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Barlows, Glass Path Correctors, Focal Extenders, and Reducers

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#1 Lentini

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Posted 23 June 2022 - 07:56 PM

I was comparing a GPC to three Barlows, using them to find a way to extend my focal point away from the scope. 
 

I noticed that they had very different clear apertures (comparing diameters of the glass), but more importantly my FOV seemed twice as big in the GPC vs the Barlows. 
 

Many of these devices are oriented to using Binoviewers, many are for astrophotography, but I’m looking at visual for single eyepiece views… to extend that focal length. 
 

Why is the FOV of the GPC twice that of a Barlow of similar magnification? How would a focal extender or reducer compare for FOV? Is there anything I should watch out for with regards to the clear aperture? 
 

Thanks!



#2 Bkoh

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Posted 23 June 2022 - 09:34 PM

GPCs and barlows are intermediate devices in the optical train. They should not affect the FOV of the eyepiece. You may notice that some barlows have rather small clear apertures, but still work fine. I am not an optical designer, but would guess that reducing the clear aperture to the minimum needed could help with stray light control. It could also require less glass, making the barlow smaller and lighter.

#3 Lentini

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Posted 23 June 2022 - 11:06 PM

I should clarify. I used my Omegon GPC and three Barlows on the scope side of my diagonal. 
 

The GPC reduced the FOV to about 2/3rds and the Barlows to about 1/3rd what I could see with nothing added. The FOV of the eyepiece may have covered the same range in terms of where you could look left/right/up/down in the eyepiece, but the actual subject field shown was very reduced. And what I’m surprised at is how much more the Barlows reduced what could be seen vs the GPC.

 

So am I right to say the AFOV is unchanged, but the TFOV is reduced by those amounts? I think that’s it. Anyhow… Hoping to learn some basics. Is a tele-extender the same thing as a GPC? How does a reducer affect the optical path? Etc


Edited by Lentini, 23 June 2022 - 11:43 PM.


#4 Bkoh

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Posted 23 June 2022 - 11:43 PM

Maybe the GPCs and barlows in question are only designed to be used after the diagonal. Try them that way, if they are fine then use them like that. If they still vignette the view, then something is wrong in the optical train. It seems unusual though, many members here routinely put their barlows in front of the diagonal without issue.

Does the issue occur with all your eyepieces or only certain ones?

#5 DRodrigues

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Posted 24 June 2022 - 05:09 PM

...

 

Why is the FOV of the GPC twice that of a Barlow of similar magnification? How would a focal extender or reducer compare for FOV? Is there anything I should watch out for with regards to the clear aperture? 
 

Thanks!

The indicated GPC magnifications are for the light path of the binoviewers and the indicated barlow magnifications are for the use with single eps. To get the indicated GPC magnifications with single eps you must add extension tubes within the GPC and the eps, with similar length of the ligth path of the BVs. There are "modular" barlows that range from 1.6 to 2.5x by adding sections equivalent to extension tubes. See an example of differences on increase factors at https://www.cloudyni...1#entry11950099
 


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