Re: ATM Eye relief

[" Gianluca Carinci" <g.carinci@iol.it>]

-----Messaggio originale-----
Da: Talbert Malone <TM85010@UAFSYSB.UARK.EDU>
A: atm@shore.net <atm@shore.net>
Data: giovedì 29 luglio 1999 18.02
Oggetto: ATM Eye relief


>
>I wear glasses and often times take them off to observe because I have
trouble
>getting close enough to the eyepiece. Can someone tell me how to choose
>eyepieces that give good eye relief ? How is eye relief determined ? Is
>there a formula or is it determined by the maker of the eye piece in his
>design work ?


Hi, Talbert
expecially for classical designs general rules are reported such as the
ratio between the focal lenght of the eyepieces and its eyerelief (tipical
eyerelief are 0.2 to 0.3 time the focal lenght for the Huygens, 0.4 to 0.6
times the focal lenght for Kellner, MA, Or...
This may work for the simplest types as, even if manufacturers don't give
exact data for them, their characteristics are limited by parameters such as
glasses types and lens number: for this we know that always an Huygens can't
perform well over more than a few degree apparent FOV, that a tipical plossl
has a 50° apparent FOV...
    Improved design can boast of wider FOV or better performance at the very
edge, thanks to special glasses or additional optics (Zeiss Or, Super
Plossl...): these are anyway deverging from classical types and are more
expensive.
Newest designs have been introduced just to pump up eyerelief and edge
definition: these types are more difficult to analize without a quite exact
data sheet: it is simple to optimize an Huygens or a Kellner starting from
one or two glasses types: try to imagine the possible combinations between
air gaps, glasses types, and ROCs of an eight lenses eyepiece!
    To cut short, from my little experience and preferences:
- for a datum type, maior eyerelief are due to maior focal lenghts
- as an eyeglass wearer, I prefer Or and Pl types starting from 12mm F.L.
- for Higer magnification, I use a good quality Barlow (avoid brand)
- I've bought from a serious sellers (avoid name) that on his web site has
data tables for his medium to hig quality eyepieces listing also eyerelief
as reported by the manufacturer
- if you're not like me (i.e., on a budget ...) try Nagler and Radian type
at least for the shortest focals as they are reported to be the quality
choise between amateur who wants long eyerelief
- if you are not astigmatic but only miopic or presbyopic, a little focus
adjust can make you observing without glasses
- at higher magnifications, the exit pupil is little and even a defective
crystalline should perform quite well even if defective and without glasses
Does anybody agree?
Clear Sky to you all