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Webster Telescopes Forum • View topic - Cutting Truss Poles to Correct Focal Length
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 Post subject: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:40 am 
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To find the correct focal length on most truss Dobs, you keep cutting off a 1/2" of truss at at time, slowly working your way down to the point where your eyepieces come to focus. If you overshoot and cut the trusses too short, you get to buy another $400 of aircraft aluminum, and start again.

Our hardware lets you find the focal point without any cutting. There is no risk of overshoot. In fact, some customers just leave the trusses a little long so they can have some extra play in the future.

Here is our guide to the easy way to find focal length.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:42 am 
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First, start with your trusses a few inches too long. You can always cut a few inches off, but you can not add them back on.

In order to use the Index Pin, leave 1" of extra material proud of the bottom of the truss clamp. A Measuring Square makes quick work of this task all the way around the scope:


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:46 am 
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At the UTA (Upper Tube Assembly), start with the truss poles at full length. Of course, all of the truss poles need to be the same length when you start.

No need to crank down on the clamps, just gently snug them up.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:52 am 
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Using a laser collimator, do a rough collimation.

Roll the scope outside and aim it straight up at zenith. The scope does not have to be aimed at anything, there are plenty of stars up there.

Place your most infocusing eyepiece into the scope. We use the 31mm Nagler along with the Paracorr as our worst case scenario.

Rack the focuser in and out, and you will see the blobs of unfocused stars get smaller as you rack inward. You will quickly get a knack for how much you need to lower the UTA clamps to get the stars to focus.

Cover the mirror (in case you drop a tool), loosen the UTA clamps, and start lowering the UTA.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:56 am 
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In about 2 or 3 tries, you will have the stars focused.

Now look inside the UTA and see where the focuser tube is in relation to the back of the focuser board. We want the Focuser Draw Tube to be flush with the backside.

In this example, we still need to shorten the truss length about a 1/2".


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 2:57 am 
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In this example, we have overshot our length and we actually need to lengthen the trusses.

Note, because we have not done any cutting yet, our overshoot has not caused us any monetary loss.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 3:04 am 
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In this example, the truss length is correct and the Draw Tube is more or less flush with the backside of the Focuser Board.

Why is this the correct position? Because the Draw Tube is not protruding into the lightpath, and since we know the worst case eyepiece combo comes to focus, all the other eyepieces will focus backwards from that point with plenty of travel.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 3:12 am 
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At this point, double check that all of the trusses are tight at the exact same length. From the inside of the Mirror Box, tighten the bolt that holds the bottom clamps in place. This will fix the truss angle (with no math needed!).

Do a real collimation and make sure that your eyepiece comes to focus with the Draw Tube flush.

You can now safely cut the truss poles without worry! Install the Retention Pins and your scope is ready to go.

If you really want to earn extra credit, now that the scope is complete and collimated, you can adjust the sling to match the exact Center Of Gravity distance from the back of the mirror. It is probably pretty close, but why not take 2 minutes and really get the maximum performance from your mirror?


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Fri Feb 04, 2011 3:17 am 
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Hopefully this makes sense to everybody. Post any questions and I'll keep revising the text and pictures until it does.


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 Post subject: Re: Cutting Truss Poles to Correct Focal Length
 Post Posted: Tue Apr 12, 2011 2:31 pm 
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Jim,

Hey, very nice explanation. However, I thought that if I buy a scope from you guys, you would already have cut the truss poles to the correct length as part of the build and I would not need to worry about that. And the COG of the mirror in the sling would be done already also.

Is this just for those who change to a different primary mirror (of the same focal length) and need to make the truss poles lengths match the new mirror, since every mirror will have variation in focal length?

Also, presuming you cut the poles to the correct length as part of the build, do you always use this process you have outlined here. So, even if I don't have a Nagler 31mm now, my scope will come to focus with this lens in the future if I buy one?

FYI, I'm slowly working towards having Webster build me a 20" f/5. This is going to be a long journey, but the dream will come to life some day!

michael


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