Exit Pupil

[staffop]

Hi, hopefully it's not possible to get banned by asking too many dumb questions, but as a very new starter i'm trying to familiarise myself with the complexity of optics. Ny new found understanding is that on top of focal length, aperture and magnification is the issue of exit pupil. 

As an almost 50 year old, with a 4" refractor can i assume a maximum EP of 5mm (maybe 6mm?) - i wear glasses so my vision is not great. Does this mean that using calculations such as http://rechneronline.de/optical-instruments/telescope-binocular.php, any magnification lower than approximately 28mm is pointless as it's all wasted light (i.e. would result in an EP greater than 6mm)?

I think my fast scope is best suited for wide field DSOs but don't want to get an EP which might be too low in magnification to be of any practicable value...

[Alien 13]

This is one topic that has allways confused me, if your maximum pupil size was say 5mm in the dark im sure it would be totaly different when peering through an eypiece which is throwing light at it and sky brightness becomes more apparent at low magnifications so I dont know but there must be an optimum at some point.

Alan

[JED-E3]

You are correct that you will be wasting light in terms of losing some effective aperture of you scope, however it can be useful to use a wider eyepiece (with a refractor) if you want to frame a large DSO such as the veil nebula.

The bigger issue is that once you start getting an exit pupil larger than about 5mm, unless you've got a very dark sky with no light pollution, the background sky starts to look bright and washed out.

James

[John]

I'm 55 and stick with exit pupils of 6mm or less. I think my largest is currently 5.8mm. I can frame the whole of the Veil Nebula with that 31mm eyepiece in my F/6.5 102mm refractor. In that scope the exit pupil that the eyepiece develops is 4.7mm.

I find the same as James describes with larger exit pupils causing a washed out background sky which does not help with fainter deep sky objects.

[Andy Mitchell]

Can you use an eyepiece with a wider apparant field of view and a shorter focal length to get the same actual field of view and shrink down the exit pupil to get all the light into your eye? Or does this introduce other problems, apart from costing lots more money

[John]

Can you use an eyepiece with a wider apparant field of view and a shorter focal length to get the same actual field of view and shrink down the exit pupil to get all the light into your eye? Or does this introduce other problems, apart from costing lots more money

Thats the approach I take. My 31mm is a Nagler with and 82 degree field. To get such eyepieces to work well with faster scopes is challenging so they are not low cost items.

There are lower cost options than the Nagler though but even those are not exactly cheap.

[staffop]

Thanks all. I can get to the Brecons easily enough so can take advantage of the dark skies and hopefully the slighter larger mag, whilst not pushing it when home and light polluted . That said has anyone had a good experience with the zoom lenses?

[ronin]

First is to find out what your pupil size is at night in the "dark".

One person asked their optician and was "surprised" when they gave them a figure of around 7mm or 8mm. Not the 5mm or less they were expecting.

In one question about pupil size and exit pupil it seemed several members didn't have a pupil, people were claiming less and less agian then the previous.

Easy option: Put an eyepiece in, if it works don't worry about it.

[John]

Well it's going to work, but a bit of care selecting a sensible exit pupil will give better results.

Try viewing a faint extended DSO such as M101 with an eyepiece that gives a 7mm exit pupil with another that gives 5mm with the same scope. Personally I found the smaller exit pupil delivers a brighter and more contrasty view of the galaxy. You mileage may vary though, as they say

[Charic]

Matching the exit pupil to your entry pupil maximises the ammount of light transfer that hits the retina.

to me its not a precise and clear cut issue!  Try Star hopping then view the Moon. The brightness can be  blinding, although not damaging, but there goes your maximum entry pupil until your pupils are dark adapted again, so do you now choose another eyepiece to get the best from a smaller exit pupil, or persist until your eyes adapt? Your pupils are constantly auto adjusting, but trying to match exit to entry is a good thing, stepping slightly eitherside still works!

Exit pupils of a certain size are better suited for certain applications. A 1mm - 3mm is supposedly good for Planetary work/detail.

A 6mm and/or 12mm EP gives me  between 1mm and 2mm exit pupil, this works well for my needs, especially on a reflector telescope.

There is a lot more information here http://www.skyandtelescope.com/astronomy-equipment/choosing-your-telescopes-magnification/

[brantuk]

It was my understanding that the human eye extends it's pupil up to 7.5mm only in your teens - and it reduces gradually from there until you're an old git like me and can barely achieve 6mm - more usually 5 point something or sometimes less. This is one of the reasons I always regret not starting astronomy when I was a kid.

I only mention it in case it affects your choice of eyepiece and how much you want to spend. But like women - I'm past caring and I'll look through anything at anything so long as it's pleasing on what's left of the eye. lol

[YKSE]

That said has anyone had a good experience with the zoom lenses?

Baader Mark III zoom is a very good one, my short comparison with BST8mm and BST 12mm sent BSTs to home. There're many more discussions on CN:

http://www.cloudynights.com/topic/481585-is-the-baader-hyperion-zoom-that-good-optically/page-1

http://www.cloudynights.com/topic/495140-yet-another-baader-mk-iii-zoom-thread-initial-impressions/page-1

http://www.cloudynights.com/topic/504808-baader-hyperion-zoom-ep

Considering the fact that most eyepieces are clearly less expensive in the States than in Europe, while Baader zoom is comparable as new or less as used here, that makes the Baader zoom a bargain IMHO.