W/A low mag eps-is exit eye important?

[ericc (Eric)]

Hi all--Another newbie question:My exit eye pupil is about 5mm.I will soon be using a 10" f5 dob.For a low power and w/angle ep,should I take my 5mm exit eye pupil into consideration,or am I geeting just too excited?
What would this really mean to my viewing of DSOs etc(as an example) if I had a pick of one of the following:

A-Panoptic 27 @5.4 exit pupil

B-Panoptic 24 @4.8 exit pupil

Thanks in advance --if anyone can enlighten me, that would be great?
Cheers-Eric

[Wavytone]

ericc,

There really isn't much to choose between these, both will be fine.

The real question is what's your eye pupil, in your typical seeing conditions and allowing for the fact that with the scope putting some light into your eye, the pupil won't be as wide as it could be in complete darkness. I would normally suggest a 5mm pupil is a fairly safe bet, 6 may be being optimistic. Hence I'd say the 24mm will be optimal if you are observing from suburbia, or the 28mm if you use a dark sky site.

[ericc (Eric)]

I would say that 90% of my viewing will be in the back yard(yep--in the burbs)---with the occasional trip to dark sky!
Thank you for taking the time to explain how it all works--!!!
Cheers-Eric

[Paddy (Patrick)]

Hi Eric,

I've yet to quite get my head around exit pupils, but found a 24mm Pan was an ideal wide field ep for my 12" f5 GSO dob and would think it would be perfect for a 10" f5.

[ericc (Eric)]

Looks like a pan24 is the go?--

[Paddy (Patrick)]

I haven't compared one to a 27 mm Pan, so I don't know if it would be better, but I think you'd be happy with it. I chose a 24 pan over 27 as 1 1/4" filters are much cheaper than 2" and having all 1 1/4" eps made changing a little easier.

Mind you, there are lots of people here with much more knowledge of eyepieces than me, so other more informed opinions may well emerge.

[astro744]

Depends what you want to use your low power eyepiece for. If it is for viewing DSO's with maximum contrast and a good wide field then yes, get an eyepiece that will give you a smaller exit pupil. Both 27mm and 24mm Panoptics are excellent choices. Dont worry about the 0.4mm extra on the 27mm over your ideal 5mm exit pupil for your eye.

If you want to get maximum field of view for general scanning and star hopping then a 41mm Panoptic will give you that with a nice 2 degree field but with an 8.2mm exit pupil. Since you are only looking at field stars the larger exit pupil will be of no concern. The Pan is of course expensive and a 55mm Plossl will give you the same true field but with an even greater exit pupil which again is not a problem when star hopping. The sky will however be quite grey with the 55mm Plossl.

Now after saying all of that a little bit of contrast and magnification is preferable so the 41mm Pan is a better choice over the 55mm Plossl since you get a bit more magnification to help spot some elusive DSO's and better contrast. The 35mm Pan would also be a good choice and will give you about 1.7 degrees TFOV.

Now if you have a good finder you don't need a wide low power scanning eyepiece so go with either the 24mm or 27mm Pan. If you dont have a good finder or prefer the Telrad, go the 41mm Pan since its field is equal to the middle circle of the Telrad (2 deg).

As an alternative to the 24 or 27 pan, consider the 20mm Nagler which will give you a nice 4mm exit pupil and the field of the 24mm Pan (1.2deg). Perhaps at a later date you can complement the 20mm T5 with a 31mm T5 for your low power scanning eyepiece.

Whatever you choose, enjoy!

[ericc (Eric)]

Great advice people--this is making alot more sense now--thanks heaps!!!!