[ATM] re: Aperture and Magnification

["Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>]

Hi Chuck
    If you are young your pupil will open up to seven millimetres maximum,
as you get older your pupil may not be the muscle it used to be so I use 5
millimetres as a general rule. 
    This governs several things, most important is the fact that if you have
a telescope aperture of 280 mm you will not see anything brighter beyond a
magnification of  56. Things will start to get pretty dim when your pupil
closes down to 2.5 mm or you are using an eyepiece which equals a
magnification of 112.
    The other factor which is sometimes overlooked is in the eyepiece. A
wide field eyepiece with a relief of 12.5 mm should have the glass closest
to your eye at least 12 mm in diameter or more or 1/2 inch. I have seen many
pricey eyepieces that do not meet this standard. 
    Unfortunately to get a reasonable eye relief and a good virtual field
angle it takes a lot of good design and glass to meet the expectations we
have of our telescopes.
    There are some terrific guys on the list who I hope will pipe in on or
off list, but the best place to start is Peter Smith's web page at
http://www.users.bigpond.com/PJIFL/ .
	There are a few of us who believe that a telescope should be
designed with the eyepiece to match since each type of scope has a different
curvature of field and different types of aberrations. Do not judge an
eyepiece by the name , design, or the number of elements in it, you can get
quality viewing with a well made Kellner eyepiece these days. I have a
beautiful Erfle military surplus eyepiece 25 mm fl which works extremely
well with photographic objectives but makes you dizzy when you use it on a
big Newtonian because it was designed for a different optical system. I have
a 25 mm eyepiece that weighs 2 pounds that has a field lens 70 mm in
diameter and can only be used on one telescope because few telescopes have a
field over 25 mm.
	Once you become familiar with the sky you will find very few times
when you can use an 8mm Nagler and most of your viewing will be done with
the lower powers.
	And don't overlook filters particularly if you want to view a spot
on the moon.
	Magnification is a overused word by hustlers - - it is the light you
want to see.
	When you go out viewing use as small a light as you can get away
with, a Maglight with two pieces of Wratten 58 ( I think ) deep red will not
affect your night vision nor will red leds the new high intensity ones.
Avoid viewing with the guys and gals who insist on using regular flashlights
- - they are not couth.


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