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large-ish, high-ish AFOV EP only in 2", why

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#1 smasraum

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Posted 25 January 2021 - 04:02 PM

Can someone explain why it seems like most EP once they get above around 24-25 if they are 68º or more, only or mostly come as 2" EP? 

 

Does it have to do with the AFOV and the field stop and vignetting or ...?

 

I thought that as long as the EP is 31mm or smaller, that it could be a 1.25".  I'm assuming that only applies at certain AFOV.

 

For instance, the Panoptic 27mm which has a 30.5mm field stop, all of which sounds like it should work as a 1.25" comes as a 2" EP.

 

Nagler 31 and 22 and Ethos 21 and even the 17mm are all 2" only.

 

The Hyperion 24 and 31 both do come with adapters to make them 1.25", but both are portrayed as if it's expected to use them as 2" EP.  Is there a limitation so that if they are used as 1.25" they'd not work as well as when used as a 2"?



#2 ravenhawk82

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Posted 25 January 2021 - 04:04 PM

My understanding is that you can't physically get that higher AFOV in a smaller tube. Beyond a point you'll just start vignetting and not getting anything out of the extra AFOV but I don't know the math that determines what that point is.


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#3 CrazyPanda

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Posted 25 January 2021 - 04:12 PM

AFOV for a given focal length is limited by the field stop. It’s not geometrically possible to have say, 70 degree AFOV and 30mm focal length in a 1.25” barrel.

There’s a geometric formula that I forget which someone can post, but it shows you from first principles why you can’t get a certain AFOV in a certain focal length in a certain barrel size.
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#4 smasraum

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Posted 25 January 2021 - 04:13 PM

My understanding is that you can't physically get that higher AFOV in a smaller tube. Beyond a point you'll just start vignetting and not getting anything out of the extra AFOV but I don't know the math that determines what that point is.

OK, that's kind of what I was expecting.  It makes sense.  The light path comes in as a cone, so you may not be able to fit part of the cone through a small opening.


Edited by smasraum, 25 January 2021 - 04:19 PM.

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#5 rkelley8493

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Posted 25 January 2021 - 04:14 PM

Their field stop diameters are wider than 1.25" [31mm]. The glass wouldn't fit inside the barrel.

 

Edit:  Technically anything with a field stop diameter wider than 27mm would have to be in 2" format because the barrel itself takes up a few millimeters. Think of it like rims on the tire of a vehicle. The tire has to be wider than the rims in order for it to roll like it should.


Edited by rkelley8493, 25 January 2021 - 04:25 PM.

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#6 Echolight

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Posted 25 January 2021 - 04:15 PM

I have a 40mm 1.25 inch eyepiece. So there's that too.

 

Also, I've always assumed the 1.25 figure was on the outside of the barrel. Not the inside.


Edited by Echolight, 25 January 2021 - 04:18 PM.


#7 rkelley8493

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Posted 25 January 2021 - 04:17 PM

Some 2" eyepieces [such as Explore Sci 100º 5.5mm and 9mm] are made that way by convenience and not necessity. However, most 2" have to do with the field stop diameter & focal length + field of view of the eyepiece. Here are a few examples.

In a 100º FOV eyepiece, the longest focal length that can be achieved in 1.25" format is 13mm [13 Ethos]. A 14mm 100º [Explore Sci] would have to have a 2" diameter barrel.

In an 82º FOV eyepiece, the longest focal length in 1.25" format is 16mm [TeleVue Nagler]. The 17mm Nagler has a 2" barrel [as well as the 18mm Explore Sci 82º].

With a 76º FOV [such as the 1.25" Baader Morpheus], the longest focal length is 17.5mm.

A couple other wide field, long-ish focal length 1.25" eyepieces are the 17.3mm Delos at 72º and the 20mm Pentax XW at 70º.

For a 68º FOV, an eyepiece can go up to 24mm which gives you the widest True field of view in 1.25" format. Examples are the TeleVue 24 Panoptic and Explore Sci 24mm 68º.

Hope this helps!

 

RK

 

https://www.cloudyni...ces/?p=10397973

 

[Quoted from another similar topic]


Edited by rkelley8493, 25 January 2021 - 04:18 PM.

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#8 Echolight

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Posted 25 January 2021 - 04:23 PM

What's really puzzling, is, why would a shorter focal length eyepiece be longer than a longer focal length eyepiece....scratchhead2.gif



#9 smasraum

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Posted 25 January 2021 - 04:30 PM

Thanks all, lots of good info!



#10 EuropaWill

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Posted 25 January 2021 - 04:33 PM

What the others said. But simply put, the field stop size determines how much TFOV you will see. The 2" eyepieces have larger field stop potential (up to 46mm) vs 1.25" eyepieces (up to 27mm). The AFOV is just how it is "presented". An example of two max TFOV eyepieces in 2" format:  Meade 5k SWA 40mm or the Tele Vue 55mm Plossl.  They both have roughly the same size field stop (around 46mm) so they will show you the same TFOV through your eyepiece. Just that the Meade's design presents it in a higher power larger AFOV of 68° while the Tele Vue's 55mm plossl presents it as a lower power 50° AFOV. 



#11 smasraum

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Posted 25 January 2021 - 04:33 PM

What's really puzzling, is, why would a shorter focal length eyepiece be longer than a longer focal length eyepiece....scratchhead2.gif

waytogo.gif
And why is is the gear ratio in the differential of a car that is 4.88, 4.56 or 4.10 considered lower than a 3.55, 3.08 or 2.56 ratio which is considered taller?  grin.gif



#12 Jon Isaacs

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Posted 25 January 2021 - 04:34 PM

In general, the field stop has to be smaller than the inner barrel diameter.

 

The ID of a 1.25 inch barrel is 28mm, a 2 inch barrel about 48 mm. There field stop needs to be somewhat smaller to avoid vignetting.

 

Generally the numbers are about 27.5 mm max for 1.25 inch, 46 mm for 2 inch.

 

With negative-positive eyepieces (optics in the barrel), this should be reduced as well.

 

A 20 mm Nagler has a 27.2 mm field stop but the optical path would be blocked by the retaining rings etc.

 

Jon



#13 spaceoddity

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Posted 25 January 2021 - 04:36 PM

I have a 40mm 1.25 inch eyepiece. So there's that too.

 

Also, I've always assumed the 1.25 figure was on the outside of the barrel. Not the inside.

Yep, I have a 40 mm plossl too that came with my 8se. A 40 mm plossl is only around 44(?) degrees though instead of the normal 50-52 degrees of plossl's 32mm and lower. A 32 plossl actually has around the same FOV as a 40.


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#14 Echolight

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Posted 25 January 2021 - 04:37 PM

And then there's this 14mm LHD 80 degree, which Don calculated to have a 19.something mm field stop. But 2 inch anyway, and is kinda one reason I chose it. Lessens the filter and reducer demands.

AA55EE62-6850-458D-B706-8B0180EF0106.jpeg

 


Edited by Echolight, 25 January 2021 - 04:42 PM.

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#15 EuropaWill

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Posted 25 January 2021 - 04:39 PM

What's really puzzling, is, why would a shorter focal length eyepiece be longer than a longer focal length eyepiece....scratchhead2.gif

They are typically shorter in height on eyepieces that don't employ a barlow in the nosepiece. Plossls, orthos, kellners, and RKE's, are good examples of the shorter FL's having a shorter overall form factor.  Though for eyepieces that employ a barlow in the nosepiece (which are many these days, especially in the long eye relief designs),  they are typically taller the shorter the FL gets because barlows magnification factor gets stronger the more space you put between them and the rest of the eyepiece elements. So the more you space them out (the taller) the more power you get (the shorter the effective FL). 


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#16 spaceoddity

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Posted 25 January 2021 - 04:41 PM

What's really puzzling, is, why would a shorter focal length eyepiece be longer than a longer focal length eyepiece....scratchhead2.gif

Yeah that one's puzzled me as well, why in some ep lines the shortest fl are bigger/longer than a mid fl. My guess is that they have a smythe lens which is kind of like a barlow in the barrel and it needs to be farther away from the other lenses to achieve the proper magnification. Just a guess.


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#17 Jon Isaacs

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Posted 25 January 2021 - 04:44 PM

What's really puzzling, is, why would a shorter focal length eyepiece be longer than a longer focal length eyepiece....scratchhead2.gif

 

A 4 mm Plossl is much shorter than a 40 mm Plossl. It also has only about 2.5 mm of eye relief.

 

Short focal length eyepieces with a reasonable amount of eye relief are what's called negative-positive eyepieces. They have a Barlow-like negative lens in the barrel and then a positive magnifying eyepiece.

 

A 16 mm and a 5 mm eyepiece with 60 degree AFoV and 15 mm of eye relief might both be based on a 20 mm 60 degree positive section with 15 mm of eye relief. 

 

The negative section will be different. The 16 mm only needs a 1.25x Barlow section, that can be very short. The 5 mm will need a 4x Barlow section, that will need to be much longer.

 

Thus the 5mm will be longer than the 16mm.

 

Jon


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#18 dan_h

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Posted 25 January 2021 - 04:48 PM

I have a 40mm 1.25 inch eyepiece. So there's that too.

 

Also, I've always assumed the 1.25 figure was on the outside of the barrel. Not the inside.

The 40mm offerings in a 1.25" format have a narrower field of view,. Some refer to this as looking through a soda straw. I don't think it is quite that bad. More like looking through a toilet paper roll.  You cannot fit a 50 degree field into a 1.25" barrel with a 40mm focal length.   

 

The 1.25" barrel size is the measurement for the OD of the barrel. Because the barrel walls have some thickness, the field stop on the inside of the barrel is limited to a maximum of 27mm or so.  You can go smaller but anything bigger simply doesn't fit.

 

dan


Edited by dan_h, 25 January 2021 - 04:50 PM.


#19 Echolight

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Posted 25 January 2021 - 04:51 PM

I see. With the Smythe/barlow lens, kinda like the difference in old school drafting pencils with fat lead. Where you want a .9mm line it has a shorter sharpened area. But to get a .5mm line you need a looong lead to the point.


Edited by Echolight, 25 January 2021 - 04:53 PM.


#20 smasraum

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Posted 25 January 2021 - 05:10 PM

Lots of good info folks, thanks.



#21 photoracer18

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Posted 25 January 2021 - 05:21 PM

Many current eyepiece designs date back to the early TV Nagler design  as a starting point with a tele-negative (Smythe) group in the bottom. This is what allows wider field eyepieces along with the rest of the design.  Its also why a higher power eyepieces gets larger and longer instead of shorter and narrower. One reason is to maintain a fixed amount of eye relief. Current designs usually fall into groups based on eye-relief more than anything else. Once you get over the old designs from the past which generally top out at 50-55 degrees, except for a couple of quasi WF designs like the Erfle and Konig which reach 60 degrees you get to the 65-70, 80-85, and 100 and over groups. If you use glasses you need about 20mm of ER, and in that range you are limited to the 65-70 group. In that group a 1.25" eyepiece can't have an FL that is over about 26mm or the barrel will vignette the field. Example: the Pentax XL 28 had a FOV that is only 55 degrees in 1.25" instead of 65. Later they came out with the XW set which had 70 degree fields. They moved the 28 to 30mm and made it a 2" so the entire set now had 70 degrees and 20mm eye-relief.

And if you are older with a narrower exit pupil than when younger you won't be able to see the entire field of an eyepiece wider than about 70 degrees without moving your eye around. And those wider eyepieces have ER of 15mm or less in general. I worked for a dealer for several years and tried a lot of eyepieces. I never bought anything wider than the 65-70 degree group and I am fine with that. YRMV



#22 MitchAlsup

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Posted 25 January 2021 - 05:22 PM

There are two issues:: both purely optical:

 

1) An objective creates an image of a certain scale based solely on the focal length of the objective.

 

If an EP wants to deliver an image to the eye of the observer, the image at the focused scale must fit inside the barrel of the EP (DUH !)

 

2) There is a "light path" through an EP and if this light path needs to be wider than the barrel, then either the part of the light path larger than the barrel has to be above the top of the draw tube--OR the EP needs to use a bigger barrel !! Otherwise the extreme edgges of the light will be vignetted (DUH again !).

 

In the case of the 9ES120, the light path is larger than the 1.25" barrel at the top of the focuser draw tube.

 

To get a touchey feely view of the light paths of various EPs; look here::

https://www.telescop...berration_2.htm

and

https://www.telescop...ce_raytrace.htm

 

ETA:: The reader will observe that those EPs that use highly diverent Smythe lens groups as the "field lens" use significantly larger light paths than the EPs from the 1800s and before.


Edited by MitchAlsup, 25 January 2021 - 05:24 PM.


#23 TOMDEY

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Posted 25 January 2021 - 05:25 PM

I graphed it all out here... it is because of the size of the draw-tube 0.965, 1.25, 2.00, 3.00 inches. The trends are quite obvious.   Tom

 

~click on~ >>>

Attached Thumbnails

  • 13 EYEPIECE GRAPH absolute field drawtube 11 jpg Tom Dey.jpg

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#24 smasraum

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Posted 25 January 2021 - 06:39 PM

Many current eyepiece designs date back to the early TV Nagler design  as a starting point with a tele-negative (Smythe) group in the bottom. This is what allows wider field eyepieces along with the rest of the design.  Its also why a higher power eyepieces gets larger and longer instead of shorter and narrower. One reason is to maintain a fixed amount of eye relief. Current designs usually fall into groups based on eye-relief more than anything else. Once you get over the old designs from the past which generally top out at 50-55 degrees, except for a couple of quasi WF designs like the Erfle and Konig which reach 60 degrees you get to the 65-70, 80-85, and 100 and over groups. If you use glasses you need about 20mm of ER, and in that range you are limited to the 65-70 group. In that group a 1.25" eyepiece can't have an FL that is over about 26mm or the barrel will vignette the field. Example: the Pentax XL 28 had a FOV that is only 55 degrees in 1.25" instead of 65. Later they came out with the XW set which had 70 degree fields. They moved the 28 to 30mm and made it a 2" so the entire set now had 70 degrees and 20mm eye-relief.

And if you are older with a narrower exit pupil than when younger you won't be able to see the entire field of an eyepiece wider than about 70 degrees without moving your eye around. And those wider eyepieces have ER of 15mm or less in general. I worked for a dealer for several years and tried a lot of eyepieces. I never bought anything wider than the 65-70 degree group and I am fine with that. YRMV

Lots of good stuff here, thanks tons for the write up.



#25 smasraum

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Posted 25 January 2021 - 06:40 PM

There are two issues:: both purely optical:

 

1) An objective creates an image of a certain scale based solely on the focal length of the objective.

 

If an EP wants to deliver an image to the eye of the observer, the image at the focused scale must fit inside the barrel of the EP (DUH !)

 

2) There is a "light path" through an EP and if this light path needs to be wider than the barrel, then either the part of the light path larger than the barrel has to be above the top of the draw tube--OR the EP needs to use a bigger barrel !! Otherwise the extreme edgges of the light will be vignetted (DUH again !).

 

In the case of the 9ES120, the light path is larger than the 1.25" barrel at the top of the focuser draw tube.

 

To get a touchey feely view of the light paths of various EPs; look here::

https://www.telescop...berration_2.htm

and

https://www.telescop...ce_raytrace.htm

 

ETA:: The reader will observe that those EPs that use highly diverent Smythe lens groups as the "field lens" use significantly larger light paths than the EPs from the 1800s and before.

Cool, thx




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