Widest - Longest eyepiece

[koputai (Jason)]

Hi Folks,
I've read in Texereau that the longest eyepiece you should use is f x 7mm, so for my f6 8" that means 42mm. That being said, you can have a 42mm ortho (45 ish degrees) giving a TFOV of around 90 arcseconds, or a Panoptic (41mm) giving TFOV of 140 arcseconds. If the limitation is based on light beams in the scope, is it possible that a wide field 42mm eyepiece would be vigneted or suffer some other catastrophic uselessness?
Seeing as Texereaux was last revised in 1984, is there more modern thinking on what the widest view should be from a Newtonian?

Thanks,
Jason.

[Starkler (Geoff)]

The limitation expressed above refers to the exit pupil size which is eyepiece focal length divided by the f-ratio of the scope.

The reasoning is that a fully dilated pupil would not allow in any more light and your eye becomes the limiting factor in vignetting available light. The 7mm figure relates to a young person and decreases with advancing years.
There is nothing to stop you using longer fl eyepieces but as exit pupil size increases the field background as viewed with the eye becomes brighter and brighter, especially where light pollution is a factor.

Width of field is independent of exit pupil size. What sets field size is the size of the field stop in the eyepiece, with the maximum being the internal barrel diameter of the eyepiece eg for 1.25" format eyepieces you can get up to about 27mm field stop. You can get that maximum field in a 32mm, 50degree plossl, or you can get the same field in a 24mm 68degree panoptic and enjoy a darker sky background.

[koputai (Jason)]

Thanks Geoff, got that.
So, what is the widest practical field of view for Newtonian?

Cheers,
Jason.

[Starkler (Geoff)]

Originally Posted by koputai

Thanks Geoff, got that.
So, what is the widest practical field of view for Newtonian?

Cheers,
Jason.

Well my little 5" newt with a focal length of 650mm achieves a field of 3.9 degrees with a 31mm nagler in it.

[OneOfOne (Trevor)]

At a guess, the widest possible view would be the angle from the centre of the mirror to the edge of the tube, anything wider that this would cause vignetting of the image caused by the tube itself. My trig is a little rusty but an F5 8" would be something like 1000mm focal length, aperture is 200mm which gives a triangle from the centre of 1000 on the adjacent side, 100 on the opposite side. The angle would be the arctan of .1 which is about 5.7 degrees as a ball park figure, or about 11 Moons!

[koputai (Jason)]

Yep you're right, 5.8 degrees, giving a widest possible field of 11.6 degrees!!
Of course, you really need to do the trig from the edge of the mirror for unobstructed viewing, and on the 8/f6 that means the widest full light field is 1.72 degrees. You get this with a 30mm 68 eyepiece, or a 26mm Nagler. Anything wider than these and you're not getting full illumination.

Cheers,
Jason.

[CoombellKid]

Originally Posted by Starkler

Well my little 5" newt with a focal length of 650mm achieves a field of 3.9 degrees with a 31mm nagler in it.

What's heavier? the nagler or the scope

regards,CS

[koputai (Jason)]

So, what's the widest eyepiece you are are all using in your 8 inch f6 Newts/Dobs?

Cheers,
Jason.