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Reminds my of the old saying..."one robin does not herald Spring..."

n of 1 is not a significent sample...period.

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Hi Tim,

I would say you have to test them on the planets. I have 13.1" dob yep old coulter but Carl Zambuto refigured the mirror in 1995 and it's 1/11th wave where before it was 1/2 wave. My scope performs better on planets 312x is easy and have pushed it to over 700x before. It rivals 6" APO refractors on planetary detail. It also does better on globulars and detail on nebulas.

I am not surprised the discovery scope was brighter it has more aperture in the review and more surface area. Carl Zambuto does make some fine mirrors and never seen a bad one. You can get to that 50x magnification per inch if the seeing is right with his optics. But with a dob without tracking sometimes that is hard to do.

Good article enjoyed the review. One of our club members has a discovery premium dob 17.5" and enjoys it and it does have good optics too.

James Bielaga

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I have read several 'good' reports as to Discovery's mirror quality, however the 14" I bought several years ago is nothing to brag about. Oh its fine for deep sky, which is what I use it for, but fails badly on planetary.

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In the ones that were optimized, though, I've generally seen that the star images are a little tighter and have a little less "fuzz" in the edges and/or diffraction rings in the Zambuto-mirrored scopes--obviously less light scatter.

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Justifying Zambuto's decision not to use enhanced coatings. Apparently his rationale for using only standard coatings. Or at least that is what he used to say. Is he still recommending standard coatings only? I belong to the ZOC yahoo group, but have not kept up with the most recent posts.

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Carl Z told me: "We use a standard aluminum with one layer of silicon dioxide overcoat. Its a good coating. Provides 92% in the visible
and it preserves our contrast."

I believe one concern might possibly be that enhanced coatings don't last as long and are harder to remove for recoating, sometimes requiring refiguring (which defeats the whole purpose for the excellent figure) The single overcoating keeps the micro finish from getting too bulky / lumpy? and that would defeat contrast.

Darrell Abrahams.

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Carl may be right about the multi-layer aluminum "enhanced coatings" of the 1980s. But today's dielectric-layer enhanced coatings (usually 2 or more transparent layers added to the silicon dioxide) will not fail earlier than a standard coating.and are no more difficult to remove. The thickness of the coatings will still be in the small number of nanometer thickness area, so what it really boils down to is the quality of the coating, and most coaters who offer enhanced coatings are pretty good at it. The ones we've tested here were very good.

What is a matter for debate in the physics arena is the nature of how multiple dielectric layers handle off-axis light. There is the possibility of increased scatter and constructive/destructive interference with off axis rays.
I haven't seen lab reports that show spectral results or photometry of light scatter, so I think those "reports" are anecdotal and suspicious from the standpoint of the physics of reflective surfaces and likely to be related to a justification to keep costs down rather than a true examination of the results of the enhanced coatings.

From the standpoint of failure of the aluminum surface, enhanced coatings should be MORE durable than simple silicon monoxide (which becomes silicon dioxide) overcoating.

What is true, though, is that the slight difference in reflectivity for the enhanced coatings would be sub-visual (i.e.not noticeable), and might not be justifiable from the cost standpoint.

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.... so what it really boils down to is the quality of the coating, and most coaters who offer enhanced coatings are pretty good at it. The ones we've tested here were very good.

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