Eyepiece induced aberrations?

[TheTalescopeMan]

Hi folks,

In a recent thread I asked whether moving the primary mirror on my Vixen VMC200L 'inwards' a fraction via the collimation screws might introduce problems such as an increase field curvature. I asked this because the view did not seem to be fully flat when using a Telescope Express 2" 70 degree, 38 mm eyepiece in the scope. See here:

TS WA38 ERFLE Weitwinkel Okular - 38mm - 2' - 70° Gesichtsfeld

I know that this is a budget eyepiece, and it effectively costs only 50 Euros or so when bought as part of a package with a diagonal. It does give a wide field of view, a bright image and perhaps 80-90% of the view looks very sharp. However, the stars right at the edge of the field of view do show little 'wings' and are not perfectly in focus. I was thinking that this was something to do with the 'scope itself, but now feel it might be the eyepiece. I don't get this effect if I use the 25 mm plossl out of my Tal-1, but then again the FOV with this is much narrower, so even if the aberration is coming from the 'scope, it might not show up.

Is it likely that the eyepiece is is the root cause of this problem? Is this a case of there being 'no such thing a free lunch', in that you can't expect to have a perfectly aberration-free wide field of view 38mm, 2" eyepiece for this price, even when used in a f9.8 'scope? The description does say that it 'offers pinpoint stars up to the rim'. But if this were true, why would anyone spend 380 Euros on something like a Televue Panoptic?

Would a 'TMB Paragon' be likely to offer a significant improvement, or is it more likely to be 'scope related after all?

Thanks!

[TheTalescopeMan]

P.s. As far as I can tell the collimation was pretty much spot on, as judged by a star test.

[Mr Spock]

The erfle design suffers significant astigmatism near the edges, that will be what you are seeing. That is what spending a lot of money eliminates!

[brianb]

The erfle design suffers significant astigmatism near the edges

Well it does if stretched to 70 deg AFOV - it's perfectly satisfactory at 55 - 60 deg, and that used to be considered "wide field" (and is still about as wide as you can view without moving your eye).

[TheTalescopeMan]

Thanks for the replies.

What has been said makes sense, seeing that I have read that issues such as astigmatism are much harder to eliminate in a wide field design. I guess the claim that the eyepiece offers 'pinpoint stars up to the rim' is unrealistic and as such provides grounds for returning the eyepiece. Then again for 55 Euros, which is what I effectively paid for it, it works pretty well. Thank goodness that I didn't start messing about with my scope in an attempt to cure the problem!

Anyone know if the TS / TMB Paragon ED would be significantly better?

TS PARAGON ED 40mm 2' Superweitwinkel - 68 Grad - 6linsig

How about the 55mm Televue Plossl?

Having a wide-field view is nice, but I find having these imperfect stars around the periphery to be very distracting.

[AndyH]

I think the 40mm Paragon should be very nice in the VMC. I do remember when they first came out, Thomas Back was very pleased with the results.

[TheTalescopeMan]

Hi Andy,

Having looked at some reviews of the Paragon, I think that I may well return the TS ERFLE in part exchange for a 40 mm Paragon, or just sell it on. I guess that the old adage about long FR 'scopes not being sensitive to eyepieces is only true to a limited extent, especially when it is the eyepice itself that is introducing the aberrations. I also realise that some are less bothered by such things, and for owners of a 'fast Newtons' edge of field aberrations are just a fact of life!

It also goes to show what a great starter scope a Tal-1 is. Nothing in the design pushes any barriers, so the resultant views are very pleasing. In my mind, if a Tal-1 can give a nice flat view with pin-point stars right across the field, something costing five times the price should be way better in every respect. It is a pity that things don't always work this way and it seems that that the cost of a 'perfect' view increases exponentially with the aperture of the 'scope.

[TheTalescopeMan]

Hi folks, Just a quick update.

I had another look last night and found the problem I am having is definitely field curvature. The 38 mm TS eyepiece will give pin-point stars right to the periphery, but I cannot get the whole field to be perfectly in focus at the same time.

I am still not sure what the real source of this issue is as I have read that ERFLE eyepieces should give a pretty flat field, even at 70 degrees. I have also read that some compound scopes, especially Schmidt Cassegrains, have more field curvature that a typical Newtonian. Then again, Vixen claim that field curvature is 'well corrected' in the VMC 200L.

Perhaps I am just being too demanding, and I have read that even the TMB Paragon /Aero ED 30 is not free from field curvature and has a tendency to show 'seagull' stars around the periphery. Apparantly, a 31 mm Nalger is much better, but at ten times the cost of the TS eyepiece!

[brianb]

I have read that even the TMB Paragon /Aero ED 30 is not free from field curvature and has a tendency to show 'seagull' stars around the periphery.

Yes ... I found the Aero EDs to be very disappointing in a f/10 SCT, they do however work quite well in an f/7 triplet apo which has more pronounced field curvature ... the curvature of the EP and objective cancelling each other out to a large extent, I suppose.

Ordinary refractors have a field curvature radius approx. 1/3 of their focal length. Newtonians 1/2 of their focal length. Cats very variable with design but 1/3 of their focal length is typical ... of course the focal length is likely to be a lot greater than a Newt or frac of the same aperture.

Apparantly, a 31 mm Nalger is much better, but at ten times the cost of the TS eyepiece!

And about 4 times the weight, too - a serious consideration if you intend to use the thing in a Dob or in a fork-mounted SCT without a sliding counterweight for tube balance ... but the Nag is likely to be worse in some scopes because its field curvature (not much) reacts with that of the objective in an unhelpful way ...

[Peter Drew]

With the greatest respect , I suggest you don't get too bogged down with peripheral field eyepiece issues. For visual work a decent 60 degree FOV is probably as much as you can concentrate on in comfort. Imagiing is another case all together, but being as there is usually no eyepiece involved, FOV issues are dealt with by other means. .

[brianb]

With the greatest respect , I suggest you don't get too bogged down with peripheral field eyepiece issues. For visual work a decent 60 degree FOV is probably as much as you can concentrate on in comfort.

Indeed. If only the eyepiece manufacturers would show similar common sense.

I love monocentrics for planetary work ...

[TheTalescopeMan]

With the greatest respect , I suggest you don't get too bogged down with peripheral field eyepiece issues. For visual work a decent 60 degree FOV is probably as much as you can concentrate on in comfort. Imagiing is another case all together, but being as there is usually no eyepiece involved, FOV issues are dealt with by other means. .

I can see your point, but when manufacturers promise views with pinpoint stars to the edge of the field, and that is not what one sees, it is natural to think that something is amiss. I know that manufacturers are in business to turn a profit, but it is a pity that 'overselling' the abilities of astro gear seems to be so common.

[great_bear]

I know that manufacturers are in business to turn a profit, but it is a pity that 'overselling' the abilities of astro gear seems to be so common.

- and borderline "criminal" IMO. I've not seen misleading advertising claims elsewhere being anywhere near as pronounced as they are in astro gear promotional material.

[brianb]

I've not seen misleading advertising claims elsewhere being anywhere near as pronounced as they are in astro gear promotional material.

Aw, c'mon ... Apple Ipad ... all those wrinkle creams ... Head & Shoulders for Men: I bought some & I was very upset not to find 6 nurses in the shower with me ...

[michael.h.f.wilkinson]

I have a Paragon 40mm with my C8 and am very happy with the results. Yes there is a bit of field curvature, but whether that stems from the EP or the scope is another matter. The EP is definitely better than what I get with 2 Erfles I have/had, aberration-wise.