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Field Curvature for Visual

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#26 CHASLX200

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Posted 07 January 2020 - 07:11 PM

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 

Try sub F/5.5.



#27 Wildetelescope

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Posted 07 January 2020 - 09:54 PM

Try sub F/5.5.

I take your point, which is why most of my gear is f8 or so:-).   Actually, I have an f6 80 mm on the way.  Jon issacs recommended test on Polaris for curvature is top of my to do list:-). 

 

Jmd 



#28 Jeff Morgan

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Posted 07 January 2020 - 10:24 PM

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

 

Jon

 

Have not had that scope on Orion yet (acquired last May). I'll make a point to look during the next session with it. 

 

Distance from Mintaka to Alnitak is only 2.75 degrees. 27 Panoptic would put each on the near the edge with Epsilon Ori centered. Or I could use the TMB 130 with the 40 Paragon.

 

Not sure if I look forward to the test or dread it ... 



#29 Jon Isaacs

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Posted 07 January 2020 - 10:40 PM

Have not had that scope on Orion yet (acquired last May). I'll make a point to look during the next session with it. 

 

Distance from Mintaka to Alnitak is only 2.75 degrees. 27 Panoptic would put each on the near the edge with Epsilon Ori centered. Or I could use the TMB 130 with the 40 Paragon.

 

Not sure if I look forward to the test or dread it ... 

 

It would be better to use a scope that could put Mintaka near the edge and Alnitak near the center.  You TMB is on the long side for really seeing the curvature.

 

I have an extra ST-80 with a 2 inch focuser I can send you. Short and fast is the recipe for seeing the field curvature. 

 

If you really want to see it, I have a 50 mm F/4 finder objective set up to take 2 inch eyepieces. With the 21 mm Ethos its about 10° at 10x and loads of curvature.

 

50mm with 2 inch diagonal.jpg

 

Jon



#30 Tyson M

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Posted 07 January 2020 - 11:17 PM

I am 34 and field curvature isnt too bad.  I have observed with a few non-petzval F5.5 refractors and havent had too much of an issue so far.  I know an older astro friend had a peak at one of my scopes and couldnt stand the FC at all.  This was with a 27 panoptic I believe and a 72mm f5.5 apo.

 

I would look at the TSFlat2 like it was previous mentioned if I was bothered by FC if the scope had a decent amount of backfocus.  I tend to use corrected eyepieces for the fast scopes but I have used Brandons as well.

 

Ultimately, I like both slow and fast scopes for what they have to offer.  If I was budget-conscious on my eyepieces and worried about the outer field abberations then a longer focal length design makes more sense. 


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#31 YAOG

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Posted 08 January 2020 - 12:20 AM

I'm using all TV eyepieces

Me too, except for the set of Circle-T UO Volcano Top Orthos and a set of vintage Japanese Meade Series 4000 Super Plossls and... lol.gif ,   



#32 nicknacknock

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Posted 08 January 2020 - 12:56 AM

I am 34 and field curvature isnt too bad.  I have observed with a few non-petzval F5.5 refractors and havent had too much of an issue so far.  I know an older astro friend had a peak at one of my scopes and couldnt stand the FC at all.  This was with a 27 panoptic I believe and a 72mm f5.5 apo.

 

I would look at the TSFlat2 like it was previous mentioned if I was bothered by FC if the scope had a decent amount of backfocus.  I tend to use corrected eyepieces for the fast scopes but I have used Brandons as well.

 

Ultimately, I like both slow and fast scopes for what they have to offer.  If I was budget-conscious on my eyepieces and worried about the outer field abberations then a longer focal length design makes more sense. 

I have used this on my Borg 55fl (f4.5) with a Pan 24mm and it gave a perfectly flat field Vs the "mess" it was before. I strongly recommend this particular flattener!



#33 SteveG

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Posted 09 January 2020 - 08:37 PM

Try sub F/5.5.

Again, it's not the ratio!  It is the focal length. So a 6", f5 refractor at 750 mm would show very little field curvature, but a 3" f5 at 400 mm will show a bunch, provided field curvature bothers you.


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#34 SteveG

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Posted 09 January 2020 - 08:40 PM

I had the Megrez 110 (655 mm) and a Zenithstar 80 (545 mm). When using a low power wide-field, the field curvature was very noticeable on both, the 80 being the worse of the two. My solution was a longer focal length refractor. My 750 mm ED 120 shows very little field curvature, enough not to bother me at all. YMMV



#35 RadioAstronomer

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Posted 09 January 2020 - 09:20 PM

I used to own a 60mm f/5.5 doublet (A Stellarvue produced by Sharpstar I believe) and FC was noticeable. At the beginning I thought I could live with it, but then what was seen could not be unseen...


Edited by RadioAstronomer, 09 January 2020 - 09:20 PM.

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#36 junomike

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Posted 10 January 2020 - 01:03 PM

I purposely acquired a Bresser 102 F4.5 (460mm F/L) for the purpose of RFT using a 31T5.  Haven't been disappointed and FC is never an issue however I've never noticed it as much as other aberrations like CA and Coma.


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#37 WyattDavis

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Posted 11 January 2020 - 06:48 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. 

I didn't know this. Thanks for posting it Thomas.



#38 CHASLX200

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Posted 11 January 2020 - 08:34 AM

Again, it's not the ratio!  It is the focal length. So a 6", f5 refractor at 750 mm would show very little field curvature, but a 3" f5 at 400 mm will show a bunch, provided field curvature bothers you.

I tried a 6" F/5 and at low power i was swimming with a bending FOV.  Way too much FC.


Edited by CHASLX200, 11 January 2020 - 08:35 AM.


#39 organge

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Posted 11 January 2020 - 08:44 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!

I find FC an issue with 60/360 but with 72/420(SW) I can live with it. With 60/600 and 80/560 I don't see it.


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#40 WyattDavis

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Posted 11 January 2020 - 08:49 AM

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.  

Jon, how are you calculating the distances between center focus and focus at the edge of the FOV?



#41 WyattDavis

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Posted 11 January 2020 - 01:54 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. 

Radius of field curvature for my scopes (if I am doing this right):

 

Screen Shot 2020-01-11 at 1.53.17 PM.png

 

Yes, the Newtonian is FAR flatter.



#42 Axunator

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Posted 11 January 2020 - 04:34 PM

Again, it's not the ratio!  It is the focal length. So a 6", f5 refractor at 750 mm would show very little field curvature, but a 3" f5 at 400 mm will show a bunch, provided field curvature bothers you.

You are correct, of course, in that the focal length (together with the design) determines the radius of curvature. However, a faster f-ratio makes the curvature more visible (when comparing scopes with similar radii of curvature), since depth of focus is shallower and therefore the diameter of defocused blur at similar field angles away from the optical axis will be larger.


Edited by Axunator, 11 January 2020 - 04:39 PM.


#43 CHASLX200

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Posted 11 January 2020 - 05:42 PM

Scopes at F/7 and slower are fine with my eyes.




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