Reading Collimation Pattern

I recently read the thread about aligning the focuser of a refractor to the lens. So tonight I tried it with my ST80. The two circles were off. So I loosened the three screws that held the focuser to the tube and was able to get the two circles to line up a lot closer. I still need to get the slop out of the draw tube and it should be even better.

Here's my question... I also tested my Meade 390 an noticed something completely different. I saw 3 circle patterns. One was violet-purple, one was clear and the other was smaller and clear positioned between the other two. The pattern was positioned from 10 o'clock, centered, 4 o'clock.
I loosened the focuser to see if I could get them lined up and I couldn't. What's causing this? Could the doublets be positioned incorrectly in the cell?

Thanks in advance,

Tim Hornbeck

Timothy,

Although I can't answer your question, I seem to have the same 3 circles that you see. This is a Meade DS 90 f/11, very similar to your Meade 390 f/11. I too, cannot get the circles to line up either. Same exact position "10, centered and at 4 o'clock" Is their anyone that can help us with this ? In most refractor doublets I see only two donuts not three. But I do get some very nice detailed views of jupiter with it. Hardly any false color, I use it alot.....

I'm no expert, but this is how I aligned my 4" refractor and a friends 4".
I used a laser to shoot a beam out the front through a piece of paper cut to fit the lens. A small hole was cut in the very center of the paper. Then adjust the focuser to center the laser in the small hole. This ensures your focuser is square with the lens.
Then I used a chesire tester to check the colimation of the objective. That is where you see the donuts and you have to move the colimation bolts on the objective to correct any displacement. If you don't have collimation bolts, you would have to check that the elements are square, not cocked somehow, and not held too tightly by the retainer ring.

Walt

Found this from a Google search:

http://www3.telus.net/public/dfell1/astro/collimate.htm

SNIP ...
Some light will be reflected from each lens surface within the objective, although the reflection from the rear surface will probably be brightest. If the elements of the objective are accurately centered and collimated with respect to each other and have no wedge, all the reflections will coincide, forming one reflected spot. If there are multiple reflected spots, it is an indication of alignment problems within the objective. The collimator could then be used to diagnose and correct the inter-element misalignment.
SNIP....

This answers my question. I am going to try and rotate the objectives to get those two rings to join. I know someone with an auto-collimator device that aligns objectives. I may have them look at it too.

- Tim