Re: [ATM] Baffles

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

John and list:


> For the Newtonian consider the entire tube as a "sunshade" extending from
> the primary. As such the rear portion must be as dark as possible (carry
> the argument to the extreme by considering the situation of an aluminized
> tube).

Yes, it will be, indeed! Take a cardboard tube, stand it on the ground, lok
down it - the ground inside is *much* darker, as well as the tube wall. Lift
it towards the sky and peek inside, see the grazing reflections, brighter
the closer you come towards the outer end.

This, I think, is an illustration to the difference between refractor
baffling and Newtonian baffling - not only is the baffling of a refractor
more straightforward, it is also much more important. (You can apply 2
principles here: shade all of the tube wall from the FOV, or shade all that
isn't shaded by the baffle before it! The former is perhaps the more common,
but the latter reduces the number of baffles needed).

The refractor tube will pick up grazingly reflected light from a far larger
part of the sky than the Newtonian. As for baffling against veiling glare
from diffuse skylight, I just don't believe it will matter at all in a
Newtonian with full tube, and with reasonable black coating of the relevant
parts of the inside of the tube, there is in practice nothing to be gained
by baffling here.

The open, unshrouded (lightweight) Dob-type OTA is something else - the FOV
may receive significant light from a mirror box top illuminated from a large
part of the sky - and very often, I believe, not adequately blackened (for
aesthetic reasons, perhaps - but if you like the furniture look, you could
put a detachable velvet baffle here during use!). Also, you may have to
observe near streetlights possibly illuminating the box top, you may need to
use at least a short "skirt" over the lower end of the truss. Also, an open
mirror cell may let plenty of light through beside the mirror from the
ground - you will have to baffle here in some suitable way. The "skirt" will
help if your secondary is dimensioned so that light outside the mirror box
could reach the FOV. Look thru the empty focuser and see what parts can
contribute!

This said, the light-proofing of the upper tube, including baffling of the
focuser, is likely much more important...

Nils Olof


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