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Dobsonian Mirrors

Started by astroboy314, 11/04/2004 12:52AM
Posted 11/04/2004 12:52AM Opening Post
Okay, so I'm thinking about a Dob now...

I have a quick question about mirrors. I have gone over past posts, and realize that everyone will have different opinions about the mirrors in top quality Dobs (Obsession, Starsplitter,...), but I noticed that they list different coatings. The Obsession mirrors claim to have an enhanced primary coating at 96% and 98% (primary and secondary, respectively), while Starsplitter lists 88% and 96%. Does this mean that the Obsession mirrors will give a brighter image? Am I missing something here?

Thanks!
Posted 11/04/2004 05:48AM | Edited 11/04/2004 05:49AM #1
Yes, the more enhanced mirrors will give slightly brighter images (note that the percent increase is small enough that you would need to have the scopes side-by-side to tell the difference). The enhanced secondaries are used by most of the big dob makers, but some feel that enhanced coatings on primaries degrades the image quality (by slightly altering the figure of the mirror).

It seems to be a QC issue - Zambuto stopped using enhanced coatings because he found that about 1 in 3 mirrors had problems with the coating and had to be recoated. Since that comes out of his pocket (he retests the mirror after coating), he decided it wasn't worth the improvement in reflectivity.

On the other hand, that means you have a 2 in 3 chance of success, so if you are willing to give it a shot and pay for a recoat if you're not happy, go for it.

It could also be that it was problem specific to one or a few coating companies, and they may have improved their process by now.

Jarad
Posted 11/04/2004 06:37AM #2
David,

What Jarad says is pretty much the answer. However, I would say that you would not only have to have side by side instruments, and perfectly matched everything else, and a lot of experience, and maybe some fancy instruments to tell the difference. In other words, there is not that much difference in the coatings in practice. Where it starts to count, though is when you have multiple surfaces. The difference between 98% x 98% and 88% x 88% is the difference between 96% total transmission and 77%. Curiously, unless you really know how to look, even this is not too noticeable. One compromise is to have the primary with a standard coating and the secondary with an enhanced.

Alex
Posted 11/04/2004 08:27AM | Edited 11/04/2004 08:27AM #3
One thing that has not been addressed is scatter. I have been told by opticians that Fully Enhanced coatings tend to scatter more light than Standard coatings. This is because of the thickness of the overcoats that are used. This scatter can cause loss of contrast, with an increase in overall brightness. The results can be less actual detail and more background light. A good Standard coating will produce about 90-92% reflectivity in the 450-650nm band, with the 89% being below or above this bandwidth. This range is where the eye is most sensitive, so that is all that matters really.
Hope this information helps.
Floyd

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 11/04/2004 04:15PM #4

I think everyone has the right idea as far as the standard verses enhanced coatings goes. Floyd really hit the nail on the head as far as the light scatter goes. I'm not really down on enhanced coatings, but if I was getting my enhanced 20" mirror recoated it would be with standard coatings. The scatter bothers me that much.
Posted 11/05/2004 09:22AM | Edited 11/05/2004 10:45AM #5
Does anyone have experience testing Discovery's premium primary mirrors? Any links to interferometric data, etc. would be very helpful. Discovery calls these mirrors "diffraction limited" but doesn't provide any more info on their web site. Anecdotal info from star testing, etc. would be helpful also. Thanks so much!

Best regards,
Tim