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Why 'knife edge' baffles.

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#1 oldtimer

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Posted 22 December 2009 - 06:20 PM

I have fabricated a number of refractors from 2.4 to 8" over the years and have always used plywood to make my baffles. I really don't understand the advantage of thin razor sharp baffles. To me once the light is cut off, its cut off. Please enlighten me!

#2 Mirzam

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Posted 22 December 2009 - 06:24 PM

Some light can reflect off the flat inner surfaces of thicker baffles. Not a big deal though in my opinion unless the surfaces are very thick.

Jim

#3 Happy-Idiot

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Posted 22 December 2009 - 06:34 PM

My two cents, ;) If you are going to put the effort into baffles why not make them more efficient with knife edge baffles. As Jim mentioned there is less surface area for the light to reflect deeper into the tube. I did an experiment using a laser pointer and shined it into a few of my tubes to see how the beam would react with protostar,krylon ultra flat, and baffles. All worked well but i rate them from best to less than best as follows bare tube protostar, 4 knife edge baffles, and bare tube krylon ultra flat. But the best of the best was an old Unitron dewshield with hundreds of internal threads that acted like hundreds of mini knife edge baffles WOW! There was barely any reflection at all,SUPER shielded! But it is almost impossible to internal thread a OTA. I think the more knife edge baffles the better. Again just my two cents sorry to get off topic.

Without too much more effort, i use a circle cutter on my drill press and reverse the blade. Simple method :)

#4 oldtimer

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Posted 22 December 2009 - 09:15 PM

I negleged to add that I wrap aluminum window screening painted flat black around my plywood baffles creating an inner tube. Each little square in the screen is a light trap.

#5 GlennLeDrew

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Posted 22 December 2009 - 10:10 PM

oldtimer,
Just to be sure the picture is clear, a baffle of non-zero thickness will have its own 'tube wall' section. The thicker the baffle ring, the more surface area this effectively short tube will have, thus potentially scattering more light in the same way a non-baffled tube would.

Yes, this area is *physically* very much smaller than the main tube being baffled. But if there are several such 'thick' baffles installed so that as seen from the focal surface their edges are nearly in line, the *angular* extent of these scattering surfaces is now relatively large when seen in projection, closely surrounding the objective aperture.

One way to easily make knife-edged (or nearly so) baffles is to use a thicker material for support but which has a slightly oversized hole. A much thinner, correct hole size material can then be attached on the objective (skyward) side. In this way the thin sheet shadows the thick material's own 'tube wall' section.

#6 polaraligned

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Posted 23 December 2009 - 08:06 AM

Glenn, when you use baffles that have an angle (taper) to the knife edge, is there a preferred way to have the taper facing. And a preferred way to have the completely flat side of the baffle facing. I am thinking it does not matter.

#7 Jared

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Posted 23 December 2009 - 04:03 PM

I don't know what is best, but on my A-P refractor (which has machined in baffles rather than traditional knife edge), the taper faces forward and the flat size faces the focuser. It is certainly quite effective.

#8 GlennLeDrew

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Posted 23 December 2009 - 05:02 PM

The AP focuser baffles, while having a quite gradual slope on the objective side, still have rather sharp edges. The sloped forward faces are designed so as to be not seen from any position within a 2" (and probably larger) diameter circle on the focal surface. Therefore *only* the fully shadowed back surfaces are 'seen' at all.

#9 photonovore

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Posted 27 December 2009 - 02:30 PM

Here's a graphic explanation as to the "Why?" of knife edging baffles: Posted Image

The angle creating the knife-edge is on one side of the baffle and is usually located on the focuser-side of the baffle to avoid back scatter of stray light towards the objective.

#10 lightfever

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Posted 27 December 2009 - 02:40 PM

All you need to do is bevel the I.D. of the baffle with a 30 or 45 Deg. router bit and put the sharp edge forward.


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