Comparing Quality of Mirrors

[Paul73]

Hi Folks

I’m a bit at sea on the subject of mirror quality. I have read about 1/10 PV, Strel ratio and smoothness. What measure is most important?? Or, just point me to the idiot’s guide!😁

Paul

[Ships and Stars]

Loads of varying opinions out there. I've 'heard' that any mirror quality higher than 1/8th PV is not discernible to the average eye, so in my book 1/6 to 1/8 will do. I think most mass production primary mirrors are around 1/4th PV.  Strehl ratio is something I'm learning more about slowly. Then there is astigmatism. Also I've heard that simple lack of secondary and primary collimation, along with mirrors not at optimal temperature often rob more viewing quality than any mirror shortcomings. I just posted a question about a boundary layer fan for my dob which blows a steady stream of cooler air across the mirror face, as opposed to cooling the traditional way with rear mounted fans. The boundary layer seems to be more important in the eyes of some. A mirror within 1C of ambient temp is good, within 0.2C is ideal. 

Then there are the ways that manufacturers measure their mirrors which have caused heated debate. Some come with paperwork stating such and such wavefront and Strehl ratio, then the mirror was tested by an independent lab and returned much, much lower results. Then I've heard the mirror cell which supports the primary must be well-made, or the mirror will slightly deform...

I'm sure you will get some more answers soon.

Fun eh?😁

[jetstream]

52 minutes ago, Paul73 said:

Hi Folks

I’m a bit at sea on the subject of mirror quality. I have read about 1/10 PV, Strel ratio and smoothness. What measure is most important?? Or, just point me to the idiot’s guide!😁

Paul

Paul, there are a few places that give good info, Lockwood is one- a trusted, excellent optician.

http://www.loptics.com/articles/fourlessons/fourlessons.html

[niallk]

Here's another on strehl from Royce:

http://www.rfroyce.com/standards.htm

[John]

One thing that I've learned is to place more reliance on what your eye sees under decent seeing conditions than whats printed on bits of paper

 

[andrew s]

Just be aware that Strehl ratio is dependant on the size of the Airy disk . For a given focal length a small aperture has a larger Airy disk than a larger one so can have a higher Strehl ratio than the larger one even if the linear image size is the same!

Regards Andrew 

Edited July 1, 2019 by andrew s

[vlaiv]

6 minutes ago, andrew s said:

Just be aware that Strehl ratio is dependant on the size of the Airy disk . For a given focal length a small aperture has a larger Airy disk than a larger one so can have a higher Strehl ratio than the larger one even if the linear image size is the same!

Regards Andrew 

I'm not sure what is it that you wanted to say, but somehow it does not seem right

Strehl ratio is ratio of energy or number of photons that should land in ideal optics (meaning perfect wavefront) in Airy disk compared to actual number of photons / energy that is landing inside Airy disk. As such it has nothing to do with physical size of Airy disk - as it represents comparison between ideal scope (of certain Airy disk) and actual scope (of again - same Airy disk). Strehl ratio does not give you any sort of indication of size of aperture and or focal length used. Smaller scope can have both larger and smaller Strehl compared to larger scope (in terms of aperture and/or focal length).

Out of all measures of mirror I would say that two are most important - Strehl ratio, and associated MTF (used for better understanding of performance) and micro smoothness of the mirror / coating.

First one is obvious - how sharp image will look with said mirror. Second one is related to general scatter of light and is important in the same sense baffling and flocking of tube, position of focuser (for newtonian scope) and such - it impacts how dark sky looks thru the scope. We might say that first measure is that of planetary performance, while other one is that of deep sky.

Having said all that - main mirror quality is upper bound of system performance, and in practice it is modulated by other things - collimation and quality of secondary for example, and flocking / baffling / position of focuser (to minimize stray light)...

If you are interested in sharpness of the system - then there is a simple way to measure whole scope as a system. When I say simple - I mean you don't have to take scope apart, have interferometers for mirrors and such - simple planetary camera will do the job. You can choose to use artificial star or do real star testing with it - there is software made for this purpose and it is called WinRoddier, and it enables you to do Roddier analysis of scope, which involves pretty much the same thing as shooting a planetary image - you take in/out focus images, stack them in certain way, load in software and it will give you results.

[andrew s]

14 minutes ago, vlaiv said:

I'm not sure what is it that you wanted to say, but somehow it does not seem right

Strehl ratio is ratio of energy or number of photons that should land in ideal optics (meaning perfect wavefront) in Airy disk compared to actual number of photons / energy that is landing inside Airy disk. As such it has nothing to do with physical size of Airy disk - as it represents comparison between ideal scope (of certain Airy disk) and actual scope (of again - same Airy disk). Strehl ratio does not give you any sort of indication of size of aperture and or focal length used. Smaller scope can have both larger and smaller Strehl compared to larger scope (in terms of aperture and/or focal length).

Out of all measures of mirror I would say that two are most important - Strehl ratio, and associated MTF (used for better understanding of performance) and micro smoothness of the mirror / coating.

First one is obvious - how sharp image will look with said mirror. Second one is related to general scatter of light and is important in the same sense baffling and flocking of tube, position of focuser (for newtonian scope) and such - it impacts how dark sky looks thru the scope. We might say that first measure is that of planetary performance, while other one is that of deep sky.

Having said all that - main mirror quality is upper bound of system performance, and in practice it is modulated by other things - collimation and quality of secondary for example, and flocking / baffling / position of focuser (to minimize stray light)...

If you are interested in sharpness of the system - then there is a simple way to measure whole scope as a system. When I say simple - I mean you don't have to take scope apart, have interferometers for mirrors and such - simple planetary camera will do the job. You can choose to use artificial star or do real star testing with it - there is software made for this purpose and it is called WinRoddier, and it enables you to do Roddier analysis of scope, which involves pretty much the same thing as shooting a planetary image - you take in/out focus images, stack them in certain way, load in software and it will give you results.

I agree but what I am saying is that if you have the same focal length then with the same Strehl ratio a larger aperture mirror will give a smaller image than a smaller one.

Thus you can simply compare mirrors of the same size, but when you compare mirrors of different sizes and focal lengths you need to be aware of difference in Airy disk size. That is all.

I could have a 1cm diamter fl 500mm mirror/lens with a Strehl ratio of 99.999 would it be better in resolution that a 100cm diameter fl 500mm mirror/lens with Strehl ratio of 98.9?

Not obvious to me one way or the other without doing the calculations. 

All else being equal the simplest way to increase resolution is to increase the aperture!

Regards Andrew

Edited July 1, 2019 by andrew s

[vlaiv]

13 minutes ago, andrew s said:

I agree but what I am saying is that if you have the same focal length then with the same Strehl ratio a larger aperture mirror will give a smaller image than a smaller one.

Thus you can simply compare mirrors of the same size, but when you compare mirrors of different sizes and focal lengths you need to be aware of difference in Airy disk size. That is all.

I could have a 1cm diamter fl 500mm mirror/lens with a Strehl ratio of 99.999 would it be better in resolution that a 100cm diameter fl 500mm mirror/lens with Strehl ratio of 98.9?

Not obvious to me one way or the other without doing the calculations. 

Regards Andrew

Very good point - it did not occur to me that someone trying to understand the concepts for the first time might mistake resolving power with Shrehl ratio, after all Strehl ratio is mentioned in very similar context in discussion topics on the internet (detail resolved).

Worth noting then that there are several "types" of Strehl values as well - in its original form, Strehl value is very unambiguous - it is ratio of energy in airy disk between perfect aperture (wavefront) and aberrated wave front. However sometimes Strehl ratio is used in different context - as for example with mirrors / or system Strehl. In this case it represents ratio of energy contained in Airy disk for perfect aperture of a given telescope design vs real sample.

For example, I tested my RC scope to be higher than Strehl 0.94. This number is misleading if we assume original form of Strehl, as it represents ratio of energy in airy disk of perfect scope with 50% central obstruction. Any scope with central obstruction of 50% can't have more than 0.8 Strehl ratio according to laws of physics (effect of 50% CO is very similar to 1/4 wave). But number given as result of the test says that my scope is "94%" as good as perfect scope with 50% CO (well this is my interpretation of Strehl - one can't really say that number translates to percentage of goodness but it's close enough) - or energy in airy disk is 94% of that of perfect 50% CO scope. Btw that is great figure for two mirror system - as in two mirror system individual Strehls multiply to produce system Strehl.