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Posted by Steven Reecy/">Steven Reecy on October 18, 1999 14:50:02 UTC
Hope you can help me. I just built my first (sonotube-style) dob. I keep having the same problem, and I think its my primary mirror. I collimated the dob very well (on more than one occassion), like my factory built Orion 4.5" reflector. I have tried two different secondarys with two different secondary mirrors (its not my secondary), my focuser has plenty of travel, and my tube is oversized with an aluminum cell for minimal tubecurrents. The inside of the tube is painted flat black, as is the spider. The main mirror is also siliconed to the cell, (so no pinched optics).
Yet still, I get the same problem. My problem is more pronounced when looking at brighter objects, especially planets...or at high power above 60x. I should note that I can see the Orion nebula pretty well, and Andromeda looks normal, but both objects are dim.
The problem is that I can't focus stars to a point. As I try to bring a star in focus, the fuzz becomes a fuzzy point, and then gets worse as I continue turning the focuser. Furthermore, When I try to focus Saturn, I get a bright image, with two ghost-like images swirling around it. The moon looks reasonable at 40x, but beyond that, it becomes blurry (like a double image of a poorly registered artists print).
My mirror is an F5.3 10" mirror. (Really, its 9.75")
So my question is...is this a problem with my mirror, or with the coating, or both?
Secondly, my Dob is adjustable, so the mirror could be refigured into a F5.0 through a F5.8, or somewhere in that neighborhood.
If my mirror is messed up, can you suggest someone who might refigure it for me, and give me an idea what the costs would be? (mailing, refiguring, coating).
It should be cheaper than buying a new mirror, I would think, because its almost shaped right.
Any advice or help, or even solicitations, are welcome.
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