[AZ-Observing] Contrast and glare reduction [Was: Scope color

To expand a little on this (hoping the dreaded Net Nanny doesn't do me 
in for Too Many Lines....) let me share some comments I sent earlier to 
Jimmy Ray:
In selecting color for scope tubes, accessories, cables, and etc. think 
about what the color will be to the eye under a RED flashlight.  RED 
objects will appear colorless, or white.  Think maximum CONTRAST in the 
dark.   I prefer WHITE color or paint, or even strips of reflective tape 
on anything that can DROP on the ground. Black stuff, including cables, 
looks great in the photo studio for the catalog shots, but doesn't work 
worth a hoot on a dark night!!  For instance, I always buy WHITE lamp 
cord for any auxiliary power cables I make, and make 'em much longer 
than the stupid manufacturers provide.
But some say, all CATS (cat-eye-di-optrics?) are BLACK in the dark.....I 
have a folding table that came with a black vinyl top, and in the dark, 
setting stuff down on it is like dropping things INTO a pool of 
water.....you can't locate the surface!!   Due to reflected light 
spoiling some piggyback photos, I put a sheet of black flock paper over 
the OUTSIDE of the highly reflective, dark blue tube on my 25-year old 
Meade 2080 --  It's still on there.
BTW, in re: Stan Gorodenski's earlier comments about contrast, most 
commercial scopes rate poorly because of inattention to details of 
masking and blackening.  Take a good look at the inside of your eyepiece 
holder, focuser accessories, etc. and eliminate ALL unblackened 
surfaces.  Remember that "black" anodized objects are actually surfaced 
with sapphire -- aluminum oxide, a form of glass -- which may be hard, 
good looking and resistant to scratching, but is highly reflective at 
low angles of incidence.  Take out the eyepiece and place your eye at 
the edge of the focuser, and observe all the sources of reflected light 
-- then cover, paint, or mask those!  Look over (into) your expensive 
bino viewer and check that, too. Black felt or flock paper is 
inexpensive and works well, and lasts a long time.
This is esp. important in observing the Moon.  The unfocused light from 
the Moon when it is 1-1/2 to 2 degrees out of the field can get right 
into the eyepiece, reflecting off the inside of the main baffle and 
leaving a ghost or glare image.  (Try that out -- take out the eyepiece 
and look into the baffle tube and move the Moon around slightly 
 >outside< the field of view and look for the light beam on the inside 
of the baffle as it sneaks up on the eyepiece.)  I was having glare 
problems in observing lunar occulations as the Moon moved up to the star 
-- long before it was in the same field.  Turns out the inside of the 
main baffle tube on my older Meade 8 inch SCT is very reflective at 
those low angles, peaking when the Moon was about a degree outside the 
field.  You see, the ID of the baffles in the commercial SCTs are sized 
to pass a beam the full size of a 35mm camera negative -- way bigger 
than most 1-1/4 inch eyepiece field lenses.   I added a glare stop or 
baffle plate with a hole the same size as (or slightly larger than) the 
field lens in my favorite eyepiece.  A 16 mm f.l. eyepiece has about 
(you guessed it!) a 16 mm field lens.  This seems to be the rule for 
most regular eyepieces.  This can just be cut out of cardboard or 
something, and the edge of the hole doesn't need to be perfect, because 
the plate or stop is way out of focus.  It's placed right at the back 
plate of the telescope, in the little filter recess, where the threaded 
accessory collar screws on.  Cuts out all that unwanted light.  A 
similar stop could be fitted to Newtonians or refractors, if needed.....
In the case of a binoviewer, most observers will be using it with the 
attached Barlow lens in the nose, which restricts the beam.  In any 
case, ALL the available binoviewers are based on microscope optics 
heads, which only pass about a maximum 16 to 18 mm beam (due to the 
prisms and mirrors inside to separate and fold the beams).  (No 
microscopes use eyepieces with f.l. longer than about 20 mm.)
I have enjoyed using my binoviewer on the Moon and other objects. It 
allow s you to see the whole field (with both eyes and your brain), 
without having to scan your eye around with averted vision.  The price 
paid is about 1 magnitude, due to the added light loss, which is 
somewhat made up by using both eyes (about the square root of two gain, 
by actual experiment.)
In Newtonians, you have to also review the inside of the tube opposite 
the focuser, and also the area around and behind the mirror - can you 
see any reflected light in those areas??  Refractors need to be fitted 
with proper baffles inside.
I did a talk at RTMC in 1988 on "Eliminating Stray Light in 
Schmidt-Cassegrain Telescopes" -- maybe its time to update that for a 
new generation....
Have fun!
Gene Lucas
(17250)


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