Download - Hidden Hollow 2012 - Pre-Talk
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Newtonian
TelescopeDesign
Pre-Talk
Michael Abrams
Hidden Hollow 2012
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Newtonian
Telescope DesignPre-Talk
Recoating my mirror
Collimating a Newtonian Telescope
Putting a center mark on the mirror
Red LEDs
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Recoating my Mirror
A Sad Looking Mirror
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But when you look at it straight on,it is not so frightening.
And optical performance did not seem affected.
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But it is a puzzle
The mirror was only six years old and not used that often.Tammy's 12.5 mirror on a Meade Telescopeis 20 years old and she said it looked good
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Jeff Decker at Majestic Optical recoatedmy 12 mirror and seondary for $145, shipping included
I received my mirror back nine days after sending it
It looked great
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I shipped my mirrorDouble boxed A box in a box.
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Collimating aNewtonianTelescope
A three step easy method using the
Cheshire Eyepiece &Crosshair Sight Tube
http://www.atmsite.org/contrib/Carlin/collimation/kolli2.htm#mkcenterspot
Credit to
Nils Olof Carlin
http://www.atmsite.org/contrib/Carlin/collimation/kolli2.htm#mkcenterspothttp://www.atmsite.org/contrib/Carlin/collimation/kolli2.htm#mkcenterspot -
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1. Focuser and Secondary Mirror should be in line
Use a Crosshair Sight Tube
2. Focuser points to center of mirrorUse a Crosshair Sight Tube
3. Mirror axis points to the center of the focuserUse a Cheshire Eyepiece
Collimating a NewtonianTelescope
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Crosshair Sight Tube
With peep hole at one end and crossed threads at the other
PeepHole
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1. Focuser and Secondary Mirrorshould be in line
Looking down the barrel of the focuser the secondarymirror should appear concentric with the focuser.
Use the Sight Tube to confirm this.
Mirror is adjusted up-down or left-right.This is generally a one time adjustment done at
the factory and not critical. Looking down the sight tube
focuser
focuser
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2. Focuser points to center of mirror
Use the Crosshair Sight Tube. Adjust the angular position of thesecondary so that crosshairs aim at the center of the mirror(or in this case a red washer glued to the mirror)
This too is a noncritical adjustment, often done just once at the factory.
Looking into the Crosshair Sight Tube
Crosshairs over mirror center --a good adjustment
A terrific adjustment. See reflection ofcrosshairs in mirror centered in the red
washer.
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3. The mirror axis should point to the center of the focuser
Use a Cheshire.
The parabolic mirror has a central axis that is impossible to actually see but amarker on the center of your mirror allows you to make this adjustment.
Make an angular adjustment to the mirror to align.All Newtonian telescopes will have mirror adjusting screws.
Unlike steps one and two this is a critical adjustment
especially if you have a f/5 or smaller mirror.
Red WasherCenter Spot
Cheshire
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Adjusting the mirror axis with a Cheshire is
No different than adjusting a hand mirror to see your face
The central perpendicular axis of the hand mirror
will point directly to your face
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45 notch to allow light oncardboard with donut hole
Peep Hole
The Cheshire Eyepiece
Elipical donut hole
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Donut as seen from end of Cheshire
The Field Stop is the OD of the donut
Peep Hole
Field Stop
The Cheshire Eyepiece
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When you look into the Cheshire aimed
at my mirror you will seethe red washer over the donut.
Adjust the mirror as well as possible tocenter the red ring on the donut.
Looking intothe Cheshire
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If you make your ownCheshire Eyepiece or main mirror spot
You can make a field stop act as a built-in tolerance
gauge
Make the mirror spot or the Field Stops of the Cheshireper table shown
Now, if you collimate and make the mirror spot appearfully inside the bright field of the Cheshire, you know
that the collimation error is acceptably small and youwill have no trouble with coma.
focal X Xratio (mm) (inch)
f/4 0.22 0.017f/5 0.90 0.035f/6 1.54 0.06f/7 2.4 0.10f/8 3.6 0.14f/10 7.2 0.28
MirrorMirror
SpotSpot
CheshireDonut
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Coma is enemy number one for
Newtonian reflectors...Nils Olof Carlin
What is considered theAllowable Collimation
Erroris very tight for telescopes of f/5 ratiosand smaller.
I think, considering all the possibilities that canintroduce error that it may be almostimpossible to achieve a good collimation.
It is easy enough for my telescope to go outof collimation with just moving it around.I frequently check it.
More on collimation later....
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Putting a center mark on the mirror
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What you need
The tweezers are revised to hold without pressure -you need to open them to release.
A red hose washer is my center marker.
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Tie a weight on one end of a thread.
Tape the other end to the edge of the mirror andstretch it across the top. The weight will put an eventension in the thread.
Center a ruler on the mirror, then....
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The mirror shows a reflection ofthe thread, except when you are
directly over it.
...Center the thread by shifting the weight
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Using a bridge (styrofoam shown here) to rest yourhand, place the center marker under the threads using
tweezers and adhesive (Goop works fine).
Note the tweezers are revised to hold without pressure.
You need to open them to release.
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Center the marker directlyunder the cross threads.
In this photo the marker is a little to far to the right.
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Red LEDs(Light Emitting Diodes)
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Blinking red LEDson my telescope
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Converting anincandescent flashlightto a LED flashlight
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Using the insides of a key chain flashlight.
A red LED (probably 20ma and 1.7v typ to
2.4 v max)
and a CR 2016 3v Lithium Battery, aka buttoncell or watch battery
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