What are good optics?

[Alchemy (Clive)]

I couldnt resist posting this quote from Alan Adler (us sky and telescope issue jan 2002) a respected contributor to sky and telescope both US and AUS for many years i have enlarged a particularly interesting section relevant to this discussion

The full potential of your Newtonian can be realized with only a small investment of time and money. By Alan Adler interest in thermal management began when S&T associate editor Gary Seronik played me a video made by Bryan Greer that showed heat waves slowing rising from the front surface of a 6-inch mirror. F or years reflecting telescopes have taken a back seat to refractors for high-magnification views of planets and double stars. The reflector's central obstruction is most often blamed for this shortfall. However, a strong body of theoretical and experimental evidence has shown that central obstructions of 15 percent (perhaps even as high as 20 percent) of the diameter of the primary mirror are not visually detectable. Another often-cited scapegoat is surface accuracy. Although errors on reflector optical surfaces must be one-fourth those of refractors to achieve the same results, such accuracy is quite common in good reflectors. So what's holding back well-made reflectors? I am convinced that it is not a central obstruction, and it's not optical quality -- the problem is heat waves off the mirror surface. This layer of warm air behaves like a weak lens of very poor quality right in front of the mirror. I believe this is the main reason reflectors have always been regarded as poor cousins to refractor telescopes. Taking the Heat from Your Mirror Thermal management in your reflecting telescope can yield astonishing improvements in resolution and contrast.

[Peter Ward]

This has been a fun thread!

Mark Sutching has quite correctly pointed out the possible shortcomings of interferometric testing, and I'd put Peter Ceravolo (my optical testing mentor), has also stated provided you are aware of the pitfalls Mark mentions, quantitative testing can be equally valid. (i.e numbers!!)

The only way you are going to know a mirror has a Strehl of 0.99 is if someone actually took the time to apply some sort of quantitative test...ie
measure the bugger!

I am not saying, nor have ever said, an excellent optic can not be made without said test.

But now after the measurements have been taken you have a few possibilities....

1) They are falsifying the data
2) They are incompetent
3) The optic is indeed excellent

Assuming 1) and 2) are unlikely with reputable optical artisans (hey, you know the names) then it's reasonable to also assume the optic is indeed excellent.

Originally Posted by wavelandscott

Can you have a bad mirror with a .99 strehl (or .95 or .96 etc. etc.)?

[Gama]

Originally Posted by Alchemy

I couldnt resist posting this quote from Alan Adler (us sky and telescope issue jan 2002) a respected contributor to sky and telescope both US and AUS for many years i have enlarged a particularly interesting section relevant to this discussion

Yada yada yada.

This was describing comparisons of refractor over a newtonian/mirror in terms of visual quality.Indeed this is what causes the most prolific problems with mirror based scopes.
But what is being discussed here is different.
As an ex Test Equipment Calibration Manager, i have seen my share of numbers. Further, what some say about definitive numbers over relative numbers is correct.
The test equipment we used for out testing is calibrated and tested against some one elses known standard. But if i test and get a value of "X" for a piece of equipment, this does not mean it will produce the same numbers if the unit was tested by, lets say H.P Calibration Labs.
There needs to be a reference point to be able to establish a common point.

Until then, you say tomatoe and i'll say tamatoe !!.

Theo