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XLT Eyepiece Performance Tests

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#26 Eddgie

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Posted 12 October 2019 - 04:10 PM

When I owned the 12T4, which was more than a decade ago, I also had a Non-Edge/ACF 8" SCT.  I remember the same for that scope with the 12T4 & 13 Hyperion.  Their off-axis was only very slightly distorted when you looked for it (with the 12T4 being very slightly tighter), so below the level to be visually distracting.

Yeah, this is the same with reflectors and refractors as well. At some point the field just gets narrow enough that the aberrated blur diameter gets small enough that it is not distracting.

 

I was out with my new to me AT 106LE the other night and the 106LE only has a focal lenght of 690mm, so I was expecting that the edge of field would be slightly out of focus, and it was no surprise when that turned out to be the case. With the 12mm Nagler, though, the field appeared pretty sharp to the edge. 

 

So at some point the part of the field at the edge becomes close enough to the best focus of the eyepiece that it stops being distracting.

 

Now, coma was still there in the C14 even in the 12mm, but the coma was very low and it was not a blur because close to the axis, the scope was in much better focus.  In the C14, off-axis brighter stars were not a spurious disk surrounded by an even ring.  Even half way to the edge of the field, this is what I would see on a steady on brighter double stars about half way to the edge of the field of 12mm Nagler:

 

coma C14.jpg

Now it would take a night of excellent seeing in the C14 to see this, but I have seen it many many many times.  At higher powers, even 10mm off axis, you can easily see the classic comatic blur start to develop but this close to the optical axis, it is very close to perfect focus.

 

As it gets further off axis though, it gets more comatic, and it starts to swell, but it falls outside of the field of the 12mm before it gets bad enough to be bothersome, but even by the edge of the field, you can see the coma and a very tiny amount of defocus. 


Edited by Eddgie, 12 October 2019 - 04:11 PM.

 

#27 Eddgie

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Posted 12 October 2019 - 04:28 PM

The 22mm T4 was an interesting case.  Whatever the field curvature is for the 22mm T4, it was not a good match for the C14.

 

Due to the magnification and the level of defocus, the abberated blur was so large that I could actually resolve the Fresnel pattern of bright stars at the edge of the field.  Now it was not huge, but it was from form being tiny.  I could actually see the secondary shadow and rings the defocus was so bad. 

This is what it looked like right at the very edge of the 22mm T4. No other eyepiece produced this much expansion and defocus in the C14.   This model is pretty close to what I would see. Not even close to a pinpoint. 

 

I could not model the exact magnification (177x) but this is pretty close.

 

22T4.jpg

 

 

In fact, I found that this same eyepiece would show a bit more defocus than most others in the C14, so I became less inclined to use it.   The 20mm T5 was much better in the C14, but I still preferred the 27mm Panoptic.  Even in the EdgeHD 8", I thought the 22mm T4 was more defocused at the edge than the 20mm T5, which I bought exactly for this reason, thinking that the 20mm might show a more in-focus edge of field, and the 20mm T5  was excellent in the EdgeHD. Stars were well focused right to the edge of the field.

 

These are models of what I could see. Anyone using the same eyepieces in a standard SCT should be able to replicate these observations under good seeing conditions.   


 

#28 Eddgie

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Posted 12 October 2019 - 04:49 PM

And of course the model makes this blur appear much larger than it was at the eyepiece.  We are talking about 5 waves off defocus though and defocus a star a couple of millimeters at 177x and one would get about what I could resolve at the edge of the field in the C14.


 

#29 mclewis1

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Posted 12 October 2019 - 05:26 PM

 Is your comment about a 12mm Nagler in last paragraph above describing the view through an XLT SCT? If so that matches my experience with a Nagler 11mm, T6 on a C-11. It is one of my favorites - one "premium eyepiece" of two I own.  ....

My experience with a 13mm T6 is about the same as well. The exit pupil and overall comfort are just about perfect for me. So this has become my favourite eyepiece in both of my SCTs.

 

I also had a 35mm Panoptic, which was wonderful in my C11. I sold it because I just didn't use it much after getting a TMB refractor for wide field work. The Pan worked very well in the refractor as well (love the overall feel and performance of the eyepiece) but I also have a 40mm Paragon that I decided to keep (the rubber grips have degraded lowering it's overall resale value so I could get substantially better money for the Pan).

 

Overall my experiences with both Televue eyepieces in regular/classic SCTs closely mirrors the comments above. I've briefly tried other newer wider field eyepieces and in the short viewing time they haven't impressed me as much as the Panoptic and T6 Naglers in these scopes.


 

#30 Dave Bush

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Posted 12 October 2019 - 06:40 PM

Of course the Panotics were equally as dazzling in the EdgeHD but why use a Panoptic when you can use a Nagler?  Well, the answer to that question is "when it gives a more pleasing edge of field view and that matters to you. If it does not really matter to the individual observer, then I don't see why they would bother reading this thread.  I know a lot of people that say that they only care about the center of the field and if that is true, then the big reveal of the results of the test will be wasted. If an observer honestly does not care about off axis performance, I would assume they will tune all of this out. 

So do you feel that the Panoptics perform equally as well in the EdgeHD as do the Naglers?


 

#31 Eddgie

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Posted 12 October 2019 - 10:40 PM

So do you feel that the Panoptics perform equally as well in the EdgeHD as do the Naglers?

I would say that they were quite similar as I recall but since the EdgeHD was so excellent with the Naglers, that is pretty much all I used in that scope most of the time so I did not really do a lot of comparisons between them.  I did use the 41mm Pan in the EdgeHD a fair amount and it was quite excellent. 


 

#32 Rustler46

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Posted 15 October 2019 - 03:16 PM

Hi All,

Further tests of various eyepieces on my C-11 XLT still need to be done. A weekend camping trip to the Oregon coast and an upcoming week of unsettled weather have put these on hold. But at next opportunity I'll be focusing on more subjective tests. One eyepiece has been added, a Celestron 26mm Silvertop Plössl. Someone pointed out that eyepieces in the 20-30 mm range at f/6.3 give the same magnifications as 30-40 mm eyepieces at f/10. Since there has been a lot of discussion about performance of premium eyepieces on a variety of SCTs, I may well add my (premium) TV 11mm T6 Nagler to the mix. Both of these fall outside Daniel's original inquiry of 30-40 mm non-premium eyepieces. The low magnification of such long ones makes it harder to see the flaws if not too severe. Also Ed (Eddgie) has reported on how reduced FOV with shorter eyepieces contributes to better edge performance. So both shorter focal length eyepieces mentioned above could provide interesting comparisons.

 

I would suggest that the most useful eyepiece comparisons will come from individual observers' subjective take on the matter. Each of us has our own tolerance for less than perfect performance. We also have differing ability to compensate for field curvature mismatch. What is more we have differing propensity for ignoring edge of field aberrations as we place the object of interest in the center of the FOV. So even if certain eyepieces have been reported on by others, we would be best served by each of us reporting on performance of our own eyepieces, seen through the paradigm of personal perception and preferences. 

 

So what have others experienced with different eyepieces on our XLT SCTs? Meade SCTs of the non-ACF variety would be of interest as well. It isn't mandatory to report as many different parameters as were displayed in the OP. Something like eyepiece model, f-ratio and percent usable field radius would suffice.

 

What has been reported on by EdgeHD owners has me desiring a 5- or 6-inch EdgeHD with a rack of premium eyepieces. Seems like such a combo would provide performance similar to a smaller APO along with a bit more light-gathering power and resolution. But I digress. Let's hear it about eyepieces on run-of-the mill SCTs!

 

Best Regards,
Russ


Edited by Rustler46, 16 October 2019 - 01:57 PM.

 

#33 jjack's

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Posted 17 October 2019 - 01:56 PM

Hi all

I have a question for specialists : 

Are all eyepieces  extremely sensitives about FC of the scope or some eyepieces could be less sensitives than others ?


 

#34 Eddgie

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Posted 17 October 2019 - 04:07 PM

The eyepieces typically have very little (compared to the telescope) field curvature, but the field curvature of both components can compliment of aggravate one another.  

 

In other words, some eyepieces will work better in some scopes and worse in others, and usually it is the field curvature of the scope that is most at fault.

 

For example, a standard doublet or triplet refractor that had a focal lenght of 540mm would have a field that curves to a radius of about 180mm, and even high quality eyepieces like the 31mm Nagler would show field curvature easily.

 

The 540mm though was not some random figure.  It was selected specifically for this reason. The Televue 101 family has what is basically a built in focal reducer and field flattener that gives a final focal lenght of 540mm.  When the same 31mm Nagler is used a scope from this family, the edge of the field is in very good focus

 

In most cases the dominant FC is from the scope and not the eyepiece.

 

When a very long focal length reflector is used, because the curvature is about equal to the focal length, the field will be in practical terms, quite flat. Since a 12" f/5 reflector would have a field that is only curved to a radius of 1500mm, then any serious curvature seen is very likely coming from the eyepiece. 


Edited by Eddgie, 17 October 2019 - 04:11 PM.

 

#35 Rustler46

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Posted 17 October 2019 - 07:20 PM

The eyepieces typically have very little (compared to the telescope) field curvature, but the field curvature of both components can compliment of aggravate one another.  

 

In other words, some eyepieces will work better in some scopes and worse in others, and usually it is the field curvature of the scope that is most at fault.

 

For example, a standard doublet or triplet refractor that had a focal lenght of 540mm would have a field that curves to a radius of about 180mm, and even high quality eyepieces like the 31mm Nagler would show field curvature easily.

 

The 540mm though was not some random figure.  It was selected specifically for this reason. The Televue 101 family has what is basically a built in focal reducer and field flattener that gives a final focal lenght of 540mm.  When the same 31mm Nagler is used a scope from this family, the edge of the field is in very good focus

 

In most cases the dominant FC is from the scope and not the eyepiece.

 

When a very long focal length reflector is used, because the curvature is about equal to the focal length, the field will be in practical terms, quite flat. Since a 12" f/5 reflector would have a field that is only curved to a radius of 1500mm, then any serious curvature seen is very likely coming from the eyepiece. 

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?


 

#36 Rustler46

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Posted 22 October 2019 - 06:22 PM

Further tests of various eyepieces on my C-11 XLT still need to be done. A weekend camping trip to the Oregon coast and an upcoming week of unsettled weather have put these on hold. But at next opportunity I'll be focusing on more subjective tests. One eyepiece has been added, a Celestron 26mm Silvertop Plössl. Someone pointed out that eyepieces in the 20-30 mm range at f/6.3 give the same magnifications as 30-40 mm eyepieces at f/10. Since there has been a lot of discussion about performance of premium eyepieces on a variety of SCTs, I may well add my (premium) TV 11mm T6 Nagler to the mix. Both of these fall outside Daniel's original inquiry of 30-40 mm non-premium eyepieces. The low magnification of such long ones makes it harder to see the flaws if not too severe. Also Ed (Eddgie) has reported on how reduced FOV with shorter eyepieces contributes to better edge performance. So both shorter focal length eyepieces mentioned above could provide interesting comparisons.

Tomorrow night looks promising for clear skies.

Screen Shot 2019-10-22 at 4.10.49 PM.png

 

The recent "cloudy nights" have given me opportunity to set up my C-11 for use of the f/6.3 focal reducer. I wanted to use my JMI EV1 Crayford focuser because it does away with mirror tilt with changes in focus direction. Also its hands-off adjustment via motorized focuser makes for easier focusing.

JMi EV1 Focuser-1.jpg

 

The easiest way to attach the focal reducer was to just insert it into the eyepiece side of the focuser. But that would add about 2 inches to the spacing from the SCT rear port. I recall some of Eddgie's insight on how spacing of SCT focal plane affects residual spherical aberration. So after some examination of the mechanics I was able to put the focal reducer ahead of the JMI EV1, to better match the SCT design geometry. I didn't want anything to affect eyepiece performance in further testing. The tests will be done without using my contact lenses. That's another factor that could skew results. I hope the skies cooperate.


Edited by Rustler46, 23 October 2019 - 03:19 PM.

 

#37 mclewis1

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Posted 23 October 2019 - 03:17 PM

The problem with your setup with the reducer in front of the focuser is that every touch of the focuser affects the more important spacing back to the sensor. IMHO it would be more beneficial to keep the reducer to sensor spacing at it's most optimal point rather than worry about the extra 50mm of positioning behind the rear cell ... particularly since you are not doing planetary imaging with the reducer in place.


 

#38 Rustler46

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Posted 23 October 2019 - 06:59 PM

The problem with your setup with the reducer in front of the focuser is that every touch of the focuser affects the more important spacing back to the sensor. IMHO it would be more beneficial to keep the reducer to sensor spacing at it's most optimal point rather than worry about the extra 50mm of positioning behind the rear cell ... particularly since you are not doing planetary imaging with the reducer in place.

Thanks for the insight, Mark. I would be interested in other's take on this subject. At the present I'm focusing on visual uses (no pun intended). But either placement is easy to accomplish. What is the optimal spacing - reducer to sensor? For non-photo use I have a 1-1/4 inch Baader prism diagonal upstream from the eyepiece. So that would affect spacing for visual use.

 

Edit:

I've switched the reducer to the eyepiece position on the focuser - testing tonight.

 

It remained clear for the night. All tests on the C-11 were at f/6.3, both 1-1/4 and 2-inch eyepieces. I still need to test the 26mm Celestron Silvertop Plossl and the 11 mm Nagler at f/10. My thoughts on each eyepiece were captured on a small digital voice recorder. I'll be posting my results in the next few days.

 

Does anyone else have some eyepieces that could be tested on your XLT type telescope? Any of the non-EdgeHD or non-ACF SCTs would be good candidates. It would be good to have others findings on the matter. My personal paradigm is to be quite tolerant of aberrations. Others with a more critical eye could provide another take on XLT eyepiece performance.

 

Best Regards,

Russ


Edited by Rustler46, 24 October 2019 - 01:32 AM.

 

#39 mclewis1

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Posted 24 October 2019 - 07:54 AM

Russ,

 

Sorry about that, I always seem to discuss precision spacing of the reducer in terms of imaging requirements. The published spec is 110mm of spacing (from the 220-230mm focal length of the reducer) but there is some difficulty in determining the exact point within the lens elements to use and measure from. So most folks will use 105mm from the bottom edge of the reducer and 110-115mm from the lip or edge above the threads.

 

In terms of convenience, where you mount the reducer obviously also makes a difference but you seem to be ok with that. Trying a number of different wider field eyepieces with the reducer would also place the field stops at different distances which could make using the setup (or trying to optimize and standardize the setup) a pain. Here having the focuser in between would seem to me to make more sense. With eyepieces it's often really difficult to accurately measure the position of the field stop when the eyepiece is in use, so getting spacing numbers accurate down to a few mms is almost impossible, I think +- 5mm would be a reasonable goal.

 

For most imagers eking out every bit of optical quality across the widest field from the reducer setup is a common goal, as is reproduceability, so keeping the spacing fixed is highly desirable. For serious low/med magnification visual work I think the spacing could vary a little bit without becoming a really big problem.


 

#40 Eddgie

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Posted 24 October 2019 - 10:58 AM

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?

You should try to find a copy of Ray Hutchinson's SCT ray trace.  This has graphs that show the spot size for the various standard SCTs.

 

Ray does allow reproduction for amateur use, so I am posting an example here.

 

C8 spot size.jpg

 

Ray does not have spots for the reducer, but this is what we know about reducers.  If the reducer did nothing but reduce the size of the image, the spot size would be reduced by .63 in size because that is what reducers do.

 

And we can infer from the instructions that Celetron provides that the standard reducer does not make the field flat though it probably does improve it.  The key to knowing this is that if you can actually find an original instruction sheet for the f/6.3 reducer, it said that the scope should be focused on a star half way from then center to the edge of the field and this what you do almost always when imaging with a curved focal surface. While the blur of the stars at the center if the field is increase in side, the blur at the outside of he field is decreased in size, so that stars across the field are now going to be more uniform in size rather than have small blurs at the center and giant blurs at the outside. 

 

If the only thing the reducer did was reduce, then the blur would be smaller, but the magnification would be less. If though, you used a higher power eyepiece to make the magnfication the same, if the reducer only reduced, then the blur would be the same angular size in the scope when used at the same magnification.  This is in fact the big trick that focal reducers perform for imaging.  Even if it does not flatten the field, it reduces the angular size of the blur.  Now most modern reducers made for refractors are also field flatteners, but you have to pay attention to how it is advertised. If it is only advertised as a flattener, it won't reduce, and if it is advertised as only a reducer, it may not flatten.   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.  My own results with the reducer were such that I thought it was better to simply use 2" eyepieces in the larger scopes.  With increased focal speed, the eyepiece aberrations start to come into play, yes??? See, you can' say if the aberration you see is coming form the flattened and reduced telescope, or if it is coming from the eyepiece working with a faster light cone.. There is a lot to consider. 

 

Now maybe Celestron has changed the reducer since that document was published, but that tells me that the reducer flattens the field, but does not make if truly flat.

 

But I can't answer the question of how flat it is not. 

 

Also, the above chart shows the 8, but as the scopes get larger, the blur diameter gets proportionally smaller but at the same time, for a given eyepiece the magnification gets much higher, so if you are using a C11 with a 31mm Nagler, the bloated star is going to be magnificence by something like 38% more than it would be in the C8.  So, the blur is bigger in the C8, but for the same eyepiece, the magnification is higher in the C11, and while the blur is smaller at the focal plane, the extra magnification makes it appear to swell more than it would if it were viewed at the same power.  

 

So, it is pretty complicated, but in the end, in the standard SCT, while the blur diameter gets smaller at the focal plane with aperture, for a given eyepiece, it will be about the same angular size. 

 

Again I am only posting the one for the C8, but if you can find a copy of the Ray's ray trace, it is packed with valuable information like this....

 

And as can be easily seen, the field curvature of the standard SCT is pretty huge. The field of the EdgeHDs would produce stars that were one third the size at the edge of the field, which is a remarkable improvement.  Celestron gives us this figure (three times flatter) in the white paper, so the Edge HD 8" would produce a blur diameter of only 13.3 um at the corner of an APS-C size sensor, while in the standard 8", the blur would be 40 um.  That is a huge improvement.  See, even the EdgeHD is not truly "Flat." but no telescope really is. 


Edited by Eddgie, 24 October 2019 - 11:11 AM.

 

#41 Eddgie

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Posted 24 October 2019 - 11:18 AM

And to be fair, the field curvature of an 800mm standard refractor is about the same as a standard C8.  The difference here is that the magnifications for any given eyepiece are far lower, so the refocused blur is smaller until you get well outside of the true field limits of the C8.  

 

Now in a 600mm refractor, if you use a 31mm Nager, you can get far enough away from the center of the field so that even though the magnfication is only a little over a quarter of what it would be in the EdgeHD, the curvature is worse, and the field is wider, so you start to see the defocused blur and astigmatism of the eyepiece.  

 

It is all about magnification.  Until the blur diameter gets to be about 3 arc minutes of apparent field, it will still look pretty sharp. It is only when it expands this much that the eye can see the aberrated blur.  If the blur were perfectly circular (no coma, no astigmatism) it would have to get bigger than this to actually see that there was something other than a point there.  We see asymmetry more easily.    


 

#42 Rustler46

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Posted 27 October 2019 - 10:52 PM

I had an opportunity for a few more eyepiece tests with a Celestron f/6.3 focal reducer on my Celestron-11 XLT.

 

Screen Shot 2019-10-27 at 10.06.17 PM.png

 

In general the 1-1/4 inch eyepieces which had been tested at f/10 performed poorer at f/6.3. I did add my 2-inch eyepieces for testing at f/6.3. Still to do is test at f/10 a couple of the 1-1/4 inchers and the 2-inch eyepieces.

 

Note:

For comparison I added two 1-1/4 inch eyepieces (11mm & 26mm) and a 2-inch 20mm.


Edited by Rustler46, 28 October 2019 - 12:12 AM.

 

#43 Rustler46

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Posted 29 October 2019 - 02:00 AM

So here are final tests on eyepieces from 30-42 mm with a 26, 20 and 11mm thrown in for comparison.

Screen Shot 2019-10-29 at 12.06.17 AM.png

 

My tolerance for error is fairly high. Of the eyepieces tested my favorites are:

  • MaxView 40 @ f/10
  • Military Surplus 32mm Erfle @ f/10
  • GSO 31mm @ f/10
  • Celestron 26mm Silvertop Plössl 
  • Televue Nagler 11mm T6

Some observations:

  • Except for its extreme 100° AFOV, I was unimpressed with the Explore Scientific 20mm. The outer 75% of radius had perceptible aberration. This is a big beast of an ocular that will likely go up for sale.
  • I might try reducing the field stop diameter for the 32mm Erfle. That might improve edge of field performance and reduce reflection from the inside of the 1-1/4 inch barrel. This 70 year old eyepiece is over 2 inches in diameter with a custom brass adapter to 1-1/4 inches. A smaller field stop could easily be added to the inside of the adapter.
  • I would like to try another Televue, maybe a 35mm Panoptic.

 

That's all I have to offer testing eyepieces with a C-11 XLT. My set is a mix of premium and mediocre eyepieces of various manufacturers. What have some of you found as for performance with your eyepieces on XLT SCTs? Share what you've found.

 

All the Best,

Russ

 


Edited by Rustler46, 29 October 2019 - 02:07 AM.

 

#44 Eddgie

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Posted 29 October 2019 - 05:15 PM

The 35mm Pan is a great match for the standard SCT. 

 

Now I have always wondered "Why a 35mm Panoptic?"  Well, I think I know the answer.   I do not think it is a coincidence that the field stop of the 35mm Panoptic is 38mm, which is the same size as the rear port of the standard SCT.  Coincidence, or intention?, I don't know, but before getting Naglers, the 35mm Pan was the widest field eyepeice I had used that time in an SCT (NexStar 11) and it was only after I went to the Naglers that I started to become more aware of the off axis performance limits of the standard design.   At this time, I had also acquired a Televue 101, and this scope was fantastic with the Naglers, and the more I compared the off axis performance of them, the more I came to value a field that is very highly corrected right to the field stop.   

Under dark skies, the combination of the flat field Televue 101 coupled with the 31mm Nagler was a richest field scope that was unmatched in terms of pure excellence of the quality of the field right out to the edge.  Only the EdgeHD 8 did as well.  

 

I have not owned a Parcorr, but I have used them and this again will produce an essentially perfect wide field with eyepieces like the 31mm Nagler or Ethos. 

 

Yeah, once you get used to it, it is hard to accept an aberrated field unless you are in the school of being that only cares about the exact center of the field, and in that case, none of this matters. 


 

#45 Viktorious

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Posted 30 October 2019 - 06:28 AM

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)?

- 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?

As mentioned in the first question, I went the T-2 + reducer path instead of 2" for diagonal. Had to send my mount back due to some wobbles but once I'm up and running I can hopefully add some comparisons of my own!*

*Still haven't acquired a 20-24 mm eyepiece to go with the reducer since I was waiting to read more results here. However, I can test it with the Baader Zoom, though this isn't really a widefield eyepiece as such.


Edited by Viktorious, 30 October 2019 - 07:38 AM.

 

#46 Vla

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Posted 30 October 2019 - 08:37 AM

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."

Don't know where that description comes from, but on Celestron's site there is no mention of "flatter" in any shape or form. For what is known, the f/6.3 reducer/corrector is a 4-element arrangement consisting of two cemented doublets. Such configuration can correct coma, but it adds some astigmatism of the same sign as the system's Petzval, which means it makes field curvature stronger. Here's illustration of what it likely looks like (toward page bottom). There is no possibility to flatten field, fully or partly, with such arrangement.


 

#47 Dave Bush

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Posted 30 October 2019 - 08:54 AM

Don't know where that description comes from, but on Celestron's site there is no mention of "flatter" in any shape or form. For what is known, the f/6.3 reducer/corrector is a 4-element arrangement consisting of two cemented doublets. Such configuration can correct coma, but it adds some astigmatism of the same sign as the system's Petzval, which means it makes field curvature stronger. Here's illustration of what it likely looks like (toward page bottom). There is no possibility to flatten field, fully or partly, with such arrangement.

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.  


 

#48 Vla

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Posted 30 October 2019 - 09:49 AM

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.  

Are you talking visual or photographic field?


 

#49 Dave Bush

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Posted 30 October 2019 - 10:06 AM

Are you talking visual or photographic field?

Visual.  That's what I and countless others have observed.  But it's also claimed to do so photographically.  


 

#50 eklf

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Posted 30 October 2019 - 10:41 AM

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.  

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.  


 


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