Old 20-01-2012, 07:10 PM
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mldee (Mike)
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mldee is offline
Join Date: Aug 2008
Location: Near Warwick, Qld, Australia
Posts: 514
First Test of C8 with Hotech CT collimator

After reading Marc's excellent DIY article on fixing his C11 collimation and mirror centering, I purchased a Hotech CT laser collimator on IIS. Thanks Troy!

I finally got around to setting it up yesterday in my kitchen to check my C8 in standard SCT configuration, after previously checking and more firmly gluing the primary mirror to the focus bracket and adding foam to provide some rear mirror support. All that had seemed to go OK, but collimation was not great.

Yesterday I did things by the Hotech instructions, which seemed a bit clunky in setting up and overall adjustment technique, so after many hours of thought, I decided to try a different tack today. Key to the Hotech is to initially ensure that its "bullseye" target surface and the OTA are in perfect co-planar alignment, ie, no slant in any direction between them, so the three led beams are shooting dead straight onto the OTA primary mirror, like three stars.

After realising that there were also three perfectly good reflections of the LED's returning from the corrector plate surface to the bullseye, I reasoned that if those reflections returned exactly onto the led's (ie, disappeared into the little led holes, similar to a newt laser collimator) then surely the two surfaces were coplanar. Sure enough, that worked. See pic 5. After also setting the centre cross-hair alignment led to sit exactly on the centre of the secondary mirror holder (see pic 2), I reckoned I had the two things pretty coplanar and centred, and easy to keep an eye on them as well for any accidental movement

If at this point the three beams didn't exit the rear visual-back hole, as seen by putting a white surface (book) behind it, then it was easy to adjust the three secondary mirror screws until they did all appear. Looking down the front of the OTA will quickly show any beam that's hitting the baffle tube edge due to gross misalignment.

After getting the three dots on the book, see pic 3, reinsert the reflecting mirror and voila! you will see the three big reflected dots on the Hotech main bullseye. see pic. Now adjust the secondary screws to make them concentric (see pic 5) and all should be fine.

My only problem so far is that when collimated, the three dots are not centred on the rear hotech mirror (see pic 6). I suspect this is due to angular misalignment of my primary mirror, and would require me to do the same mod that Marc did to his C11. I'll try some star tests before proceeding further on this.

Pics 7 & 8 are just info of setup and book at focus plane.

Might check the RC8 next, if the C8 results are good.
Attached Thumbnails
Click for full-size image (Setup 1.JPG)
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Click for full-size image (Setup centred.JPG)
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Click for full-size image (Book 3 leds.JPG)
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Click for full-size image (Display 1.JPG)
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Click for full-size image (Display 2.JPG)
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Click for full-size image (Mirror 1.JPG)
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Click for full-size image (Setup book.jpg)
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Click for full-size image (Book focussed.JPG)
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