Author Topic: homemade truss dobson collimator  (Read 1119 times)

John Pierce

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homemade truss dobson collimator
« on: November 07, 2016, 04:51:21 PM »
I recently acquired a 17.5" Discovery Truss Dobsonian...   Naturally, this requires collimation each time you assemble it.    The first step of secondary collimation is easy enough, I have a decent quality laser... but the primary mirror collimation is a pain with my peep sight cheshire equivalent... I've read about barlowed lasers in the past, and recently found out about Howie Blatter's "The Blug", but didn't want to spend the $75 or whatever on one.   

Hey, look here, I've got an old(!) Orion variable barlow, the kind you slide the lens up/down the tube to go from 2X to 3X...  its fully multicoated... so I remove the lens slide element from the tube, measure it, and use OpenSCAD to build a Blug clone that will hold this barlow...  My friend 3D printed it for me last night, and it works great.

 



I was going to put an o-ring in the groove around the part that goes into the back end of the focuser, but its such a snug fit as-is, I think I'll leave that out.

here's the SCAD file that generates it...

$fn=200;

inch=25.4;    // mm per inch

scale([inch,inch,inch])
{
  difference()
  {
    union(){
        translate([0,0,-3])cylinder(h=3,d=2.3); // main cylinder
        translate([0,0,0])cylinder(h=.2,d=1.98);   // 2" insert fudged to 1.98 for tolerance
        translate([0,0,.2])cylinder(h=.1,d=1.90); // groove for 2" OD 3/32" thick O-ring
        translate([0,0,.3])cylinder(h=.15,d=1.98); // rest of 2" insert
        translate([0,0,.45])cylinder(h=.05,d1=1.98,d2=1.93); // bevel
    }
   
    union(){
        translate([0,0,-.75])cylinder(h=1.26,d=1.175);   // barlow inset, fudged for tolerance
        translate([0,0,-3.0])cylinder(h=3.5,d=.2);      // laser hole
    }

    translate([1.25,0,-3]) rotate([0,45,0])cube(3.5,true);      // 45 degree truncate
  }
}

Marko

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Re: homemade truss dobson collimator
« Reply #1 on: November 07, 2016, 05:01:16 PM »
For any new scope I get in this catagory I recommend a full check as follows.

- Get hold or make a sight tube and this will allow you to properly position the secondary mirror in terms of distance from the primary and rotation in 2 dimensions.

There are two things to be evaluating here.   

1) You must be able to see the full secondary edges centered in the view through the sight tube.  This is assisted by placing a white paper on the other side of the secondary cage or as I did paint with whiteout or white paint 4 or more dots at the very end of the secondary metal housing IF you have such a housing.  Of course never paint on the actual secondary, use paper behind it to see it's edge.

2) You must be able to see the full primary edges also within the mirrored part of the secondary or you have to adjust tilt and rotation of secondary.  If you do have to adjust tilt or rotation you may have to redo part 1 above in an itterative way till you obtain 1 and 2

Only after 1 and 2 are setup need you even try to do typicall normal field collimation methods using laser or cheshire (which I like).

Often people jump right to collimations and if 1 and/or 2 are out it is often a hopeless situation in some way or another.

Marko
Let me roam the deep skies and I'll be content.

John Pierce

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Re: homemade truss dobson collimator
« Reply #2 on: November 07, 2016, 06:57:15 PM »
yup, already done that.   I have the peep sight that came with my Farpoint-made Orion Pro laser (a model Orion no longer sells).

last night I finally straightened a pair of the truss brackets that I'd slightly bent the first time I setup the scope, so hopefully now there will be better consistency from setup to setup.   next time I set it up, I'm numbering the poles, too, so I always set them up in the same order/place.

« Last Edit: November 07, 2016, 10:19:31 PM by John Pierce »

John Pierce

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Re: homemade truss dobson collimator
« Reply #3 on: November 12, 2016, 10:43:01 PM »
the other night I setup the scope in my hallway, and went through the whole collimation again.   as I mentioned, I'd straightened two of the angle bracket ends that are on the truss rods, and indeed, the upper tube assembly went on much easier than it had last time when they were still sligthly bent (which had the effect of making those two truss rods about 1-2mm too long)....    double checked the secondary centering, and it was a bit rotated, so I tweaked it back as best I could visually, did the sheet of paper thing to verify the secondary was centered in the peepsight...

setup the secondary adjustment using my laser, rotating the laser while holding it against the focuser seat made a 2-3mm circle, about the size of the hole in the donut sticker.     then popped in my polyBlug, and WOW, that was easy to adjust the primary, a couple quick tweaks and it was spot on, so I double checked the secondary adjustment, gave it a slight extra tweak, double checked the blug, good to go, so put my peepsight back in, and the scope was spot on centered like you see in the drawings.   I also numbered the legs so I'll be reassembling them in the same order next time, lets see how close the collimation is when I do that.

I need to get it out for a star test, I think our next star party is in 2 weeks.
« Last Edit: November 12, 2016, 11:07:44 PM by John Pierce »