Enhanced coating vs. Standard Coatings and 6%

I have been having a long discussion with another person concerning the claims being made about the 6% difference between Enhanced coatings and Standard coatings. Some people are making claims of great differences in effective aperture and the ability to see object that are not seen with standard coatings.
What is bothering me about these claims it this.
If a 14" mirror was coated with 90% reflective coatings and has a surface area of 154 square inches, then a 6% increase in reflectivity would add 6% to the effective surface area ( according to the math) for a total of 9 square inches. This would equal a 14.4" mirror in surface area.
Now even if this were true, the effective gain would be less than .1 magnitude in light grasp.( Doubling the surface area of a 14" scope requires a 19.8" mirror and that only gives you .7 mag increase in light grasp. ) It is my understanding that this would not be perceptible by most observers. Even if it was, .1 mag would not make much difference in what objects would be visible. General rule, a star is 2.512 times brighter than a star one magnitude less. But .1 magnitude is very difficult to judge when looking at two stars, one mag 5 and one 5.1 mag, don't you agree?
So how is it possible for people to state that they are seeing mag 15 and 16 stars under light polluted skies after the Enhanced coatings were applied if the increase in light grasp is no more than .1 mag?
I can not see any real difference between a 14.5" scope and a 15" scope, can you?
I am a bit miffed about this because I believe that there is a lot of hype being distributed concerning this. It seems to me that there just can not be evidence to support the increase in effective aperture and light grasp making visual differences that are being claimed.
What do you think about all of this? :S

Floyd,

If I were strapped for money, I would put the difference between coatings into a better diagonal or better eyepieces or ANYTHING where the $100-$300 really made a difference. The $125 difference paid between UHTC and standard coatings doesn't give you much as far as visual observing goes in my SN8, but it would cut down long photo times a bit. So for photography, possibly worth it. It does nothing for resolution, and your eyes don't know 10% difference in light levels unless switching back and forth quickly and repeatedly. Also, most dielectric reflective coatings that I have checked out get rid of IR, which would hurt photography that needed IR wavelengths. I think that the difference between a cheap $50 diagonal and a $175 diagonal is easily noticeable, and I put the $125 difference into a Stellarvue 2" dielectric diagonal for my LOMO Mak-Cass. I can't tell the difference between straight in and with the 2" Stellarvue dielectric diagonal in place at 400X, but with a cheap prism diagonal it is very apparent there is something else in the optical path. When I can clearly and easily see a difference, plus upgrade to 2" EP capability, then it's worth paying for. You can always get mirrors and lenses recoated down the road if you really like the scope and decide to keep it long term.

Some people buy the enhanced coatings only for resale value later on, and it's a valid point if you like to play around with different scopes and end up trading often. I just see my SN8 as a cheap workhorse that has to stay portable, and if I wanted more light all that bad, I would have spent the extra money for an SN10 for about the difference in coating upgrade price. I spent money on a 2-speed JMI focuser instead, and it is a HUGE difference in practical utility. Driving a short distance to a dark sky site gets me AT LEAST as much performance boost as 4 extra inches of aperture. Most people would do well to upgrade their mount before thinking about fancy coatings, because most mounts can't handle much wind at all before getting shaky. Get in the dark, get focused/collimated and get steady. Then you will start to see things. Installing fans on a big Newtonian will do more for visual observing than fancy coatings, and it costs a lot less. Fancy coatings in astronomy is somewhat analogous to "magic wire" in audio. Wire has to be garbage before it really starts degrading sound. Connectors are where much more audio signal is lost, as it is where breaks take place in the signal. Eyepieces and diagonals represent breaks in the visual signal, and it's best to spend money there first and get it compatible with your scope.

Also, getting a good laser collimator that's actually collimated is more important than fancy coatings if you own a reflector scope, and especially a fast Newtonian. I also used my precision American built laser collimator to align my Mak corrector into the best position. LIGHT THAT IS FOCUSED TO A POINT IS BRIGHTER than light that is spread out over a larger area. I have seen the central star in M57 with my 8" SN8 on one particularly clear and steady night. It's rarely possible, but doable if the scope is well collimated, which my f/4 SN8 is. Nearly a textbook perfect star test,and collimated totally indoors with my old AstroSystems laser collimator built back sometime in the 90's and bought for a lot less than a coating upgrade for a small reflector. A small miscollimation will cost you more light than going from enhanced to standard coatings when looking at very small and dim stars.

Also, lucking out and getting near perfect optical figure is better than fancy coatings on second rate glass. My SN8 has managed to do things that only the best 8" CAT's have done, and it shows a near textbook star test, so why worry?

Phil

Hello:
Actually, the difference in performance should be even smaller than you state. With a 6% gain in reflectivity, you get 6% more light from the star AND 6% more from the sky. A 15" mirror with 89% coatings acts like a perfect 14.15" mirror, whereas one with 95% coatings acts like a 14.62, roughly a 1/2" gain in effective aperture. If this calculation is applied to both the primary and secondary mirrors, then the 15" with 89% coatlings acts like a 13.35" while the one with 95% coatings acts like a 14.25". A difference in an inch of aperture is getting to the point where it can be noticable. I read an account of a side by side test of a 20" and a 22", and the 22" went noticably deeper (I don't know if they had the same coatings, equally effective baffleing, etc.). If you have something like a cassegrain with 3 reflections, then the efficiency of the coatings starts to matter (12.59" vs. 13.88"). The scope may appear to go deeper just after recoating since the coating is new and clean, and doesn't have all of the little scratches, pin holes, dust and all of those other things that scatter light that the mirror had before it was shipped off to the coaters.

Cheers
Mike Connelley

>>>What do you think about all of this?
----

Floyd... Personally I think it would be very difficult to do a side by side with the old coatings and with the new coatings. In the bicycle business, all sorts of small changes are reputed to make large gains in speed but even real improvements only make small differences.

My thinking...

If both mirrors are coated with with 96% coatings then my math says this provides a 16% better "through-put" than a scope with 89% coatings on both mirrors. This corresponds to a 0.17 magnitudes, not too much, maybe perceptable by a very skilled observer...

Put another way, both mirrors coated, the 96% will be the equivilent of a perfect 14.4 inch scope, the 89%, of a 13.35 inch scope....

This is a small gain but certainly not going to put you magnitudes deeper..

Then the question arises if there is some difference in scattered light between the two, everything I have heard suggests if there is, the 89% performs better in this regard.

I also have a certain doubt that 96% coating are really 96% across the spectrum.... I remember Roland suggesting that this was wishful thinking and that even protected these degraded rather quickly.

Bottom line:

When I had my secondary recoated by L&L, I had them do it in 96% as the cost was not much more....

But great differences in performance between the two, math says no. Many people want to see improvements when they spend money on something, whether it is $2000 for a pair of bicycle wheels or a few hundred for a mirror.

In my thinking, one goes faster on the bike by sitting in the saddle more, as Eddy Merckx said "Ride Lots" when asked about his training secrets... With astronomy, the same applies, you want to see deeper "keep your eye to the eyepiece."

Jon Isaacs