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Optica and Parks Orthos and Erfles

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#1 spartan1

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Posted 07 June 2020 - 03:46 AM

I am looking for fov's on these four ep's which I am guessing are from the 1990's.  The 32mm is a 2".  Any help much appreciated.   Thanks, Philip

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#2 Astrojensen

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Posted 07 June 2020 - 05:24 AM

If these eyepieces are in your posession, why don't you just measure the field of view???

 

 

Clear skies!
Thomas, Denmark


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#3 Ernest_SPB

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Posted 07 June 2020 - 10:33 AM

I see that all of the eyepieces have open field stop (FS). So just measure diameters of their FS and then calculate effective AFOV = 57.3*FS/FL, where FL - is focal length in the same units


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#4 spartan1

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Posted 07 June 2020 - 08:48 PM

I see that all of the eyepieces have open field stop (FS). So just measure diameters of their FS and then calculate effective AFOV = 57.3*FS/FL, where FL - is focal length in the same units

Thanks Ernest,  Philip



#5 spartan1

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Posted 17 June 2020 - 05:00 AM

The 40mm 1.25" Plossl pictured has an unusual zebra knurled ring and the FS is 26mm (there is little change to the inside diameter of the ep) 

So 57.3*26/40 gives an AFOV close to 38 degrees,which seems really narrow?  Any ideas what the ep is?



#6 Starman1  Happy Birthday!

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Posted 17 June 2020 - 10:00 AM

The 40mm 1.25" Plossl pictured has an unusual zebra knurled ring and the FS is 26mm (there is little change to the inside diameter of the ep) 

So 57.3*26/40 gives an AFOV close to 38 degrees,which seems really narrow?  Any ideas what the ep is?

That makes sense, since a 40mm eyepiece with a 27mm field stop has about a 40° field.

The eyepiece probably is a 2:2 configuration Plössl, as it's labeled.

 

In 1.25" eyepieces, a 40mm typically has a 40° field, a 32mm about 50° and a 24mm a 68° field if the field stop in each is 27mm.

There is a little variation--those are approximate.

 

You can derive a true field from the field stop:

True field = (field stop / telescope focal length) x 57.3

but you cannot derive the actual apparent field from the field stop without knowing the distortion characteristics of the eyepiece.

 

Ernest's formula is very approximate, though it works quite well on eyepieces with narrow fields, like this one.


Edited by Starman1, 17 June 2020 - 10:01 AM.


#7 spartan1

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Posted 18 June 2020 - 04:33 AM

That makes sense, since a 40mm eyepiece with a 27mm field stop has about a 40° field.

The eyepiece probably is a 2:2 configuration Plössl, as it's labeled.

 

In 1.25" eyepieces, a 40mm typically has a 40° field, a 32mm about 50° and a 24mm a 68° field if the field stop in each is 27mm.

There is a little variation--those are approximate.

 

You can derive a true field from the field stop:

True field = (field stop / telescope focal length) x 57.3

but you cannot derive the actual apparent field from the field stop without knowing the distortion characteristics of the eyepiece.

 

Ernest's formula is very approximate, though it works quite well on eyepieces with narrow fields, like this one.

 

Many thanks for this. Explains a lot. I have depleting peripheral vision and the really wide angle ep's are probably wasted on me.  I have a Panoptic 27 for example...which I still love but I am searching around the view now and it will only get worse.  So whilst others are expounding the virtues of 80-100deg fov, I am thinking that I maybe better finding some good, perhaps older, narrower fov ep's for the future.    



#8 25585

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Posted 18 June 2020 - 05:54 AM

Isn't a 2:2 a symmetrical?



#9 Starman1  Happy Birthday!

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Posted 18 June 2020 - 11:36 AM

Isn't a 2:2 a symmetrical?

not necessarily.  The doublets can be quite different.




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