For Collimation: Big Doughnut, Big Mistake... to collimate an SCT you need to collimate on the (tiny) diffraction rings at increasingly higher magnifications, not the big doughnuts. Don't even start with a doughnut unless you are going to eat it. They don't do anything for collimation and just make you fat in the meantime.
Bob's Knobs rule.

David Moody

>
> Date: 2004/11/18 Thu AM 09:30:32 EST
> Subject: [M]: Collimation (2nd try)
>
> I seem to be having trouble with posts getting through, so
> will try again...
>
>
>
> I was aware that collimation was important with SCT's, but I
> never realized how important until recently. I'd been
> struggling to get proper focus and accurate tracking in
> order to get good round small well defined stars on my
> astrophotos, and I just wasn't getting the results I was
> hoping for. In other words, I hadn't taken care of first
> things first. I'd done a text book (Meade manual)
> collimation a while ago, but what prompted me to check again
> was a chance photo (digital) of a bright but very unfocussed
> star. Out came the big classic doughnut, and there it
> clearly showed the off center "hole". That's when I also
> discovered that using a photograph rather than a diagonal
> and eyepiece was easier, and more precise. For one thing,
> eyeballing that doughnut and judging whether it's properly
> centered is a judgment call that leaves me wondering if it
> really is as centered as it can get. Having a photograph,
> and being able to zoom in on it and literally measuring the
> pixels when it gets down to the fine points leaves me
> feeling a lot more confident that yes, that hole truly IS as
> centered as it can get. I'm sure some of you more
> experienced ones know this, but I also discovered that the
> center of the photograph (or eyepiece view for that matter)
> is not necessarily the place where the doughnut should be
> examined for proper centering of the hole. Moving the star
> to the edges of the photograph caused the hole to be off
> center, but triangulation of the direction of the offset
> aided in determining the true center of the field of view, -
> where the doughnut should be set and checked. Again, it was
> a lot easier to do this with a photograph where I could more
> easily triangulate, and then definitively place the doughnut
> at intersection X and Y. When I finished collimating this
> time, the subsequent astrophotos revealed that I'd been
> trying to improve tracking when actually the culprit was
> poor collimation. Suddenly, focusing became a lot easier,
> and the tracking looked like it had improved dramatically.
>
>
>
> I also did a web and archive search for "Collimation" and it
> brings up a number
> of good articles. One thing was clear in most of them, the
> instructions in the Meade manual are not all there is to the
> story. Although a necessary first step, is actually more
> like
>
> a rough collimation. There is more to a very fine
> collimation, and a fine collimation is one of the necessary
> ingredients to get fine results.
>
>
> I maybe should mention at this point that I was using a
> digital camera at prime focus that downloads photographs to
> my computer where they can be easily examined. I don't have
> an illuminated reticule eyepiece. I know now to take a quick
> "collimation" photo check each time before getting down to
> business.
>
> Hope this helps someone else out, and emphasizes the
> importance, and ease, of a quick collimation check from time
> to time
>
> Keith
> W90N45
>
>
> Astronomy Website
> http://www.willowberry.net/keithnk_m42/
>
>
>
>

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