The classic qualitative test done wrongly, sure. But if you do it correctly you don't stop here, you come closer to focus and tweak until it's good there, possibly finishing off in focus if the seeing is good enough, looking at the appearance of the first diffraction ring. I didn't invent this, it's been done for ages, see e.g.
But it requires a lot of experience (and luck). i can't do it well if I'm tired, to give just one example. And it's really hard to quantify anything you're doing and when you should stop, so a tool that works 100% of the time and gives you quantitative data is indeed invaluable.
But since the brain is definitely a neural network, should I be worried about infringing on your patent when I star test ? ;-)
You are correct, I totally agree.
My comment was essential about raising the level of awareness related to some limitations when using human inspection or alignment tolls based on the defocused star symmetric or/and optical path pattern. I would consider this a a fist coarse step, which is useful for starting close to the solution.
When doing collimation using only human qualitative evaluation it certainly important to do more, such as comparing intra and extra focal images as well as looking at the final PSF (in focus and near), on and off axis, for fine tuning.
Yet it maybe hard to spot a some small amount of aberration even under a good seeing, where its impact is the most, especially when the CO is offset.
Below an actual experience from a DK 20" scope in Chili under 0.6" of seeing:
The collimation was done by the local support team using the traditional star test (qualitative collimation).
As one can see the defocused star pattern looks quite good, the PSF (in focused) seems good as well, but if one looks more carefully we can spot some horizontal coma, yet this is hard to tell since the seeing effect is changing from frame to frame and it may be even harder if the seeing is just a bit worse.
The SR is 67%, below is the 3D plot of the scope MTF.
For comparison I have added also, below this, the DL MTF and the DL defocused star with for this scope.
I have to say, to be honest, that when I have started using WF analysis, first a Shark-Hartman (SH) analyzer and then our AI based WF sensor technology, I was surprised to see this quite common situation where the CO is mechanical offset.
Like most of us I was expecting and looking for a nice symmetrical defocused star pattern with concentric rings only to find out that the end result, the WF, was not as good as it could have been.
The dominant aberration of a misaligned optic is usually coma on axis. In the context of a perfectly mechanically centered obstruction (and baffles, ...) coma leads to an offset CO in the defocused star pattern.
When perfectly collimated the CO shadow should be centered indeed. If the CO is mechanically offset on the other hand this is not true. It is quite natural to correct the perceived offset CO shadow in the focused star when using the classical star test. As matter of fact this is doable, but at the expense of some coma. Assuming the symmetry of the star pattern and shooting for a centered CO shadow during the collimation could be misleading.
The most important aspect is the intensity pattern gradient, it should be uniform (from an axis-symmetry stand point), which what the ITE tell us, (see my other post on this matter). That is more relevant than the pattern shape itself but under seeing conditions the scintillation may limit how well one can assess the intensity gradient, at least for short time exposures or when using an eyepiece.
Having say that before using any WF sensing technique I would recommend to aim for a symmetrical pattern (or using a collimation tool for that matter) as a coarse step. WF is used for fine tuning then.
SKW should be seen as a WF sensor tool without any dedicated hardware, such as a SH. When using meteorology to quantify a scope alignment one needs to be realistic since we'll always find some error, no scope is perfect.
One should manage expectation and consider the task relative to the local seeing which at the end of the day limit the long term exposure performance (excepted for LI).
As mentioned before doing WF analysis across the field (multi-star) at once opens the door for optimal collimation, basically active optics.
One can use this information to infer the mirrors misalignment parameters, angle and offset values, which in turn are used to control actuators (active optics) or provide quantitative guidance to the user (which knob to turn in which direction).
The goal of SKW is to eventually provide field dependent WF/aberration and correction feedback on top of quantitative scope performance data. We are working to implement this in future SKW updates after validation using the VLT collimation strategy.
Edited by Corsica, 11 June 2021 - 10:37 AM.