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Wide angle limitations in TeleVue 85?

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#1 Birder

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Posted 08 April 2020 - 03:58 PM

As one of my friends is considering to buy a TeleVue 85 for birding, to use with hyper wide angle eyepieces such as Televue Ethos 21mm 100 degree, APM 20mm 100 degree, Explore Scientific 17mm 92 degree or Nikon NAV-HW 17/14mm 102 degree, I wonder if it will be possible to view the entire field of view of these eyepieces?

 

Myself I use a TeleVue NP101, where there are no problems at all, but this is said to be able to show a real FOV of 4.9 degree, where the TV 85 has a limit of 4.4 degree.

 

I thought it was only the eyepiece, which determined the real FOV and not the telescope, so I'm a bit confused about this 4.4 degree limitation. Anyone who could please explain this to me, as my friend will have to buy it from abroad, so better to be sure that it will work!

 

Also, does anyone know if this refractor will be able to use all of these eyepieces with a 45 degree correct image prism? On my NP101 I can't focus to infinity with my 17mm Nikon NAV-HW and also had to make a modification to be able to use the APM 20mm 100 degree.

 

Thanks, Henrik.



#2 havasman

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Posted 08 April 2020 - 04:12 PM

TFOV  =  (eyepiece field stop diameter  X  scope focal length)  /  57.3



#3 Eddgie

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Posted 08 April 2020 - 04:17 PM

Henrick

The true field is set by the focal length of the telescope and the size of the field stop of the eyepiece.

 

For example, the Televue 85 has a focal length of 495mm.  The 21mm Ethos has a field stop of 36.2mm.  To know the true field, divide the field stop size by the focal length and then multiply the result by 57.3.  In this case, the true field would be 4.19 degrees.

 

Now one thing to consider though is that while a lot of these eyepieces offer very wide fields of view, many of these super-wide field eyepieces will have very pronounced rectilinear distortion  pincushion or barrel distortion).  This means that sweeping a field with these can be quite unpleasant for many people though when the scope is still, the pincushion usually is not bothersome.

 

I would be wary of this though. Some birders do not at all like using a scope with pronounced rectilinear distortion.  Many bird watching forums have reviews of spotter scopes and one of the things they usually mention is how much rectilinear distortion a particular scope has.  Now many of these scopes have very narrow field eyepieces because as the field of the the eyepiece gets wider, this aberration gets harder to control.  

 

Also, some astro wide field eyepieces produce a fringing on the near the outer part of the field.  At night you might not see these, but during the day, they could be annoying.  The 31mm Nagler is well know for this and the condition is called "ring of fire" on the forums. 


Edited by Eddgie, 08 April 2020 - 04:43 PM.


#4 sg6

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Posted 08 April 2020 - 04:25 PM

In a way, No. The eye sees something like 62 or 68 degrees say 70 to cover everything.

So what happens is the person sits there looking, and sees 70 degrees, to see the other 15 either side they have to move their head, to sort of look around the side.

 

Some may not mind it, some may dislike it.

Some may never really use the 100 degrees but do like having no field stop making a visual limit while looking.

 

Always seems funny that the one item that ultimately creates an imge - the output of a scope does not form an image - is the eye and that is generally ignored and forgotten.



#5 Eddgie

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Posted 08 April 2020 - 04:25 PM

And for the kind of money involved, I would go with the number one choice recommended by Birdwatching.com, which is the Swarovski Optik HD-ATS-80 equipped with the BTX eyepiece module. Using both eyes hugely improves the amount of detail one can see on a distant bird.

 

If birdwatching was the primary use, a scope like the Swarovski Optik HD-ATS-80 with the BTX is a fantastic tool. Nothing beats using both eyes and having zoom eyepieces makes it far easier to locate the subject. While this kind of gear seems expensive, I think it is not much more than getting a Televue 85, 21mm Ethos, and some shorter focal lenght eyepieces, and the diagonal you need to get a correct image and comfortable position. 


Edited by Eddgie, 08 April 2020 - 04:29 PM.

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#6 Birder

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Posted 08 April 2020 - 05:12 PM

Thanks for your comments, didn't know you have to use the field stop to make such calculations, just a problem that the field stop is normally not mentioned in the specs of eyepieces. But the TV 85 is not 495mm, it's 600mm, so the AFOV would then be only 3.46 degrees. At least that should give lots of head room up to 4.4. degrees!

 

But then I totally disagree that the Swarovski BTX is good for birding, at least for migrating birds, as both my friend and I practice a lot. To find the bird in the telescope, for instance a bird moving fast on a blue sky or the sea, where you have no points of orientation, the angle is FAR too narrow. It's OK for watching birds not moving, very nice optics, but then you only have half amount of light, since there is only one OTA to feed the two eyepieces. If you then want max. light and go for the 95mm body, it has an enlargement of 35X, very often too much because of haze in the air, even the 30X on the 85mm body is often too much. Most of the year you can only use 30-35X early morning and evening, 25X or even 20X is MUCH better to use most of the daytime. And then if you follow a flying bird in the BTX, you can only follow it for a very short distance, if you want to keep both eyes in the right position. With a single eyepiece you just turn your head, when the telescope is panned to the sides, this way you can follow a bird over a distance around 3 times as long as in the BTX.

 

Swarovski makes very nice optics for sure, but their new telescopes are not very good for birding, since they only come with BTX or zoom eyepieces, no fixed wide angle eyepieces as for instance the old Leica Apo Televid or Nikon EDG. But for migrating birds TeleVue or similar refractors, where you have a choice of super wide angle eyepieces, are FAR superior, except for the portability. But if you can drive close to the observing spot and stay there for the whole day, this is no problem. I use a small Pentax 65ED as 'grab and go scope' when I have to carry my telescope, not a lot of light and not the very best optics, but I can use 100 degree eyepieces, and in full daylight at least, the light is no problem.


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#7 TOMDEY

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Posted 08 April 2020 - 05:28 PM

Another significant consideration is that the inside of the 2-inch (?) drawtube and (especially) if stacked with a 2-inch Star Diagonal... will begin to vignette. The eyepieces that you mentioned at least don't completely push the limit like The 55mm Plossl or 41mm Panoptic would. Your NP101 has a 2.4-inch thru-hole so doesn't suffer that as much, especially is used without a 2-inch Star Diagonal. I got a refractor with a 3-inch drawtube and 3-inch star diagonal... which completely overcomes vignetting, even with the 46mm field stops.    Tom



#8 Spikey131

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Posted 08 April 2020 - 05:29 PM

TV 76 and ES 100 20mm in loon watching mode.  Rect-a-linear distortion? Probably.  Field curvature?  Certainly.  The loons still look like loons.  I am not “uncomfortable” panning.

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#9 Birder

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Posted 08 April 2020 - 05:50 PM

Yes Tom, vignetting might of course be a problem with the narrower TV 85. Now I never use star diagonals for birding, but with my 45 degree correct image prism I have no vignetting problems with either of the 4 mentioned eyepieces on my NP101 at least.

 

Spikey, the TV 76 is of course very portable, but 480mm I find a bit short, I find 540 optimal for birding, since you get 32X enlargement with 17mm eyepieces, where Nikon NAV-HW and Explore Scientific 92 degree are probably the nicest af all eyepieces to look in; excellent eye comfort, which is very important for birding, where you both move the telescope and your head a lot. Then I'm also afraid the balance is not the best on such a short scope with a super heavy eyepiece as ES 17mm 92 degree.



#10 davidmcgo

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Posted 08 April 2020 - 09:21 PM

The 17 Ethos works very nicely in the TV85 but it get really back heavy and hard to use on the Panoramic mount without a lot of tension on the altitude knobs.

My favorite ultra wide view with it is an old Celestron 50mm Axiom.  Gives 12x and a 4.5 degree field and is nearly free of any rectilinear distortion so planning is completely natural.  It didn’t have a separate field stop, just a slight ridge in the grooving in the barrel and they moved the filter threads to the eye lens side to avoid even that small restriction in field.  It is pretty fun tracking satellites and helicopters with it, plus hawks.  Hard eyepiece to find Used these days though.

 

Dave



#11 Jon Isaacs

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Posted 08 April 2020 - 10:20 PM

But the TV 85 is not 495mm, it's 600mm, so the AFOV would then be only 3.46 degrees. At least that should give lots of head room up to 4.4. degrees!

 

I think, Edggie just made a typing error.

 

The maximum TFoV of the TV-85 can be achieved with the 41mm Panoptic, the same eyepiece you probably use with your NP-101 to get the 4.88 degree TFoV.

 

For the TV-85, the TFoV max looks like this:

 

TFoV max = 57.3 deg/rad x 46/600 = 4.39 degrees.  

 

Vignetting is complicated but during the day, the exit pupil of these eyepieces will far exceed your pupil diameter so probably vignetting is not an issue.   With the 41mm Panoptic and reasonable daylight, the NP-101 is probably only using 26 mm of the objective.  

 

Jon


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#12 Birder

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Posted 09 April 2020 - 04:59 AM

Thanks Dave, when the 17mm Ethos works fine with the TV85, I suppose the ES and Nikon 17mm will as well. Then the question is if there'll be any vignetting with 20 or 21mm 100 degree eyepieces, but according to Jon, this should probably not be a problem at daytime.

 

As I understand it now Jon, the 4.4 degrees is not a limit set by the TV85, but just by the eyepieces available at the market today, that's also what I originally thought, got just confused when I saw the max. FOV was smaller than on my NP101. So am I right that the only difference according to wide field view between NP101 and TV85 will be possible vignetting in TV85, if the future will bring eyepieces with even wider angles, so maybe more than 5 degrees FOV becomes a possibility?

 

I'm surprised the TV85 gets very back heavy with a relatively 'small' eyepiece as Ethos 17mm, how would it then act with an almost double weight monster as the ES 17mm 92 degrees? I had this concern with the TV76, but for birding it has to be possible to balance the telescope, so you don't have to have any tension on the altitude knob at all! I use a long dovetail on the NP101, so even with the ES 17mm 92 I can have the telescope 'floating' freely around without any tension on the knobs. I feared this might be a problem on TV85 with ES 17mm 92, but not for 'normal' size wide angle EPs as the Ethos 100 degree line. But maybe the panoramic mount you are using Dave is differently balanced than a standard Manfrotto video head?


Edited by Birder, 09 April 2020 - 06:21 AM.

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#13 Jon Isaacs

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Posted 09 April 2020 - 06:31 AM

As I understand it now Jon, the 4.4 degrees is not a limit set by the TV85, but just by the eyepieces available at the market today, that's also what I originally thought, got just confused when I saw the max. FOV was smaller than on my NP101. So am I right that the only difference according to wide field view between NP101 and TV85 will be possible vignetting in TV85, if the future will bring eyepieces with even wider angles, so maybe more than 5 degrees FOV becomes a possibility?

 

 

That is not how it works.  

 

The field of view is determined by two things, the focal length of the telescope and field stop of the eyepiece.  

- The field stop is a ring you see as the edge of the field of view looking through the eyepiece.  If you turn the eyepiece over, with many eyepieces you can see the field stop.  With the 41mm Panoptic, you can see it as very narrow ring (1mm wide) near the top of the barrel.  

 

For a 2 inch eyepiece, the inner diameter of the barrel is 48mm so 46mm is just about the largest field stop possible.  If the field stop is smaller in diameter, then the field of view is reduced.  

 

The objective produces an image at the focal plane and the field stop is at the focal plane of the eyepiece and determines how much of the image you see in the eyepiece.

 

- The objective produces the image at the focal plane and the image scale, the size of the image, depends on the focal length of the telescope.  The longer the focal length, the larger the image is, this is why magnification is proportional to focal length. The larger image means a smaller field of view.

 

The difference between the TV-85 and the NP-101 is the focal length. The 600mm focal length of the TV-85 means that with the same eyepiece, the magnification will be:

 

600mm/540mm = 1.11x greater than the NP-101 and the field of view will be:

 

540mm/600 = 0.90x as wide.  

 

- This is relationship between focal length, field stop and TFOV is expressed in the equation:

 

TFoV = Field Stop / Focal length telescope.  The result is in radians so to convert to degrees, it much be multipled by 180 degrees/Pi Radians = 57.3 deg/radian. 

 

TFoV = (57.3 deg) x (Field Stop)/(Focal length telescope)

 

- So, the difference in field of view is related to inherent limitations in the 2 inch eyepiece format.  With a 3 inch focuser and a 3 inch diagonal, wider fields of view are possible,

 

Jon



#14 Birder

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Posted 09 April 2020 - 06:48 AM

Thanks a LOT for this explanation JON, I'll study it thoroughly and try to understand the physics of this important subject. But I know the real field of view of the same eyepiece will be smaller on the TV85, then the enlargement will be larger as well, so same AFOV. Just a pity that the field stop is not normally specified in the standard specs of eyepieces.

 

Regards Henrik.



#15 Jon Isaacs

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Posted 09 April 2020 - 07:22 AM

Thanks a LOT for this explanation JON, I'll study it thoroughly and try to understand the physics of this important subject. But I know the real field of view of the same eyepiece will be smaller on the TV85, then the enlargement will be larger as well, so same AFOV. Just a pity that the field stop is not normally specified in the standard specs of eyepieces.

 

Regards Henrik.

Henrik:

 

For TeleVue eyepieces, the field stop is always supplied, both on the page for the eyepiece itself as well as the Televue eyepiece specifications page:

 

41mm Panoptic Specifications

 

TeleVue Eyepiece Specifications

 

The current model eyepieces are listed at the top of half of the page, the discontinued eyepieces at the bottom.

 

Jon



#16 Birder

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Posted 09 April 2020 - 07:51 AM

Yes Jon, I found that very nice Televue specification page already, I'm more thinking about other brands, would be nice if they included that in their standard specifications.

 

Henrik.



#17 25585

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Posted 09 April 2020 - 09:46 AM

Very interesting site http://www.pt-ducks....-telescopes.htm



#18 Birder

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Posted 09 April 2020 - 10:23 AM

Thanks for the link, will have a look into it later waytogo.gif 

 

Henrik.


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#19 m9x18

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Posted 09 April 2020 - 01:40 PM

We can always depend on Jon to school us in the art and science of our wonderful hobby. Thank you Jon.


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#20 MarkGregory

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Posted 09 April 2020 - 03:12 PM

Thanks Dave, when the 17mm Ethos works fine with the TV85, I suppose the ES and Nikon 17mm will as well. Then the question is if there'll be any vignetting with 20 or 21mm 100 degree eyepieces, but according to Jon, this should probably not be a problem at daytime.

 

As I understand it now Jon, the 4.4 degrees is not a limit set by the TV85, but just by the eyepieces available at the market today, that's also what I originally thought, got just confused when I saw the max. FOV was smaller than on my NP101. So am I right that the only difference according to wide field view between NP101 and TV85 will be possible vignetting in TV85, if the future will bring eyepieces with even wider angles, so maybe more than 5 degrees FOV becomes a possibility?

 

I'm surprised the TV85 gets very back heavy with a relatively 'small' eyepiece as Ethos 17mm, how would it then act with an almost double weight monster as the ES 17mm 92 degrees? I had this concern with the TV76, but for birding it has to be possible to balance the telescope, so you don't have to have any tension on the altitude knob at all! I use a long dovetail on the NP101, so even with the ES 17mm 92 I can have the telescope 'floating' freely around without any tension on the knobs. I feared this might be a problem on TV85 with ES 17mm 92, but not for 'normal' size wide angle EPs as the Ethos 100 degree line. But maybe the panoramic mount you are using Dave is differently balanced than a standard Manfrotto video head?

Interesting observations. I have a TV-85 on a TV Panoramic mount/head. I pop on the monster ocular - 31mm Nagler - and have no problem with balance. Having said that, I like your description of having your scope float freely. I am going to have to play a bit and see if I can get that effect. Mark



#21 Birder

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Posted 09 April 2020 - 04:29 PM

Now I haven't seen or tried that TV panoramic mount, Mark, so don't know how it works, looks a bit more basic than the Manfrotto Nitrotech N8 video head I'm using. That works great for enlargements up to around 60X, when it's not too windy, on a Manfrotto 535 carbon fibre tripod.

 

With the large and heavy ES 17mm 92 I have to move the long dovetail almost completely forward, with a small Masuyama 26mm 85 I then move it a lot backwards. When the NP101 is in complete balance with no tension at all, I lock the dovetail and just tighten the tilt-knob slightly, so it will stay in position when panning. If I then want to raise or lower the telescope, it's just a gentle little push. I'm very pleased with that setup, but of course it only works for low power enlargement.

 

Henrik.



#22 davidmcgo

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Posted 09 April 2020 - 09:31 PM

I don’t like sliding the tube to re balance between eyepieces and frequently use a little 3-6 Zoom on the planets and doubles, so I find the big Ethoses and Nagers not much fun on it and usually stick with a 19 Panoptic tha focuses and balances with the lighter higher power ones.  I like a light enough drag on my mount that I can smoothly finger tip follow objects at high power.  So unless I purposely am sticking with low power I just don’t use the big glass heavy eyepieces on it.  They work well but the overall hassle factor changing and rebalancing to see a high power view and then reverting back isn’t worth it to me.  Birding would be different, daytime seeing usually doesn’t let you go to such magnification anyway and a13 Ethos would probably be the high side for not showing atmospheric turbulence.

 

Dave



#23 Jon Isaacs

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Posted 10 April 2020 - 05:25 AM

I don’t like sliding the tube to re balance between eyepieces and frequently use a little 3-6 Zoom on the planets and doubles, so I find the big Ethoses and Nagers not much fun on it and usually stick with a 19 Panoptic tha focuses and balances with the lighter higher power ones.  I like a light enough drag on my mount that I can smoothly finger tip follow objects at high power.  So unless I purposely am sticking with low power I just don’t use the big glass heavy eyepieces on it.  They work well but the overall hassle factor changing and rebalancing to see a high power view and then reverting back isn’t worth it to me.  Birding would be different, daytime seeing usually doesn’t let you go to such magnification anyway and a13 Ethos would probably be the high side for not showing atmospheric turbulence.

 

Dave

 

For me, balance is a mount issue.  I setup my mounts so I can use any of my eyepieces (2 pounds +) without needing to rebalance.  For refractors, I go with mounts with slow motion controls so I can track and center at high magnifications with ease.  Slew and sweep with the scope, center and track with the slow mo.  

 

Jon



#24 StarAlert

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Posted 10 April 2020 - 09:37 AM

Yes Jon, I found that very nice Televue specification page already, I'm more thinking about other brands, would be nice if they included that in their standard specifications.

 

Henrik.

I have always been able to find the field stop measurement for an eyepiece... 90% of the time I’ll find it at either Agena or Highpoint, or the manufacturer’s website. 



#25 Birder

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Posted 10 April 2020 - 03:26 PM

Thanks, at least Agena Astro seems to have relevant information on field stops, I'll keep that in mind.

 

Yes Dave, 13mm is certainly on the high side, especially on TV85, which is 600 mm. On my NP101 at 540mm I rarely use more than 17mm, then there has to be VERY little disturbance at least.

 

Henrik.




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