From the above, I calculate from the ratio of the obstruction:
obs = 1.09/1.46 = 0.75 and thus the spherical wave error is 1-0.75 = 0.25 waves. (see note below about f number of the etx125)
My guess is that the spherical wave error is between 0.2 to 0.25 waves.
Here's the aberrator simulation:
which looks pretty close to what I measured.
So, is 0.25 waves of spherical error typical of Maksutovs? Perhaps this is to be expected, see
First, as mentioned, is astigmatism visible as classic 90 degree ovals on both sides of your defocused images and the plus star image at best focus from your video. You can have about 0.37 waves PV of astigmatism and be diffraction limited at 0.8 Strehl. I have no idea how much is present in your video. Need to look at that harder, but an initial look a Suiter seems to suggest it's not terrible. Not too dissimilar to his sims of 0.37 PV waves. The worse the astigmatism is, the more it will be seen with oval patterns at larger defocus. You might just barely see it in your defocused WinRoddier star test images.
The difficulty with your Aberrator sim is you dialed in 0.25 waves of primary spherical aberration (3rd LSA) which is not going to give good results (although you certainly got close). That would be fine if you were testing the primary alone as the source of lower order spherical, but this scope is a system. You will not see spherical aberration from the primary alone. The reason is because the steeply curved negative meniscus lens generates a lot of higher order (secondary) spherical aberration. This is balanced by adding lower order spherical aberration of opposite sign generated on the primary mirror. Primary and secondary spherical aberrations do not have the same waveform so one cannot fully correct the other, however the correction can be pretty good depending on the residual error present. So, there is spherical aberration present, that much is pretty obvious. It's just difficult to tell what form it takes and how severe it is because it is a mix of aberrations and not the pure form of a single lower order aberration we're all familiar with.
There is going to be some residual higher order spherical aberration (from the meniscus) present in the star test. What we are actually star testing is not a pure 3rd LSA primary spherical wavefront as produced in Aberrator, rather we are testing a hybrid (balanced high/low SA) wavefront that only approximates a spherical one to a varying degree. I recall Vla explaining (in one of your links above) the balanced form is actually represented in Aberrator as 5th HSA input. This residual secondary aberration will affect the star test which is very sensitive close to focus and it may show both characteristics of over and under correction (which your later WinRoddier images show). And the star test images will differ on both sides because of the residual error and that makes it more difficult to evaluate. Since some of the residual error is in the paraxial zones near the secondary obstruction, there is a chance this will affect the shadow breakout test as well. The shadow breakout test seems best suited for the pure lower order spherical aberration with a smooth 3rd LSA curve and a 0.3D obstruction as Eddgie often explains. However, this is not the actual wavefront you are testing, so you're results are likely off. I cannot simulate your patterns in Aberrator. Doing so may be complicated by the apparently under corrected medial zone (shown in Winroddier images) and a mix of additional errors not allowing us to test for the pure balanced high/low pure form of spherical or even the 0.25 LSA results above.
But, reading the star test above may be possible without quantifying it. Based on you later images, they do not look like 15 waves defocus to me. At large defocus, you should see more than one ring between the paraxial and marginal rings. I see only 1 ring and part of another (which is a consequence of slight miscollimation). The first thing readily apparent is a bright medial ring inside focus (and the same ring is dimmer outside focus) suggesting an under corrected zone as shown in Winroddier (and somethign Aberrator cannot simulate). Notice in the Winroddier "corrected images", that bright medial ring is missing. I don't know what Winroddier is doing, though, other than maybe showing what it should look like. The next thing is the shadow diameters appear to indicate over correction. As Pinbout often says, the larger shadow indicates the correction meaning larger shadow outside indicates over correction and visa versa. This is likely a result of the meniscus adding over correction of opposite sign (convex wave toward the primary) relative to the normal under correction from the primary. If you are indeed near the same defocus, and I suspect you are, the outside image is also a tad larger indicating some small amount of under correction, too. This is confirmed by the slightly brighter marginal ring outside focus. So, the ETX apparently has both and probably a consequence of its residual higher order spherical aberration. How good is it? I dunno, but I suspect astigmatism is the biggest problem.
So, saying the wavefront error is 0.25 waves of spherical aberration is kind of misleading and not typical of MCTs. An MCT may well have 0.25 waves PV wavefront error or more somewhere, but it will almost certainly not be the same pure form of 3rd LSA seen in star test simulations generated by Aberrator. That amount of error may be present at either the center or the edge or only in a small portion of the wavefront, and even then we have to understand the nature of the aberration causing the PV error. For example, you may have as much as 0.37 PV of astigmatism. A MCT can have as much as 0.4 waves PV of residual 5th HSA and still be diffraction limited. (Scroll down: https://www.telescop..._spherical.htm)
Edited by Asbytec, 08 September 2019 - 02:35 AM.