Re: ATM Focal Ratio Question

["Frank Ward" <thewards@pop.mindspring.com>]

Dave,

P.S.

I didn't answer your question on magnification.

First rule, There isn't enough light collected by most mirrors
to magnify the circle (the circle of confusion) more
than 50 times the diameter of the mirror.

A 10 inch mirror can under best possible conditions
give you a useful image at 500X

A 20" mirror can under best possible conditions
give you a useful image at 1000X


Now, you can use extenders and eyepieces to magnify
the 10" mirror to 1000X but you'll only get a blob.


Magnification is a factor of the focal length of the mirror (lens)
divided by the focal length of the eyepiece.  A long focal
length lens divided by a short focal length eyepiece will
give you maximum magnification.  A short focal length
mirror divided by a long focal length eyepiece will give
you minimum magnification.

When we are looking at the extended Messimer objects
there is no use magnifying them.  You won't see any more.
Therefore we use a short focal length mirror and a short
focal length eyepiece (but one made to give a wide field
of view.

When we look at the planets we want a narrow field of
view and a long focal length on the mirror.  Then we
magnify that view with short focal length eyepieces.


There, how's that? Now you're more confused, but
now you too can join the "circle of confusion"

Frank Ward
-----Original Message-----
From: dhill@mail.usa.com <dhill@mail.usa.com>
To: atm@shore.net <atm@shore.net>
Date: Monday, November 16, 1998 1:04 PM
Subject: ATM Focal Ratio Question


>
>I understand that shorter, faster focal ratios are better for viewing large
star clusters and nebula and that slower ratios are better for observing
planets and the luner surface.
>
>What I do not understand is why?
>
>All the F/ratio really is is the ratio of the apature to the aparent focal
length. The aparent focal length is really nothing other than an expression
of the angle at which the light converges. If I have an otherwise fast
system and the last thing I do is run the light through a negative lense
system before it exits so that the folcal point is extended back farther, I
suddenly have a slower system. It seems we are almost talking about degrees
of magnification here, which I understand to be a variable function of the
chosen eyepiece. Magnification is, after all, the angle at which the
incomming light intercepts the eye.
>
>Can somebody help me understand why different focal ratios are better for
viewing/photographing different objects, and how focal ratio differs from
magnification power?
>
>Dave
>
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