sign me up for testing! I've been using the GoldFocus mask on my RCT and it's not an terribly enjoyable experience!
Also, one thing to note is that as I am collimating, the star moves, how would the software handle this?
Similarly, on and off axis collimation with an RCT usually requires that the star(s) under test being variously be on axis (primary) and in the corners (secondary). Can it deal with both degrees of freedom or is it better suited to CDKs and SCT's with spherical mirrors?
Final question - the "trick" with collimation of long focal length trains is often dealing with the other elements of aberration eg: focal length being out of spec (mirror spacing), focuser centricity, focuser collimation, sensor tilt etc - are these all going to need to be eliminated before the tool has any useful data to share?
Not trivialising the great work you've done Gaston, it's a gnarly problem and I'm trying to understand what to expect.
Many good questions!
I'll attempt to answer some.
SKW is a stand-alone software which takes FIT files as inputs, therefore it does not connect to any hardware, nor does it control the mount (there is no driver involved). FIT files are either manually selected or automatically loaded by SKW when a new frame becomes available in an user defined folder.
Tracking/slewing/centering and imaging is done by what ever application the user may have.
Having said that SkyGuide and SkyGuard (SKG) will both eventually support the SKW-lite capability as a specific tab in the GUI. In this case our full frame guiding technology will be used to track the defocused star and recenter it in between collimation adjustments, as long as the star remains inside the imager's FOV. SKG can guide with objects of any type and shape as long as there is something else than noise in the frame, it does not need a guide star nor does it use any centroid.
As far as collimation (reflector) goes there are two aspects, one in the alignment of the mirror optical axes (offset and tilt in the general case), the other one in the correct spacing between the mirrors.
With a secondary spherical mirror we do not care about optical axis tilt (at least as long as vignetting is not an issue), only offset matter since any radius of a sphere is an optical axis. In this context the tilt/tip of the secondary is used to control its offset for bringing its optical axis (one of the mirror radii) coincidental with the primary mirror optical axis. This makes collimation much easier since we have only 3 degrees of freedom to deal with, tilt/tip and spacing.
With a RC scope, for instance, we have two hyperbolic mirrors having each an unique optical axis (unlike a sphere). This means that we have now 5 degrees of freedom to deal with, offset (X/Y), tilt/tip and spacing. To make matter worse most amateur RCTs do not provide any secondary (and primary) offset adjustment, only tilt/tip.
As a result if the secondary mirror has any offsets relative to the primary (so far I assumed that the image plane in squared with the primary mirror) the inevitable induced aberrations (mainly coma) can only be mitigated by tilt/tip adjustments.
The good new is that tilt/tip of the secondary (relative to the primary) can correct most of the offset error, up to some limit though, for making the scope DL (SR>80%). This is one of the reason why amateur RCTs usually do not offer any offset adjustments, unlike high end scopes (RCOS, ...) do. I have seen many RCTs with secondary mirrors and/or mounts offset by few millimeters, this is a very common occurrence.
The trick, with an offset secondary, is that such DL collimated scope will exhibit a de-centered central obstruction shadow in the defocused star pattern. When doing collimation we usually aim to centrer the secondary shadow, either during the initial day time coarse collimation step, using lasers and/or the Takahashi collimation telescope and similar tools, or during the night fine collimation step with a defocused star. This can make RCT collimation confusing and difficult without having direct access to the telescope actual optical performance as a feedback (aberrations, wavefront, ...), see an example below.
Accessing the telescope related wavefront (WF) and therefore its aberrations by types and magnitudes not only provides quantitative information it also allows to clearly separate the problem in interdependent parts. I have collimated many RTCs using a Shack-Hartman (SH) WF analyzer and I can tell this makes a fundamental difference in the process, speed and quality of the result.
Collimation using the WF is a quite different approach than the traditional collimation. However using the WF makes sense during the fine collimation step, which I would consider as the actual optical collimation, the coarse one is all about the mechanical alignment of the optical components, mounts and baffles. From an optical stand point this is a coarse collimation but it is an important step which should be done first whatever the tool one may use for the optical collimation, with an actual star on the sky or an artificial one in the lab.
First, especially with a RCT, you want to make the two mirror spacing right. Looking at the scope actual focal length provides a general idea but since there are inevitable tolerances on the mirror figures the resulting FL is usually off by few percents relative to the scope specification.
In professional telescopes (RCOS to pick an example) the master optician will edge the side of each primary mirror with the back working distance (BWD) value between the primary back and the image plane, this is the key parameter. When proper spacing is archived than the FL may be (and will be) different than the nominal one and this is fine. Using the FL as a predictor for mirror spacing collimation is useful for a coarse adjustment step, but not enough.
There are various optical methods for checking for the mirror spacing with higher accuracy, one is the use of a dedicated Ronchi ocular, PWI uses this for their scopes, another one is the WF. From a WF one can compute aberrations, typically one may want to use the Zernike polynomials.
When the Zernike primary spherical (balanced 3rd order spherical) is as minimum as possible than the mirror spacing is optimal. The beauty of the Zernike polynomials is that one can look at a give type of aberration, for a given propose, regardless of the others, one can separate them. This can be traced back to the orthogonal nature of the Zernike decomposition, a very useful property.
It is important to understand that SKW is a true WF analyzer, we have shown (see SPIE proceedings and conferences, initial post) that our technology competes with interferometry and SH WF sensors and alike. SKW provides the WF and related aberrations without the need of any dedicated WF sensor using your imager instead. Traditional WF sensor/analyzers, such as a SH FW sensor, cost at least several $1000 and require removal off the user setup to be replaced by the WF sensor hardware. SKW offers the same performances at the fraction of the cost in the order of few $100 without touching the user setup at all. SKW Lite and Pro version both use the same WF engine the difference begin the GUI and level of information/capability offered. Yet even the basic version (SKW Lite) is providing WF driven collimation feedback.
I am working on some tutorial material for SKW, I do not want to add too much more on this already long post, but here is an example showing what value a WF driven collimation adds.
As I mentioned before there are many amateur level RCTs with very good optics facing some mechanical limitations, such as mirror offsets.
Below the defocused star image of a RCT (taken under seeing limited conditions) having an offset secondary mirror and central obstruction. This scope is well collimated and DL (SR = 99.43%):
One can clearly see that the secondary obstruction shadow is not centered and some of the rings are not concentric, yet the scope is perfectly collimated, I have faced this situation may time over.
Below its WF retrieved using our AI based WF sensing technology (to know more about WF please visit our education page at https://www.innovati...opecollimation/ ):
Now if we do not have access to the scope WF and related aberrations/SR one may conclude that scope is badly collimated and need some adjustment.
Without any other feedback we would usually aim at centering the secondary obstruction shadow while making the outer rings as concentric and circular as possible, using the classical qualitative collimation method. Below is the result after tilt/tip correction of the mirrors:
This looks much better than the first initial defocused star image. However the resulting WF analysis tells us a different story:
The collimation adjustments have creates about 0.12 wave rms of vertical coma leading to a SR = 57.84%, we can see below the comatic resulting PSF, the scope is not DL limited anymore:
I think this may illustrate the value of using WF (or similar tools) and aberrations to express collimation results.
This kind of situation can make RCT collimations very difficult and frustrating.
As you mentioned there maybe many difference sources of error in the optical paths. We already talked about using the Zernike polynomial spherical aberration to set the correct spacing between both mirrors, which I would recommend to do first, solving one problem for itself. Another good example would be about any possible tilt of the image plane, for any reason.
An RCT perfectly collimated does not have any primary coma in the field (ON or OFF axis), this is one of the fundamental properties of RC scopes (assuming correct spacing between both mirrors though). The RC design trades coma for astigmatism, therefore without any corrector a RCT exhibits astigmatism off axis as well as field curvature (defocus), but no coma. Since having access to the WF allows to get each aberration for itself one can use this on our advantage when collimation a RCT, for instance.
Even if the imager is tilted resulting to the centered star on the chip be off axis relative to the scope, one can still collimate it looking only at the coma aberration term from the WF. If we are off axis there may be some astigmatism but we do not care about it just yet a this stage, we just collimate the scope until the coma is removed (remembers RCTs have no coma on AND off axis).
Then we can look at the astigmatism (as well as field curvature) across the field (in the comers) if it is not symmetrical we know that our imager is off axis indeed and we can act accordingly, say by adjusting a tilt/tip correction device in the optical path.
Such a strategy, looking by type of aberrations for solving one problem at the time, can only be implemented if one has access to the WF/aberration in a quantitative way, this is a fundamental difference versus traditional collimation. It s also the way optician aligned complex optics. For instance we have the DoD as one of our customer using a SH WFS (StarWave Product) for collimation, this one was bought before the design of our AI based WF technology but it does the same thing providing quantitative WF information and aberrations.
I hope this, a big long post, provides more inside on the value of using quantitative aberration feedback for collimation.
Edited by Corsica, 11 March 2021 - 06:21 AM.
