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XLT Eyepiece Performance Tests

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#51 Dave Bush

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Posted 30 October 2019 - 11:15 AM

My experience is based on using 22mm and 13mm LVW in an XLT C8 with and without the 0.63 reducer/corrector.

 

I found a distinct improvement in the middle third of the field.  The central third was pretty sharp either way. 

Based on what i had read at the time I assumed that the the improvements were due to the decrease in focal curvature.  However CNer Frank ( frankstar8..i think) described the theory behind the improvement that it corrects for coma and not focal curvature per se.  So while the aberrations are indeed reduced, it was not a "flatter" field per se..which would have require a decrease in FC.  Rather, the improvements are due to a decrease in coma. 

 

I believe this is also what Vla is stating.  

Well, yes, it does reduce coma a little but it also flattens.  And by that, I mean, without it, you find stars in focus at the center but out of focus at the field edge.  With the r/c in place this is nearly eliminated.  And this is what I and others refer to when we say it "flattens" the field.

 

If you care to look, there are threads here where this has been debated over and over.  Some argue that it can't do that and they base their argument on optical theory and such.  But there are legions of users who from actual visual experience will attest to that fact that it does indeed flatten the field (by my definition above) 


 

#52 Viktorious

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Posted 30 October 2019 - 11:27 AM

Well, yes, it does reduce coma a little but it also flattens.  And by that, I mean, without it, you find stars in focus at the center but out of focus at the field edge.  With the r/c in place this is nearly eliminated.  And this is what I and others refer to when we say it "flattens" the field.

 

If you care to look, there are threads here where this has been debated over and over.  Some argue that it can't do that and they base their argument on optical theory and such.  But there are legions of users who from actual visual experience will attest to that fact that it does indeed flatten the field (by my definition above) 

The important thing for me is that both of you indeed report improvement. Be it from flattening or coma correction, I'm a happy camper either way!

Also feel like we've drifted off topic. So as to not get the thread locked by a moderator, any answers to my previous post which was on the topic of the eyepieces (#45)? smile.gif tongue2.gif


Edited by Viktorious, 30 October 2019 - 11:30 AM.

 

#53 eklf

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Posted 30 October 2019 - 11:34 AM

Well, yes, it does reduce coma a little but it also flattens.  And by that, I mean, without it, you find stars in focus at the center but out of focus at the field edge.  With the r/c in place this is nearly eliminated.  And this is what I and others refer to when we say it "flattens" the field.

 

If you care to look, there are threads here where this has been debated over and over.  Some argue that it can't do that and they base their argument on optical theory and such.  But there are legions of users who from actual visual experience will attest to that fact that it does indeed flatten the field (by my definition above) 

What confounds an objective interpretation of a subjective observation regarding focal curvature is that different people have different accommodations to it.

 

Maybe you and others have very good adaptation - so that once the assymetric shape of coma is removed, the eye is better able to focus the bloated but symmetric out-of-focus star into a better point. 

 

A good test of this would be to see photographically if the focal curvature at the edge is nearly eliminated.  This would remove the subjective accommodations of the observer out of the equation (but then, a flat sensor is not a curved eyeball..so ..).  I dont do AP so i can't speak to this this, but maybe others who have can chime in.  Although it appears to me that if this were to be true, then there would not be a need for edge HD.


Edited by eklf, 30 October 2019 - 11:39 AM.

 

#54 Vla

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Posted 30 October 2019 - 11:43 AM

Yes, that's why I asked to clarify. Many people mistakenly call visual field "flat" when they don't see blurring in the outer field. It may and may not be. Well corrected field aberrations-wise can be strongly curved and appear "flat" visually in that sense, because eye accommodates (refocuses) as it goes beck and forth between the mid field and periphery. I wouldn't be surprised if Celestron's marketing department used that terminology, either because of ignorance, or calculated effect.


 

#55 Dave Bush

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Posted 30 October 2019 - 12:17 PM

What confounds an objective interpretation of a subjective observation regarding focal curvature is that different people have different accommodations to it.

 

Maybe you and others have very good adaptation - so that once the assymetric shape of coma is removed, the eye is better able to focus the bloated but symmetric out-of-focus star into a better point. 

 

A good test of this would be to see photographically if the focal curvature at the edge is nearly eliminated.  This would remove the subjective accommodations of the observer out of the equation (but then, a flat sensor is not a curved eyeball..so ..).  I dont do AP so i can't speak to this this, but maybe others who have can chime in.  Although it appears to me that if this were to be true, then there would not be a need for edge HD.

Well, indeed some can accommodate to it better than others.  But that doesn't negate the vast number of reports that support the flattening.  All I might add, by people like me who know the difference between coma and FC

 

Also, this flattening has been reported photographically.  Again, do the research in the forums. 

 

Oh, and just to be clear, the r/c doesn't eliminate FC but rather greatly reduces it.  Thus the EdgeHD has its place.


Edited by Dave Bush, 30 October 2019 - 12:19 PM.

 

#56 Dave Bush

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Posted 30 October 2019 - 12:25 PM

Yes, that's why I asked to clarify. Many people mistakenly call visual field "flat" when they don't see blurring in the outer field. It may and may not be. Well corrected field aberrations-wise can be strongly curved and appear "flat" visually in that sense, because eye accommodates (refocuses) as it goes beck and forth between the mid field and periphery. I wouldn't be surprised if Celestron's marketing department used that terminology, either because of ignorance, or calculated effect.

See my reply above.

 

As for Celestron, the designer of the r/c, Jim Riffle, is a well respected optical designer with no-doubt greater credentials and knowledge than most of not all in this forum 


 

#57 Rustler46

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Posted 30 October 2019 - 01:59 PM

Nice presentation Russ!

Some questions to you and the other experts following this if I may.

- First regarding your (Russ) reducer in connection to Eddgie's post #40 where I read "... Celestron never says their reducer is a flattener and in fact, the instructions hint that it is not a full flattener, though it is possible that it does improve field flatness. ...". Since I recently entered the SCT world I don't know of all equipment that has been available through the years. The only new Celestron f/6.3 reducer I can find is the reducer/corrector (#94175) which is the one I have now bought. In the manual for this is stated "In addition to reducing the focal length and f/ratio the Celestron Reducer/Corrector Lens also reduces field curvature significantly so you get a flatter, well corrected field." So does this differ from what Eddgie discusses and in that case what reducer did you use (flattening or not)?

 

Here is a partial reply to your questions. The focal reducer I use is the #94175. The literature it came with states the same as you mentioned above:

  • "In addition to reducing the focal length and f/ratio the Celestron Reducer/Corrector Lens also reduces field curvature significantly so you get a flatter, well corrected field."

Mine was purchased in 2008. So the current version may differ.


 

#58 Eddgie

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Posted 30 October 2019 - 02:43 PM

That comes right from the documentation that comes with the reducer/corrector.

 

It does flatten the field.  This is quite obvious to anyone who has actually used one.  

It does "flatten" the field but it does not make the field flat.  It "reduces field curvature significantly." That is the same with the EdgeHD.  Celestron nowhere says that it makes the field flat, it says that it reduces curvature to about 1/3rd of the standard design.

 

Some of the perceived reduction in curvature is simply a reduction of the of the blur diameter by the 37%. 

 

The instructions also say that when imaging, the field should be focused between the center and the outside edge.  This is pretty standard practice when the field is curved.

 

So, this is what I said earlier. The reducer/corrector does make the field flatter but it does not make it "flat"

  

 

No doubt that it improves the view so some degree, but I have used both and I do not think the reducer provides the same level of correction as the EdgeHD does. It is an improvement over no reducer, but at the same time, I think the field is better using a high quality eyepiece like the 41mm Panoptic rather than a reducer and something like 24mm Pan.  But that is me and everyone's experience varies, so no right or wrong there. 


Edited by Eddgie, 30 October 2019 - 02:59 PM.

 

#59 Vla

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Posted 30 October 2019 - 02:51 PM

See my reply above.

 

As for Celestron, the designer of the r/c, Jim Riffle, is a well respected optical designer with no-doubt greater credentials and knowledge than most of not all in this forum 

Not sure what "see my reply above" should mean. I saw it, and commented, along with another poster. Did I miss anything? What seems flat visually may be actually strongly curved. Common mixing blurring due to off axis aberrations and due to field curvature.

As for Celestron's optical designers, be sure they are not writing their marketing materials.


 

#60 Eddgie

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Posted 30 October 2019 - 03:10 PM

How does the field curvature of a C-11 XLT change with/without the Celestron f/6.3 focal reducer? Does the shorter focal length and wider FOV have a worse edge of field performance? What is the story with say a 32mm 1-1/4 inch eyepiece with a field stop of 27mm? Same linear field size but lower magnification must figure in somehow. What's your take on this?

My experience was that the field was not as sharp using a reducer as it was using a wider field high quality eyepiece that produce the same size true field..  For example, using a 32mm Plossl with a reducer did not produce anything close to the sharpness of using a 41mm Panoptic, but the Plossl working at a faster focal ratio is the culprit perhaps more than the field curvature but they add together. 

 

Now the 24mm Panoptic does better than the 32mm Plossl, but it does not produce the true field that one could get in a 41mm Panoptic, so here, the better comparison would be (in my opinion) to the 35mm Panoptic, which I think does a very good job in the SCTs. 

 

And back to the OP, my own experience was that no eyepiece or reducer produced a field that I found as pleasing as using Panoptics, which all worked reasonably well in the standard SCTs (and I have owned every Panoptic and tried them all), and while not as good as the EdgeHD, these were the eyepieces that produced what was for me, the most pleasing fields. 


 

#61 Viktorious

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Posted 30 October 2019 - 03:19 PM

So in the midst of all this discussion about the reducer, since I have acquired and will use it, I'll move on to eyepieces.

Perfect, new reply popped up while I typed!

 

My experience was that the field was not as sharp using a reducer as it was using a wider field high quality eyepiece that produce the same size true field..  For example, using a 32mm Plossl with a reducer did not produce anything close to the sharpness of using a 41mm Panoptic, but the Plossl working at a faster focal ratio is the culprit perhaps more than the field curvature but they add together. 

 

Now the 24mm Panoptic does better than the 32mm Plossl, but it does not produce the true field that one could get in a 41mm Panoptic, so here, the better comparison would be (in my opinion) to the 35mm Panoptic, which I think does a very good job in the SCTs. 

 

And back to the OP, my own experience was that no eyepiece or reducer produced a field that I found as pleasing as using Panoptics, which all worked reasonably well in the standard SCTs (and I have owned every Panoptic and tried them all), and while not as good as the EdgeHD, these were the eyepieces that produced what was for me, the most pleasing fields. 

We now have some eyepieces tested by Russ and this info from Edd. Can I from this gather that the Panoptics have some field curvature that make them so nice with the standard SCT? 

 

What eyepiece would you expect to give best views WITH the reducer? Asking this since I'm still looking for a ~24 mm eyepiece to give me 3-4 mm exit pupil (still with reducer then since I'm confined to 1.25"). Could be what eyepiece you theoretically think would be best and then I can try that and present my findings at some point. 

 

I have the Celestron 40 mm Plössl to use without reducer but at 43° AFOV I really feel like looking through a porthole with that. Therefore I'm looking at something around the 70° region (thinking ES 68° 24 mm).

 

 

Can also add more fun quotes from the Celestron documentation while that discussion is still going on:

 

"Although the Reducer/Corrector Lens diminishes the small amount of field curvature common to all Schmidt Cassegrain telescopes, it does not completely eliminate it."

 

"Because the field of view is very close to flat, you can leave the Reducer/Corrector Lens in place at all times. High power views will provide flatter fields all the way to the edge, both visually and photographically."

 

Then they also suggest that one does NOT use eyepieces longer than 35 mm focal length due to the short focal length of the scope (for those interested).


Edited by Viktorious, 30 October 2019 - 03:37 PM.

 

#62 Dave Bush

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Posted 30 October 2019 - 03:20 PM

Not sure what "see my reply above" should mean. I saw it, and commented, along with another poster. Did I miss anything? What seems flat visually may be actually strongly curved. Common mixing blurring due to off axis aberrations and due to field curvature.

As for Celestron's optical designers, be sure they are not writing their marketing materials.

It means to look at the previous reply that I made above as in scroll up and you'll see it in post #56.  Pretty straightforward actually.

 

And there you'll see that Eddgie is re-stating what I said, namely that if flattens the field noticeably. 


 

#63 Rustler46

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Posted 30 October 2019 - 04:55 PM

- Other questions are regarding optics and eyepieces so I accept if I'm told to go and search the internet, maybe I have misunderstood things that have been brought up here. Some may also be brought up in e.g. Daniel's thread but don't remember (and the thread digressed at times..), if so just send me back there. Still posting, in case anyone feels inclined to be bothered and answer.

  • Eyepieces struggling in faster scopes (eyepiece aberrations): the issue lies with the steeper light cone so the eyepieces simply need to be made better to cope with this, correct? I see this as the aberrations other than field curvature. As such I assume this is technically unrelated to the field curvature of the scope and eyepiece. E.g. a 12" f/5 reflector would have a quite flat field but is still a fast scope so what would be the case here? Assuming a premium flat eyepiece is definitely needed, both to cope with the light cone and since the scope field is quite flat.
  • These tests are more for investigating the field curvatures to see if an eyepiece with a specific field curvature is the best match for XLT SCT. I thought, when bringing up the reducer initially, that a premium flat eyepiece should perform better(?) at a corrected f/6.3 than at uncorrected f/10. The same eyepiece should perform equal at either ratio if the scope curvatures were the same (in both cases assuming this premium eyepiece has no issues with the faster light cone). These thoughts are connected to Eddgie's post #40 and to the question regarding what type of reducer was used here.

As a reason for the questions above I see the Nagler doesn't appear to perform worse at f/6.3, the others do. So, in connecting to what type of reducer and post #40 again; would you think this is due to the faster light cone but same field curvature, and the non-premium eyepieces struggling with the light cone (other eyepiece aberrations becoming apparent). Or is it more due to flattened scope field and mainly the field curvature of the eyepiece becoming apparent?

I wouldn't take my subjective assessments of eyepiece performance (with or without focal reducer) too seriously. These tests were made on different nights and are just an estimate of percent radius where some level of performance was noted. I didn't have any eyepiece reticle with radius percent markings. So these percents are quite rough. Also I was much more tolerant of unacceptable performance on the mediocre eyepieces than for the premium ones. For example the 5% of radius unacceptable in the Nagler was MUCH better than the 25% unacceptable in the military Erfle. And the aberrations across the FOV of the premium ES 100° were much less than the aberrations in the outer 1/2 of the Celestron 32mm Plössl. 

 

It would be most useful to have some assessments by those with a more critical eye.


 

#64 Viktorious

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Posted 30 October 2019 - 07:22 PM

I wouldn't take my subjective assessments of eyepiece performance (with or without focal reducer) too seriously. These tests were made on different nights and are just an estimate of percent radius where some level of performance was noted. I didn't have any eyepiece reticle with radius percent markings. So these percents are quite rough. Also I was much more tolerant of unacceptable performance on the mediocre eyepieces than for the premium ones. For example the 5% of radius unacceptable in the Nagler was MUCH better than the 25% unacceptable in the military Erfle. And the aberrations across the FOV of the premium ES 100° were much less than the aberrations in the outer 1/2 of the Celestron 32mm Plössl. 

 

It would be most useful to have some assessments by those with a more critical eye.

I for one am taking your findings as exactly that, subjective assessments. Also the tolerance for the mediocre eyepieces was according to me possible to read between the lines. As I say I'm still looking at something like the ES 68 for my upgrade, not throwing a thought towards a Plössl. 

 

All kidding aside with the "You know, I'm somewhat of a scientist myself", I actually am. As such I know that interpreting a single experiment is **** near impossible, you need a lot of data to reach a supported conclusion. That's why what you have done is so great and important, how can we ever find the best eyepieces for our specific scopes if no one does these experiments?! This is also why I eagerly await to see Daniel's results turn up at some point and why more people should chime in with their assessments. What's great to see here is that your assessments support the discussed theories and match what most other throw in the discussion.

 

So thank you for starting this thread and presenting your findings, and thanks to all others here who have shared their experiences! As someone who recently got his first SCT, your knowledge is of great importance and help to me in my search for accessories to my new scope! 

 

Writing this on my phone after working an evening shift, so apologies if there are weird sentences or spelling errors. 


Edited by Viktorious, 30 October 2019 - 07:33 PM.

 

#65 eklf

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Posted 30 October 2019 - 08:12 PM

 But that doesn't negate the vast number of reports that support the flattening.  All I might add, by people like me who know the difference between coma and FC

 

Also, this flattening has been reported photographically.  Again, do the research in the forums. 

 

 

When several posters who understand optics state that the 4 elements corrector/reducer like the celestron unit can correct coma but not field curvature, is taken along with the proposition that reducing coma will allow people to accommodate curvature better supports the idea that the "flattening" effect is primarily due to coma correction.

 

FWIW, my inspections of photographical comparisons as well as spot diagrams also supports this.

 

I am not including the effects of the specialized correctors like starizona which appear to be a different combination.

 

However, I am perfectly fine with your belief.  But it might help others reading this forum that there is an alternative explanation that is better supported by theory (as worthless as it might be perceived to be).


Edited by eklf, 30 October 2019 - 08:19 PM.

 

#66 Jaimo!

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Posted 31 October 2019 - 12:02 AM

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Pending moderator review.


 


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