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20mm erect image finder vs 24mm plossl magnification.

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#1 zanxion72

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Posted 12 July 2018 - 01:33 AM

It might be extremely silly, but here it goes.

I have an old Celestron AstroMaster 130 newtonian telescope and I have noticed that the 20mm erect image eyepiece delivers a lower magnification view compared to a 24mm Meade Super Plossl. Shouldn't it be the other way around? Shouldn't the eyepiece with the smaller focal length deliver the highest magnification from the two?


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#2 DLuders

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Posted 12 July 2018 - 02:43 AM

Perhaps you're being confused by the Apparent Field of View (AFOV) offered by the various eyepieces.  The wider AFOV in the 20mm erect-image eyepiece may make it SEEM like its delivering a lower magnification.

 

Magnification = (Focal Length of Telescope) / (Focal Length of Eyepiece)

 

According to the Specifications on this Celestron website, the Astromaster 130 has a Focal Length of 650mm, so the 24mm eyepiece in it produces a magnification of (650mm) / (24mm) = 27x.

 

The 20mm eyepiece produces a magnification of (650mm) / (20mm) = 32.5x.


Edited by DLuders, 12 July 2018 - 02:46 AM.


#3 DLuders

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Posted 12 July 2018 - 02:58 AM

Here is an illustration of how the Apparent Field of View (AFOV) differs between two eyepieces.  Using the Astronomy Tools Field of View Calculator, one enters the telescope and eyepiece parameters and presses the blue "Add to View" button on the bottom-right corner.  I'm guessing that the 20mm erect-image eyepiece has an AFOV larger than the 24mm Meade Series 4000 Super Plossl does (with its 52-degree AFOV).  Let's assume that the 20mm eyepiece has an AFOV of 70 degrees.  The two eyepieces' views (compared together) would make the Moon look like this:

 

Astromaster130EyepieceAFOVs.JPG

 

 

 



#4 zanxion72

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Posted 12 July 2018 - 03:17 AM

Here is an illustration of how the Apparent Field of View (AFOV) differs between two eyepieces.  Using the Astronomy Tools Field of View Calculator, one enters the telescope and eyepiece parameters and presses the blue "Add to View" button on the bottom-right corner.  I'm guessing that the 20mm erect-image eyepiece has an AFOV larger than the 24mm Meade Series 4000 Super Plossl does (with its 52-degree AFOV).  Let's assume that the 20mm eyepiece has an AFOV of 70 degrees.  The two eyepieces' views (compared together) would make the Moon look like this:

 

attachicon.gif Astromaster130EyepieceAFOVs.JPG

Thank you so much about this! It explains everything. I had the erect image eyepiece cleaned and thought that I might had put things together in a wrong way.

It has been a couple of years I left my telescope aside and it takes a while for things to come back altogether.

 

Thank you!



#5 aeajr

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Posted 17 July 2018 - 03:16 PM

It might be extremely silly, but here it goes.

I have an old Celestron AstroMaster 130 newtonian telescope and I have noticed that the 20mm erect image eyepiece delivers a lower magnification view compared to a 24mm Meade Super Plossl. Shouldn't it be the other way around? Shouldn't the eyepiece with the smaller focal length deliver the highest magnification from the two?

The erect image eyepiece has a prism of some kind inside to correct the image. This may be changing the effective focal length of the eyepiece.

 

Yes you would normally expect a 20 mm to deliver a higher mag than a 24 mm in the same scope. 



#6 Sky Muse

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Posted 17 July 2018 - 05:29 PM

The erect image eyepiece has a prism of some kind inside to correct the image. 

https://www.celestro...t-back-together


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#7 aeajr

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Posted 17 July 2018 - 11:01 PM

Seems I was mistaken. Thanks for the link Alan. 



#8 Sky Muse

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Posted 17 July 2018 - 11:21 PM

You're welcome, Ed.  It's easy to imagine that it does contain a prism, but it's just the configuration of the lenses that accomplishes that, but I don't know how it does it.  I do know that one of those will come with my very own Celestron "Powerseeker" 127mm f/8 Jones-Bird-like catadioptric when it arrives in the near-future.  I'm so excited.


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#9 aeajr

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Posted 17 July 2018 - 11:31 PM

Alan,

 

With all the scopes you have, why would be buying a Powerseeker 127mm?


Edited by aeajr, 17 July 2018 - 11:33 PM.


#10 Sky Muse

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Posted 17 July 2018 - 11:49 PM

Why, to work on it, of course.  I couldn't do too terribly much to a Schmidt in that regard.  It will be my "5SE" when I'm done with it.

 

 

Alan,

 

With all the scopes you have, why would be buying a Powerseeker 127mm?




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