Re: [ATM] Purpose of baffles/shrouds

["Peter" <peter@kitgear.com>]

Interesting discussion.

What kind of baffling do you suggest for a folded Newtonian where the
eyepiece looks upwards into the sky? I am incorporating such a design in my
large ultra light scope but I plan to use a 9" long focuser tube which will
have two movable baffles. The tube and adjustable baffles will limit the
field of view to only the secondary mirror. The secondary mirror will not
have a shield behind it.

Any comments about this idea?


Peter

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Mike Lockwood
Sent: November 21, 2005 2:00 PM
To: atm@atmlist.net
Subject: [ATM] Purpose of baffles/shrouds


Hi,

Tom Krajci wrote:
> "This telescope will be able to carry a half height or full height shroud.
> It will also be usable without a shroud for observing during windy
nights."
> Because you use a low profile focuser/focal plane position...you are
forced
> to use a large baffle disk opposite the secondary mirror.  That will
provide
> a large wind profile.  Are you sure you want to do that?
> for an example of a smaller baffle disk that's possible if you don't use a
> low profile focal plane position.  This presents a smaller wind profile.
> (And there are some other anti-wind measures covered on this page.)
> Also, why use a shroud?

An excellent question.  Just for those who may not have thought this
through, I want to mention some of the reasons for baffling/shrouds.

First of all, Tom K. has been teaching the benefits of a focuser
baffle for a while now, and it is definitely a good thing.  I
incorporate them where I can in my telescopes.  However, they can't
block all stray light.

For those of us with scopes already constructed (myself included), and
for which making room for a focuser baffle is tough, we generally rely
on larger shrouds.

So why do we need a shroud/shield?  Well, even with a focuser baffle,
there exists a light path that unwanted light can traverse and get to
the eyepiece.  If the focuser baffle is perfectly placed, it will
allow light from the entire surfaces of the optics to pass through.
(Without a focuser baffle, even more light from more angles can get to
the eyepiece.)

However, light from off-angles can get past the focuser baffle,
particularly from directly behind the secondary mirror, as viewed from
the eyepiece.  How is this possible?  Well, the focuser baffle still
allows light from angles larger than those covered by the secondary
mirror to get into the focuser.  (Stray light passing the left edge of
the diagonal can get to the right side of the eyepiece barrel.)  The
light doesn't enter at the same angle as the converging light cone
from the primary, so it is not "imaged" the same way by the eyepiece,
but it can definitely be scattered by the eyepiece, lens edges, dust,
and the interior of the eyepiece barrel.

This scattering is especially troublesome for popular wide-field
eyepieces, like Naglers.  (I suspect this may be partly why they have
slightly less contrast than other narrower-field eyepieces - I find
their contrast increases greatly with proper baffling.)  The shroud
opposite the focuser baffle, appropriately sized, will prevent much of
this scattering in dark environments.  This shield must be sized to
prevent stray light from entering the barrel of the eyepiece that has
the largest field stop (31mm Nagle, 41mm Panoptic, etc.).

In my experience, another shield is very important, especially in
environments with stray light - the baffle that keeps stray light
illuminating the secondary mirror and the inside of the light shield
opposite the focuser.  I most commonly have problems with sources of
stray light that are behind me and to my left - they can light up the
secondary mirror and the other light shield if it has any reflectivity
at all.  (My telescopes have the focuser on the west side of the scope
if it is pointed south.)  So, a shield placed below the secondary cage
on the same side as the focuser is very beneficial, too.

With these three baffles/shields (or two larger ones) in place, I
don't really worry about shielding the primary from stray light, but I
do place a shield behind the primary to block light from leaking
through around the primary.

	Mike Lockwood

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