View Single Post
  #38  
Old 08-05-2014, 09:55 PM
stephenb's Avatar
stephenb (Stephen)
Registered User

stephenb is offline
 
Join Date: Sep 2008
Location: all over the shop...
Posts: 2,098
Quote:
Originally Posted by Astro_Bot View Post
Actually, the ES 82d 14mm is the only one in the line-up that I wouldn't recommend - due to field curvature apparent in the outer 20% of the field - I used to own one. Pick any other ES eyepiece.

Forget about the relative AFOV (68, 82, 100). IMHO, that's just a personal preference for how you want your view "bordered". All that you can comfortably take in is in the inner 65-70 degrees or so; 68 degrees is enough, and beyond that is window dressing.

Having said that, I have some 82d eyepieces, but that's because I like the view at that focal length - if there'd been a 68 degree eyepiece at that same focal length, with the same performance, I'd probably own it!

BTW, Don's advice is good, if a little short on explanation. Selecting eyepieces is usually best done by picking your lowest and highest practical magnifications (based on a number of factors, including how often you will get to use it/them), selecting corresponding eyepieces at those extremes then in-filling using f/stop intervals (halving the exit pupil area), though mostly you would start with double f/stop intervals (a double f/stop below is equivalent to halving the eyepiece focal length). So, for example:

27mm Pan = exit pupil of 6mm at your focal length (a good figure - largest practical size is 7mm for young eyes and 5mm for old eyes, with 6mm being a good in-between number)

Double f/stop below 6mm is 3mm exit pupil >> 13.5mm focal length.
Double f/stop below 3mm is 1.5mm exit pupil >> 6.75mm focal length.
In-filling at single f/stop intervals also gives 19mm and 9.5mm focal lengths.

So, if the 27mm Panoptic is your starting point, you would have the following set:
27mm, 19mm, 13.5mm, 9.5mm, 6.75mm.

But, don't get too hung up on these numbers - they're only a rough guide. For example, I also have a 40mm eyepiece for wider views (not applicable to you as the exit pupil would be far too large) but didn't want more than 5 widefield eyepieces, so I omitted some focal lengths and picked others in between f/stops to cover the range.

Specifically, my ideal range would be:
40mm, 28mm, 20mm, 14mm, 10mm, 7mm

Restricting myself to commercially available eyepieces and with some focal lengths in between f/stops (in bold):
40mm, 28mm, 18mm, 11mm, 6.7mm.

This arrangement works very well for me. Your mileage may vary.
I've spent most of the afternoon digesting the latest replies, including Astro Bots and it is starting to gell with me. Yes I tend to agree, i've always thought to myself that the AFOV is not a critical element and more of a comfort element of the eyepiece.

I've taken in all your advice, comments and recommendations - cheers to you all. Today I've ordered the ES 18mm 82D from VTI Optics. This hits 131x. I was surprised that they offered a very competitive price (AUD $212) compared to the overseas stores (e.g. OPT USD $190 + shipping)
Reply With Quote