Baffled by a large exit pupil

[David Smith]

Interesting thread. I am learning lots! Based on the discussion above my current set of eyepieces cover all the bases - from high power (313x) to large exit pupil (6.4mm with a 32mm eyepiece). If that is the case then my thoughts about getting a lower power, wide field eyepiece (around 40mm FL) would be point less as the exit pupil will be 8mm.

Would it then be reasonable to conclude the only thing I could gain from new eyepieces would be apparent field of view (and quality of glass / light transmission)?

I currently have a set of Revelation Plossls ranging from 6mm to 32mm in FL and a TMB 4mm for special occasions!

Thanks

David

[Ags]

David, a wider AFOV eyepiece also shows more true FOV: an 80 degree 32mm EP shows much more sky than a 32mm plossl. So there is a real gain to be had from a wide-angle EP.

[arbacia]

The true field of view as seen through an eyepiece is realted with its field stop. This opaque "solid" ring is the physical limit of the aparent FOV and offers a direct way to compare eyepieces.

TV Plössl 55mm and Panoptic 41mm has a field stop of 46mm, both are very close to the limit of 48mm of the 2" barrels (meaning that the barrel walls of these eyepieces can have a maximun of 1mm in thickness)

Nagler 31mm and TV Plössl 32mm have field stops of 42mm and 27mm respectively.

TV Plössl 32mm has the same FS that Panoptic 24mm and both offer a field stop near the maximum of 1.25" barrels

Patricio

[catburglar]

You may get wider FoV, so could be useful as a finder, but in my experience unless you have very low light pollution, you will probably see more with an eyepiece that gives a smaller exit pupil.

Typically a 16mm 82 degree AFoV eyepiece will show about as much sky as a 32mm Plossl. However, you will get a wider FoV if you use something like a TV panoptic or SW Aero at around 40mm, but these will be 2 inch eyepieces. A 40mm Plossl in 1.25 inch fitting will show you no more than the 32mm you already have.

[David Smith]

Thanks guys. So as 32mm focal length gives me just about maximum exit pupil, an alternative eyepiece at a similar focal length that gives me a larger apparent field would still be potentially sound investment.

So why is the field stop not mentioned in any of the specs for eyepieces?

Thanks, and apologies if I have hijacked the thread for my purposes but seemed like a good place to post the question rather than start a new thread.

David

[arbacia]

So as 32mm focal length gives me just about maximum exit pupil

For 1.25" eyepieces, yes.

Some manufacturers (as Televue) give the field stops, but it is not a general rule.

Patricio

[John]

The field stop is only specificed by a few but the apparent field of view is quoted by all - and not always 100% accurately when the field stop is actually measured - which might explain why some don't give you that info

[arbacia]

That it is, John.

Aparent Field of View is a "series label" for manufacturer. However, each different focal lengh of an eyepiece series may not satisfy the general rule.

Patricio

[Chris H]

Would it then be reasonable to conclude the only thing I could gain from new eyepieces would be apparent field of view (and quality of glass / light transmission)?

The 32mm plossl with its 50 degree AFOV will give you 1.3 degrees TFOV. If you want to keep the exit pupil below 7mm (which most people do) the lowest power ep you could go for would be the Televue Nagler 31mm type 6, with it's 82 degree AFOV it would give you 2 degree TFOV. A cheaper option would be a Skywatcher Nirvana 28mm this would deliver 1.9 degrees TFOV but at a much lower cost than the Nagler and without compromising on quality.

[cs1cjc]

The field stop is a ring at the focal point of the eyepiece - it is the sharp edge you see in an eyepiece. It is this which limits the true field of view. In plossl, orthoscopic and other simple designs, it is on the field side of the eyepiece, at or near the top of the barrel, and it can be measured directly.

On more complex designs the field stop is internal and its effective diameter is modified by field lenses, however it still controls the true field of view precisely. I estimate the effective field stop on such eyepieces by viewing a ruler and comparing how much I can see of it to eyepieces with a measured field stop.

The common approximation of AFOV/Magnification for the true field of view can be out be up to about 20%, particularly for wide field eyepieces because of distortion.

See: TELESCOPE EYEPIECE

[David Smith]

Thanks again all.

By my calculations the Explore Scientific 35mm with a 70° AFOV would give me a 2.0° True Field and the exit pupil would still be just 7mm (scope focal length of 1250mm), granted probably a bit close for my eyes now. But this eyepiece is half the price of the Skywatcher and much more like my budget!!

Feel free to correct me as I am still figuring this stuff out.

Cheers

David

[michael.h.f.wilkinson]

For those who have not seen the secondary in the case of an oversized exit pupil: try it during day-time. A 4.5" F/4.4 with 28mm Kelner is OK at night with its 6.34mm exit pupil, but in daytime it is useless due to the complete mismatch between the eye's 1-2mm pupil and the massive exit pupil.

I got a 31mm Nagler exactly because it gets the most out of my 80mm F/6, whereas the 40mm Paragon gives slightly washed out views with its 6.7mm exit pupil (not too bad, but the 31mm Nagler is distinctly better).

[Moonshane]

I agree with this. even with a 24mm Panoptic, I used to see the secondary on my f5.3 dob when observing the moon in daytime. not often a problem at night though.