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Celestron Ultima LX - distortion

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#1 JonathanK

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Posted 04 February 2013 - 05:17 PM

I recently picked up a Celestron Ultima LX 22mm on a popular auction website :) I've used it a couple of times in my Sky-Watcher 100ED and I'm a bit surprised at the extent of the distortion. Halfway out from the centre to the edge stars won't come to focus, and 70% of the way out they no longer look like stars. The problem seems to be astigmatism, in that a point source focuses to a line orientated either radially or circumferentially, but not to a point.

I didn't pick up on this as an issue with this eyepiece when I was reading reviews online. Did I get a dud?

#2 GlennLeDrew

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Posted 04 February 2013 - 05:30 PM

Can you adjust for a better focus off axis (thereby defocusing at the field center)? In other words, is the problem primarily field curvature?

When I read the thread title, I thought you were referring to distortion, which is a change of image scale with field radius. What you are referring to is aberration.

#3 maroubra_boy

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Posted 04 February 2013 - 05:47 PM

Hi Jonathan,

Duds are rare with branded pieces. Do you have another non-refractor scope that you could try it out in? If you don't, a friend with a scope, again that's not a refractor?

There's another fresh thread on the Ultima LX series, you'll find it here. One poster mentions his 22mm, and he found it fine - its the fifth post in the thread. He used his in an f/6 Newtonian.

I am surprised that it should be performing so poorly. I can't say I've heard of any eyepiece performing so poorly in such a "safe" scope. I'm not standing up for the brand, but trying to solve a problem.

If it is a dud, I hope you're able to return it and get a refund. If the folks you got it from on the auction site is a business rather than an individual, and you got it new, bring it up with them and maybe look for an exchange.

I hate to say it, but I'm assuming all the bits and pieces on the scope itself are all in good order - no focal reducers in the light path? These can cause havoc with eyepieces.

#4 Scanning4Comets

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Posted 04 February 2013 - 05:52 PM

I just listened to an audio test I did into my digital camera while on video mode from Nov 21, 2012. I use this recording technique because I can fall back on it if I need to hear what I saw.

In using a 17mm Ultima LX I saw field curvature 50% of the way out when looking straight into the EP. If I tilted my head it went away. I know ~ weird! 3/4 of the way out showed FC and coma. I don't know if that is possible in an eyepiece?

Near the end of the recording I tried testing again and here is what I saw:

1/2 way out FC starts to show, but is tolerable.
3/4 of the way out FC starts to get bad.
Out to the edge of the eyepiece it was a total mess.
Eye relief was nice and can be used while wearing glasses.
Coatings were a dark green color.

In the 14mm range, I would recommend a Denkmeier over an XW, unless you can't afford it, then I would recommend a used 13mm Vixen LVW. In the 17mm Range I would recommend a 17mm Vixen LVW.
If you're looking for a 22mm, I would recommend a 22mm Vixen LVW over the Ultima LX and a 22mm TeleVue Panoptic as the 22mm LVW has way better edge correction in a fast scope, and the 22mm Vixen LVW is the BEST 22mm eyepiece I have ever tried.

IMO, the 17mm Ultima LX would be a nice EP in a long focal length scope.

Cheers,

#5 Eddgie

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Posted 04 February 2013 - 06:40 PM

Just about all eyepeices have astigmatism, and that is perfectly fine.

Is all the designer has to do is keep the astigmatism low enough that you can't resolve the astigmatic blur, and with modern widefields, this is almost always the case.

A Doublet refractor is usually coma-free, but Radius of Curvature (RC) of the field is usually about 1/3rd of the focal lenght.

Your scope is f/9, so the RC would be about -300mm.

This is only a bit better than a C8 (RC + -270mm), so while the field of most small refractors is coma free, it is usually curved.

If you focus your eyepeice on the center of the field, in the presence of field curvature from either the scope or the eyepeice, or both, and the stars at the outside are not sharp you are seeing the residual astigmatism in the eyepeice. Try re-focusing on the outside of the field.

Agian, even Naglers show this residual astigmatism. It is almost impossible to eliminate, but at best focus, it should be so small as to be impossible to resolve visually. But when it is defocused because of the telescope's curvature, you see the residual astigmatism.

If the stars do re-focus to points, try to then look at the center of the field and see if your eye can automatically re-focus. This is called "Visual Accomdation." If you focus the outside of the field at infinity and then let your eye try to re-focus on the center, it is like looking at a distant object and then putting a newpaper in front of your face. Your eye will try to re-focus on the newspaper. The same thing will happen when you have a scope with nagative field curvature and focus on the outside of the field. When you move your eye back to the center, it will attempt to automatically find the sharpest focus.

This works for younger observers, but usually not for observers over about 40 (the time people start needing progressive glasses or bi-focals).

As for your eyepiece, I had a simialr scope (Vixen 100ED SF) and could not keep longer Naglers focused across the field. The 31mm Nagler in particular did not give very satisfying result. I could not quite fully accomdate the field so I could never get the entire field sharp.

Now using the same eyepiece in my 6" f/8 (RC of about -400mm), the field appears very sharp all the way across.

So, it could just be that the field of the refactor is curved enough that you are seeing field curvature in the scope itself.

Some could be coming from the eyepiece though. Not really any way to know, though the scope itself does indeed have some field curvature.

In fact, it is more complicated because the field curvature of the scope and the field curvature of the eyepiece can compliment or aggrevate the problem.

If the field curvature of the eyepiece is exactly opposite that of the telescope it is used in, the defocus will cancel out and the scope/eyepiece will give perfectly focused fields all the way across.

If on the other hand, both have outward curving fields, it will add together makeing the stars appear even more out of focus.

And this is why most eyepiece designers are going to use a flat field design and allow some astigmatism (Naglers do this, but again the blur is too small to resolve) or a design that curves toward the observer so that it will slightly reduce the defocus caused by the telescope itself.

Bottom line.. Your scope does have a bit of field curvature and if you are sensitive to off axis performance, then you may see it when many observers may not.

Try using visual accomdation.

Again, I could not accomdate the 22mm or 31mm Nagler in my 100EDSF, but Panoptics worked beautifully (for the same size true field, the blur is less magnified making it harder to resolve the blur), but both eyepecies give perfect result in my 6" f/8.

Not saying the eyepeice could not be contributing, but at the same time, more often when we see field curvature, it is just as likely that it is coming from the scope.

#6 maroubra_boy

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Posted 04 February 2013 - 07:24 PM

Eddgie, what a wonderful explaination! You've phrased things in a way I've been struggling to put to words. Excellent!

Alex.

#7 JonathanK

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Posted 05 February 2013 - 04:58 PM

Thanks for the really interesting replies. I should have spotted the other thread! Oops.

I hadn't come across the idea of field curvature before, but your explanations make sense to me. In fact my impression was that stars just wouldn't come to focus at all around the edge of the field, but I'll go back and have another look as soon as I get some clear sky...

To complicate the issue further, I do have some built-in astigmatism of my own, but I don't think this is a big part of the problem as it's only just noticeable in a TV 25mm Plossl.

I'll let you know!

#8 Scanning4Comets

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Posted 05 February 2013 - 06:31 PM

I think the best thing ANYBODY could buy, (including myself), is a Paracorr. The Paracorr cleans up field curvature etc in fast scopes.

Cheers,

#9 jjack's

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Posted 07 February 2013 - 05:40 PM

Hi all
do you think Ultima LX are made to compensate field curvature on Schmidt- cassegrain, so they don't work properly with refractors or newtonian ?

#10 Jon Isaacs

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Posted 08 February 2013 - 06:44 AM

Thanks for the really interesting replies. I should have spotted the other thread! Oops.

I hadn't come across the idea of field curvature before, but your explanations make sense to me. In fact my impression was that stars just wouldn't come to focus at all around the edge of the field, but I'll go back and have another look as soon as I get some clear sky...

To complicate the issue further, I do have some built-in astigmatism of my own, but I don't think this is a big part of the problem as it's only just noticeable in a TV 25mm Plossl.

I'll let you know!


Jonathan:

In your first post, you mentioned that the off-axis stars were astigmatic, i.e. that they focus to lines that presumably change orientation as you pass through focus. Your description does seem to fit astigmatism perfectly. Off-axis astigmatism is the predominate aberration in wide field eyepieces, typically one pays big money for eyepieces that are free from off-axis astigmatism particularly in the longer focal lengths, it is not exactly a surprise that you might see some.

Off-axis astigmatism and field curvature are two different aberrations. If the focal plane is curved rather than flat, the stars are just defocused and not astigmatic. With a curved field, the stars can be brought to focus and should be nice and round but the center and edge cannot be brought to focus simultaneously. Of course it is possible/likely that this may be a combination of both astigmatism and field curvature. Your scope is F/9 so one would normally not expect too much astigmatism but from your description, that does seem to be a problem.

Looking at the Ultima LX line, one thing I noticed is that the 22mm and the 32mm are 6 element designs where as the shorter focal length designs are 8 elements and actually heavier than the longer focal length eyepieces. This indicates a change in the design from the Negative-Positive design uses essentially a built in Barlow to help with off-axis correction to something simpler.

As far as the contribution from your eye is concerned, this should be present across the field of view and not just off-axis.

Jon

#11 Starman1

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Posted 08 February 2013 - 11:25 AM

The story of an eyepiece:
TeleVue 35mm Panoptic, used in a 12.5" f/5.
With a focal length of 1587mm, field curvature should be invisible.

The eyepiece showed severe coma, with the stars at the edge looking like stars in the movie Star Wars when the Millenium Falcon went to light speed.

So I bought a Paracorr coma corrector (this, by the way is ONLY for newtonian reflectors, not refractors or catadioptrics), and this eliminated the coma.

But now I could see the star images at the edge, though round, were defocused. The eyepiece had field curvature itself! The FC had made coma appear worse by flaring the images to a larger size than they really were.

Could I use the eyepiece? I tried focusing not in the center but about half-way to the edge of the field. To my pleasure, I could now accommodate the entire field and everything appeared in focus. But if I focused on the center, the edge of the field appeared defocused.

And so it goes with many widefield eyepieces. And if the eyepiece has astigmatism, which appears likely from your description, and field curvature is visible from your scope and maybe some from the eyepiece, making the astigmatic image appear even worse (larger), then you have just discovered why people say, about eyepieces:
"Widefield, well-corrected, inexpensive. Pick any two, because you can't have all three."

Though do remember that telescope field curvature matters. People with reflectors of 2000mm + focal lengths are unlikely to see any field curvature, ever, unless the eyepiece has tons. And astigmatism in eyepieces is more likely in short focal ratio scopes because of the eyepieces' inability to handle the more oblique light rays near the edge of the field.

In your case, I would look for anastigmatic widefields if you like widefields (the aforementioned Panoptics qualify) and try focusing off-center to mitigate the effects of field curvature. Try the off-center focusing first before you discard the eyepiece.

#12 GlennLeDrew

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Posted 08 February 2013 - 01:38 PM

Are not at least some eyepieces made to accommodate the field curvature typical of most telescopes? To be sure, not all eyepieces are made for a flat image surface only. It would be most handy if all eyepieces were provided with the designed field curvature and recommended minimum f/ratio.

#13 Starman1

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Posted 08 February 2013 - 02:25 PM

Are not at least some eyepieces made to accommodate the field curvature typical of most telescopes? To be sure, not all eyepieces are made for a flat image surface only. It would be most handy if all eyepieces were provided with the designed field curvature and recommended minimum f/ratio.


The only field curvature graphs I've ever seen for an eyepiece line is the ones Pentax once published for the XW line. Otherwise, I'm assuming that many if not most eyepieces have some form of field curvature inherent in the eyepiece. Whether that field curvature has the same sign as the scope is coincidental, since reflectors and refractors typically are different.

The best choice for an eyepiece designer is to produce a flat field and let the field curvature of the scope be the only one detectable since the designer has no clue what kind of scope the eyepiece will be used in. Plus, you don't want a scope without field curvature to see some in the eyepiece either.

But take the Pentax XW line, highly-revered by CN eyepiece buffs:
40mm=+2 diopters positive FC (worst-case curve) and 1 diopter astigmatism (difference between Sagittal and Tangential curves at the edge)
30mm=+2 diopters positive FC and 1.5 diopters astigmatism
20mm=+4 diopters positive FC and 2 diopters astigmatism
14mm=+4 diopters positive FC and 2 diopters astigmatism
10mm=-5 diopters negative FC and 5 diopters astigmatism
7mm=-5 diopters negative FC and 2.5 diopters astigmatism
5mm=-5 diopters negative FC and 0 diopters astigmatism
3.5mm=-5 diopters negative FC and 4 diopters astigmatism
Yet these are highly thought of eyepieces.

Now, the astigmatism I mention is the maximum deviation between the sagittal and tangential curves, not the +/- deviation or the deviation in the center 50% of the field. And the field curvature is not the Petzval surface but the worst of the two curves. The figures would look better if the Petzval curves were used in the illustrations.

But it shows how even high-end eyepieces aren't perfect, and how tolerant we observers can be of these issues, and how not every eyepiece in a line has identical characteristics with every other focal length. Anyone who has collected an entire line of a particular series of eyepiece and compared them knows that.

If eyepieces DO compensate for the field curvature in some telescopes, it is serendipitous, not by design. No eyepiece designer wants his eyepieces to be good only in one type of scope.

As for the Critical F/Ratio threshold, this will be an unlikely piece of relevant information to find anywhere. First, because the company selling the eyepieces won't want to exclude a fair number of customers and second because this is a grey area for the observer. It's well-known that an eyepiece designed for f/8 may not do well in a scope of f/4 because of the angle on the lateral rays, but what about f/6, where some people might be OK with what they see and others would think the eyepiece is a dog? It's a little like coma: some want coma correction at f/5.5 while others aren't bothered by it (or don't see it) at f/4! If the eyepiece corrected coma, the first individual would love it, while the second wouldn't care. And the refractor owner would find it horrible.

So, it's useful to know a company has designed their eyepieces to work well at f/4 and slower (TeleVue comes to mind, but it may not be the only one), but that STILL doesn't say how much FC there is or what the percentage of RD and AMD are at the edge of the field or even if the eyepiece is anastigmatic.

As observers continue to age, their abilities to accommodate the defocus of field curvature diminishes. I think this is a desirable characteristic of eyepieces to know, like focal length and apparent field. An astute observer would know when a potential purchase might exceed his ability to accommodate, and it would definitely prod the manufacturers to design for flatter results.

#14 GlennLeDrew

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Posted 08 February 2013 - 04:18 PM

Don,
The *vast* majority of telescopes used by visual observers inherently have field curvature of the same sign, it being concave toward the sky. There can be a not inconsiderable range, but it should be 'safe' to make an eyepiece so that its own field of best focus is curved to fit a typical or mean radius.

I'm surprised at the reversal (and degree) of curvature for the shorter f.l. eyepieces in the list you just posted.

#15 Jon Isaacs

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Posted 08 February 2013 - 04:32 PM

Are not at least some eyepieces made to accommodate the field curvature typical of most telescopes? To be sure, not all eyepieces are made for a flat image surface only. It would be most handy if all eyepieces were provided with the designed field curvature and recommended minimum f/ratio.


Glenn:

So, what is the field curvature typical of most telescopes? I think TeleVue is about the only one that says much about field curvature and focal ratio. Way back when, they built the MPT, a 5 inch F/4 refractor with a built in field flattener. It was and still is used to test eyepieces. The approach TeleVue took was to build eyepieces with flat fields of view and build telescopes with flat fields of view. They do work nicely together.

They also work nicely in Newtonians because Newtonians have less field curvature than other designs.

I remember reading some conjecture that Pentax eyepieces were designed to match the field curvature of their spotting scopes...

Jon

#16 Starman1

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Posted 08 February 2013 - 06:19 PM

Don,
The *vast* majority of telescopes used by visual observers inherently have field curvature of the same sign, it being concave toward the sky. There can be a not inconsiderable range, but it should be 'safe' to make an eyepiece so that its own field of best focus is curved to fit a typical or mean radius.

I'm surprised at the reversal (and degree) of curvature for the shorter f.l. eyepieces in the list you just posted.

I believe many eyepiece designers try to have the average between tangential and sagittal curves be as close to flat as possible. Of course, this means that the field curvature in the objective will be quite visible if of sufficient diopter to exceed the eyes of the observer. I believe I read accommodation in young people is about 3 diopters and shrinks to less than one for older observers.
Probably explains the growing popularity of flatter field scopes and eyepieces, since we're all getting older.

I would also want flat field eyepieces to be a general rule if I were designing a coma corrector or barlow of some sort. Otherwise the results might be a little unpredictable (or at least not optimum).

Astigmatism and field curvature are "brothers in design". Some designers solve for one, some the other, and some try to make both as negligible as possible. That 35 Panoptic I mentioned earlier was anastigmatic, but had some FC. It wasn't too much for me to accommodate at the time, but might be now since I'm older. It was easier for me to accommodate the field of the 31 Nagler (which replaced the 35 Pan).

If you knew all the characteristics of a particular objective, and wanted to make eyepieces usable only in that scope, you could design eyepieces to work together with the objective to cancel any issues visible to the observer. The coma-correcting eyepieces made back in the '80s died, I think, not because they were expensive, but because they didn't have a wide enough field of view. Plus, at that time, not many observers yet had short f/ratio scopes. Perhaps today we have resurrected the idea of aberration cancellation but simply separated coma correction from the eyepiece. I note that at least the Paracorr also provides some field flattening--good for us crotchety types.

#17 jjack's

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Posted 09 February 2013 - 11:35 AM

Yes but for me Celestron and Meade manufacturers have make a big mistake.
they could build one or two spécial eyepieces especialy made for their best selling SC 8" and 11-12" around the world (near 20mm 70°field 1"1/4 and 30mm 70° 2").I bought them because i prefer a good eyepiece dedicated for my Sc and another one for my newton than have only one with fair performance for the two...

#18 JonathanK

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Posted 20 February 2013 - 01:54 AM

So I finally got the chance to check the EP for field curvature and astigmatism. I think it has both. When stars are in sharp focus in the centre of the field, stars towards the edge are radially oriented lines. By changing the focus, I can make stars near the edge into circumferential lines, or round blobs. The minimum distance from the centre at which stars cannot be brought to acceptable focus is less than half the distance from centre to edge.

What do you think?

#19 Starman1

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Posted 20 February 2013 - 10:02 AM

Sounds like, in your scope, the eyepiece exhibits both astigmatism and field curvature. The 22mm does have 2 fewer elements than the shorter focal lengths. That might be key.






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