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fetoma
Pooh-Bah
Reged: 09/26/06
Posts: 1098
Loc: NW Ohio
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Hi group,
I have a Meade SN-8 f4 mirror and cell. Can I make a regular newt out of it or does it need the corrector plate to be worth doing anything with it?
Thanks,
Frank
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Frank in NW Ohio
Vixen CF VC200L and R200SS
Meade SN-10
Celestron 9.25" SCT
Meade 8" f/6.3 SCT
Intes-Micro M73
Stellarvue 80/480 LOMO Triplet
Takahashi FS-60C
Celestron CGE
QHY8 and SXV-H16
www.jerseydevilastronomy.com
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Don W
Postmaster
Reged: 05/19/03
Posts: 14623
Loc: Wisconsin, USA
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The mirror is spherical. You need the corrector plate.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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teast
professor emeritus
Reged: 11/10/07
Posts: 500
Loc: Kentucky
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If it's spherical, I believe you could have it parabolized by someone who is experienced in working with fast f ratio mirrors. You could then make a nice RFT with it. Just my 2 cents.
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fetoma
Pooh-Bah
Reged: 09/26/06
Posts: 1098
Loc: NW Ohio
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What's the difference between spherical and parabolic?
Would it be expensive to have that done?
Or, should I get a corrector plate from Meade?
--------------------
Frank in NW Ohio
Vixen CF VC200L and R200SS
Meade SN-10
Celestron 9.25" SCT
Meade 8" f/6.3 SCT
Intes-Micro M73
Stellarvue 80/480 LOMO Triplet
Takahashi FS-60C
Celestron CGE
QHY8 and SXV-H16
www.jerseydevilastronomy.com
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teast
professor emeritus
Reged: 11/10/07
Posts: 500
Loc: Kentucky
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I don't know what the cost for either option would be - maybe someone here could shed some light on that.
As for spherical vs. parabola, a parabola brings all the light to a focus at the same point/plane (take your pick of terminology), whereas a spherical surface does not. At high f ratios (slow scopes) there is very little difference between the two, but with low f ratios (fast scopes) the difference is significant and the parabola is what you want.
Most, if not all, modern, well-made Newtonians have parabolic mirrors. It's possible that some of the smaller diameter Newtonians are spherical but they also probably have high f ratios - 10+.
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teast
professor emeritus
Reged: 11/10/07
Posts: 500
Loc: Kentucky
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Post a new message in this forum asking about having the mirror parabolized and see what turns up.
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teast
professor emeritus
Reged: 11/10/07
Posts: 500
Loc: Kentucky
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By the way, having the mirror parabolized would give you something pretty similar to the 8" f/4 Orion Skyview Deluxe listed in your signature line.
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6090
Loc: Singapore
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Having the mirror "parabolized" would mean re-grinding it. This would destroy the surface, and coatings. You might as well buy a new mirror from scratch. Also, Meade will not sell just a corrector plate.
The reason Meade uses a spherical vs a parabolic mirror on the SN, is the cost of an f/4 parabolic mirror is higher, and since Meade can make a custom corrector plate cheaply, it is the most economical way to get a fast (f/4) mirror with less coma than an f/4 parabolic mirror.
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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teast
professor emeritus
Reged: 11/10/07
Posts: 500
Loc: Kentucky
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Is it really necessary to re-grind the mirror when it's already (supposedly) spherical? Everything I've ever read about creating a parabola states that obtaining a polished spherical surface is the last step before creating the parabola. (I've started on a mirror but not completed it, so if I'm mistaken I need to know  )
Yes, the coatings (protective and reflective) would have to be removed (and then re-coated afterwards), but that can be done using non-abrasive techniques. Mirrors are routinely re-coated and this doesn't involve re-grinding the existing surface, whether sphere or parabola.
You may be correct that it is less costly to buy a new mirror but I would not do so before at least checking out the feasibility and cost of turning the sphere into a parabola.
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