Let us hypothesize you have a chance to buy a very special mirror. In this case its a 7" F/5.6 and you want to use it for a top grade, premium, highest standard planetary-specific telescope. It is being made on a premium substrate, that being Zerodur, and thin enough to equilibrate fast.
This mirror was fabricated by an optical company who specializes in aerospace and industrial optics. It is tested on a WYKO phase shifting interferometer, so thousands of points are being sampled. The interferogram is reduced by ZYGO software and is certified to have a PV wavefront of better than 1/15 wave, and a Strehl ratio of better than .99.
This mirror happened to come to the bench at Zambuto Optical Company where it was measured on the bench by Carl Zambuto. According to Carl's measurements, he agreed the spherical aberration is indeed in the range of 1/15 wave with a corresponding Strehl ratio of better than .99. In fact, Carl Zambuto's large scale surface measurements agree very precisely with the interferometer.
Let us take a brief look at the interferogram for this mirror (we have blocked out several lines to protect the identity of the company).
Notice the Strehl ratio middle left is .997, and the PV (peak to valley wavefront) at just above center is .062 (1/16 wave). By the numbers one would conclude this is a world-class optic.
Now the question- would you want this mirror in your telescope?
Before you answer, let us take a brief look at a very simple bench image of the above mirror. The image below was taken using a 50 line Ronchi grating, and positioned where it essentially nulls the entire mirror, so that the surface looks much like a Foucault test.
In case you were wondering, the mirror in the interferogram is the image on the LEFT.
The mirror on the right is a similar size and focal ratio production mirror made by Zambuto Optical Company and imaged for comparison.
What you're seeing on the surface of the mirror on the left is a classic case of machine-induced primary ripple. -Not that all machine figuring produces it, because the mirror on the right was also machine-figured. But the pattern on the left demonstrates that machine work produced the paraboloid.
So what IS primary ripple? Primary ripple is medium-scale surface roughness which is caused by the facet pattern on the polishing lap, combined with the pattern in which the lap does its work. Polishing laps have channels, or grooves in them, to allow the pitch to slowly flow as to keep in contact with the surface, as well as to give space for the polishing compound to be uniformly distributed.
The primary ripple in the image above left was accomplished most likely by a combination of hard pitch, heavy polishing compound concentration, and heavy, fast action on the machine arm. The optician was most likely in a very big hurry. By the way, the price tag on this mirror was top dollar. At the time, its cost was about 2.5x the price of a similar size Zambuto mirror.
So what's the issue with primary ripple? It creates severe slopes that are not accounted for in surface measurement. These slopes scatter light and have the ability to severely reduce contrast. The result?
Dick Suiter says it like this,
"The net effect of mild primary ripple is to blow the scattered light into a knobby glow surrounding the image, which has its greatest brightness within a radius less than 5 times the airy disk. Such scattered light can be a bad problem because it is condensed enough to easily see" (Star Testing Astronomical Telescopes p. 237).
So what about the interferogram and the Strehl ratio?
Yes, "What about it." -That's the question. The answer is, the primary ripple (in this case more severe than "mild") is obviously not accounted for in the final numbers. The problem is, its going to blow the light outside and close to the airy disk, damaging the optical train's ability to resolve detail in planets, double stars, and so forth. However the customer will have no indication of this other than that this optic will have reduced resolving power (in this case reduced contrast) compared to an optic which does not have such medium scale surface roughness.
Let's look at the two mirrors again from another perspective. Below are two Foucaultgrams. We put each under a knifedge test and placed the knifedge at the radius of the innermost zone, to create a "null". The secondary mirror area has been blocked off. The center zone, represented by the two central openings in the masks represent 1/5 the total surface area of the mirrors.
In order for the center zone to return the reflected light to the airy disk, the surface needs to be smooth, as in the image on the right. Notice the steep ridges in the left image, and the severity of the light-to-dark aspect. Each of these ridges has slopes whose measurements are unknown and unmeasured by any method used today that will most certainly scatter light outside the airy disk, reducing contrast. Think of "you-know-what" hitting a fan. That's what's happening to the light. Such intense slopes can have scatter amplitudes which are equal to the total slope between one zone and the next, in a long focus optic.
Significant primary ripple exists in the vast majority of mirrors from all sources including mass produced as well as "premium" mirrors. And if you think the mirror above is an isolated incident, let us consider yet another example.
Below is a mirror made by another company using another phase shifting interferometer and Zygo software. This is the ultra grade model, with a corresponing price tag. The interferogram below was provided by the manufacturer. Bench images below taken by Wolfgang Rohr. http://www.astro-foren.de/showthread.php?t=6084
See the nice, straight fringes on the synthetic map. And notice once again, a Strehl ratio of better than .99.
Below are bench images as well as an interferogram taken on the same optic as the above documentation, by Rohr. Notice in particular the upper left Foucaultgram. It has moderate primary ripple throughout and a healthy turned down edge, evidenced by the bright left edge and the dark right edge. This is also seen in the interferogram lower right, where the fringes hook sharply all the way around the edge,
So the primary ripple will smear light near the airy disk, and the turned edge, which we will discuss in a subsequent criterion, will scatter a veiling glow over the entire focal plane.
But you ask, "The Strehl ratio says .99+, so what are we supposed to think?"
The Strehl ratio is a measurement of the large scale (geometric) surface. Regardless of the method of test being used, including the most advanced optical devices in existence, criteria such as medium scale surface roughness and turned edge are not taken into account. This is obviously evidenced by the results. So what you do is you have to know your optician uses their eye on the bench and you will have to know and trust that they address this issue.
The fact is, the vast majority of optics from almost all sources contain various levels of visible medium scale roughness. And that includes "premium" optics. The problem right now is, optical companies don't talk about this. Medium scale roughness has the potential to severely reduce optical performance, allowing a mirror of a given size to be outperformed by smaller aperture, and sometimes significantly smaller aperture, as we have witnessed take place again and again.
This criterion is probably the single greatest reason that Zambuto mirrors outperform the majority of other mirrors on the market in resolution and contrast, showing both better detail and definition, and outperforming most other mirrors (including a surprising number of larger aperture) on the deep sky. When we look at mirrors from other sources on our bench, what we see in most cases is primary ripple- from mild, to moderate, and even severe. It is very, very common. The truly smooth mirrors from outside this shop are rather rare, from everything we have seen in over 20 years of mirror making. The failure to successfully meet this criterion is a great contributor to loss of performance and contrast, and yet where is it to be found in the documtation of optics in this industry? People ask their mirror makers about Strehl ratios, but do they ask about medium scale surface roughness?
So if you want world class performance, one very important criterion is to have a mirror that on the bench looks just like the image of the Zambuto mirror shown above.