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Yes...another collimation question?

Started by bobblack, 02/24/2009 08:40AM
Posted 02/24/2009 08:40AM Opening Post
Having just spent 3 evenings trying to collimate my LX90 8" SCT I have decided to try a collimator. The question is what would be the best and easiest. I don't want to buy two or three to find the best one.
By the way, if it won't collimate, could there be another problem.
I met a person in my campground who has been in the hobby for years and he couldn't get it any better than myself.
I would appreciate any thoughts you might have on this subject, regards, bob
Posted 02/24/2009 10:04AM #1
The collimators that are available for scts really don't work the way they do for Newtonians. You're really best off with a star.

It shouldn't take you 3 hours to collimate once you have a better idea of what you're doing.

One problem people have is deciding which screw(s) to adjust. One way to find out is to take a half inch stick, or your straight hand, and run it from the adjustment screw to the edge of the corrector (don't touch the optics). On the out of focus star you will see a shadow which indicates this radius.

Now look at how the black hole in the out of focus star is off center. One of the three possible shadow positions will describe a diagonal that lies closest to the out of center hole. That is the screw you adjust.

Typically you work with two screws and leave the third alone. If you're spending three hours you may be going back and forth undoing adjustments that were otherwise good. The other problem is that the seeing may not give you a good "doughnut". The answer to this dilemma is that you can collimate as well as the seeing permits. Estimate the approximate shape of the deforming out of focus star and center the secondary shadow (black hole) as best you can. On a steadier night, touch it up; on a bad night where you can't collimate, you won't be able to see much fine detail on planets anyhow. The other thing to do is check it out again after a few hours as a hot mirror simulates the problems of bad seeing.

If I could figure out how to draw perfect circles with a black hole inside I would put an article up on this.

I'll have to think about it. Right now my regular desktop is down for repairs and I don't have all my usual resources.

keep the faith,
Greg N


"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin
Posted 02/26/2009 08:36PM #2
robert black said:

Having just spent 3 evenings trying to collimate my LX90 8" SCT I have decided to try a collimator. The question is what would be the best and easiest. I don't want to buy two or three to find the best one.
By the way, if it won't collimate, could there be another problem.
I met a person in my campground who has been in the hobby for years and he couldn't get it any better than myself.
I would appreciate any thoughts you might have on this subject, regards, bob


Make sure the scope is cooled down enough.... Especially an SCT. Check out this webpage that shows the effects of tube currents which may occur if the scope has not had a chance to cool to the outside temperature. The result looks almost like miscolumnation to the untrained eye....

http://astro.geekjoy.com/scopes/tube_currents.html

It is what it is...