A question about baffles
Posted 28 December 2013 - 02:52 PM
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?
Posted 28 December 2013 - 03:25 PM
Minimum baffle method
Posted 28 December 2013 - 04:05 PM
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
Posted 28 December 2013 - 04:49 PM
Posted 28 December 2013 - 08:52 PM
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.
Posted 28 December 2013 - 11:32 PM
Posted 29 December 2013 - 05:28 AM
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.
Posted 29 December 2013 - 06:15 AM
Posted 29 December 2013 - 10:14 AM
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.)
Posted 29 December 2013 - 10:28 AM
Posted 29 December 2013 - 07:07 PM
Agreed. Thanks Glenn for weighing in on this
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!
Posted 02 January 2014 - 09:57 PM