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Best Eyepieces for FC on Cats...

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#1 Dave Bush

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Posted 03 January 2019 - 08:41 PM

Which eyepieces tend to show/reveal the least field curvature in an SCT?



#2 Neptune

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Posted 03 January 2019 - 09:51 PM

Look into PENTAX XW's.  I believe one end of the focal lengths helps counter the curved field. It's either the shorter focal lengths of the longer ones (3.5mm - 20mm).



#3 Ryan555_1

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Posted 04 January 2019 - 12:51 AM

Nikon 17/14 HW does very well in this regard.

#4 junomike

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Posted 04 January 2019 - 08:08 AM

IME It's all dependent on the AFOV and F/L ot the eyepiece as well as one's personal tolerance for FC.

A long F/L EP with a 70° FOV will show less than the same F/L with 82° FOV.

Also, a shorter F/L EP (15mm) will show less than a longer F/L EP.

I find the 31T5 excellent but there are several others who find the FC bothersome using it.



#5 jjack's

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Posted 07 January 2019 - 05:12 AM

Not totaly right Junomike. Eyepieces with integrated smith lenses are more tolerant  about FC. It is my opinion.



#6 tmaestro

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Posted 11 January 2019 - 10:20 PM

This might be sidestepping the question, but $100 on the 6.3x reducer/flattener and you get a flat field in any eyepiece you'd care to use.  That's probably cheaper than upgrading your eyepiece collection.


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#7 Jon Isaacs

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Posted 11 January 2019 - 11:40 PM

Not totaly right Junomike. Eyepieces with integrated smith lenses are more tolerant  about FC. It is my opinion.

 

An eyepiece has a focal plane (curved to some extent).   A point on that focal plane is either in focus or it's out off focus.  To be tolerant of field curvature implies that it can be in focus at two different distances.  

 

I think eyepieces that are best suited for scopes with field curvature also have curved fields and are free of their own aberrations so they do not add aberrations to the field curvature.  

 

The 31mm Nagler has integrated Smyth lens.  In a flat field scope, it shows nice, tight, round stars right to the edge.  In a telescope like a short focal length refractor, the field curvature is apparent. 

 

Field curvature is the second power of the off-axis distance.  This means that if you double the field of view, the curvature at the edge of the field is quadrupled.  Field curvature is less visible in narrower field eyepieces, lower power eyepieces.  

 

An example is the 31mm Nagler versus the 35mm Panoptic. Very similar eyepieces but the Nagler has a 8.5% wider field and provides 13% more magnification.  This means that at the edge of the field, the Nagler has 18% more curvature and that defocused imaged is magnified 13% more.  The result is the Nagler's defocused blur is 33% larger in diameter and more visible.

 

Jon


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#8 junomike

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Posted 12 January 2019 - 09:03 AM

An eyepiece has a focal plane (curved to some extent).   A point on that focal plane is either in focus or it's out off focus.  To be tolerant of field curvature implies that it can be in focus at two different distances.  

 

I think eyepieces that are best suited for scopes with field curvature also have curved fields and are free of their own aberrations so they do not add aberrations to the field curvature.  

 

The 31mm Nagler has integrated Smyth lens.  In a flat field scope, it shows nice, tight, round stars right to the edge.  In a telescope like a short focal length refractor, the field curvature is apparent. 

 

Field curvature is the second power of the off-axis distance.  This means that if you double the field of view, the curvature at the edge of the field is quadrupled.  Field curvature is less visible in narrower field eyepieces, lower power eyepieces.  

 

An example is the 31mm Nagler versus the 35mm Panoptic. Very similar eyepieces but the Nagler has a 8.5% wider field and provides 13% more magnification.  This means that at the edge of the field, the Nagler has 18% more curvature and that defocused imaged is magnified 13% more.  The result is the Nagler's defocused blur is 33% larger in diameter and more visible.

 

Jon

Thanks Jon for placing the theory and example behind this.


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#9 Eddgie

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Posted 12 January 2019 - 04:21 PM

I recommend Panoptics.   For a similar size true field, these will give the appearance of sharper stars at the edge than you would get with Naglers. 

 

Past the 20mm focal lenght range, you can skip to the 13mm or 12mm Nalger.  By this time, the FC is not such an issue because you are pretty close to the center of the field.   Stars still won't be super sharp at the edge, but pretty good.   Focal lengths longer than about 20mm though and it is better to stick with Panoptics (in my opinion) if you want a field that is decently sharp fairly far out.


Edited by Eddgie, 12 January 2019 - 04:24 PM.

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#10 jjack's

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Posted 12 January 2019 - 07:25 PM

I have heard the nagler 20T5 does very well on SCTs, but i really dont know. I never try it. It is a rare eyepiece.

Anybody to tell us ?



#11 jjack's

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Posted 31 January 2019 - 09:36 AM

i made some experiment with eyepieces i have : Nagler 13 and Panoptic 19.

Pan 19 have a wider field than the N13 and stars into the Pan are not pinpoint to the edge into my C8.

But the question is if they have the same TFOV are one or the other better corrected ?

So i put two craterlets at the opposite of the field into the N13 and they are perfectly defined.

watching at the same distant craterlets into the Pan19 and they are not as clean and i saw little coma and field curvature.

it is why i can say eyepieces with smyth lenses (maybe not all) are more tolerant about FC, coma, astig, because dispite more power, the same apparent field have less aberrations than looking thru the Pan 19.


Edited by jjack's, 31 January 2019 - 09:40 AM.


#12 Dave Bush

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Posted 31 January 2019 - 09:50 AM

i made some experiment with eyepieces i have : Nagler 13 and Panoptic 19.

Pan 19 have a wider field than the N13 and stars into the Pan are not pinpoint to the edge into my C8.

But the question is if they have the same TFOV are one or the other better corrected ?

So i put two craterlets at the opposite of the field into the N13 and they are perfectly defined.

watching at the same distant craterlets into the Pan19 and they are not as clean and i saw little coma and field curvature.

it is why i can say eyepieces with smyth lenses (maybe not all) are more tolerant about FC, coma, astig, because dispite more power, the same apparent field have less aberrations than looking thru the Pan 19.

This is very interesting.

 

I would be curious to know what you would find with stars.

 

Which generation (type) Nagler is that?



#13 Erik30

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Posted 31 January 2019 - 02:44 PM

I have kind of the same question but only with a GSO 10" RC (Truss) 

 

I have a 10" dob that is an f4.5 and after talking to some people and actually trying out a few eye pieces, I settled on the 12t4 Nagler and the 20t2 Nagler. (I have a lot of other eyepieces, but they on the lower end of the scale)  The 20t2 give that true spacewalk feeling in the 10" dob, I wonder if it will in the 10" RC at f7.9??  If it wasn't -40 below windchill I would actually go out and try it myself..  lol



#14 jjack's

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Posted 31 January 2019 - 05:59 PM

Hi Dave. it's a T6 i bought ten or eleven years ago. On a star i can see the airy disk to the edge. but circles around are drifted on one side opposite to the on axis (coma begin to disturb). No field curvature detected.


Edited by jjack's, 31 January 2019 - 06:00 PM.


#15 Eddgie

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Posted 01 February 2019 - 09:07 AM

I have heard the nagler 20T5 does very well on SCTs, but i really dont know. I never try it. It is a rare eyepiece.

Anybody to tell us ?

Not on the standard SCT.   Amazing on the EdgeHD.  Have used it on both types.  In my C14,and standard C8 as it was not nearly as sharp as 27mm Panoptic but in the EdgeHD 8", it was brilliant. In that scope my progression was 31mm Nagler, 20mm T5, and 12mm T4.  In the C14, it was 41mm Panoptic, 24mm ES, 12mm Nalger.

 

Now I will say that the 20T5 was sharper than the 22mm T4 in SCTs.  The 22mm T4 seems to have a field curvature that is not well matched to the C14.  When I got the Edge HD, I did not think the 22mm T4 was as sharp as the 20mm T5 in that scope, though they were pretty close.  Now one must remember that the field was bigger with the 22mm T4, so that alone could account for the difference.  Both though were by most peoples standards very sharp in the EdgeHD.

 

I know a lot of people question the value proposition of the EdgeHD scopes, but this thread says it all.  The standard SCT design was intended for 1.25" eyepieces where these issues were not really a big concern.  The EdgeHD is a modern design that works beautifully with modern eyepieces to produce a very wide and very sharp field.   That only matters if it matters to the observer, but once you have used a scope with pinpoint stars from sea to shining sea, it is hard to go back to a scope that leaves more of the stars in the field aberrated than it shows as being sharp. 


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#16 Dave Bush

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Posted 01 February 2019 - 10:33 AM

... but once you have used a scope with pinpoint stars from sea to shining sea, it is hard to go back to a scope that leaves more of the stars in the field aberrated than it shows as being sharp. 

 

Yup.  That's why after reading all the comments here I've decided to stick with using my f/6.3 r/c on my C8 because that's what I get with it.

 

Once I retire and have more time to spend (maybe in a couple years) I'll probably upgrade to a 9.25" (or maybe 11").  At that point I'll pop for an Edge edition.


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#17 jjack's

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Posted 01 February 2019 - 12:03 PM

Dave, can you tell us how work your ES 9mm and your ES 12mm on your C8 ?

For shure an SCT is not as well corrected than edgeHD. But i see pinpoint star field to the edge with the nag 13 with (100×) or without (166×) the reducer. So edgeHD or not, SCTs are not made for wide field. You can be satified with up to 100x and use a really complementary good wide field scope like a newton or refractor.

The other way is to sell your C8 and by an 8" F4 newton with a paracorr. You got the same obstruction and if you buy a good mirror it can compete the C8 on planetary views and be better on wide field views.



#18 jjack's

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Posted 03 February 2019 - 09:31 AM

I know now the nagler 13 is a nice eyepiece for a C8, we need to try others 82º field eyepieces between it and the N20.

N16, meade 18, ES 18, delos 17.3 or morpheus 17.5 for exemple. Are they better at the edge than the 19 pan ?

Feedbacks please.



#19 jjack's

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Posted 09 February 2019 - 07:12 PM

Seen at telescope optic.net : 

 "For meaningful correction of eyepiece astigmatism it is necessary to introduce a negative (Smyth) lens, which induces astigmatism of opposite sign to that of the positive lens group. It also induces Petzval field curvature of the opposite sign, resulting in both astigmatism and field curvature minimized. Best known brand of this kind, the Nagler, has astigmatism reduced up to several times vs. comparable conventional eyepieces."

 

So if a smyth lens contribute to reduce some aberration, perhaps the same field is cleaner with nagler types eyepieces than others dispite some coma.



#20 gnowellsct

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Posted 11 February 2019 - 12:38 PM

30 and 40 XWs if you can get 'em, but the 41 Pan Optic is what I would get if heaven forbid something happened to my XW.  

 

I am not terribly partial to the Kidney Bean line (most of the Televues 82s and on up) but there are a few that I like.  The Pan 41 is excellent and under-appreciated I think as a wide field option.  

 

Remember in an SCT low powers achievable in an f/4.5 Newt or even an f/6 ish refractor just are not achievable.  This makes them a good choice for people with problems such as native astigmatism.     And you get less field curvature....but there's still enough to bother many users.  They tend to gravitate to the focal reducers.

 

I used a focal reducer upon the recommendation of people who like them and found that I preferred higher end eyepieces.   In a way that's unfortunate, as the eyepiece collection cost more than the c8 and c14 combined.  

 

Greg N




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