As I said on one side the rings are clean and sharp and the other side there are no rings to be found. Brightness of the defocused pattern was the same as was the size, its just one side you could see the rings and the other you could not. The instrument had the sharpest view of mars I have seen. He was told by AP when he purchased the unit it was .99 and then again when he sent it back to AP after many years because he noticed oil around the edge of the lens. The images in the scope are very, very sharp but this has been the results when star testing the unit on many occasions.
You keep skipping the vital piece of data : is this with a narrowband green filter? If yes, then someone is twisting the truth big time. There is no way an APO with Strehl of 0.99 will form Fressnel patterns like you described ("one side you could see the rings and the other you could not".).
It's an interesting discussion, a bit off topic, but interesting.
As I understand what Roland is saying we use a narrow band filter to isolate a specific wavelength (or range), but not to hide the effects of higher order SA.
Here's Roland on filtering...:
"If the outer rings are identical on either side of focus, there is no significant spherical aberration. What you see in contrast difference on the inner rings is due to chromatic effects. Use a green or yellow filter (in fact, use them both at the same time) to isolate the middle of the visible spectrum."
On the differences inside and out due to residual HSA as I suspect Powell argues...
"In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront...The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high. In other words, the optic will deliver a very high contrast image, consistent with the high wavefront rating. When tested on the night sky, the inside and outside diffraction patterns will be quite different."
I suspect this is what he is seeing, along with some color further reducing contrast in the de-focused image. My ~30% obstructed MCT (struggling to return to topic) with lesser correction exhibits similar behavior, except outside focus the rings are still seen with reduced contrast. Scared the bejeezuz outta me. Inside focus is tack sharp, though.
But, if you look closely, you can tell the outside ring patterns are nicely presented in relative brightness to one another and fairly similar both sides (to the extent one can tell such things.) The patterns either side are different in that contrast has fallen off outside focus, but in-focus is clean as a whistle when seeing allows the pattern to calm down nicely...exactly as Roland and Powell describe.
"I have recently finished exhaustive tests of different 10" F14 Mak-Cass systems, some with these inherent aberrations left in, some with them meticulously removed. All the optics tested between 1/10 and 1/12 wave with Strehl ratios of 98% or better."
Unless Roland is wrong, a premium scope with higher order left in, including a fast APO, can have a very high Strehl (maybe at a single wavelength or all of them in focus) and markedly different patterns either side. Some of what Powell is seeing is residual higher order SA and the rest of it may be color.
After some discussion on the central zone and the CO...
"The other half of the defect occurs at the outer zone. If this is left uncorrected, you will see fuzz in the in-focus image which is highly destructive of contrast. The inside and outside patterns will look the same, causing you to conclude that the optic is textbook perfect."
And from http://geogdata.csun.../startest1.html
"I have an 8" SCT that shows perfectly identical inside vs. outside diffraction patterns, but tests only 1/4 wave..."
Which doesn't make a lot of sense to me, but back in his comments on complex optical system he states:
"...should we do our best to produce smooth high contrast optics, or should we please the star test crowd and do some hand aspherizing to get a more pleasing out-of-focus star image? I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast."
"The real test of an optic is not so much how the diffraction pattern looks outside of focus, rather, how much extraneous junk is floating around a star when it is in focus."
I have modelled just about every possible combination of defects in Zemax and it simply does not exist. Not if aberrations are kept to level of 0.99 Strehl.
I really do not know what to make of the argument, just saying I think it's possible what Powell is seeing is not only chromatic effect. And that if his patterns are different (lower contrast outside), it might well have a very nice Strehl if we believe what Roland is telling us about his star test experience along with his IF work.
Edited by Asbytec, 15 August 2014 - 09:15 AM.