...

(I know I have one lol)

Edit: They're on his actual site here: http://www.skystheli... eyepieces.html

Anyone with ideas of buying it your too late, lol. An absolute steal and I wonder if he makes money on that. It even includes postage

Bit his hand off and I will have next week for some erm (looks out through window at clear bright skies ) DSO viewing...

Baz

[Spec-Chum]

Anyone with ideas of buying it your too late, lol. An absolute steal and I wonder if he makes money on that. It even includes postage

Bit his hand off and I will have next week for some erm (looks out through window at clear bright skies ) DSO viewing...

Baz

Cool, let us know what you think

[michael.h.f.wilkinson]

I hope it does not arrive with a box full of clouds.

Clear skies!

[John]

..... I wonder if he makes money on that. It even includes postage

I think it's because his wholesale price is a lot lower still Baz

I hope it does a good job for you but, assuming your 12" is an F/5, thats a tough challenge for a low cost 80 degree eyepiece.

[bomberbaz]

I think it's because his wholesale price is a lot lower still Baz

I hope it does a good job for you but, assuming your 12" is an F/5, thats a tough challenge for a low cost 80 degree eyepiece.

He must have a good wholesale contact, I Looked abroad at prices and they were nearly double, so the man must have good connections.

Regards the chalenge, TBH John I am not expecting the world, I am just hoping that it gives me something a little more than my current BST 32mm which is a nice piece for planetary work, but not good enough for the larger DSO stuff. I will post some findings once i have had chance to try it out. Arrives on Wednesday so watch this space.

Baz

[Ajohn]

When people start talking Naglers etc and very expensive eyepieces I am inclined to think of making purses out sows ears as the old saying goes. Personally I wonder about skywatcher optical quality and on top of that a netownian reflector is basically an axial device not wide field and even more so as the F ratio gets faster. Coma correction can do something about that but it tends to be an expensive option.

One thing for sure once F ratio's get down to F6 stars should snap in and out of focus. I wonder how many do but in real terms on a 12in telescope seeing conditions need to be pretty good too.

John

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[John]

He must have a good wholesale contact, I Looked abroad at prices and they were nearly double, so the man must have good connections.

Regards the chalenge, TBH John I am not expecting the world, I am just hoping that it gives me something a little more than my current BST 32mm which is a nice piece for planetary work, but not good enough for the larger DSO stuff. I will post some findings once i have had chance to try it out. Arrives on Wednesday so watch this space.

Baz

Chinese imports Baz - order enough the the price comes right down. I'd be surprised if he was paying more than £30 a unit for the 30mm 80 degree eyepieces on the wholesale market.

It will show more sky than a 32mm 1.25" eyepiece of course but the question is how much of that field of view is sharp enough to be pleasant to view.

@ John (Ajohn): If you get a chance, try some of the premium wide field eyepieces and see for yourself. If there was nothing to them apart from "a name" I'm sure that the many smart folks on here who own them would not have parted with their hard earned cash and would have a case full of "Moonfish clones" instead

[Ajohn]

Hi John (Staff) having sort of been there very briefly due to light pollution problems - 11in F4.2 I think I would approach it on the basis that for 0.100mm of radial fanning of a star the max field size for a 12in F5 telescope is 0.95ins dia, 1.35ins for an F6 covering 55 or 64' respectively and then going on to the size of the 2ndry mirrror in case that wouldn't cover this field anyway and then spend my money on this basis.

I do own one high end eyepiece. A 28mm UWAN. Sadly little used as the weather at the time when it arrived along with a scope drove me purely into microscopes for several years so I can't really compare it with the moonfish. Now I'm venturing back in again the weather seems worse than ever. I did use the moonfish on a 127 F9 apo regularly and found it to be rather good. I also own the Nagler zoom. I've compared that with Seben ED eyepieces of all things. I also have some older Vixen true plossl's and couple of TeleView ones. Those were mostly used on a C8. The Sebens on the F9 appo. I would say all of these are a fair step up from the usual so called super plossls etc contrast wise. I did gains a few of those along with a Skywatcher refractor. The Seben's probably loose a little contrast compared with the Nagler zoom. Hard to call and I haven't had much call to use the longer focal lengths. I was gob smacked by the views of DSO's and star fields the APO gave. Last time I looked Seben had drastically reduced the range. I should be trying those on a 110mm F6 APO when I am up and running again - weather permitting.

John

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[Spec-Chum]

Hi Baz, did you get your MoonFish?

What did you think of it?

[Ajohn]

There are a couple of other eyepieces that are of interest for low magnification and shorter F ratios especially the erfle in that respect. This is what the wiki has to say about them

Erfle

An erfle is a 5-element eyepiece consisting of two achromatic lenses with extra lenses in between. They were invented during the first world war for military purposes, described in US patent by Heinrich Erfle number 1,478,704 of August 1921 and are a logical extension to wider fields of four element eyepieces such as Plössls.

Erfle eyepieces are designed to have wide field of view (about 60 degrees), but they are unusable at high powers because they suffer from astigmatism and ghost images. However, with lens coatings at low powers (focal lengths of 20 mm and up) they are acceptable, and at 40 mm they can be excellent. Erfles are very popular because they have large eye lenses, good eye relief and can be very comfortable to use.

The König eyepiece has a concave-convex positive doublet and a convex~flat positive singlet. The strongly convex surfaces of the doublet and singlet face and (nearly) touch each other. The doublet has its concave surface facing the light source and the singlet has its almost flat (slightly convex) surface facing the eye. It was designed in 1915 by German optician Albert König (1871−1946) as a simplified Abbe[citation needed]. The design allows for high magnification with remarkably high eye relief — the highest eye relief proportional to focal length of any design before the Nagler, in 1979. The field of view of about 55° makes its performance similar to the Plössl, with the advantage of requiring one less lens.

Modern versions of Königs can use improved glass, or add more lenses, grouped into various combinations of doublets and singlets. The most typical adaptation is to add a positive, concave-convex simple lens before the doublet, with the concave face towards the light source and the convex surface facing the doublet. Modern improvements typically have fields of view of 60°−70°.

The erfle in particular was aimed at faster F ratios than the older designs. There used to be a smaller UK astro dealer that mainly sold scope making bits and pieces in but not grits, mirror blanks and grinding materials etc that sold a decent range of these. Can't remember the name but I have seen many favourable comments about them from people with rather fast scopes, I'm pretty sure they were made by antares. I meant to go in that direction for an 11ins F4.2 dob I sold recently but it had a fairly open pack down structure and there is too much light pollution really for that where I live. Full thickness mirror too so a lot to carry around.

Konigs pass - I don't think there is a UK source and feel that they are for high mag work anyway.

There may be a need to wonder about "modified" erfles too.

John

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[John]

I've owned a couple of Erfles - an Antares 40mm, and an old Celestron (made by Vixen) 32mm in the 1.25" size. They were nice eyepieces to use with F/10 refractors and SCT's but lost edge sharpness quite quickly as the focal ratio dipped below around F/7. I don't think I'd have enjoyed them at F/5. The 1.25" one was still limited to a 52 degree apparent field of course, despite the adverts for it claiming 60 degrees or more

The Tele Vue Wide Fields were Al Nagler's attempt to improve the Erfle design I believe and he did manage to improve edge field sharpness in faster scopes though it was still not perfect by any means. The Wide Fields were eventually replace by the Panoptics which are extremely sharp across the whole field in fast scopes but carry a relatively high price tag too.

I reckon a number of the Super Wide eyepiece models around today are adaptations, to a lesser or greater extent, of the Erfle design.

[Ajohn]

I read some time ago when people gave rather old descriptions of eyepieces that erfles were aimed more at fast focal ratios. F5 was mentioned from memory. Never owned one so can't say really. I would have to wonder about the modified aspect of what is around now. Maybe they had been better if left alone. Having some old Vixen plossls that might even be copies of Clave's I wonder about "improvements" to those as well.

I still have a 10in F4.5 scope - in bits - for low power work which is what that sort of scope is for really I might still give them a try. Sometimes aberrations in the eyepiece help those in the objective. TeleVue by several accounts from when I regularly read sky&telescope tend to make eyepieces to suit their apo's. Something entirely different comes out of a newtonian.

John

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[LukeSkywatcher]

I have very fond memories of using a 32mm GSO.

[John]

...... TeleVue by several accounts from when I regularly read sky&telescope tend to make eyepieces to suit their apo's. Something entirely different comes out of a newtonian......

So Tele Vue eyepieces don't work well with newtonians then

And why did they develop the Paracorr coma corrector - it's expressly for newtonians ???

[Ajohn]

Pass John I don't know. Just repeating what I have read several times. The focal plane of a newtonian is entirely different to a typical higher end apo and as I mentioned before elsewhere people who use 2in eyepiece on fast newtonian really aught to use a coma corrector. There might be some point using a very high end eyepiece then, It's a personal thing anyway really but that's how I see it.

As to why this is the spot diagram for a 10in F5 at about the limit for a 2in eyepiece. It shows the semi angle and the radii - the real field of view is 2 degrees etc. I just happened to have this up on my screen - comparing it with something I may make some day where the scale bar comes up as a tiny fraction of a mm not 1/10 of an inch. Being fair though that one is better than most apo's but has other problems.

That to me puts me off buying a very expensive wide angle eyepiece for a newtonian. An apo is a different matter but each to his or her own. I do own 2 of their barlows that I am pretty sure are really aimed at refractors - just from their size.

John

[Naemeth]

Coma is a problem in fast Newtonians, but, that is what a Coma Corrector is for, it is because the TV eyepieces lack any other aberration (apart from pincushion or barrel distortion, can't remember which) that the coma is noticeable.

I'm certainly not going to get into the Reflector vs Refractor debate.

Are some of the high end Meade SCT's built in with a Coma Corrector?

[John]

I think the issue of coma becoming visible is not restricted to Tele Vue eyepieces. There are a number of wide and ultra wide eyepiece brands that are well corrected enough for coma to become apparent. Pentax, ES, Skywatcher Nirvana's, William Optics UWAN's, Explore Scientifics, Meade UWA's - all these are well corrected enough to allow the coma to make itself apparent when the focal ratio is F/5 or faster.

[Naemeth]

Just out of interest, does a Coma Corrector completely eliminate coma in an F/4, or just reduce it to what would be seen, in say, an F/6?

[michael.h.f.wilkinson]

Apart from coma, the difference between a fast APO and a fast Newtonian is the field curvature, which in the Newtonian has the opposite sign of that of a refractor. A field flattener for a fast refractor will increase the curvature on a Newtonian. As I understand it, TV eyepieces are corrected for a flat field. A Paracor (and other coma correctors) also acts as a field flattener. By contrast, eyepieces for spotting scopes may be corrected for the curved field of the scope they are used in.

[Ajohn]

The eye copes with field curvature fairly well - if that's all it is. We sort of automatically refocus the image. It isn't just field curvature though so we see small blobs. This effect are easy to see on a microscope. There some one may have a straight achromatic objective save there money for a plan achromat and not see any difference at all. They will when they take photo's with it. Very little in some cases though. Microscope manufacturers do include the eyepiece in the design of the whole optical train and use them to compensate for problems in the objective. How important it is on scopes I don't know but it would be a bit silly of TeleVue not to consider performance on their own scopes.

What I didn't show on that oslo shot is the effect the aberrations have on resolution. Might be best not to know. Don't let it put you off. What this shows is resolution along the bottom and contrast on the vertical axis. The contrast scale is what comes out for 100% in.

You can see it's pretty severe. I have no idea how much a coma corrector cleans this up. The other point is that ray traced telescopes are perfect. The real ones aren't.

I don't think the graph should make people throw their newtonians away and buy another type of scope either. Longer slower focal ratio's help. This is what happens at same size but F10. Notice that the scale bar is a lot shorter.

The MTF graph doesn't look a lot different though. I wonder about the accuracy of MTF graphs when the effects shown are so large but it has to be said that they are realistic for smaller aberration. The 2 spot diagrams show why an F10 SCT or a similar mac are likely to be better in this respect but neither are likely to cover a 2 degree field. A 10in F10 newt is unlikely to either. These can be better for the simple reason that they are F10. The SCT may be the best of the lot. Not sure I want to ray trace one to find out.

As to APO's you will probably find similar graphs on Tak's and Astro Physics sites. The catch here though is that visual limiting magnitude and axial resolution are down as few people could afford a 10in apo even if they were made. Photographic limiting magnitude is entirely different so small apo's can be great for that. I would also say that a decent 5in apo is likely to be better than an 8 in reflector of the usual types. 4in better than a 6in etc. Costs though are a lot lot higher.

One thing for sure Texereau wrote an amazing book. It's unique as far as I know. The above partly explains why he came up with the design he did. Might be conservative but it makes a lot of sense. I reckon the design is a step up from a home made dob too.

John

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[Moonshane]

I don't really understand a lot of the graphs or even the discussion but I can say that when I look through my 16" f4 0.984 strehl newt with a paracorr and my 26mm Nagler, or my 13mm Ethos, the views are pretty damn fine. When I look through the same scope with an aperture mask creating a 170mm f11 with no obstruction at e.g. Jupiter, the views are incredible, especially at the outlay of around 15% of the cost of a 150mm Takahashi.

I tend to look through a telescope rather than theorise about what it should and should not be able to do, or be like; I'm happy and believe that the star shapes at f4 with Paracorr are similar to what I'd expect at f8 i.e. pretty much sharp across the field. Not suggesting that John or Texereau don't look through telescopes by the way.

[Damo636]

Have to admit I'm pretty lost here too? When it starts to get all technical I usually stand clear because I know I'll come off sounding like a numty! All I'll say is, I use ultra wide fields in an f5 newt and wouldn't part with them, no matter what the graphs say :-)

[bomberbaz]

This has all got very deep and I am lost. I have not as yet had chance to try my moonfish due to those fluffy grey things but as soon as I do, I shall let you know my experience and thoughts.

Baz