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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
And it can easily be flexed to a parabola later, to sharpen things up 
Owen
Absolutely! 
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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I reiterate. Choose a focal ratio that can be functional as a sphere. Attempt the parabola. If successful you have a fine optic and can confidently consider making a larger mirror. If the results of a parabola are not good, then spend a few minutes returning it to a sphere. Try the parabola again but only go ½ or 2/3 of the way. This will yield a mirror that is much better at high power than if left spherical and avoids having to hit the correction spot on. If all else fails, polish back to a sphere. You are no worse off than if you had settled for a sphere at the outset and you have knowledge of the issues in making and testing a parabola.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Ed wrote:"your mirror will be elliptical but will be 1/10 wave accuracy and would be an excellent mirror. This is perhaps wat Mark was saying. "
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Yup, that's about it. A smooth mirror that's slightly undercorrected should perform very well. Especially if you get one of those clear nights where the temp is dropping constantly all night. The mirror tends to go slightly deeper in the center, etc. adding to the correction a bit. Such a mirror is ideal for that scenario.
Mark
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So many projects, so little time!
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