Help a dad and daughter pick a scope!

The larger eyepiece will show a larger field of view, but be aware that it will result in a larger exit pupil. If the sky is not pitch black, the pupil will not dilate fully, and not all light will hit the retina.

I know it's a bit technical, but if you have a magnitude 5 night sky at the city and a 7mm exit pupil, the combination is not ideal. The sky background will look gray-ish, the contrast is noticably lower. Under these xonditions a 30mm will be a better choice.

A mag 6 night sky would be pretty dark, meaning a 6mm exit pupil would work. exit pupil = eyepiece focal length divided by telescope aperture ratio.

So for a 8" dobsonian (with typically f/6 aperture ratio) a 36mm woulg give you 6mm exit pupil. A 30mm -> 5mm.

Now with a 10" telescope and f/5, the same 30mm eyepiece has 6mm exit pupil. A 40mm would give you 8mm exit pupil, typically unusable in our light polluted regions as city light spreads for dozens of miles... Also not all eyes still reach 8mm (depending on age and so on).

I often recommend the 30mm with 80 degree ultra wide angle as the exit pupil is not top large and the wider apparent field of view almost matches the field of a 36mm Erfle.

But all math aside, with a 10" or larger telescope things get more complex. These typically have f/5 or even f/4.7. Cheap, simple eyepieces will only be sharp in the center. While the 30mm80degree is decent at f/8 and still tolerable at f/6, it's performance is weak at f/5. Other wide angle eyepieces in the (sub) $100 range do not perform that well either.

This is the reason why some buy $300 or even $1000 eyepieces ;-)

The Aspheric/Hyperion perform a bit better at f/5, http://agenaastro.com/baader-31mm-hyperion-aspheric-eyepiece.html ...but the difference to a simpler eyepiece is not as great as the price tag would suggest. The 30mm explore scientific performs much better, almost as good as far more expensive ones. http://www.optcorp.com/eyepieces/all-eyepieces-not-sets-kits/explore-scientific-30mm-82-degree-waterproof-eyepiece.html

I still observe with a cheap 30mm 72 degree erfle eyepiece on my 10"f/5 telescope. I tolerate the outer field being not sharp, but when trying to find faint fuzzies it can be bothersome. One of these days I will buy the ES30mm, even though it almost costs as much as a 8" dobsonian :-)

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