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Field Curvature for Visual

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#1 helpwanted

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Posted 06 January 2020 - 10:20 AM

Hello,

 

Question about field curvature. I've seen this statement here on CN and elsewhere on the internet:

 

Field curvature in a scope depends on its radius of curvature, essentially its focal length (not ratio).  The smaller the radius, the greater the field curvature. 

 

The reason I am asking is the Orion 110mm f6 660mm focal length (same specs as the WO 110 Megrez). 

When you search for reviews people mention FC. But if the above statement is true, then the WO 72 f5.8 with 420mm focal length should have worse FC. I have the 72 and it is a great scope with no noticeable FC to me. Does this mean I shouldn't notice FC in the 110?

 

Thanks for your help,

 

David


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#2 Benach

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Posted 06 January 2020 - 10:50 AM

For visual FC is irrelevant more or less. It is usually corrected in your eye by automatically accomodating. For photographic purposes, it matters since a chip is unable to accomodate nor change the curvatures in your optical train (eye included).



#3 salico

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Posted 06 January 2020 - 10:58 AM

I find FC very annoying visually in my Lomo 80/480 Binoscope, in the Nagler 12mm. Outer half of FOV is pretty much damaged by FC. In the ED120/900 BT it is a bit better in the N22, in the N12 minimal.


Edited by salico, 06 January 2020 - 10:59 AM.


#4 Astrojensen

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Posted 06 January 2020 - 11:01 AM

Actually, the radius of the field curvature in a refractor, without a field flattening element, is roughly 0.35x the focal length. This, with very minor variation, is true for ALL refractors, regardless of lens design. 

 

For a newtonian, the radius of field curvature is 0.95x the focal length and in an old plain vanilla SCT, it's 0.25x. This means that of the three most common telescope types owned by amateur astronomers, the newtonian has BY FAR the flattest field, . Sadly, it's plagued by coma instead. 

 

But to return to your question: Yes, a 72mm f/6 has much worse field curvature than a 110mm f/6. IF you don't perceive it as a problem in the 72mm f/6, then you won't in the 110mm either, as long as you use the same eyepieces. If you use longer focal length eyepieces with a wider true field, then you might notice it. 

 

For visual use, how annoying field curvature is, pretty much depends on how annoying YOU find it. Some can't stand the merest hint of it, others are very forgiving. 

 

 

Clear skies!
Thomas, Denmark


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#5 wrnchhead

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Posted 06 January 2020 - 11:17 AM

Using my C8 last night, and having a couple years observing experience under my belt, I noticed it as I was panning across the double cluster. It's a strange effect. Though the source, scope or EP is possibly debatable.  I don't have the knowledge to determine. May have to look up that astigmatism source document/post that Starman made, I think it covers this. 



#6 helpwanted

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Posted 06 January 2020 - 11:17 AM

Thank you all, I was getting discouraged from the 110 because of those reviews, but then it dawned on me about the 72.

 

BTW, aperture fever is nothing compared to APO fever!



#7 k5apl

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Posted 06 January 2020 - 11:29 AM

I combat FC in my refractors by choosing eyepieces that give me sharp stars at the edge of the FOV.  If there is any curvature, I tend to not see it.  Might be psychological, but it works for me.


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#8 helpwanted

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Posted 06 January 2020 - 11:43 AM

I'm using all TV eyepieces



#9 Jon Isaacs

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Posted 06 January 2020 - 12:06 PM

I'm using all TV eyepieces

 

With a 31mm Nagler, focus at the edge of the field in your 72ED is about 1.5 mm inward of center focus.  If you want to see how much field curvature there is, focus sharply on a star at the edge of the field and then look at the center of the field.  Bright stars are best... 

 

I calculate that the edge of the field of using a 31mm Nagler in a 110mm F/6 would be about 1.0mm inward of center focus.  

 

BTW, aperture fever is nothing compared to APO fever!

 

The 110 mm F/6's push the definition of "apo" pretty hard.  Back in the day when the WO 110 mm F/6 was on the market, people were using minus violet filters with them.  

 

Jon



#10 Erik Bakker

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Posted 06 January 2020 - 12:08 PM

Actually, the radius of the field curvature in a refractor, without a field flattening element, is roughly 0.35x the focal length. This, with very minor variation, is true for ALL refractors, regardless of lens design. 

 

{...}

 

 

Clear skies!
Thomas, Denmark

For clarity, it does depend on design. 4-element Petzval refractors, like the TeleVue Genesis, NP101 an NP127 or the 4-element Takahashi FSQ85, FSQ106 and FSQ130 to name a few,  have a built-in flattener and a flat field. 


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#11 Jon Isaacs

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Posted 06 January 2020 - 12:14 PM

For clarity, it does depend on design. 4-element Petzval refractors, like the TeleVue Genesis, NP101 an NP127 or the 4-element Takahashi FSQ85, FSQ106 and FSQ130 to name a few,  have a built-in flattener and a flat field. 

 

I think Thomas meant to include the modified Petzvals with the mention of a flattening element but in any event it's good to clarify it.  

 

I will say this:  If you want to see field curvature in a refractor, spend some time with a scope like the NP-101 and some TeleVue eyepieces.  

 

When you go back to a standard refractor, you will see it's field curvature. 

 

I should add this:

 

Field curvature is very dependent on the TFoV.  The 16mm Type 5 Nagler has about 1/2 the TFOV of the 31mm Nagler.  That means it will have about 1/4 the field curvature.  The magnification is doubled so the defocused star is larger but the net result is still a reduction in curvature of a factor of 2.

 

Jon 


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#12 Jeff Morgan

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Posted 06 January 2020 - 12:37 PM

I think Thomas meant to include the modified Petzvals with the mention of a flattening element but in any event it's good to clarify it.  

 

I will say this:  If you want to see field curvature in a refractor, spend some time with a scope like the NP-101 and some TeleVue eyepieces.  

 

When you go back to a standard refractor, you will see it's field curvature. 

 

Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

 

Sounds like the Tail wagging the Dog.

 

It seems to me that for refractor owners that can no longer accommodate small amounts of field curvature, the task becomes finding a field flattener.

 

Or finding eyepieces that have focal surfaces which are better suited to their telescopes. See Figure 211, and the following graphic 1B:

 

https://www.telescop...errations_1.htm


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#13 Jared

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Posted 06 January 2020 - 12:49 PM

I

 

Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

 

Sounds like the Tail wagging the Dog.

 

It seems to me that for refractor owners that can no longer accommodate small amounts of field curvature, the task becomes finding a field flattener.

 

Or finding eyepieces that have focal surfaces which are better suited to their telescopes. See Figure 211, and the following graphic 1B:

 

https://www.telescop...errations_1.htm

I think you can substitute “most high quality eyepieces” for “TeleVue eyepieces”.  Sure, eyepieces can have their own field curvature,  but most modern designs are intended to have flat fields in their own right.  One can always try to find an eyepiece design and focal length whose field curvature matches that of their chosen telescope (thus cancelling it out), but that seems like an exercise in frustration. I don’t think the previous poster intended to suggest TeleVue eyepieces are unique in their flat field designs, just that they are of consistently high quality.


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#14 Erik Bakker

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Posted 06 January 2020 - 01:51 PM

So far, I've never had an issue with field curvature in my refractors. The TV Nagler 31 is the most glorious deep sky eyepiece I've ever used in that scope. Very sharp across the field. But as Thomas said, sensitivity to FC is a personal thing. Wouldn't potty too much about it. And if you really desire a flat field instrument after all, the cure is simple: get a good flat field refractor.

 

Just an hour or so ago, I observed the Pleiades with my Nikon ED82 spotter and Nikon 30x Widefield eyepiece. What a glorious wide field sight. And no issue with FC. Just enjoying that gem of our universe.

 

And no issue means: no issue. As in virtually invisible. Was it there: of course in the outer field. But not nearly enough to distract from the incredible view presented to my eye and heart.



#15 BillP

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Posted 06 January 2020 - 02:18 PM

Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

 

Sounds like the Tail wagging the Dog.

In a sense, it very much is.  There is nothing "magic" about the 31T5 performing well in an NP-101.  Both gear are produced by the same company so logical that they would build them to work well together.

 

The scope is of course the elephant in the room.  Most people choose that first.  But not all eyepieces out there are made for flat fields.  One has to realize that.  If all one wants in their life is the Tele Vue take on an eyepiece, then no issue getting Petzval flat field scopes.  But if one wants more variety and spice in their observing life, then they will enjoy the variety of doublet and triplet refractors out there, as well as SCTs, and yes even Dobs with their generously supplied coma aberration!  So all depends where you want your focus in the hobby.  For me, I really enjoy the wide range of eyepieces that are available as each has its own unique strength.  And of course competition in the marketplace keeps it healthy so would never limit myself to just one branding.  And I also like the old vintage stuff as well and even use on occasion 100+ year old eyepieces!  Given all that, I tend to keep the focal ratio longer on my scopes, around f/8.  At that point FC not much of an issue visually for me.  It is still there, but tiny, and in Thomas' example I am the more forgiving type...with scopes and people I like to think.


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#16 wrnchhead

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Posted 06 January 2020 - 02:33 PM

Related, I posed this question in another thread and I don't think anyone had the answer. But what I glean from here is that my GTF flat field refactor should have none, is that right? And any that I see is induced by the EP itself? 



#17 Rich V.

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Posted 06 January 2020 - 02:42 PM

How old you are affects your perception of FC a lot. Youth has its benefits. 

 

With widest field eyepieces, older folks using the common doublets or triplets are quite susceptible to FC because our eye's lenses have hardened and lost their ability to accommodate the varying focus across the field.  

 

With my 70 yr old eyes, when I put my 21E in the 80/480 triplet, the FC is quite obvious but switching to the 13E presents a view that is sharp to the edge for me.  In my 130/910, the 21E is sharp like the 13E is in the smaller scope.

 

Enjoy your focus accommodation while you have it!   gramps.gif

 

Rich


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#18 Jon Isaacs

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Posted 06 January 2020 - 03:57 PM

Quote
Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

Sounds like the Tail wagging the Dog.

It seems to me that for refractor owners that can no longeraccommodate small amounts of field curvature, the task becomes finding a field flattener.


This thread is about understanding field curvature and how it relates to focal length and focal ratio.

Field curvature is an aberration, it's one that can be corrected if you want to.. it's like coma, spherical etc, people generally put up with some..

Field flatteners are a possible solution but they must be properly spaced and there's only one i know of that can be used with a diagonal. The advantage of a Petzval is that it's always properly spaced because the Petzval elements are fixed instead of moving with the Focuser. They're also matched for other aberrations, the TeleVue and Taks have ED glass front and rear.

Field curvature is primarily a low power wide field phenomenon and quickly disappears with narrow fields since it's related to the square of the off-axis distance.

I suspect most people use a strategy similar to mine. Low power, wide field means short focal length and fast. That means eyepiece's that are well corrected for off axis astigmatism. There's not much use in having a flat focal plane if the stars are astigmatic.
That's where it starts. The 31mm Nagler is the classic.

And the 31 Nagler and other similar eyepiece's do very well in longer scopes.

I happen to have a scope corrected for field curvature but I do a lot of wide field observing with shorter focal length scopes, an ST-80 with a 2 inch Focuser. It'll do 6.6° with the 41mm Pan. From the center to the edge, there's about 2 mm of difference. It's very obvious.

I just accept it, it comes with the territory. The NP-101 view has an extra special quality because the stars are in focus across the field but such perfection is unnecessary for enjoyment of the universe.

Jon
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#19 CHASLX200

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Posted 06 January 2020 - 07:10 PM

FC drives me nuts with any fast refractor faster than F/5.5.  Same as coma does in fast Newts under F/5.7. Sweeping at low powers shows bloated stars coming and going out of the FOV.  Makes me sea sick. At least a Paracorr helps with the Newts.  We need a FC eater type of Paracorr for fast fracts and that's the truth ruth.  The ST80 was just not my cup of tea for sweeping as FC ruined it for me. Higher powers in the center of the FOV were fine, but sweeping was a killer.


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#20 213Cobra

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Posted 06 January 2020 - 07:12 PM

Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

 

Sounds like the Tail wagging the Dog.

 

It seems to me that for refractor owners that can no longer accommodate small amounts of field curvature, the task becomes finding a field flattener.

 

Or finding eyepieces that have focal surfaces which are better suited to their telescopes. See Figure 211, and the following graphic 1B:

 

https://www.telescop...errations_1.htm

No. Just add a flattener, ala TSFlat2 or 2.5, or any other flattener that also accommodates visual use. And if you truly like some FC in your views, or you have eyepieces that aren't optimized for flat fields, you can always remove the flattener.

 

Phil


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#21 CHASLX200

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Posted 06 January 2020 - 07:13 PM

Actually, the radius of the field curvature in a refractor, without a field flattening element, is roughly 0.35x the focal length. This, with very minor variation, is true for ALL refractors, regardless of lens design. 

 

For a newtonian, the radius of field curvature is 0.95x the focal length and in an old plain vanilla SCT, it's 0.25x. This means that of the three most common telescope types owned by amateur astronomers, the newtonian has BY FAR the flattest field, . Sadly, it's plagued by coma instead. 

 

But to return to your question: Yes, a 72mm f/6 has much worse field curvature than a 110mm f/6. IF you don't perceive it as a problem in the 72mm f/6, then you won't in the 110mm either, as long as you use the same eyepieces. If you use longer focal length eyepieces with a wider true field, then you might notice it. 

 

For visual use, how annoying field curvature is, pretty much depends on how annoying YOU find it. Some can't stand the merest hint of it, others are very forgiving. 

 

 

Clear skies!
Thomas, Denmark

I am a FC hater. Even SCT 's have it bad while viewing the moon at lower powers.



#22 Jeff Morgan

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Posted 07 January 2020 - 02:53 PM

How old you are affects your perception of FC a lot. Youth has its benefits. 

 

With widest field eyepieces, older folks using the common doublets or triplets are quite susceptible to FC because our eye's lenses have hardened and lost their ability to accommodate the varying focus across the field.  

 

lol.gif

 

I often tell my younger friends "Your body has many surprises in store for you with age, and none of the surprises are pleasant".

 

So far FC is not an issue for me using many eyepieces (two of them Tele Vue) across different telescope types.

 

But I know I am living on borrowed time ... 


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#23 salico

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Posted 07 January 2020 - 04:21 PM

I m 36, and FC drives me crazy...



#24 Wildetelescope

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Posted 07 January 2020 - 07:06 PM

Does this mean that owners of non-Petzval refractors (about 98% of us on this forum) should dump their refractors so they can use Tele Vue eyepieces?

 

Sounds like the Tail wagging the Dog.

 

It seems to me that for refractor owners that can no longer accommodate small amounts of field curvature, the task becomes finding a field flattener.

 

Or finding eyepieces that have focal surfaces which are better suited to their telescopes. See Figure 211, and the following graphic 1B:

 

https://www.telescop...errations_1.htm

I use my tv eyepieces in all my scopes.  Never notice field curvature unless I look for it.  Of course most are f8 or longer.   For that matter, I never notice it with my Brandons either, unless a planet drifts to the edge.  Of course my astigmatism is so bad, that might have something to do with it:-). Seriously, even with my observe’n glasses, it is not something that jumps out at me.  My eyes are going on 52.  Different for different folks.  

 

‘’Jmd 



#25 Jon Isaacs

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Posted 07 January 2020 - 07:11 PM

lol.gif

 

I often tell my younger friends "Your body has many surprises in store for you with age, and none of the surprises are pleasant".

 

So far FC is not an issue for me using many eyepieces (two of them Tele Vue) across different telescope types.

 

But I know I am living on borrowed time ... 

 

So how clean and sharp are the three brightest stars in the belt of Orion when viewed in the same field?

 

Jon




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