Quote:
Originally Posted by ColHut
Well funny this should come up. I have a 10" f5 dob and I cannot find the optimum UWA eyepiece. You see My eyes are not young and a maximum exit pupil of 6mm would be best (less better still), But in a 2" barrel, The best field stop I might get is only 48mm which woulds allow 2.2 degrees of TFOV. My maths says a 30mm lens would be 1250/30=41.67 magnification. As AFOV=MagXTFOV then AFOV = 41.67X2.2 = 92 degrees. This does not really come close to the 31mm Nagler, and both it and the Moonfish have restrictive field stops - The Nagler according to reviews noticeably so. Really an exit pupil of 5mm would be better so lens is 25mm, mag is 1250/25 = X50, and AFOV = 50X2.2=110 degrees. I would settle for a 28mm with a 98 degree AFOV which will give me the same 2.2 TFOV and a 5.6mm exit pupil. I am sure this would be possible!. Surely Televue should make this a priority. Just keep an eye on the field stop thanks. Those with 12" dobs would have to shift for themselves....
cheers!
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Hi mate,
Your best bet at this point in time is a Nagler 26mm T5 with 82 degrees AFOV. Exit pupil= 5.2mm and will pull in 1.71 degrees of sky. I know it's not 2.2 like you have mentioned, but exit pupil is very important regarding useability, especially in town.
A William Optics 28mm UWAN with 82 degrees AFOV may be an option, although I'm not sure about the edge correction for a semi fast reflector.
I have a Pentax 30mm XW 70 degrees AFOV and love it. Gives me an exit pupil of 6mm and 1.68 degrees of sky and the edge correction is pretty good. Still get the space walk feel. To me the difference between the 82 and a 70 degree EP is not much when looking through it, well a Pentax compared with a Nagler anyway IMO.
I have the same scope as yours 1250mm 10 inch F5 and I researched alot of EP's before I settled on the 30XW. I looked through plenty and was never really satisfied till I had a go with a Nagler 26T5 and the Pentax 30XW. You also need to consider eye relief. Around 15 to 20mm is good, 20 being better for the long haul. A combination of ER and AFOV and exit pupil is what you need to work out, whats best for your eyes. My eyes are not real old, but I love the comfortable viewing through the Pentax.
PS. If you want to get the real space walk feel, get an ED80 with a 26-31mm 70 or 82 degree EP, wow, gives you about 3.5 to 4.5 degrees (depending on focal length), second thoughts, just get some binoculars.
and was diappointed by the (relatively) narrow field stop reported for the Nagler. If they improved their 31mm Nagler Field stop to 48mm that would allow 2 degrees in my scope which would be fine! Thanks for your serious comments though - I am thinking about where to from here.
