Re: [ATM] Baffles

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

hello John et alii,

> We are probably splitting hairs here of course, but what else is this list 
> for? Anyway, the reflection from the baffle is darker than the reflection 
> from the tube in that the light is no longer grazing. >

My baffling scheme will take care of grazing reflections going *towards* 
you, as they are in general strong. But you have baffled for grazing 
reflections going *away from* you - do you think they are as strong? It 
should be obvious that grazing reflections going down the tube will not be 
reflected by the primary to the EP - from the focuser, you should never see 
the inside of the tube reflected in the primary.

As to what ASTRONOMY magizine advocates I am unsure of what you mean. I 
wrote the article I think you make reference to and advocate the method as 
outlined in Telescope Optics WITH a larger tube in the rear for maximum 
darkness.

So am I, I recall an Astronomy mag aricle mentioned in this thread . I only 
have a vague recollection of having read an article advocating (what I claim 
to be an incorrect) refractor-type baffling for reflectors. I have no 
recollection of the author or if I still have the issue available - what 
issue was your article???

> Has anyone bothered to measure darkness of the paints we use? I know I 
> have asked this before but I am hoping to stimulate someone to examine 
> flatfields with and without baffles?

Being inspired by this, I just did an experiment!

I took 2 pieces of slightly rough wood, painted with a not entirely flat 
black. One larger piece I held to see it from a grazing angle (acting tube 
wall), and a narrow strip held at right angles to this (acting baffle). I 
held a strong lamp beside my head, shining at a similar grazing angle away 
from me. Here, it was very obvious that the "tube wall", reflecting light 
away from me, was significantly darker than the "baffle" itself. Repeating, 
this time in front of a mirror reflecting the light from the lamp at a 
grazing angle *toward* me, it was even more obvious that this grazing 
reflection was far brighter than the baffle (though still illuminated from 
its visible side). This was the case for a not too narrow angle of view.

Thus, with a minimum of material and in a few minutes I could demonstrate 
that for *this* admittedly not the best material,
1) John's baffling scheme is, as I suspected, counter-productive,
2) Anthony's assumption of specular grazing reflection is too "liberal" and 
that my more conservative assumption that a grazing reflection will add 
light even if a specular reflection would miss the secondary, is indeed 
justified.

Too bad I left my camera out of reach - I'll try take a few shots tomorrow 
to show this simple experiment. It should be repeated with other materials 
of course - volunteers?


For a telescope, I would suggest taking a photo through the empty focuser 
opening (or simply checking visually) during daytime to see if any 
significant stray light is visible, and if so, identify the path.

Nils Olof



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