My eyepiece has field curvature does it?

[Dave47tuc (David)]

Your eyepiece has field curvature you say?

Well tell me this then!
One eyepiece lets use a 14 mm Pentax xw for example. Many on the Internet forums say this eyepiece has very bad field curvature!! Lets do two examples and see if anyone can answer why?

Example 1.
First scope a 10” F5 FL1250.
Put eyepiece in focuser then look at bright star.
Bring to sharp focus.
Then move star to edge of field. You have to refocus to get pinpoint again.
Re center star in the middle of field, you have to re focus again! Again repeat above seems to be the same. Focus on edge is different to middle! Sweep star from side to side of field on it seems to move in an ark a bit! And goes out of focus from middle to edge of field. Also star seems slightly out of focus at edge when you have focused in the middle of field.
Happens in above eyepiece and even a 13 nagler.

Example 2.
Scope a 5” F8 FL 1025 Apo. Any make but lets say Astro Physics.
Again same eyepieces.
This time focus is the same in the middle of field as on the edge! star is pinpoint in any part of the field.
Sweeping shows no sign of curvature or defocusing. !!

Above examples say that both scopes are perfect for what they are.
The Newtonian is perfectly collimated and seeing is 10/10.
The above is hypothetical test and I wanted to see what your answer would be?
Does the eyepiece have field curvature or is something else going on here?
Look forward to a response.

[cjmarsh81]

I am by no means an expert on optics, but I would think that if the eyepieces work fine in the 5" scope then the error lies with the 10" scope. If the eyepiece performs perfectly in the 5" I don't see how there could be any problem with it.

Is it possible that the 10" scope is not capable of the wide angle the EP needs. I think the 13mm Nagler is an 85deg FOV, and the pentx xw would be something similar. Maybe the 10" cannot handle that wide a FOV. I have this exact problem in my 10" scope with 52deg FOV EP's. I do however suspect the EP's as they are cheap ones but I have never tried them in another scope so who knows.

[Starkler (Geoff)]

My understanding is that a newtonian and most other scopes inherently have a curved focal plane. Im not sure if thats called positive or negative curvature.
My other understanding is that some of the longer fl pentaxs have a focal plane curved in the opposite manner, exacerbating this field curvature issue.

If the scope is one designed with a flat field, the issue would be less noticeable.

FWIW, in my 14mm pentax its noticeable if I go looking for it , but its not intrusive to me. I have seen far worse examples of this with the meade s5000 plossls being shockers for this abberation at f5.

[janoskiss (Steve H)]

Dave, are you reporting actual findings or is it all as you say "hypothetical"? How about with an f5 + paracorr?

I saw field curvature in Geoff's XL14 in the ED80, which is an f7.5. I read others say what Geoff is saying: that field curvature in EP can enhance or cancel that of the scope. So one unit of FC one way can be bad, one unit the other way not so bad. Even though they both amount the same degree of aberration, one EP will be judged superior to the other, because it is better suited to the aberrations of the telescope it is being tested in. Two wrongs do make a right sometimes.

[Dave47tuc (David)]

Originally Posted by janoskiss

Dave, are you reporting actual findings or is it all as you say "hypothetical"? How about with an f5 + paracorr

"hypothetical" but why test a eyepiece in a scope with a coma corrector, meaning fixing the scopes problem coma!
And in that. Is that the problem that people see on how the scope is and not the eyepiece

But I'm asking myself and have been for a long time ( I hope JB can help with this one )
People complain about there eyepieces in there scopes, but its seems to me its not the eyepiece its the scope
look I'm no expert on optics. But I'm just confused with so much talk on field curvature, what do people really see
A eyepiece problem or a telescope flaw

Please take note newbies and all I'm not talking about cheap eyepieces. I'm talking on eyepieces that are not suppose to have problems.
Just trying to get some informative talk on what i think is a over rated problem with eyepieces.

[janoskiss (Steve H)]

Originally Posted by Dave47tuc

"hypothetical" but why test a eyepiece in a scope with a coma corrector, meaning fixing the scopes problem coma!

Precisely because it does fix the coma. Julian uses a paracorr in his big Dob and it is nothing to be ashamed of. ... Same reason you would add a third lens element to the objective of a refractor to correct for false colour: turn it from an achro into an apo (i know it's not quite that simple but you get the idea).

Is that the problem that people see on how the scope is and not the eyepiece

But I'm asking myself and have been for a long time ( I hope JB can help with this one )
People complain about there eyepieces in there scopes, but its seems to me its not the eyepiece its the scope
look I'm no expert on optics. But I'm just confused with so much talk on field curvature, what do people really see
A eyepiece problem or a telescope flaw

Without a telescope the eyepiece is not much use. Obviously people are testing the combination of telescope and eyepiece. People want eyepieces that will work in their telescopes. In many reviews you read that an EP worked well in one type of telescope not so well in another. Or it worked well in fast Newt with a coma corrector but not without. But another EP might work well even without the coma corrector. People are reporting on the performance of combinations of optical components.

Someone here (Mick P maybe?) bought one of my Stratus EPs for use in an SCT. It worked well in my f6 Dob so I assumed it would work even better in the slower SCT. Apparently not. The owner reported severe blackout problems.

That's the way the cookie crumbles.

[Dave47tuc (David)]

I have no problems with people using parracors or anything that helps improve there scopes preformance.
I'm not saying anything against that.
Nor am I saying that what works in that scope may or may not work in others. Thats to be worked out for each scope.

And yes you get the eyepiecs that will work in YOUR scope best.

But what I'm finding hard to get is some write ups on eyepiecs say that this "nagler" say is no good because of its field curvature. Then I read the scope was a 4.5 such and such. But it work really well in my F8 Apo
Tell me thats the eyepiecs fault

[Starkler (Geoff)]

Well I guess the main lesson is that when reading eyepiece reviews, or indeed reviews of anything, pay careful attention to the test conditions.

Yes some people get very opinionated about certain things without considering all factors involved.

[janoskiss (Steve H)]

By the way, if you want non-telescope-contaminated info, XW field curvature plots, showing deviation of focal surface from an ideal focal plane, here: http://www.pentax.co.jp/japan/tech/xo-xw/64.html

And in case you're interested in transmission vs wavelength, it's here: http://www.pentax.co.jp/japan/tech/xo-xw/63.html

The amount of technical data Pentax discloses about their EPs is impressive.

[astropolak (Joe)]

I am so glad someone had raised the issue here.
In my 8" LX90 (F10 SCT) I have field curvature affecting any eyepiece above 10mm FL. Whilst meeting some great people at SPSP I got a chance of trying different eyepieces in my scope (I mean to say great eyepieces like Panoptics and Naglers) and they ALL showed field curvature. Thanks to Don from Bintel and the Iceinspace clan.

I tested them against Nagler 17mm and Uwan 28 I own and yes they show it too. I have also tried my eyepieces in someone else's LX90 - same problem.

I think I have reached the limit of my LX90 (price / performance limit that is).
I am told that the new RC scopes from Meade perform better but they are expensive...

RGDS Joe

[janoskiss (Steve H)]

Originally Posted by astropolak

I think I have reached the limit of my LX90 (price / performance limit that is).
I am told that the new RC scopes from Meade perform better but they are expensive...

How about trying a field flattener?

[astropolak (Joe)]

Originally Posted by janoskiss

How about trying a field flattener?

I am told field flatteners are for astrophotography and will cause vignetting in low power wide angle eyepieces.

Please correct me if I am wrong here but there is no such thing as dedicated field flattener - there are focal reducers that flatten the field as a by-product..

RGDS Joe

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[janoskiss (Steve H)]

COuple of nights ago I was comparing EPs in my f12-13 Mak. The 19mm Panoptic, 25mm Antares Elite and Meade 5000 26mm widefield Plossl all showed the same aberrations in the same places in the true FOV. Beyond about the 0.9 degree true field mark stars begin to not focus properly. And at out at edge of a 1.2 degree field the aberrations become very distracting. This is obviously showing shortcomings of the optics of the scope because every eyepiece tells the same story. This also tells me that a focal reducer will probably not help me get wider views because anything in the outer portion of the wider field will be unusable.

OTOH if you were to test an EP in a number of different telescopes and showed the same problems in the same places in the apparent FOV then there is more likely something wrong with the EP itself.

Dave, I would guess that the example you give would indicate that there is no significant field curvature in the EP, if indeed you get an edge-to-edge practically perfect view in a scope with a perfectly flat field, like a premium apo. But we will have to wait and see what the actual results are when you do test out that EP in the AP apo.

[Dave47tuc (David)]

Originally Posted by janoskiss

Dave, I would guess that the example you give would indicate that there is no significant field curvature in the EP, if indeed you get an edge-to-edge practically perfect view in a scope with a perfectly flat field, like a premium apo. But we will have to wait and see what the actual results are when you do test out that EP in the AP apo.

I have not had a chance to do any night tests. In the daylight the Pentax was very sharp and clear. I saw no field curvature.
I have done a night test with a 12 nagler and 22 panoptic in my old dob and a 5"Tak.
I short the eyepieces where perfect in the Apo but did show coma in the F5 Dob.
The last 3rd Of the field in the Pano did seagull a bit but the stars where pinpoint in the apo across all the field.
I have no doubt that in my MK-67 all the eyepieces i have will be super sharp and keep me happy, thats what counts I suppose.

[ausastronomer (John Bambury)]

Dave,

This was something I alluded to recently in this post on the 14mm Pentax XW

http://www.iceinspace.com.au/forum/s...ead.php?t=8815

Originally Posted by ausastronomer

FWIW, the 10mm Pentax XW is a slightly better performer than the 14mm Pentax XW IMO, particularly in fast newtonians *** and is one of the best eyepieces I have ever used in any telescope. You can stack whatever Nagler you like up against it. In addition the 7mm Pentax XW is just as good as the 10mm.

*** The 14mm Pentax XW when used in a fast F-Ratio scope having a curved focal plane such as a Newtonian, exhibits mild field curvature. Its minor and not an issue IMO. This is not evident to any degree with the 7mm and 10mm Pentax XW's. The 14mm Pentax when used in a telescope design having a flatter focal plane such as an apochromatic refractor does not exhibit this field curvature.

ENJOY !!!!!! sounds like you're already converted

CS-John B

I saw your thread the other day, but haven't had time to reply in the required detail, it gets quite complicated and necessitates a lengthy reply to explain it all properly. I may not have the chance to do this for another couple of days but will do so when I have more time. In the interim, this book contains a lot of the answers if you have access to it at your local library or Astronomy Club Library. Its actually a book worth owning IMO, if you're interested in what makes all this stuff tick

https://www.willbell.com/tm/tm6.htm

CS-John B

[Dave47tuc (David)]

Originally Posted by ausastronomer

Dave,
I saw your thread the other day, but haven't had time to reply in the required detail, it gets quite complicated and necessitates a lengthy reply to explain it all properly. I may not have the chance to do this for another couple of days but will do so when I have more time. In the interim, this book contains a lot of the answers if you have access to it at your local library or Astronomy Club Library. Its actually a book worth owning IMO, if you're interested in what makes all this stuff tick

https://www.willbell.com/tm/tm6.htm

CS-John B

Thanks for the link John.
Was wondering if you would respond.
Anything you mite add will be of great interest

Cheers.