Hidden Hollow 2012 - Pre-Talk

7/31/2019 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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Hidden Hollow 2012 - Pre-Talk

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