exit pupil

I'm looking to purchase a 27mm or a 35mm Panoptic. The exit pupils compute to about 5.7 and 7.4 respectively.

Can someone help me out and explain the practical significance of this? I've seen a couple of comments in reviews that the 27mm is a better choice because the 35mm exit pupil is too large. Will the smaller exit pupil provide better contrast, brightness, etc.?

It will be used in a 10" XTi (f/l-1200mm , 254mm aperture).

Hi Dennis
Here is a link to an article by Al Nager from the Televue site that explains it very well. About half way into it is where the exit pupil starts to come into play but the entire article is a good read. It also expels some myths.

http://www.televue.com/engine/page.asp?ID=140

Hope this helps
Buck

>I'm looking to purchase a 27mm or a 35mm Panoptic. The exit pupils compute to about 5.7 and 7.4 respectively.

Can someone help me out and explain the practical significance of this? I've seen a couple of comments in reviews that the 27mm is a better choice because the 35mm exit pupil is too large. Will the smaller exit pupil provide better contrast, brightness, etc.?
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The Article on the TV website is informative but there is a great deal of information there. Here are some of my thoughts....

1. If your eye pupil dilates to 7.4 mm, then your eye will not be acting as an aperture mask and reducing the effective aperture of the scope. If you eye only opens to 5.7 mm (or less)then the 27mm and the 35mm will have the same brightness, just different fields of view.

Getting that wider field of view can be worth it, both as help in finding targets and just to enjoy the wider field of view... Trading field of view for effective aperture is desireable for some targets or for just roaming around the night sky.

2. Large exit pupils show the defects in your eye...

Like any optic, the center is the best part of your eye's lens, as you use more and move towards the edge of the lens, its own aberrations will show, often this is astigmatism which you would not normally see and that an eye doctor would not normally correct for.

But when the eye is wide open and the exit pupil large, it can manifest itself as spikes, quite bright eminating from bright and not so bright stars.

When I bought a pair of 11x80 binoculars, I thought the optics were rather poor because of the spikes but then I found that when I rotated my head, the spikes rotated with me, indicating the problem was in my eye rather than in the binos...

3. Bottom line:

Everyone needs a low Power widefield eyepiece. A 35mm Panoptics is a nice place to be, in your 1200mm scope it will provide 2 degree TFOV, nice for viewing things like the Veil Nebula and nice for scanning the Milky Way from a dark site. It is an eyepiece that will allow you to take advantage of the 2 inch focuser and give you a nice widefield of view.

Also, when buying eyepieces, it is good to look to the future. Currently you have a 10 inch F4.72 scope, in the future it is likely you will have others scopes as well. One might be a small relatively fast refractor with a Focal ratio somewhere between F/6 and F/7.5

At that point, you would be glad to have that 35mm Panoptic, it would give you some amazing widefield views...

Jon

Not only does the 35 Pan have a large exit pupil, it also has large eye relief. As Jon mentioned, when you use it with a fast refractor it appears that you are immersing yourself into a crystal ball full of stars. It really is am amazing view. You will get somewhat of the same effect with your present scope.

The best thing would be for you to go to a star party where you can see the difference for yourself. But then again, that could be VERY dangerous for your wallet.

Thanks for the advice. I have a better idea now of the practical effects. Nagler's article was a good primer.