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Detecting 8" SCT field curvature by refocusing

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#1 Vla

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Posted 01 November 2019 - 10:50 AM

In the thread "XLT eyepiece performance test" it was described by a poster that by refocusing he could detect field curvature (presumably of the image of the objective, assuming no significant eyepiece field curvature) in the C8 SCT at about 50% of the field radius, and higher. That was part of the argument that the f/6.3 SCT reducer/corrector "significantly flattens the field", since with it, according to the poster, significantly less refocusing was needed. I intended to post this in the mentioned thread, but since it was locked, I have to do it this way.

 

Raytrace simulation below shows coma in the widest visible field in the C8 (the poster wouldn't specify which eyepiece he was using, so we'll assume the widest field, when the effect of field curvature is the greatest) on both, flat and best curved field (R=-245mm). Assuming 30mm f.l. eyepiece - a well corrected Nagler-like with ~80deg AFOV - so the Airy disc angular radius (4.6/P, P=exit pupil diameter in mm) is 1.5 arcminutes, the sagittal coma - lower 1/3 of the comatic image seen in the eyepiece with sufficiently bright stars - is about three times larger (nearly 5 arcmin) at 50% radius, and six times larger at the field edge. In either case it is large enough to be seen as a blur, so no significant refocusing is possible. There is some difference in the blur shape, but at the actual scale it is indistinct, more so considering that eye cannot see the flat field, since being near-instantly accommodating to near its limit of accommodation (in other words, the blur before and after refocusing - if it would be possible at all - would be even more similar).

 

Accommodation required for the 50% field radius is 0.14mm, which with 30mm f.l. translates to less than 0.5 diopters, i.e. near-instant accommodation, making any refocusing impossible. Even at the edge, needed accommodation is less than 2 diopters, to which most eyes will readily adjust (average eye with the smallest distance of distinct vision of about 25cm has 4 diopters accommodation power). Evidently, field curvature in the C8 cannot be detected by refocusing.

 

In the case of observer with much lower than average accommodation power, effective refocusing would be impossible simply because of the overall similarity of the flat vs. curved field blur. So even the fact that f/6.3 reducer, by making the image field correspondingly smaller, significantly reduces apparent field curvature (if it remains nominally unchanged), wouldn't change nothing in the fact that coma prevents effective refocusing w/o reducer. Since reducer does correct coma, or most of it, its field should look cleaner. But that does not equate "less curved". Raytrace shows that reducer of this configuration actually makes field curvature somewhat worse, but the combined effect of smaller field and corrected coma makes the visual field look more "in focus" or, as people often mistakenly describe, "flatter".

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#2 Pat Rochford

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Posted 01 November 2019 - 02:12 PM

Maybe it's just me - and if so, please accept my apology - but is it really necessary to take this topic to the extreme of using ray tracing to prove a point? 

 

The C8 has been around for right at 50 years, the 6.3 reducer/corrector for about thirty.  I would say they've been the proverbial success story, seeing as both are still in production.  In the thirty-five years I've owned my C8, I've never lost any sleep over any field curvature or coma lurking in the outer field of view.  The 8" F/8 Newtonian I owned previously (w/flat field and negligible coma), was a beast to transport and set up.  The C8 got used ten times as much because it was so much easier to use.  And the views were still extremely satisfying.  When Jim Riffle's (Celestron's) reducer/corrector became available, it made a great product even better.  I doubt I'll ever forget the first time I looked through Rod Mollise's C8 with an R/C.

 

I have no training in optical theory so I'm not going to dispute any of the diagrams in the above post.  I have  been involved in this hobby for slightly over fifty years though, and am experienced enough to recognize good views of celestial objects in an eyepiece.  The already excellent image put up by the C8's design and implementation is dramatically improved by the R/C.  It's wider and it appears flatter.  How it accomplishes this and to what degree seems a bit inconsequential to me.  And to others as well, judging by the many dozens of related posts over the years on these forums.  

 

Arguing to this degree over the of improvement of the reducer/corrector - to me anyway - takes away from the enjoyment of hearing about others' experiences with this combination of optics, which are both affordable and available to the average amateur astronomer.  All in favor of taking your C8 out tonight and looking through it without a copy of Rutten & van Venrooij's book sitting in your lap, raise your right hand.


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#3 eklf

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Posted 01 November 2019 - 02:51 PM

Thank You Vla for taking the time and describing the optical physics behind the improvement effects of the 0.63 celestron reducer/corrector.  Having some interest in the underlying theory behind optical improvements, its helpful to have this level of description made easy to assimilate.  


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#4 eklf

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Posted 01 November 2019 - 03:07 PM

Maybe it's just me - and if so, please accept my apology - but is it really necessary to take this topic to the extreme of using ray tracing to prove a point? 

 

The C8 has been around for right at 50 years, the 6.3 reducer/corrector for about thirty.  I would say they've been the proverbial success story, seeing as both are still in production.  In the thirty-five years I've owned my C8, I've never lost any sleep over any field curvature or coma lurking in the outer field of view.  The 8" F/8 Newtonian I owned previously (w/flat field and negligible coma), was a beast to transport and set up.  The C8 got used ten times as much because it was so much easier to use.  And the views were still extremely satisfying.  When Jim Riffle's (Celestron's) reducer/corrector became available, it made a great product even better.  I doubt I'll ever forget the first time I looked through Rod Mollise's C8 with an R/C.

 

I have no training in optical theory so I'm not going to dispute any of the diagrams in the above post.  I have  been involved in this hobby for slightly over fifty years though, and am experienced enough to recognize good views of celestial objects in an eyepiece.  The already excellent image put up by the C8's design and implementation is dramatically improved by the R/C.  It's wider and it appears flatter.  How it accomplishes this and to what degree seems a bit inconsequential to me.  And to others as well, judging by the many dozens of related posts over the years on these forums.  

 

Arguing to this degree over the of improvement of the reducer/corrector - to me anyway - takes away from the enjoyment of hearing about others' experiences with this combination of optics, which are both affordable and available to the average amateur astronomer.  All in favor of taking your C8 out tonight and looking through it without a copy of Rutten & van Venrooij's book sitting in your lap, raise your right hand.

I don't think the post is meant to minimize the improvement effects of the corrector.  On the contrary, it underscores this quality by detailing the physics behind the improvement.  Understandably, it may not be of universal interest; but its  easy for those not interested to move on from such posts.  Not as easy, for those interested, to find such detailed descriptions of aspects that one values.  

 

Speaking only for myself, I enjoy reading these and similar posts from experienced CNers who take the time to educate others interested in understanding optics.  And since this information is archived on these forums, it provides a rich treasure trove for future readers interested in these topics.

 

I hope these CNers continue to post and educate.


Edited by eklf, 01 November 2019 - 03:14 PM.

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#5 Vla

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Posted 01 November 2019 - 03:12 PM

Maybe it's just me - and if so, please accept my apology - but is it really necessary to take this topic to the extreme of using ray tracing to prove a point?

It is not proving the point, it is saying it as it is. There's nothing in it anti-Celestron, or even anti-SCT, merely showing C8's optical characteristic (or Meade's, for that matter), i.e. whether its field curvature can be detected by refocusing, or not. The coma does look horrific, but as you and many others know, it is unintrusive for most observers in the actual use. I can understand that you are not interested in how it all works, but the solution in that case is very simple: just skip it. I'm pretty sure some others (beside me) find it interesting.



#6 Vla

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Posted 01 November 2019 - 03:18 PM

Thank You Vla for taking the time and describing the optical physics behind the improvement effects of the 0.63 celestron reducer/corrector.  Having some interest in the underlying theory behind optical improvements, its helpful to have this level of description made easy to assimilate.  

Looks like I can't help it wink.gif  Anyway, appreciate your support. 


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#7 bbqediguana

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Posted 01 November 2019 - 06:58 PM

What I see often on these type of threads is a lot of technical information - which I agree is of interest to many people. I think where it becomes an issue is when poster A says "Yeah, I'm so happy with my f/6.3 focal reducer - it makes my view nice and flat and my stars are all in focus", and then poster B says "Oh, you're so wrong. The theory states this shouldn't be".

 

Personally? I don't care either way. The other night I was looking through my Meade 8" SCT with a wide angle 24mm eyepiece and I focused the view in the middle and looked out at the edge and thought to myself "Yep, those stars aren't in focus." I then focused the outside stars and looked in the middle and thought "Yep, those stars aren't in focus." I then shrugged and continued to enjoy my telescope.

 

Do I read the technical posts to educate myself? Yes. Do I enjoy them? Absolutely! Do I sell off my 8" SCT because someone may imply it's crap based on theory? Nope.

 

BTW: This isn't directed at the OP - this is a common theme I've seen over and over. It also happens in the eyepiece forum. Don't tell anyone in there that you love Plössls! They'll go ape... um... crazy. smile.gif

 

Cheers!

 

Rick


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#8 Eddgie

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Posted 01 November 2019 - 07:49 PM

Thank you Vla, that was very interesting and more or less what I had said elsewhere in a post.

 

I had thought that much of the improvement of the corrector was simply due to the fact that the blur diameter was decreased and maybe some improvement in coma but when I used a corrector, I did not feel that the edge of field was as good as using a 2" eyepiece with excellent correction.  Some of the benefit of the coma reduction is probably negated by the astigmatism of the eyepieces (slightly out of focus) working with the faster light cone.

 

Actually, older observers can have less than .5 dipopters of visual accommodation, and those with lens implants have almost none (and I fit both of those categories). A great many CN members are probably of the age where they have very limited visual accommodation.

 

When I was younger, using eyepieces like the 35mm Panoptic, I found the field to be fairly sharp at the edges.  As I got into my late 50s, I had more and more trouble and at 60, I was not able to accommodate hardly any curvature.

 

I could make the field of my C14 reasonably sharp by focusing half way out from the center (41mm Panoptic) but with the 31mm Nagler, even though the field was a bit narrower, even focusing half way out would not produce tight stars.

 

The same eyepieces in the EdgeHD were near perfect at the edge of the field. 

 

I for one am very pleased that you took the time to trace these out.  It may not settle anything in the larger picture, but I like having the data.


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#9 Vla

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Posted 01 November 2019 - 09:11 PM

I don't think the post is meant to minimize the improvement effects of the corrector.  On the contrary, it underscores this quality by detailing the physics behind the improvement.

It should be enough just to look at the coma, to realize that just correcting it (or most of it) has to produce significant improvement. The stronger curvature is at least compensated for by the reduced field size, and probably does actually appear flatter, in addition to coma correction. One thing that has nothing to do with the performance of the reducer is the considerably faster focal ratio. Not all eyepieces will perform well with it, so personal experiences will also vary. I never said, nor implied that the reducer does not perform well.


Edited by Vla, 01 November 2019 - 09:12 PM.

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#10 Vla

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Posted 01 November 2019 - 09:23 PM

Eddgie, I'm sure some people have pretty low or nearly non-existent accommodation. At 60, I should be content with mine, considering how much I was abusing my eyes staring at the computer screen. With rested eyes, still can focus at 35cm, or so (but certainly not as clear at any distance as I was able decade or two ago, and earlier). I found that only a few minutes a day of eye exercises does make a difference, especially when combined with giving the eyes rest every 2-3 hours (more often doesn't hurt either).



#11 MrJones

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Posted 02 November 2019 - 09:19 AM

For what it's worth I found this nice photographic comparison a few years ago done with a C9.25 that does appear to show a f/6.3 reducer/corrector is reducing coma, and possibly adding astigmatism at the edges. I always wondered what the weird shape at the edges was but it's generally been attributed to astigmatism elsewhere.

 

https://astro.ecuado...-dslr-on-c9-25/

 

Here's another photo test that people have referred to over the years.

 

https://stargazerslo...corrector-test/


Edited by MrJones, 02 November 2019 - 09:33 AM.


#12 Daniel Mounsey

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Posted 02 November 2019 - 10:02 AM

I think the measurements as well as the opinions of other observers are both important. This also expresses how observers may or may not see something.



#13 BillP

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Posted 02 November 2019 - 05:34 PM

I think the measurements as well as the opinions of other observers are both important. This also expresses how observers may or may not see something.

 

I would say it a little different...  Measurements are important when one is trying to figure something out or model it.  Observations are important when one really wants to know how the system of telescope+eyepiece will actually perform.  Measurements, which are based on modelling, which almost always does not model the complete set of all variables potentially impacting a system, virtually never tells the whole story.  Why one has field tests.  Or in the military why one has flight tests and sea worthiness tests.  The models and simulations are never trusted 100% as they cannot model what you do not know.  As the adage goes: You don't know what you don't know!


Edited by BillP, 02 November 2019 - 05:35 PM.

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