Re: [ATM] Purpose of baffles/shrouds

["Alan Scott" <alanne1956@hotmail.com>]

Mike,
  You took the words out of my mouth.

  At it's most basic, I have tested my scope under light and dark 
conditions, and it has better contrast with a shield AND shroud.  Tom has a 
good point about eyepiece baffle, but my Telvue barlow is so long that it 
pushes the baffle out of the focuser.

alan


>From: Mike Lockwood <melockwo@uiuc.edu>
>To: atm@atmlist.net
>Subject: [ATM] Purpose of baffles/shrouds
>Date: Mon, 21 Nov 2005 13:00:06 -0600
>
>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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