Quote:
Originally Posted by Allan
Hi guys. I've got a 31 Nagler which gives me 1.7 degrees FOV and 48x, and a 13 Ethos which gives me 0.9 degree FOV and 115x. I always saw a 21 Ethos in my collection, but now that I've got these I'm wondering how much use it will get in my 12" Dob.
Anyone out there with a similar setup or experience care to comment. Thanks for your help.
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I have a 12.5" and use a 31 Nagler and 21 Ethos.
(next step is a 13mm).
I have discovered some things over the time I've owned both:
--the field of view of the 31 is wider, but not by a huge amount.
--at 62, my dark adapted pupil seems to show more natural astigmatism with the exit pupil of the 31 than with the 21, so the 21 gives generally sharper images to my eye, from edge to edge.
--the 21 yields a darker sky background in the image (noticeably), so the contrast in star fields is greater in the 21, making a more pleasing image quality.
--though the 31 sees a greater extent of nebula than the 21, the detail in the nebula is superior in the 21. I've used both on the Veil, and the 21 yields superior detail and slightly fainter nebulosity than the 31 when both are used with the same filter.
--the 21 gives superior views of nearly everything except the largest clusters. These tend to loose their "clusterness" and look more like random field stars with the increase in magnification.
I find myself using the 21 a lot more than the 31. I haven't brought myself to sell the 31 yet, but I really doubt I'd miss it. Both the 31 and the 21 are parfocal and use the same Paracorr setting, so there is no disadvantage to owning both. It depends on your age and your eyes. If you're older than 60, the 21 would be a great choice as a low-power eyepiece.
As for the possibility of the 25mm Explore Scientific 100 degree, which is a lot less expensive than the 21 Ethos, the transmission seems lower, the correction less, and the eyepiece a little more difficult to use. It is obviously a lower-priced eyepiece than the 21 Ethos, but does fill a niche.
One point: the wider the apparent field, at every focal length, the more visible is coma. If you are not using your scope with a coma corrector (and it sounds like you are not), coma will become obtrusive in the outer field in a 100 degree eyepiece. If you plan to stick with no coma correction, from that standpoint a 22 Nagler would be a better choice for the "in between" eyepiece.