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#1 bowersox1

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Posted 30 September 2020 - 12:18 AM

I am still new. I am compiling a putting together a list of items to add for purchase over the next months. I am looking at adding more EP's. My XT8 has a 2" cray-ford focuser. I'm trying to get a understanding of the AFOV and FOV and so forth. I know each scope has a "max limit" for FOV and it is depending upon many things. So, my question is, so to get the best viewing and not push the limits what is the cut off for EP FOV for a XT8? Example, (just using numbers here to show examples) if i would buy a 2" 100Deg FOV 40mm EP would it be a wasting my money cause it is too much for the telscope? Would i not be getting the full benfit of the EP? I read where they say the change over for EP is 32mm to increase to 2". I just don't want to drop money on a EP with a wide FOV and be expecting it to one thing but get it and find out it's over my scopes limits.   I have a University Optics UW80 16mm that came with my scope. I use it a lot but it doesn't feel right to me that it's to wide for that high of power, but it might just be me yet still learning. I hope my question makes sense. Thank you for the replies.


Edited by bowersox1, 30 September 2020 - 12:53 AM.


#2 CrazyPanda

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Posted 30 September 2020 - 12:49 AM

if i would buy a 2" 100Deg FOV 40mm EP

There is no such thing. The widest apparent field of view in a 40mm focal length in a 2" barrel is going to be about 68 degrees simply due to geometry. A 40mm 100 degree eyepiece would require a barrel larger than 3". A 30mm 100 degree eyepiece requires a 3" barrel, and it weighs 5 pounds and it requires a secondary mirror large enough to illuminate it (or a refractor that can illuminate it)

 

The longest focal length 100 degree eyepiece in a 2" barrel is the Explore Scientific 25mm 100 degree offering.

 

 

I'm trying to get a understanding of the AFOV and FOV and so forth.

 

Apparent field of view is a property of the eyepiece that describes how immersive it feels. A 100 degree eyepiece means you essentially don't see the field stop easily - it's way out in your peripheral vision - so you feel like you're floating in space.

 

The *true* field of view is how big the slice of actual sky you can see is, and is a function of the magnification you're operating at, and the apparent field of view (more accurately, the effective field stop of the eyepiece). For very rough back-of-napkin math, you can approximate true field of view by dividing apparent field of view by magnification.

 

So if you had a 100 degree eyepiece with a 6mm focal length in your 1200mm XT8, that would give you 200x magnification. 100/200 = 0.5 degree true field of view, even though apparent field of view is 100 degrees.

 

 

 

I know each scope has a "max limit" for FOV and it is depending upon many things

 

It's only dependent on two things: the focuser size (1.25" , 2" etc..), and the focal length of the telescope.

 

The formula is this:

Max True Field = Max Eyepiece Field Stop Size / Telescope Focal Length) * 57.3

Since your scope supports 2" eyepieces, and a 2" eyepiece has a max field stop size of about 46mm, and your telescope focal length is 1,200mm, then plugging in the numbers means the max possible true field of view in your scope is going to be 2.2 degrees.

 

What eyepieces will let you achieve that? Any 2" eyepiece with a field stop of roughly 46mm.

 

Some options:

 

  • 55mm Tele Vue Plossl (50 degree apparent field of view) - not recommended as the exit pupil would be way too big, just wanted to show this as an example so you can see the pattern
  • 41mm Tele Vue Panoptic (68 degree AFOV)
  • 40mm Explore Scientific 68 (68 degree AFOV)
  • 40mm Pentax XW (70 degree AFOV)
  • 38mm Orion Q70 / Agena SWA (70 degree AFOV)
  • 31mm Celestron Luminos (82 degree AFOV)
  • 31mm Tele Vue Nagler (82 degree AFOV)
  • 30mm Explore Scientific 82 (82 degree AFOV)
  • 25mm Explore Scientific 100 ( 100 degree AFOV)

(these don't all have exactly 46mm field stops, but they are in the general category of "widest possible true field of view in a 2 inch barrel at varying focal lengths and apparent fields")

 

Note that even though the 55mm Plossl is much lower magnification than the 30mm Explore Scientific 82, it doesn't really offer any wider true field of view because the limitation is governed by field stop size, NOT focal length or magnification.

 

So, my question is, so to get the best viewing and not push the limits what is the cut off for EP FOV for a XT8?

 

There is no cut off for field of view in terms of pushing any limits in either AFOV or TFOV. However, there is a cutoff of usable exit pupil, hence my warning above.

 

Exit pupil is literally a virtual "hole" formed by the eyepiece that you look through in order to see the field of view. It's no different from punching a hole in a piece of paper with a hole punch and looking through it. The bigger the hole, the more light it lets through. The smaller the hole, the less light.

 

Well, if the hole is bigger than your eye's pupil, your eye is not able to receive all the light from the telescope. It's falling on the iris and getting wasted, effectively reducing the working aperture of your telescope.

 

You can calculate exit pupil most easily by dividing eyepiece focal length by telescope focal ratio:

Exit Pupil = Eyepiece Focal Length / Telescope Focal Ratio

alternatively:

Exit Pupil = Telescope Aperture / Magnification

Your telescope has an F/6 focal ratio, so in the above example, the 55mm Tele Vue Plossl means 55/6 = 9.1mm  exit pupil. The average pupil size for a young adult is typically around 7mm. So that 9.1mm exit pupil means a lot of light is never actually making it to your retina.

 

Assuming your pupils do indeed dilate to 7mm, then inverting the formula means 7 * 6 = 42mm as the longest useful focal length in your telescope.

Max Eyepiece Focal Length = Telescope Focal Ratio * 7 (or whatever you know your max pupil diameter to be)

Given you have a 2" focuser, if you did want to get something around 42mm in focal length or a tiny bit less, then you would be limited to about 68 to 70 degrees AFOV.

 

If you wanted something wider than 68 to 70 degree AFOV, you would have to use shorter focal lengths. Going down into the 30mm-ish focal length gets you an 82 degree AFOV. Going down into the 20-25mm focal length will let you get a 100 degree AFOV.

 

So aside from the exit pupil limitation, the only thing you really have to worry about is finding the right combination of focal length and apparent field of view that suits your desires (easier said than done).

 

 

I have a University Optics UW80 16mm that came with my scope. I use it a lot but it doesn't feel right to me that it's to wide for that high of power,

 

Not sure what you mean, but it's definitely not "too wide" for that magnification. That's a perfectly standard ultra wide eyepiece. The real question is *do you like it?*. Some people don't like ultra wides. Some people think even 80 degrees is too narrow. It's really just a matter of your personal preference (and that preference may change over time, and it may even vary from focal length to focal length or target to target)


Edited by CrazyPanda, 30 September 2020 - 10:21 PM.

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#3 Taosmath

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Posted 30 September 2020 - 01:19 AM

In an 8" F6 dob like yours, the biggest True Field of View (TFOV, i.e. the angle you see when you look through the scope) you can get with a 1.25" barrel eyepiece is 1.2 degrees.  You can get this TFOV with a 24mm 68 degree eyepiece like a Televue Panoptic.    However you can also achieve that TFOV with other 1.25" eyepieces such as a 32mm plossl.  The Panoptic will give you 1.2 degrees at a magnification of about 50x.  The 32mm plossl will give you 1.2 degrees with a magnification of about 37x . So if you look through the scope using both eyepieces one after the other, you would see the same amount of the sky in each one, but the 'Image circle' you saw with the 24mm would be bigger than with the 32mm plossl.  The 24mm image would probably let you see some more details on brighter objects, but the 32mm image would appear brighter (same amount of light gathered but it's spread over a smaller circle) so you might see fainter objects more easily. 

 

The biggest diameter barrel you can use in your scope is a 2" barrel and the largest TFOV you can get with a 2" barrel in that scope is about 2 degrees.  You can achieve this TFOV many ways including a 31mm 82 degree eyepiece (which is what I use) or a 38mm 68 degree eyepiece or various other combinations, including a 100 degree 25mm.

 

My preference to get the largest possible TFOV in my 8" dob is to use a a 31mm 82 degree EP, but these are large, heavy and expensive eyepieces (anywhere from $200 to $600).

 

If you want to save some money and some weight, I find the views through a 32mm 70 degree eyepiece, such as the Agena SWA pleasing for about $100, new.  The TFOV is a little less at about 1.8 degrees. However these are not premium eyepieces and some people find they have distortions in the outer part of the field of view.

 

Since I use these wide field eyepieces mainly for sweeping around and finding things, I don't use the outer part of the FOV in these eyepieces for detailed observation.  If an object is at the edge of the TFOV I move the scope to center the target in the EP where the optical performance is fine.  Hence I am happy with these slightly lesser quality EP's.  Many others disagree, so you need to find out if you find these imperfections both detectable and distracting.

You'll save a lot of money if, like me, you're not so picky !

 

Clear skies !


Edited by Taosmath, 30 September 2020 - 01:21 AM.

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#4 sg6

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Posted 30 September 2020 - 02:17 AM

Well you have the assorted numbers and how to get from A to B now.

My input is on the sort of practical side. Mainly a 2" eyepiece is big, you are likely sat thinking 1.25 to 2 isn't a large increase. It is when it comes to eyepieces.

 

Take one of your 1.25" eyepieces and go find a standard drinks can. The drinks can will be the size of most 2" eyepieces. I know I have one, a 20mm 2". The box for my 2" eyepiece can fit 4 TV Plossl boxes in it. Which oddly makes sense:

 

1.25 to 2 is a changes of 1.6x. That 1.6x takes place in all 3 dimensions so a 2" eyepiece is 1.63 times the volume = 4x.

That also carries over to weight, and in some ways cost - making a bigger lens costs more then the simply change in size ratio.

 

In practical terms I use only 1.25" eyepieces. And on all possible scopes I use 2" diagonals. Part reason is that 2" is for wide field and say you put a 2" in and find Lyra and then the dot that is M57. Now what? You need higher magnification so it is swap over time. Take out the 2" lump, put the 2" lump somewhere safe, find the 2" to 1.25" adaptor and out that in, get your 1.25" eyepiece and put that in, change focus.

 

2" work well if you use only 2" all the time, the outreach here is an example, also there are other reasons (the scope). However magnifications will be low. I am slightly unsure for one person looking at objects the evening. Also nice if you want an hour outside just scanning around.

 

I don't know how I got the one I have, I have no idea why I got it, but one thing I can say is I cannot recall ever using it. So in 20+ years it would seem that having/using a 2" eyepiece is not exactly high up the list in priority.

 

Why do you want or think you want 2" eyepieces?

If you have read of people having 2" eyepieces and think you therefore have to have one, or two as well. Don't bother. You will do everything with a reasonable selection of 1.25" eyepieces.


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#5 25585

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Posted 30 September 2020 - 02:31 AM

Fit 1.25 to 2" adapters on your 1.25" eyepieces, then you have no problem with switching.


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#6 bowersox1

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Posted 30 September 2020 - 07:15 AM

Thank you gentlemen. This is exactly what I was needing. Now things make sense and I can understand. As always, amazing group of people with professional skillset and experience.
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#7 macdonjh

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Posted 30 September 2020 - 08:50 AM

bowersox1,

 

Welcome to Cloudy Nights.

 

CrazyPanda and Taosmath have done an excellent job with the math, so I'll make this comment:  Several years ago, when they were still fairly new, I tried a couple of the Tele Vue Ethos 100o AFOV eye pieces.  Some of my friends had bought them.  I found I didn't like the 100o AFOV exactly because I couldn't see the field stop.  It seemed like I was wasting some of the field of view I would have paid top-dollar to get.  Because of that, I haven't jumped on the 100o bandwagon.  Not so bad for me, really.  After the introduction of the Ethos, the used prices for Naglers dropped so I was able to purchase the focal lengths I wanted for less than a couple of years prior.  I still use those eye pieces, so they were good purchases.

 

Having said all of that, one thing to know is: I use scopes on mounts which track.  Those who use manual mounts, like your XT8, often like the 100o eye pieces because they can allow an object to drift across the field longer before they have to nudge the scope.  So in certain circumstances there is a practical advantage to wide AFOV which isn't related to the view per se.


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#8 MitchAlsup

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Posted 30 September 2020 - 09:20 AM

Several years ago, when they were still fairly new, I tried a couple of the Tele Vue Ethos 100o AFOV eye pieces.  Some of my friends had bought them.  I found I didn't like the 100o AFOV exactly because I couldn't see the field stop.  It seemed like I was wasting some of the field of view I would have paid top-dollar to get.  

I, on the other hand, just love these 100º+ EPs. 

I, personally, can see the 120º Field stop laterally but not vertically in my 9ES120 (first series) and I see the entire FoV of my Ethoi.

 

Not being able to see the field stop is what gives the port hole effect.....


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#9 bowersox1

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Posted 30 September 2020 - 09:22 AM

bowersox1,

 

Welcome to Cloudy Nights.

 

CrazyPanda and Taosmath have done an excellent job with the math, so I'll make this comment:  Several years ago, when they were still fairly new, I tried a couple of the Tele Vue Ethos 100o AFOV eye pieces.  Some of my friends had bought them.  I found I didn't like the 100o AFOV exactly because I couldn't see the field stop.  It seemed like I was wasting some of the field of view I would have paid top-dollar to get.  Because of that, I haven't jumped on the 100o bandwagon.  Not so bad for me, really.  After the introduction of the Ethos, the used prices for Naglers dropped so I was able to purchase the focal lengths I wanted for less than a couple of years prior.  I still use those eye pieces, so they were good purchases.

 

Having said all of that, one thing to know is: I use scopes on mounts which track.  Those who use manual mounts, like your XT8, often like the 100o eye pieces because they can allow an object to drift across the field longer before they have to nudge the scope.  So in certain circumstances there is a practical advantage to wide AFOV which isn't related to the view per se.

This is the stuff and information that one doesn't think about and like you said makes a huge influence on benefit and appreciation. I wouldn't have thought of this and now I have something else to consider when buying EP. I thank you.



#10 macdonjh

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Posted 30 September 2020 - 09:32 AM

I, on the other hand, just love these 100º+ EPs. 

I, personally, can see the 120º Field stop laterally but not vertically in my 9ES120 (first series) and I see the entire FoV of my Ethoi.

 

Not being able to see the field stop is what gives the port hole effect.....

Indeed, I am definitely in the minority in my club for not liking the 100o eye pieces.  It's great they are available, but I consider myself lucky the Naglers weren't discontinued with the Ethos were introduced.

 

This is the stuff and information that one doesn't think about and like you said makes a huge influence on benefit and appreciation. I wouldn't have thought of this and now I have something else to consider when buying EP. I thank you.

I am a proponent of "try before you buy".  Hopefully there is an astronomy club in your area and you can attend some of their events.  You should get the opportunity to view through other observers' scopes and eye pieces.  You might even be able to borrow an eye piece or two to use in your scope for a few minutes.  That's the best way to know for sure if an eye piece meets your priorities. 


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#11 LDW47

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Posted 30 September 2020 - 10:05 AM

I am still new. I am compiling a putting together a list of items to add for purchase over the next months. I am looking at adding more EP's. My XT8 has a 2" cray-ford focuser. I'm trying to get a understanding of the AFOV and FOV and so forth. I know each scope has a "max limit" for FOV and it is depending upon many things. So, my question is, so to get the best viewing and not push the limits what is the cut off for EP FOV for a XT8? Example, (just using numbers here to show examples) if i would buy a 2" 100Deg FOV 40mm EP would it be a wasting my money cause it is too much for the telscope? Would i not be getting the full benfit of the EP? I read where they say the change over for EP is 32mm to increase to 2". I just don't want to drop money on a EP with a wide FOV and be expecting it to one thing but get it and find out it's over my scopes limits.   I have a University Optics UW80 16mm that came with my scope. I use it a lot but it doesn't feel right to me that it's to wide for that high of power, but it might just be me yet still learning. I hope my question makes sense. Thank you for the replies.

If you were to look at say the Double Cluster with your scope using say the 2” WO SWAN 40mm, 70° FOV it would hit you right between the eyes even in a Bortle 6 zone on one of those dark, perfect nites. I own 100° eps, they aren’t needed for the big $’s, 70-80° is perfect ! 1 or 2 - 2” eps are almost a must if you have a 2” focuser to get max. wide field views that can be awesome, give that ‘being there’ experience that no one should miss. Could you borrow one or two to try before you buy ?


Edited by LDW47, 30 September 2020 - 10:22 AM.

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#12 25585

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Posted 30 September 2020 - 02:19 PM

I like to see field stop, even moving eye & head, because I am getting my money's worth. ES92s are exceptional in allowing me to get a big view and by slight eye and head movement look across, while wearing glasses. Morpheus 76° & Orion LHD 80° likewise. Otherwise its 70° AFOV and less.

 

If you can't see your eyepiece's field stop at all, how do you know how much you do see, and how much you don't?  I guess buying a UWA/100° or more, just not to see edges, makes edge performance, or into your zone of view, less important. Maybe that is where cheaper UWA eyepieces come in. 


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