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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?



#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!




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