[ATM] Purpose of baffles/shrouds

[Mike Lockwood <melockwo@uiuc.edu>]

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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