Help me understand eyepieces.

[Owmuchonomy]

I think I understand a reasonable amount about this hobby, predominantly due to this forum over the years.  However, I still struggle in understanding about the different types of EP, how they compare, how to match them to scopes and the relative merits of the designs.  I use or used predominantly ES 82' EPs in my scopes both past and present and most of the time they are or have been excellent.  However, when introduced to my recently acquired SW ED120 they do seem to struggle.  Finding focus is somewhat hit and miss being quite variable across the field of view.  So I want to learn more in general and about that scenario. Is there a sticky or nice text someone can point me at so I can do my homework and understand how to maximise my EP experience? I currently own:

ES 82': 24mm, 20mm, 14mm, 4.7mm

Celestron: 15mm, 25mm, 40mm plossl

TV: 2x, 2.5x and 5x power mates (used mainly for imaging solar/lunar/planets)

Many thanks.

[John]

Hi Chris,

There are lots of pieces on eyepieces and their strengths and weaknesses around the internet. This is quite a good one:

http://www.eaas.co.uk/cms/index.php?option=com_content&view=article&id=4:a-short-guide-to-choosing-eyepieces-by-neil-paterson&catid=5:learning-zone&Itemid=8

I'm wondering if what you are experiencing field curvature from the refractor optics and / or the eyepieces ?. This would have the effect of having sharp stars in one part of the field and not quite in focus at other points

 

 

[Ricochet]

6 minutes ago, John said:

I'm wondering if what you are experiencing field curvature from the refractor optics

This was my first thought too. What other telescopes is the performance of the ED120 being compared to? I see an f10 SCT and f7.5 80ED listed in the signature. The SCT should be gentler on eyepieces but the 80ED has just as steep a light cone and with a shorter focal length should show even more field curvature than the 120ED. I don't recall field curvature being a complaint levelled at the ES82° series but I've not used any to check for myself. 

[Piero]

52 minutes ago, Owmuchonomy said:

However, when introduced to my recently acquired SW ED120 they do seem to struggle.  Finding focus is somewhat hit and miss being quite variable across the field of view.

This seems field curvature (FC) to me (as pointed out by others above).  

If I understand correctly, you cannot reach focus at the same time on the whole field of view.

Assuming that you play with the focuser, can you get pin point stars on axis (at the centre) and off axis (near the edge) when you focus? 

- If you do, this is a sign of FC. Both my refractors shows FC to some extent (the shorter TV60 shows much more!). 

- If you don't get pin point stars off axis, it depends on how they look like. A more detailed description would help. 

[Owmuchonomy]

Yes it’s highly likely the root cause is FC. This explains why the ES eyepieces performed much better in my F/5 Dob. However, as discussed in the article posted above there is some expectation that my ES eyepieces would compensate for this and they don’t appear to. My eyes are also old so maybe I have to accept it. Maybe my question should be “What EPs do folks recommend for my SW ED 120?”.

[ronin]

Could be field curvature from the objective, but the scope is not overly fast so I kind of doubt that, or spherical aberration from the objective.

120mm is a reasonable size for what is a doublet, so the edges could be bringing the wavelengths to a slightly different focal plane then those of the same wavelength that passes through the central portion. Hence no easily defined focal plane and difficult to get a focus. All the blue wavelengths are spread out a bit along the axis. Same for the rest of the spectrum.

You could jus have a "soft" one, where the 2 front elements are within specification but do not work well as a pairing.

According to a report (by TMB) an apo (triplet) should account for SA as well as CA, the wording reads as if it is a consequence not just a design criteria - have to dig into why it reads that way at some time. Suppose makes a bit of sense when you think about it. Wider spread of controlled wavelengths.

There is more to the word apo then what many manufacturers write on the box.

[Mr Spock]

I get field curvature in my 80ED f7.5. It affects my 42mm LVW 72° rendering the outer 10% unsharp. The 22mm LVW 65° is pin sharp edge to edge.

Both are fine in the f10 C9.25.

[YKSE]

As many others have mentioned above, I'suspect it's field curvature(FC) too, both from 120ED, and 14mm ES (mostly IMHO).

Refractors has it's field curvature propotional to its focal length (not focal ratio), as describe here

https://starizona.com/acb/basics/equip_optics101_curvature.aspx

The smaller radius of curvature, the more curvature it has.

If you use the 14mm ES in your 80ED, you'll be seeing more bloated stars in the edge than in 120ED if the center is in sharp focus.

The FC in 120ED is quite mild for visual,I've not read on the other site about anyone seeing the FC of 120ED using a flat field EP.

There're quite some comments on the other site about the FC in the ES 14mm which is often referred as the worst of ES 82°, even though they're quite good. The optical test I've read confirms these observations. 

C9.25 is quite flatter SCT than other f10 SCT, that you see the FC in 120ED with the 14mm are likely because the curvature of the scope and EP are of the sign, they add up here.

[Louis D]

I had to get a TSFLAT2 field flattener for my AT72ED to flatten the very curved focal plane.  I have it spaced slightly over 15mm from the front of my GSO 2" dielectric diagonal via an adapter and spacer ring.  Here's a CN thread for using it visually and here's an SGL thread for using it for photography.  It makes a world of difference when using widest field eyepieces.  I now get pinpoint stars right out to the edge.  It boggles my mind that manufacturers don't put field flatteners in their short focal length refractors.  Is it because they tend to reduce back focus?

[Stu]

5 hours ago, Louis D said:

It boggles my mind that manufacturers don't put field flatteners in their short focal length refractors.  Is it because they tend to reduce back focus?

I couldn't agree more Louis, it seems odd that we complain about and try to improve so many other abberations produced by kit to get the best views, but something as fundamental as getting objects in focus at the centre and edges at the same time is largely ignored or accepted!

I guess your last point is most likely correct, that back focus may be reduced, but cost will also be up there too. I have successfully tried the same flattener as you and it does address the problem very well.

I wish Televue would produce something which worked well for their ultrawide eyepieces in scopes with FC. Perhaps they feel that they offer their Petzal scopes which have flat fields so we should all buy on of them . Oh to have the budget available!

I always get a little confused by the longer focal length Pentax XWs which apparently suffer from FC in some scopes. Is this because they are designed to work in refractors with FC and if you use them in a scope with a flat field they show the problem?

[John]

As I understand it, the 14mm and above focal length XW's (not sure if the 2" ones are included in this ?) show one type of FC while the shorter ones show the opposite type and to a lesser extent. With the shorter ones the eyepiece FC mitigates FC that is found in many scopes so their overall performance is very good wheras the longer ones add to commonly found scope FC so it becomes a more obvious. This does seem to vary scope design to scope design and some folks are very happy with their 14mm and 20mm XW's wheras some avoid them. If we accept that the XW's were primarily designed for use in terrestrial spotting scopes (not 100% sure if this is so though ?) perhaps FC is not such an issue for that purpose or with the spotting scope designs ?

It's often said that there is no such thing as the perfect eyepiece, at any price, and I guess thats still true !

At least with forums such as SGL and CN these issues are discussed (often at length !) and the prospective purchaser can be aware of them and make a more informed choice on which compromises might be more acceptable to them than others

 

 

[Owmuchonomy]

Well, the best performing EP in my f/7.5 refractors is definitely the ES 82' 14mm.  I am being quite picky with this thread topic because I can accommodate to the effect quite easily but I like to understand why things happen as they do.  Most of the replies above are excellent and thanks for contributing to the topic.  The 4.7mm/ED 120 combo on a good seeing night is also spectacular at splitting doubles.

[Louis D]

13 hours ago, Stu said:

I always get a little confused by the longer focal length Pentax XWs which apparently suffer from FC in some scopes. Is this because they are designed to work in refractors with FC and if you use them in a scope with a flat field they show the problem?

Funny you should bring this up.  While trying to set the proper eyepiece to flattener distance, I had them way too far apart and tried my 14mm XL which has field curvature just like the XW.  To my surprise, it was sharply in focus from center to edge!  I was blown away I had counteracted its field curvature by overcorrecting the refractor's field curvature.  Since it requires unscrewing and screwing different length adapter tubes to achieve the proper distance in front of the diagonal, it's not a practical solution.  I tried pulling the eyepiece up out of the holder, but it wasn't nearly enough.  Perhaps someone could come up with a variable curvature corrector similar to the Paracorr's adjustable top.

[25585]

It seems what is needed is either a telescopic extension tube or a field flattener system that is tuneable. 

So would using a Paracorr 2's device or a Paracorr 2 itself with fracs & cats be a suitable compromise solution due to its design? TV sell separate tuneable tops...

Perhaps TV designing optics for compatibility & use with their fracs should be copied by others - eps designed to give the best results in their own models. 

 

 

[Louis D]

3 hours ago, 25585 said:

So would using a Paracorr 2's device or a Paracorr 2 itself with fracs & cats be a suitable compromise solution due to its design? TV sell separate tuneable tops...

If the field flattener didn't have such a long separation distance (120+mm for the TSFLAT2), something like the tuneable top might be workable.  As it is, the TSFLAT would need over an inch of travel to correct some eyepiece's field curvature beyond what it is doing to correct the scope's curvature.

Using the whole Paracorr 2 in a frac would induce coma of the opposite sign of a Newtonian scope.  I'll have to give my GSO corrector a try sometime in my frac.  I have a vague recollection I have tried it already and that the entire image was mush as a result.  I know I've tried the Panoptic Barlow Interface by itself, and it turns the image to mush.  That might be what I'm recollecting.