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coated/uncoated objective?

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#1 roscoe

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Posted 03 January 2014 - 10:54 PM

This morning, I took advantage of the Surplus Shed sale, and for $185, am/will be the proud (I hope...) owner of a Jaegers 6" f/10 uncoated second-grade objective. I'd always wanted a D&G f/12 someday, missed out on the Jaegers f/12's at SS, and recently decided that, for my old-guy vision, a chipped, uncoated f/10 was good enough, considering the price.

So my questions: what is the light transmission/ghosting difference between coated and uncoated? Is it cost-effective/intelligent to get it coated? If so, who would I send it to??
Thanks,
Russ

#2 plyscope

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Posted 03 January 2014 - 11:16 PM

Russ you could send it to Majestic Coatings. I think that is where D & G lenses are coated. However I don't think it is worth the cost and the risk. Lenses can be destroyed in the coating process so there is some risk involved.

I think you can make a fine telescope without having to get the lens coated.

#3 mikey cee

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Posted 03 January 2014 - 11:33 PM

Russ coatings are more in the "ego of the beholder" rather than the "eye of the beholder". I used an uncoated R.E. Brandt 8" F/13.3 lens for over 30 years and was pleased. Now I upgraded to an Istar 10" F/11 R35. True this lens is coated but I had no choice....that's what I was having to pay for anyways so might as well accept it. :grin: In a blind test study more than likely most all would fail to tell the difference and of those who could were probably just lucky. It really is just that difficult. Most would be amazed at how much of an objective lens would be covered by a cardboard mask before they could say "stop" I see a difference. Our club used to do that routinely in demonstrations amongst themselves years ago. Some so called experts say the diminishment must approach somewhere in the vicinity of 11%-15% gain before the human eye becomes sensitive to a change. I would absolutely agree with that and then some. ;) Mike

#4 bremms

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Posted 04 January 2014 - 12:04 AM

They are very good. My 6" F10 is my most used scope. Super sharp diffraction rings. E And F stars were easy tonight in the Trapezium and the seeing wasn't great. I'm so happy with mine I bought a 6" F8 with the code and a credit plus a little money.
The 6" F10 is a great all around scope. A D&G F12 may be a little better. N

#5 PhilHerring

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Posted 04 January 2014 - 02:43 AM

I believe that an uncoated surface transmits around 96% of the light passing through it, whereas a basic, MgF2 coated surface transmits more like 99%. With 4 surfaces, this means a total of about 85% uncoated versus about 96% coated.

(BTW, my 6" f/5 was listed as "uncoated", but the flint is definitely AR coated, so you might get lucky...)

#6 GlennLeDrew

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Posted 04 January 2014 - 08:31 AM

The transmission difference is practically of no import. The eye is principally a contrast detector; a 10-15% lower transmission is essentially nothing compared to the eye's huge dynamic range.

What *might* matter is the veiling glare which could result if the curves on one or more surfaces happens to be just right for causing an aperture-filling (or near-filling) ghost reflection as seen from the focus.

One way to initially test for this is to shine a penlight up the focuser, with eye and light beside one another at about the position of focus. You could ease things by using a piece of glass from a small picture frame as a beam splitter. Place the glass ahead of the light, tilted about 45 degrees. You observe the reflection off the glass, as though it were a diagonal. You're looking to see if any reflection fills a considerable portion of the objective. If the spots are small, you *should* have little to fear from veiling glare from this source.

#7 roscoe

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Posted 04 January 2014 - 09:01 AM

Thanks, folks!

I'll wait and see if any surfaces are indeed coated, and try Glen's test....ghosting is something I am a bit concerned about....but hopefully the folks at Jaegers took this into account during their design phase....
R

#8 mikey cee

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Posted 04 January 2014 - 10:52 AM

Like I said I never had any issues nary a bit of ghosting was ever seen. ;)Mike






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