Should I consider a 31.7mm EP?

[pipnina]

Based off some maths I've done, the lowest power EP I got with my scope (in sig) gives me a 3.5mm exit pupil. I thought this might be a little low for deep-sky observing (31.7mm would give me 4.6mm).

In a previous thread someone mentioned that different targets have different "Optimal" exit pupil sizes and that bigger (and therefore brighter) isn't always best.

What do you guys think? The Sky watcher SP seems to be of good value if a 31.7 would be beneficial to me. http://www.firstlightoptics.com/all-eyepieces-barlows/skywatcher-sp-plossl-eyepieces.html

P.S. what's the eye relief like on that EP? My glasses stop me seeing the full field on my 25mm.

Thanks,

    ~pip

[John]

I think 31.7mm is the diameter of the barrel of the eyepieces rather than the focal length - thats the same as the 1.25" fitting.

The plossl eyepieces have eye relief that is around 70%-80% of the focal length so, for example, a 32mm plossl will have eye relief of approx 22mm - 26mm.

If you wear glasses when observing generally you need eye relief of at least 15mm and preferrably closer to 20mm to see the full field of view.