Stray light reflected off inner tube walls in Newtonian
& Dobsonian telescopes has been a subject of increasing concern
in recent years by Amateur Astronomers. Small portions of this light
finds its way to the image plane, where it can significantly diminish
resolution and contrast. The degree of image deterioration is directly
determined by the degree of reflectance at the inner tube surface.
Therefore, in order to achieve the best possible performance, it is
critically important to have this reflectance reduced to a possible
minimium, particularly in unbaffled tubes.
Current methods to reduce this stray light have been
to flock the inside walls either fully or partially with an assundry
of various dark fabric based materials, paints, or user designed baffles,
to reduce or eliminate this stray light. In the results that follow
it has been shown that significant differences in reflectivity exist
among various commonly, and not so commonly used materials.
Thirteen members of the Astromart ATM forum participated
in an informal relative reflectance test to aid in identifying any
materials which could reduce this reflectance more efficiently than
the few commercally available flocking sources. Toward those ends,
the membership constructed a light box test fixture desgned to project
a wide angle source of weak white neon light uniformerly over all
samples to be tested.
The test fixture was constructed of common materials
in a light tight configuration. Sensing of the reflected light was
accomplished with two Radio Shack photosensors. The output of these
sensors was measured by a dual trace Oscilloscope. Digital Voltmeters
could have been used, but better measuring control and electrical
matching of the sensors was most easily done using the channel gain
controls of the Oscilloscope. The sensors were at a reasonably shallow
angleto the material holding table at apprx. 7 degrees.
The light reflectance tests were not under strict laboratory
conditions of precise angular control nor were the measurements in
absolute readings of Lux. No readjustments of any kind occured to
the light box or settings of the equipment through the duration of
the testing. The tests were conducted carefully but are intended to
only show RELATIVE reflectance at a fixed angle of sensor to table.
In the general opinion of this membership, this proceedure
has helped narrow down many flocking materials & a few baffle
designs, notably Andrew Zaharchuk's triangular baffle which measured
exceptionally well in the testing. A sample of stretch velvet found
by Floyd Blue initiated a search into fabric retail stores to identify
it. That search uncovered a variety of photon eating felts.
In support of the general validity of the test it turns
out that two members, Lee Stock and Chuck Fellows had found a particular
material (a felt fabric), in Dec. 2000, years before this test, and
had been flocking their telescopes with it ever since because of its
superior light reduction properties. That fabric was "rediscovered"
in the course of fabric store searching and is represented in the
test.
The reference material which was used in the testing
for instrument control was a sample of McMaster-Carr flocking given
by Walt Hamler. This sample was constantly compared to the test samples
to control and confirm instrument repeatability.
The graph illustrates the wide range of materials tested
and were measured under informal conditions without benefit of strict
clinical laboratory equipment designed for that purpose. The higher
readings on graph correspond to higher reflectivity and represents
our findings of their relative merit in light reduction.
Special thanks are tendered to Cloudy Nights Review
Staff, for making this information public to the Amateur Astronomer
Community for their consideration.
Clear and Dark Sky to all.
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