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How am I doing with collimation?

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Hi Lee, the secondary looks about right, but the primary is off a bit.

Using just the primary adjustments, see if you can centre the 'doughnut'

I always finish off with a star test myself.

HTH, Ed. Edit - ABs example does show the secondary offset with

a fast mirror.

Edited by NGC 1502
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Using just the primary adjustments, see if you can centre the 'doughnut'.

Something I should be able to do while star testing I presume. I have to say I am losing a whole lot of confidence in this laser coli that cost me 55 quid :) It reckons I am bang on, it did last night too, but defocussing showed it up clearly.

Edited by palebluedot
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The issue with laser collimators is ensuring they are themselves collimated but also that they are set into the EP holder square and central.

Have a look at this: http://stargazerslounge.com/primers-tutorials/84752-barlowed-laser-collimation-method.html, it is a good thread on barlowed laser collimation which seems to bypass issues with the laser needing to be collimated and square / centred in the draw tube.

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Thanks for the advice guys, I am pretty sure I have this licked now, but I am happy to be told otherwise.


This time I used the laser to collimate the secondary as well as the primary. I was using another method of aligning the secondary, which I clearly have not mastered.

So to sum up what I did:

  • Measure distance of secondary from tube and adjust to centre it using thumb screws on the tube.
  • Look down tube and adjust angle of secondary to ensure red dot falls within the marked centre of the primary.
  • Adjust primary until the laser dot reflected back into the collimator hits the centre of the reference plane.

Have I missed anything?

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I'd not read Astro-Baby's tute on collimation until just now, boy its heavy going IMHO - just be aware that all you are doing is getting two mirrors in line with each other and centred. The wording makes it sound like there is something hard about this, or somehow challenging. Don't beat yourself up - you've got it covered mate!

One thing however to bear in mind is that on fast ratio reflectors the collimation zone can be a mm...here's list of sweet spot diameters at various f ratios:

f/4 1.4 mm

f/4.5 2.0

f/5 2.8

f/6 4.8

f/8 11

f/10 22mm

With this in mind, go easy on big tweaks on your scope, and you'll be fine.

Its really sounds like you have got it. Let us know what you see next!

When I designed my telescope, I used a f 7.7 mirror as everything is relaxed.

Edited by Trull
add f ratio sweet spots.
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Hauled myself out of bed at 2am this morning to test the collimation. Boy was I not disappointed. After the scope had cooled down, I still noticed plenty of atmospheric turbulence during the star test, but that did not prevent me from having getting some stunning views of Saturn, couldn't quite make out the Cassini division, but I am pretty sure I could make out the darker cloud bands and 2 of the moons. Very crisp indeed.

No more fuzzy blobs for me :icon_eek:

Edited by palebluedot
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