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.
- 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
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.