exit pupil question

[iamjulian]

Hello

Trying to get an idea of how important exit pupil is in terms of selecting an eyepiece. All the literature seems to suggest good average pair of eyes top out at about 7mm exit pupil. And this drops as you age. Warthog's excellent primer above touches on the subject.

I am mid 30s, have 20/15 vision when corrected with glasses, but have no idea what my exit pupil is. I am looking at an EP that gives a 7mm exit pupil, but if it turns out my exit pupil is smaller than 7mm, how will it affect me? What would I see differently through the EP compared to someone who did have a 7mm exit pupil? Is it a case of not seeing as much, or something more negative, like seeing the shadow of the secondary?

Thanks!

[Merlin66]

If your pupil is less than the exit pupil of the eyepiece two things happen:

- you effectively "stop" the optical system down by the vignetting so the light grasp (effective aperure) will be reduced.

- The FOV will be reduced as the peripheral zone is being vignetted, so you won't get the benefit of those wide angle eyepieces - unless you constantly move your head around.....

Such is life - aging is such sweet sorrow.

Ken

[John]

... or something more negative, like seeing the shadow of the secondary?....

Thats the effect that I've experienced, that and blacking out of the view as my eye drifts slightly from the optimal position. My largest exit pupil is 6.5mm (Nagler 31mm plus 10" F/4.8 newtonian) which works fine, with none of these effects.

I'm 50 and can still manage to see the edges of the FoV of Naglers and Ethoi without moving my head around but I thought that was more about the eye relief than the exit pupil.

I found the troubles mentioned above started when I used 35mm and longer FL eyepieces with the F/4.8 scope.

[Merlin66]

The eye reief is the point in space behind the eyepiece where the converging beams from the edge of the field and the centre cross to form the exit pupil.

The size of the exit pupil will always be the aperture/ magnification; the eye relief depends on the design and construction of the eyepiece.

Ken

[NGC 1502]

Hi, if the dilated pupil of your dark adapted eye is smaller than the telescopes

exit pupil, then you are effectively wasting light, as not all of the light from the

instrument can reach your retina.

I love low power wide views, but this does tend to produce a washed out image

with our light polluted skies.

Regards, Ed.

[iamjulian]

Thanks for the replies. Really torn on whether to go for one that will give 3.1 degree AFOV but 7mm exit pupil, or 2.6 AFOV with 6mm exit pupil. I am 34, so hopefully have some time before my eyes fall apart

[Chris H]

You can measure you pupil size using the following method. I'm quoting from my Astronomy Hacks book.

Get a full set a metric allen keys, allow your eyes to become fully dark adapted (which may take up to an hour). Look directly at a bright star and hold one of the small allen keys along your cheek so that the long part crosses your eye parallel to and near the eyeball, move the allen key up and down until centered on you pupil. You'll see the star split into two stars, one on each side of the allen key. Gradually increase the size of the allen keys until the star no longer splits and is only visible on one side or other of the allen key. The size of this allen key is the size of your fully dark adapted eye pupil.

[NGC 1502]

Post #6, personaly, I'd go for the 2.6 degree, 6mm exit pupil. That will still show

all of large objects like the Pleiades, but with a darker more contrasty background.

Hope this helps, Ed.

[iamjulian]

Thank you Ed, sounds like sensible advice.

Chris, I am definitely going to try that!