Jump to content


Photo

A question about baffles

  • Please log in to reply
12 replies to this topic

#1 Dr. Woo

Dr. Woo

    Mariner 2

  • -----
  • Posts: 216
  • Joined: 23 Dec 2013
  • Loc: The IE

Posted 28 December 2013 - 02:52 PM

I am building a tube for a 6" lens. I have drawn a full size ray trace of the F8 light cone.

My question is when sizing the center hole of the baffles should I make the hole equal to the size of the light cone at a particular position or slightly larger like 1/16 or 1/8 inch?

TIA...........Robert

#2 plyscope

plyscope

    Apollo

  • -----
  • Posts: 1479
  • Joined: 23 Nov 2006
  • Loc: Perth, West Australia

Posted 28 December 2013 - 03:25 PM

Equal to the light cone is ideal however it is not too critical. Plus or minus a little probably won't make much difference visually. I tend to size the baffles for my largest intended eyepiece. Theoretically that makes them oversize for smaller eyepieces.

Minimum baffle method

#3 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1965
  • Joined: 10 Dec 2007

Posted 28 December 2013 - 04:05 PM

The minimum baffle method that plyscope linked to is a good process.

One important point often overlooked is that the field size at the eyepiece should not be zero, that is the light cone does not taper from the objective to a single point at the focal plane. The light cone needs some dimension at the focal plane.

When I have baffled refractors, I recognized that it is next to impossible for me to make precision holes that are accurately centred in the tube. To ensure that there were not random parts of the various baffles that were intercepting the light cone I always make the holes slightly oversized on all but one baffle. The one baffle defines the light cone and the others simply control reflection and scatter off the tube walls

dan

#4 Dr. Woo

Dr. Woo

    Mariner 2

  • -----
  • Posts: 216
  • Joined: 23 Dec 2013
  • Loc: The IE

Posted 28 December 2013 - 04:49 PM

Thanks to everyone for the advice. I'm going with more than minimum. Maybe every three inches or so with the front of the tube flocked. So I'll go with the slightly oversize hole on all but the back one. Thanks again

#5 plyscope

plyscope

    Apollo

  • -----
  • Posts: 1479
  • Joined: 23 Nov 2006
  • Loc: Perth, West Australia

Posted 28 December 2013 - 06:19 PM

This thread has some good photos and also a formula by Mike Jones to determine baffle sizes.

Making a 6" f15 refractor

#6 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10493
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 28 December 2013 - 08:52 PM

Actually, I would suggest that no baffles be 'minimum' size. Your light bundle used for the baffle dimensioning and laying out should be that which encompasses all image-forming light for the widest field you anticipate.

The only 'downside' to using larger baffle openings is a somewhat tighter baffle spacing so as to ensure the required tube wall shadowing and masking.

#7 Dr. Woo

Dr. Woo

    Mariner 2

  • -----
  • Posts: 216
  • Joined: 23 Dec 2013
  • Loc: The IE

Posted 28 December 2013 - 11:32 PM

After thinking about your recommendation, I agree, since I plan on using many baffles anyway why limit the field? At this point I only plan visual use but who knows............

#8 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10493
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 29 December 2013 - 05:28 AM

You could accommodate for the largest 2" eyepiece field stop of 46mm, which is a bit larger than the 43mm diagonal of the 24X36mm frame of the so-called full-frame 35mm format. In no way will this be any kind of compromise for 1.25" eyepieces; good baffling us good baffling, and it works as well for any smaller formats as it does for the designed-for largest.

Indeed, for apertures up to around 5"--and particularly for shorter f/ratios--I sometimes use the sawdust and flat black paint system, which can be though of as comprising innumerable, maximum-aperture baffles. And it works *very* well! And it's 'future proofed' against any increase in desired field coverage and or focuser upgrade.

#9 Cames

Cames

    Ranger 4

  • -----
  • Posts: 338
  • Joined: 04 Aug 2008

Posted 29 December 2013 - 06:15 AM

Don't the forward facing surfaces of the baffles reflect light onto the rearward facing surfaces of the objective lenses? I imagine that these unwanted reflections probably create a zigzag pattern of stray light that reduces gains of baffling. Is it so? If so, may flocking be a better way to go?

#10 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10493
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 29 December 2013 - 10:14 AM

The low albedo of the flat black baffles, in conjunction with the low reflectance of the objective lens coatings, mean that any light so double-reflected is of such low intensity compared to the direct light delivered to the image surface as to be of no import.

One can try to see if any vestige of the baffles is visible via reflection off the objective by peering up into the focuser while pointed at the daytime sky. A more severe test could be tried, to make the effect even more damaging than is normally encountered. With the scope pointed toward a very dark background, shine a light obliquely into the objective so as to illuminate the baffles' front surfaces but at the same time not be visible directly from the back end. This will afford the greatest contrast for the visibility of the reflections. Even here these reflections may not be very apparent, which gives confidence of their being too subtle to be of concern during normal use.

But in the end, whether these baffles be few or many, the diffrence may amount to little in terms of the total amount of light being sent back toward the objective. That is, the total area illuminated will not differ to any meaningful degree; a closer baffle spacing also results in the 'exposed' annulus on each baffle being reduced somewhat, thus compensating for the larger number of surfaces. An analysis of the light paths should bear this out, when the illuminated and shadowed areas are considered from the standpoint of both the front side projected area and that as seen from the focal surface (especially.)

#11 amicus sidera

amicus sidera

    Skylab

  • *****
  • Posts: 4000
  • Joined: 14 Oct 2011
  • Loc: East of the Sun, West of the Moon...

Posted 29 December 2013 - 10:28 AM

Excellent explanations, Glenn; I can see that I was under a few wrong assumptions regarding baffling, and this information will come in very useful if and when I attempt a refractor build. Thanks!

#12 Dr. Woo

Dr. Woo

    Mariner 2

  • -----
  • Posts: 216
  • Joined: 23 Dec 2013
  • Loc: The IE

Posted 29 December 2013 - 07:07 PM

Excellent explanations, Glenn; I can see that I was under a few wrong assumptions regarding baffling, and this information will come in very useful if and when I attempt a refractor build. Thanks!

Agreed. Thanks Glenn for weighing in on this

#13 Dr. Woo

Dr. Woo

    Mariner 2

  • -----
  • Posts: 216
  • Joined: 23 Dec 2013
  • Loc: The IE

Posted 02 January 2014 - 09:57 PM

Well I did a full size drawing of the lens, light path and inner tube and with 10 baffles spaced at 3" from the back of the 34" tube there should be zero tube wall visible except possibly the very front into the cell which was originally designed to be threaded onto a 5" tube but will in this case be on the inside of a 6" tube, when looking through a 2" focuser. I plan on flocking the very front tube portion, that possibly might be visible.Thanks again for the advice






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics