Working out effective Dia. for given exit pupil?

[mental4astro]

Hi all,

I am aware of the idea of looking to maintain an exit pupil to a maximum of 6 to 7mm. Anything larger & we reduce the amount of light entering through the pupil and this in turn will act to effectively reduce the objective's/mirror's diameter.

But how can this be worked out mathematically?

I'll give one of my scope's & EP's as an example to work on:

Scope: 17.5", f/4.5, fl 2000mm
EP: 65mm
Mag: 31X
Exit pupil: 14.4mm

Alex.

[Rodstar]

Alex, I think you just divide the focal length of the EP by the f/ratio of the scope.

A 31T5 in my f/5 scope produces an exit pupil of 6.2mm which is at the limits of what I should be using - much better with a Paracorr, which reduces it to about 5.3mm.

The example you have given is unusual in that you have referred to 65mm EP. I have never come across one of them!

[mental4astro]

Thanks Rod, but calculating the exit pupil isn't the thing I'm after. By having such a large exit pupil, to what degree am I reducing the diameter of my scope?

This 65mm eyepiece is a Russell Optics XL Super Plossl. Sure it's field of view is only 38* due to the 2" barrel size, but because of this I get no coma problems, & I can comfortably get the maximum FOV a 2" barrel will allow. Not for urban use as the sky glow is a killer. Its eye relief is about 4"! so not to everyone's liking, but it is unique.

[michaellxv]

Pupil @6mm gathers ~28.27mm^2
Scope/EP combo delivers ~162.86mm^2

You are getting ~17.35% of the available light into your eye.

Your 17.5" has area of ~240.53"^2
times 17.35% is 41.73"^2
which is equivallent to about a 7.3" scope with an EP that produces a 6mm exit pupil

You could refine the above by taking into account your secondary mirror size.

[mental4astro]

:thanx: how easy was that! :doh:

Thanks Michael. The effective diameter is smaller than I thought. Not going to stop me using it though. This EP is a perfect match exit pupil wise in the C8. I may get some vignetting, but I'm still to test it out with star fields in the C8. Stunning with the Moon though.