Is there any advantage in having complete darkness in an autocollimator?

[pat]

I have a xt10"f4.7. I had bought an AC thinking that it would give me perfect collimation when it was completely dark inside.

The best I could do was half dark and half bright when looked through the peep-hole.

Latter I got somewhat better at collimation. The view now is completly dark at all times.

Now I've read that this dark view is not completely true.

Does this mean the darkness of the AC is not a facter in judging the performance/collmation of the scope? Pat

[umadog]

Ok, the autocollimator is very useful and very accurate but confusing at the start. The first thing you need to know is that the autocollimator being dark means that the collimation is vaguely in the right direction. Darkness does not, not, mean you are collimated. Once you can see all four reflections then you know that you're roughly correct but still far out. You need to stack the reflections to be collimated.

What brand is the tool and did it come with instructions saying that darkness means everything is correct? IIRC there is one brand that is notorious for saying this. The Catseye ACs are known to be very good. You should make sure yours is working correctly by stacking the reflections then rotating it in the focuser whilst looking through the pupil. There should be minimal unstacking of the reflections as you do this. If there is significant unstacking then the autocolimator's mirror isn't at right angles to the drawtube of the focuser and the device should be returned to the manufacturer. Removing and reinserting the tool should provide consistent results.

You should spend some time digesting the first page of this thread: Telescope Reviews: Concise thread about autocollimators+improvements

[Tinker1947]

This sounds like something i don't want to think about..

[umadog]

I know, I know... It's actually not as bad as it first looks and it's much more accurate than the sight-tube crosshairs. My procedure for tweaking the collimation is to adjust the primary tilt with a cheshire then adjust the secondary whilst looking through the autocollimator. I get it as best as I can (if the primary tilt is out the reflections won't stack) then go back to the Cheshire. Once the primary and secondary are dialed in the reflections will stack. A better technique is known as the "carefully decollimated primary", which allows you to skip the iterative stuff with the Cheshire. It should be more accurate too, I would guess. I've not bothered with that approach yet, however.

[pat]

At best I can remember the vender told me that this complete darkening would suffice. Since Its been sometime back when I bought it. The vender is unknown and the AC is a single puple. The stacking part and the triangles in your link are somewhat confussing to me. I would hazzard a guess that "stacking" reflections means everything is concentric. Pat

[umadog]

Then I'm afraid the vendor has no idea what their talking about. You'll want to really check that the tool works before you trust it.

The mirror centre in the link I sent was marked with a triangular centre spot. You should see 4 circles instead (I assume that's what you've marked the mirror with). Stacking means that you tweak everything until all of the reflections overlap and you see just one triangle/donut. The thread contains a movie of the triangles stacking. I know it all looks very esoteric but once you see the reflections for yourself, and watch them respond as you turn the knobs, it will all start to make a lot more sense.

[pat]

Thanks for all the good advice and the donut explaination. That hit the nail on the head. I just hope that the slight play/slop in my older Orion focuser won't throw the AC off (I've taken all-complete advice to fix it ). If it's such a precision tool, I'm afraid I wasted my money on a venders advice. Pat