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Flat Fields for a 5" F5 RFT

classic equipment eyepieces refractor
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#1 Bomber Bob

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Posted 13 February 2020 - 06:58 PM

Ol' BB is looking for 2" eyepiece recommendations for a vintage (non-ED) 5" F5 triplet RFT.  I'll take flatter over wider, but a good compromise between those two traits will work.  Naglers have done very well in this old frac, but I haven't seen any in the 30mm to 20mm range.  Any tried & true favorites out there?


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#2 NC Startrekker

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Posted 13 February 2020 - 07:17 PM

BB... How about the Nagler 22T4 or the Nagler 31T5?


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#3 stevew

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Posted 13 February 2020 - 08:50 PM

Bob, tell us what eyepiece you have tried in your RFT.



#4 Bomber Bob

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Posted 13 February 2020 - 09:23 PM

In 2" format:  Erfles, Kellners, & Plossls -- all vintage.  In 1.25" format:  A-T Paradigms, Erfles, Kellners, Naglers, Plossls, RKEs, & Radians.

 

So far, the Nagler gives the largest + flattest field.  The Paradigm does well -- especially for the price -- edges out Plossls.

 

Naturally, I'm looking at used (but not abused), as I won't spend more on an eyepiece than the scope is worth.  But, it's a fun frac that pulls in a lot of stars, even in my backyard, so it's worth some upgrades.


Edited by Bomber Bob, 13 February 2020 - 09:27 PM.


#5 junomike

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Posted 13 February 2020 - 09:46 PM

31T5 would be my choice but the 35 Pan might be better for some.



#6 Jon Isaacs

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Posted 13 February 2020 - 09:49 PM

The scope itself has noticeable field curvature so that's part of the equation.

 

Generally though at F/5, the eyepiece's astigmatism overwhelms the curvature unless it's a very well corrected eyepiece. Such eyepieces are not inexpensive to say the least.

 

In my experience, the 31mm Nagler and the 35 mm Panoptic are at the top of the list. From there, a step down, are the Explore Scientific 82° and 68° eyepieces.  Still very good eyepieces.

 

Probably not much help... still expensive.

 

Jon


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#7 SeattleScott

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Posted 13 February 2020 - 09:56 PM

ES 24/82? I use mine at F4 in coma corrected newt.

#8 39.1N84.5W

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Posted 22 February 2020 - 08:41 AM

Life is short. Buy a used 31mm Nagler and be done with it. You can easily sell it later... if you take care of your gear. Clear skies.
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#9 Sarkikos

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Posted 22 February 2020 - 12:42 PM

Your triplet might have decent color correction.  But an eyepiece won't flatten the field curvature of the telescope.  If you want a flat field for your well-corrected wide-field eyepieces in your triplet, you need a field flattener.  

 

TS-Optics REFRACTOR 1.0x Flattener and Field Corrector - 2" connection

https://www.teleskop...connection.html

 

The TSFLAT2 has worked well in several of my achromats and ED/APOs.

 

Mike


Edited by Sarkikos, 22 February 2020 - 01:04 PM.

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#10 Terra Nova

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Posted 22 February 2020 - 12:47 PM

Erfles are generally poor performers in fast scopes as they exacerbate field curvature. Have you tried the BST Flat Field eyepieces? The 19mm has a 65° AFOV and does very well in my Custom-made 80mm F5 RFT.



#11 Sarkikos

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Posted 22 February 2020 - 12:51 PM

Naturally, I'm looking at used (but not abused), as I won't spend more on an eyepiece than the scope is worth.  But, it's a fun frac that pulls in a lot of stars, even in my backyard, so it's worth some upgrades.

You don't buy an eyepiece for just one scope.  You buy it for all your scopes.  

 

Likewise for the TSFLAT2.  It will probably work well in several of your refractors.

 

Mike


Edited by Sarkikos, 22 February 2020 - 01:01 PM.

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#12 junomike

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Posted 22 February 2020 - 01:52 PM

Your triplet might have decent color correction.  But an eyepiece won't flatten the field curvature of the telescope.  If you want a flat field for your well-corrected wide-field eyepieces in your triplet, you need a field flattener.  

 

TS-Optics REFRACTOR 1.0x Flattener and Field Corrector - 2" connection

https://www.teleskop...connection.html

 

The TSFLAT2 has worked well in several of my achromats and ED/APOs.

 

Mike

Interesting option.  Do you affix it to the telescope side of the Focuser?



#13 Sarkikos

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Posted 22 February 2020 - 03:24 PM

The TSFLAT2 screws onto the telescope end of the diagonal.  You attach it to the diagonal, and then the TSFLAT2 slides into the visual back of the refractor.  Here's a thread with more information:

 

Field Flatteners for Achromats, Visual Only

https://www.cloudyni...ts-visual-only/

 

Depending on the focal length of the telescope, you either screw the TSFLAT2 to the 48mm threads on the barrel of the diagonal, or you take off the barrel and screw it directly to the back of the diagonal.  If the diagonal has SCT threads, you need an 48mm/SCT adapter ring between the TSFLAT2 and the diagonal threads.

 

Mike


Edited by Sarkikos, 22 February 2020 - 03:28 PM.


#14 junomike

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Posted 22 February 2020 - 04:10 PM

Thanks for the info.  From the initial ad I thought it went on the Lens side of the Focuser (which might be difficult to do).



#15 areyoukiddingme

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Posted 22 February 2020 - 04:22 PM

FWIW, I've found the flattener works, but it's not completely consistent with eyepieces.

 

For example, in my Stowaway, the 12.5 Docter adds astigmatism at the edge when the flattener is installed.

 

I haven't been systematic about comparing all my eyepieces, but this one stands out.


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