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William optics SD66 needs collimating - but how?


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I always had the feeling that my William Optics SD66 frac was not collimated as good as it could be. I have now obtained a william optics SCT 2" adapter which has allowed me to insert my Hotech laser into the back of the scope.

I know that the Hotech is well collimated by noting that the laser dot does not move when it is rotated.

Anyway when the Hotech is inserted into the back of the frac I see that the laser dot is about quarter of an inch away from the centre of the main lens. I assume it should be in the centre!!!

Perhaps I can adjust the focuser to bring it in line? Any thoughts?

Mark

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Hi Mark, I have just googled "collimating a refractor" and lots of links came up.

Perhaps one of them could help. But if the problem is misalignment between the

objective elements ( I'm not saying there is, just a possibility ) then other than

proffesional help, I don't know. Good luck, Ed.

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Good news....

What you describe shows the focuser is not aligned to the optical axis. Loosen the screws holding the focuser to the OTA and tilt the focuser to bring the laser dot central and then re-tighten.

You'll also need a Cheshire and a bright LED torch to check the alignment of the objective.

Ken

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Thanks Ken for that information. I have now removed the screws holding the focuser and re-adjusted the focuser so that the laser dot is now in the centre. Phase one complete :rolleyes:. Currently I don't have a cheshire. I assume that the LED light is shone down the front through the objective and will create something onto the cheshire target area - am I correct?

As an alternative would a mirror place over the front of the objective create a similar exercise test?

Mark

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Small fracs usually don't have any provision for collimating the objective. If you have V shaped star images at focus or the out-of-focus image of an on-axis star is assymetric even though the focuser is properly squared on, the scope will have to go back to the manufacturer.

It's actually quite rare to have a significant collimation / centring error in the objective of a small 'frac, unless it's been dropped or used as a baseball bat.

Even if you do have collimation adjustment in the objective, if you don't have a very high quality first surface mirror at least equal in size to the objective (which will cost you a considerable amount of money) star testing on a night of good seeing is the best way to do the adjustment. You shouldn't have to wait too long to get a night with steady enough seeing to star test a 66mm scope.

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I had this issue with my 6" F/8 achro - I used a cheshire eyepiece plus star tests to determine this - the cheshire does help diagnose things:

Refractor Collimation

As my scope was an early model with a non-collimatable objective lens cell (a design that I notice the latest versions of this scope have gone back to now) I tackled the issue like this:

- Loosen the objective lens retaining ring, point the scope vertically upright (objective lens pointing up !), and gently "slap" around the sides of the lens cell to settle the objective lens. Then, gently, tighten the retaining ring but only "touch tight".

- Check that the objective cell assembly was properly mated onto the scope tube - when I examined it I found that mine had a half turn left on the threads.

- Check that the focusser was properly squared on to the tube.

- Check that the visual back (where the diagonal goes) is fully tightened onto the focuser drawtube - mine had a couple of turns left and I reckon, because of the lowish Synta machining tolerances, this imparted a slight tilt to the visual back, and anything plugged into it.

Not all the above will apply to your scope of course but the result for me was that the scope went from noticably out of collimation to collimated so it's worth at least checking these things.

Edited by John
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Mark,

So far so good...

The Cheshire actually illuminates the rear of the objective, not the front.

A series of concentric doughnuts/ circles should be seen if the objective is square to the optical axis.

This test does not show if the objective is good or bad ( you'd need a Rochi screen or star test) but ensures that the objective is not tilted or misaligned.

Ken

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