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Webster Telescopes Forum • View topic - Owner's Manual Collimation Part 2
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 Post subject: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 5:39 pm 
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Location: Detroit Michigan
This is part 2 to the new Owner's Manual Collimation Section - Laser Collimation.

Again, we are going to avoid any confusingly drawn diagrams, formulas, and deep theory. Every illustration is a real photo of exactly what you will see.

We are assuming that you are using the two standard tools of Laser collimation: The Howie Glatter 2" laser, and the B.L.U.G.

Although it will take you several minutes to read this manual, the actual collimation takes less than one minute.

Like always, feedback is welcome!

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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 5:46 pm 
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Here we have the standard Glatter two inch "holographic" laser. If you still don't own one, get the 635nm unit as it is much brighter and allows you to collimate in daylight.

We have fitted the laser with the included 1mm Aperture Stop. The Aperture Stop makes the beam smaller so it will fit into the center target on the mirror. Naturally the beam expands with distance, so the Stop really helps on those old 36" f/5 with their 180" focal lengths.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 5:50 pm 
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We install the laser all the way in until the shoulder is flat against the focuser. The Feathertouch focusers are really well made so it probably does not matter here, but it is a good habit to get into as some focuser thumbscrews cause the laser to tilt off center if they are not flat against the drawtube.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 6:27 pm 
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This picture is not an actual collimation step, but it will get some definitions out of the way and will make the next steps clearer.

First note that we have 4 collimation knobs on the back of the secondary holder. It would seem that a 3 knob unit would make more sense (as 3 points define a plane), but once you have owned both styles of holders, you will never go back to a 3 knob - the 4 knob models are much easier to control. A famous author from a national astronomy magazine used to downgrade scopes with 4 knob secondary holders. When questioned, he would always say he "had tried them both, and 3 knobs are clearly better". One day he bought a scope with a 4 knob unit and it forced him to really "try them both". He now only recommends 4 knob units. I'm not going to call him out in public, but I have a few of his old books as proof.

Next, note that the secondary holder is not centered under the spider. This is called Offset. When you read books about collimation, they always say "You can offset your secondary, but you don't really have to". These books must have been written before the fast (f/3.6 or faster) optics revolution hit the Dobsonian market, because if you don't offset the secondary, the light cone can collide with the inside of the UTA. On slower telescopes, the offset might be 4mm and not concern anybody. But on a large fast telescope the offset might be .6 inches! If you want a rule of thumb to live by we can say that if a scope is slower than f/4 it probably does not need the offset holder.

We get emails asking why we don't just make the spider itself offset, but if you do that, you end up with thicker diffraction spikes. The spikes in a 4 vane Newtonian end up reinforcing themselves. If you can't picture it in your head, think how a 3 vane spider gives SIX diffraction spikes at the eyepiece.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 6:47 pm 
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OK, back to collimation.

Now turn the laser ON and look at the Target in the center of the primary mirror. Look for the laser beam spot.

If you mirror is really clean, there might not be enough dust to see the laser spot off axis. Stand behind the secondary holder and look down to see it.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 6:55 pm 
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Now we need to move the laser beam into the center of the target.

The collimation knobs on the back of the secondary holder work in Push-Pull pairs. You turn one knob clockwise while you simultaneously turn the other counter-clockwise. This sounds complicated, but in reality it becomes second nature after a few seconds.

Turn the "A" pair of knobs until the laser beam is centered horizontally with the target. Turn the "B" pair of knobs to move the laser beam vertically.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 6:57 pm 
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Using the A and B knob pairs, center the laser beam in the target.

If you have a really long focal length telescope, or if you are collimating in full daylight, do not be afraid to walk up alongside the mirror box and get a closer look.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 7:02 pm 
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Leaving the laser turned on and still in the focuser, now install the B.L.U.G into the back of the focuser.

The BLUG does two things, it expands the laser beam so it lights up the entire Target, and it gives a display to center the Target shadow on.

Walk around to the rear of the telescope and put one hand on each of the collimation knobs. Turn the knobs SLIGHTLY and watch the reaction of the shadow of the target on the BLUG face.


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8-SHADOW-OFF-CENTER.jpg
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Last edited by Eric on Thu Apr 28, 2011 7:54 pm, edited 1 time in total.
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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 7:03 pm 
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Here we see the Target shadow almost centered on the BLUG's hole.


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 Post subject: Re: Owner's Manual Collimation Part 2
PostPosted: Thu Apr 28, 2011 7:21 pm 
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Now you see the Target shadow perfectly centered on the BLUG's hole.

You are done!


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