Eyepiece Advice for DSO's

I just purchased a Coulter 13.1" dob, and it is my first large aperature scope. It has a 2'' focuser, the f/r = F4.5 and the fl = 1497mm. I need some advice on which eyepieces to get for some DSO observing. Previously, I owned a 6" dob with a 1.25" focuser. When viewing DSO's with my 6", it's limited aperature required me to use my UO orthos to get maximum light through-put. I would usually use a 30mm Celestron Ultima to locate an object then use a 25mm or 18mm ortho to increase power. Any more magnification seemed to decrease object brightness. But now with more mirror, the new scope's ability is going to give me more FOV-resolution-magnification options for viewing DSO's, and I really don't know what eyepieces to get that would best enhance the views. Can anyone share from their experience what they like to use? I would appreciate anyone's input to help me make some good choices. It would save me some money/time on trial and error.
Thanks for any help!

Cameron Voorhees

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Can anyone share from their experience what they like to use? I would appreciate anyone's input to help me make some good choices. It would save me some money/time on trial and error.
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Cameron,

You might also post this in the "Eyepieces" Forum. Lots of opinions there !

I can't go with too small an exit pupil or the floaters in my eye start to be more noticable than the object I'm trying to see. So I try to stay with 1.5-2mm exit pupil or larger. 3-5 is the sweet spot for me. If skies are really-really dark, I'll go up to 7mm, but I doubt my eye's pupil can open up to 7mm anymore.

Exit pupil = Mirror diameter / magnification, or:

Eyepiece focal length / f# of scope.

Example for Nagler ep's and your mirror:

31mm / 4.5 = 6.88 (needs really dark skies)
26 / 4.5 = 5.77 good low power combination
22 / 4.5 = 4.9
17 / 4.5 = 3.8 nice medium power
12/ 4.5 = 2.7
... etc. etc.
4.8 / 4.5 = 1.1 about my limit. Also 300X is about all you can hope to track in a dob anyway 8O


Do you have any issues with eyeglasses, as some of these have much longer eye relief so you can keep your eyeglasses on. I think the 31,22,17 and 12mm Naglers are better for that.

Maybe other folks can disuss the Panoptics, Radians, also the Pentax lineup. But for me, the Naglers seem to get a lot of use in my dobs. I would imagine the 17mm would be a really good all around eyepiece for your scope. 88X, 3.8mm Ex-p,~1 degree FOV. I think your ortho's might be a bit narrow FOV, and they might not like the wide f4.5 incoming light cone, while Naglers seem to put up with that much better. I have also tried using the Paracor, and it helps to sharpen the edge of the field quite a bit, but the stack of metal and glass hanging off the focuser can be a bit much. I also have the old Meade 18mm SWA and like it a lot also, good FOV, sharp, and lightweight being a 1.25" ep.

Gentlemen,

I apologize for my delayed reply, but I want to thank you for your excellent suggestions. I haven’t had any real experience viewing fainter DSO’s because of my small mirror and living in light polluted areas. Mostly the brighter Messier objects. Your input offers me a lot of help.

Warren- I now have a much better idea what f/l eyepieces to start off with my new scope that will give me sufficient magnifications for viewing brighter/dimmer DSO’s. Thanks also for your input about OU Orthos vs. Televue Plossls. That’s good to know. I have a set of the Orthos- no need then for me to “upgrade” to Plossls.

David- I never really thought about the exit pupil factor when choosing eyepieces before, but it is a good point to consider along with the other variables. Your advice will give me good Ex-p guidelines to follow for this scope, for not going too low or too high. I will also keep an eye out for a 17mm as a good med. power choice. It’s 3.8mm Ex-p is about what my 30mm Ultima gives me now with my 6” and that one is enjoyable for me.

Dan- Thanks for sharing your positive experience as a former Coulter 13.1 owner. That’s encouraging as my scope is still in the mail and I haven’t had a chance to look through it yet! (But the former owner vouches for the views it gives.) I will consider the 20mm Nagler. The weight shouldn’t be a problem, as this scope has a counter-weight upgrade on it.

Thanks again everyone, for taking the time to offer your great inputs, they are well-appreciated. Looks like I’d better start saving up for some Naglers. Looking forward to getting to see some galaxies and other DSO’s with a little detail in them! Take care.

V/R

Cameron