Exit pupil matters...

[Orion_the_Hunter]

Hi, I am about to buy a low power eyepiece to use with my 8" f5.91 Dob, BUT.

Calculating my exit pupil suggests that I would need a 30mm eyepiece. But there don't seem to be any decent 30mm 2" eyepieces (preferably under £100).

So the dilemma is as follows:

Does the exit pupil have to be quite accurate?

I've been looking at a

-SW Panaview 32mm, £79, 24mm eye relief. OR would it be better to get a

-SW Aero ED 30mm, £125 (Mrs won't be too happy), 16.7mm eye relief.

-I've been offered a Celestron Ultima LX 32mm for £90, 16mm eye relief.

Or are there others which I haven't seen?

I guess this is all about which matters most; exit pupil or eye relief? (And £ of course). PS I wear glasses.

What would you do?

[John]

It matters insomuch as an oversize exit pupil vignettes the light cone as it enters the eye which has the same effect as using an smaller aperture scope.

In saying that, I don't think a mm or two difference in eye piece focal length is going to make much difference.

28-30-31-32mm would be fine I reckon - 35mm plus, perhaps not so much.

For use as a "finder" eyepiece the above is not critical - it's when you want to maximise the amount of light entering the eye that it's worth having an eyepiece that does that.

[Orion_the_Hunter]

Thanks John.

I'm veering towards the Aero ED 30mm (this also won't over balance the ota).

I'll be spending the next couple of weeks looking for a low-mid power to go with it.

Ed