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Which of these two eyepieces has the greater Actual FOV?

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#1 jffielde

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Posted 19 October 2019 - 08:15 PM

The calculation I've found for Actual FOV suggests that the ES12mm has a greater Actual FOV than the Ethos 13mm, despite the ES 12mm's greater magnification and lesser 92 degree Apparent FOV.  I do not understand how that is possible.  Would someone please explain how that is possible?  Thanks.

 

Televue Ethos 13mm 100º 13mm (Field Stop 22) (Actual FOV 0.54)

 

Explore Scientific 12mm 92º 12mm (Field Stop 28) (Actual FOV 0.67)

 

I am using this formula: (57.3 * Field Stop) / Focal Length of Telescope = Actual FOV

 

Thank you.



#2 Neptune

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Posted 19 October 2019 - 08:37 PM

It's all about field stop diameter. The larger the field stop the more field of view you are going to see.

 

Additionally, the field stop of the 12mm 92 deg is only 19.6mm (per the website)

 

The Ethos has the larger viewable field.


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#3 Sky Muse

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Posted 19 October 2019 - 08:38 PM

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#4 jffielde

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Posted 19 October 2019 - 08:44 PM

Thank you both.



#5 gnowellsct

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Posted 19 October 2019 - 09:05 PM

A quick way to determine which has the bigger field is to multiply the ocular focal length times its apparent field.

 

So for the Ethos 13x100=1300

For the ES 12x92 = you don't need to know, because if it was 12x100 it would be less than 1300, and if it's 12x92 it's even more "less."  

 

This is just a quick comparison tool to rank the eyepieces by field of view.  It doesn't tell you actual size of the field.  

 

For comparison 

 

XW 40 = 40*70 = 2800

Nagler 31 =31*82=2542

 

So the XW 40 will have more true field of view, the Nagler less than the XW 40.  And both will exceed in true field of view the 13 Ethos and ES 12mm.

 

by comparison the 41 Pan Optic is 41*68=2788.  This is so close to the XW40's 2800 that it is telling you that the two are for all intents and purposes identical in field of view.  

 

Greg N


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#6 junomike

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Posted 19 October 2019 - 09:42 PM

A quick way to determine which has the bigger field is to multiply the ocular focal length times its apparent field.

 

So for the Ethos 13x100=1300

For the ES 12x92 = you don't need to know, because if it was 12x100 it would be less than 1300, and if it's 12x92 it's even more "less."  

 

This is just a quick comparison tool to rank the eyepieces by field of view.  It doesn't tell you actual size of the field.  

 

For comparison 

 

XW 40 = 40*70 = 2800

Nagler 31 =31*82=2542

 

So the XW 40 will have more true field of view, the Nagler less than the XW 40.  And both will exceed in true field of view the 13 Ethos and ES 12mm.

 

by comparison the 41 Pan Optic is 41*68=2788.  This is so close to the XW40's 2800 that it is telling you that the two are for all intents and purposes identical in field of view.  

 

Greg N

This works as a rough estimate however some manufacturers list the AFOV larger than it really is as well as rounding off the F/L which can also vary the true FOV some.



#7 gnowellsct

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Posted 20 October 2019 - 12:00 AM

This works as a rough estimate however some manufacturers list the AFOV larger than it really is as well as rounding off the F/L which can also vary the true FOV some.

it is a rough estimate but I doubt the afov cheating is enough to change the basic structure of the comparison.  As for example 2788 vs 2800.   You'd have to get out the calipers to see which one really offered more field of view; and in the end, they are both designed to offer maximum field in two inch format, even if the 2788 "won" (more field than the 2800) it would not be by enough to matter. 

 

There are other parameters that also change.  For example my f/6.9 CFF clocks in, to be precise, at f/6.85, and my old 10" f/6 Newt was, in fact, f/6.3.  And one will find similar variations around the stated specs in other brands.  So if you have two Stowaways one might be f/6.66 and another one f/6.7, which will affect true field between two different copies.

 

When I have clocked star transits at the celestial equator (transits across the field of view) I generally get results within 5% of what you would expect from the specs on the gear.  Some of it is measurement error: it's hard to have a star enter the exact center of the ocular in such away as to cross the exact center of the field of view.  And there's some delay in clicking the stop watch, and it's hard sometimes to say *exactly* when a star has left the field.  Because there is an ever so slightly vignetted edge where the star is part visible part not.  And then there's the variation in scope focal lengths, and the variations in the oculars.  

 

Taken all in all 95% accuracy is probably pretty good, and in a context where that is the case, the calculation trick I outlined above is well within the norms of the gear we're using, taking them as total systems.

 

--GN


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