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Eyepiece baffles~ optimizing

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5 replies to this topic

#1 Glen10

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Posted 23 September 2020 - 05:06 PM

Ok, bare with me folks.

 

Experimenting with cheap ep's & a cheap lathe, hopefully to learn something, but that's hopefully where you all come in.

 

Some ep's have baffles some don't, for those that do, should the chamfer face the incoming light or trail the lightpath?~ I've Seen both.

 

If making  baffles then

what angle would be optimal for the chamfer? I can easily implement a 30,45 or 60 deg chamfer angle. Other angles are possible but more tricky.

 

Ok, I'm possibly splitting hairs here & it's all more applicable to  high end ep's.

But i hope to learn something, both from your comments & any subsequent experimentation.

 

i'm ALL ears.wink.gif Thanks.

 

Ps am i right in thinking optimum ep baffle placement would be dependant on scope focal ratio? so one size won't necessarily 'fit all'

 

 



#2 RFeaster

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Posted 24 September 2020 - 03:32 PM

I find this subject interesting.  If you haven't seen it, this thread is a good read:

 

https://www.cloudyni...rew-in-baffles/


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#3 MitchAlsup

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Posted 24 September 2020 - 04:24 PM

Baffling an eyepiece is conceptually identical to baffling a refractor.

 

EyepeiceBaffel.jpg

 

Basically, what you want to do is to prevent the field stop from admitting any light that has reflected or scattered off of the EP barrel inside.

 

In the figure, I illustrate the focal plane, the field stop, and the point at which stray light can enter the eyepiece. The baffles are positioned such that  no light from the inside of the opposite barrel surface can pass through the field stop.

 

The hard part is that many modern EPs have an optical lensing element at the very entrance to the EP itself, making the positioning of these baffles difficult if not impossible.

 

ETA:: the most important baffle is the one at the front of the EP barrel (or the one at the bottom of the draw tube--called the focuser baffle.) This baffle is sized to admit only the light cones of the F/ratio (F/4 shown) off of the secondary mirror. Everything else is to be considered stray light.


Edited by MitchAlsup, 24 September 2020 - 05:05 PM.

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#4 Glen10

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Posted 24 September 2020 - 05:28 PM

Thanks to you both.

 

   Anyone chime in on the chamfer angle.orientation  (could apply to fieldstop too?)~

I have a couple of ep's that really need new field stops,ones machined badly, the other is too big,so prepared to experiment.



#5 MartinPond

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Posted 24 September 2020 - 05:56 PM

Some helpful ideas:

 

---you will always worry about the 'edge flashing' of same diameter irises

    ...stepping them in towards the EP will keep each edge in the shadow of the other.

 

---Zeiss, for over 100 yrs, has made great use of internal threading,

     about 30 degree faces.   The stray light 'perishes"

 

---simply stepping inner diameters helps a lot...used often in short refractors.

    ....an excellent example is the inside of the Orion Tri-Mag

.    --it has a long blackened-threaded section, and then a step-out towards

      the eyepiece.  very effective.

 

----too narrow and you will vignette the field of a longer-fl eyepiece,

    BUT:

---note that you do NOT have to obey the inner diameter of an eyepiece always..

   ...you could have things flare out and back..

     Common in Explore Scientific and Televue multipliers..

  

 

----an iris up front is handy, but there are more tricks you can do

 

---any commercial eyepiece with a decent field stop has pretty good light control..

 ...usually, just a front iris will do a lot (if the fl is low) 


Edited by MartinPond, 24 September 2020 - 06:18 PM.


#6 MitchAlsup

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Posted 24 September 2020 - 06:34 PM

Thanks to you both.

 

   Anyone chime in on the chamfer angle.orientation  (could apply to fieldstop too?)~

I have a couple of ep's that really need new field stops,ones machined badly, the other is too big,so prepared to experiment.

You put the knife edge away from the field stop (the knife edge is "seen by incoming light cones"), so the EP "sees" the flat "flat black edge" and conversely does not "see" a surface which can re-reflect a ghost coming out of the EP.




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