Star testing my 8" f/6 Dob gave me pause and confusion as to what Suiter (or anyone) means by 10 waves defocus. As I understand it, there is a difference between 10 waves defocus distance and 10 waves of defocus error.
In focus, my focuser graduated scale reading is right at 30mm (I believe the measure is in millimeters) as a starting point using a 6mm TMB II. When I scroll inside to what I believe is 10 waves defocus (distance?), I get an image I am familiar with that looks similar to the +/- 10 wave simulations we see in Suiter's book and other references. That is, a bright marginal diffraction ring, a bright paraxial diffraction ring bordering the shadow, and a single ring between them. This is the pattern I recognize as 10 waves defocus, same as the simulations show. The test results look pretty good on both sides of focus. So, I am happy.
But, when I read the focuser travel on the scale, it shows I am inside focus by about 2mm with a reading of right at 28mm. Same with outside focus, the reading is 2mm beyond best focus right at 32mm. Two millimeters either side gives me the pattern I am looking for. However, according to Suiter, that amount of defocus distance should be 12 waves distance and 10 waves distance is just over 1mm. So, I scroll back toward focus so the focuser scale reads 1mm from best focus at 29mm inside and 31mm outside. I do that visually, anyway, just to see when the paraxial ring disappears on both sides. Then I take a reading of focuser travel, it turns out to be close to 1mm travel and 1mm from best focus on both sides. Again, that corresponds to 10 waves focuser travel.
At this shorter defocus distance the paraxial diffraction ring is barely discernible inside focus, but it can be seen. A touch more toward focus and the paraxial ring gives way to a bright marginal ring with rings decreasing in brightness toward the center. This happens right near where Suiter says is 10 waves defocus distance or about 1.2mm. Outside is quite different at near 1mm focuser travel. At 1mm outside best focus, the bright paraxial ring is clearly seen and brighter than the now reduced marginal diffraction ring. There are only these two rings visible outside the shadow and it does not resemble the +/- 10 wave simulations, anymore.
At 1mm defocus both sides, the test looks like a classic 1/4th PVW under correction as the paraxial ring is gone inside focus and brighter outside focus. I am not as happy, the under correction looks more dramatic (more sensitive, I guess). However, when I scroll back out to 2mm from best focus, the defocused patterns look much better corrected. They still show some indications of minor under correction. So, when evaluating the optic, which setting do I trust? The one with a measured 1mm focuser travel consistent with 10 waves defocus distance as show in Suiter's book, or the better looking ones at 2mm defocus that /may be/ 10 waves error and resemble the +/- 10 wave simulations?
I have always trusted the latter. And I still do because the better corrected patterns are consistent with the favorable shadow break out and sizes. The shadows are well broken out by 2mm and even at 1mm, and they are not grossly different with the outside being marginally smaller. I scrolled through focus several times trying to see the difference, it was not that easy to determine. But, what is going on here? Am I really at 12 waves defocus travel per Suiter and 10 waves defocus error per simulations? If so, which defocus do we test for correction error?
Oh, my obstruction happens to be right at 65mm or 33% for some reason. Very tall focuser.
Thanks in advance.
Edited by Asbytec, 08 November 2018 - 09:15 PM.