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10/23/2021
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I tested the beta version of Skywave Pro on my CDK14 under the stars. The Pro version includes analysis of a scopes wave front - including things like Modulation Transfer Function (MTF), Wavefront Analysis and control of Zernicke coefficients - as well as collimation. My primary aim was to test the collimation function. I had previously collimated using an eyepiece and SKYX that will place rings to help center the shadow. SKW analysis showed that I had done a decent job visually but the computed PSF showed some coma and astigmatism.  Here is the 3D view of my PSF. This is all on-axis.After  |
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10/23/2021
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Here is after using SKW for secondary adjustments. This was all near real-time under my seeing limited conditions.  t  |
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10/23/2021
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Here is the sqrt of the PSF when I finished collimation. You can also select the Seeing Limited PSF which is what is shown in the PSF 3D model above. Looks really good. I tested with 2.5" and 2" seeing. You can change the parameter on the fly.  |
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10/23/2021
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Here are a few other screenshots using the Pro version. These are the Wavefront Analysis and MTF of my scope under seeing limited conditions. It would be nice to use this as a bench test with an artificial star....but way too much work for me.     |
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10/23/2021 |
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That looks like really cheap software, maybe even for free?
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10/23/2021 |
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Interesting. Where do we find that piece of software?
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10/23/2021 |
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Lorenzo, It is from Innovations Foresight. It is not released yet. It is scheduled for November. Their Web site has more detailed information about the software. https://www.innovationsforesight.com/ |
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10/24/2021 |
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Ah, ok! Interesting. Maybe I'm wrong, but it seems similar to the Roddier test.
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10/24/2021 |
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Lorenzo Siciliano: But it isn't. They took the liberty of avoiding splitting the beam and make some rather strong assumptions about coherence length. Not sure it is going to replace a Shack-Hartmann device any time soon but may be useful to some. I mean, if collimating is the issue here you don't need this kit. |
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2/8/2022
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SkyWave-Collimator version is now available, it can be downloaded from here: https://www.innovationsforesight.com/support/skg_download/ The SKW-Collimator license is free, the user buys a mathematical model for his telescope. SKW-Collimator is also standard in the new SkyGuide and SkyGuard (SKG) versions. Here is a link to some documentation and tutorial: https://www.innovationsforesight.com/support/skg_skw_documentation_tutorial/ The SKW Pro version is scheduled to be released at the AIC 2022, May 20th and 21st 2022 at San-Jose CA. SKW is using our patent pending AI based wavefront sensing (AIWFS) technology. For more technical details here are two peer reviewed SPIE proceeding references related to the two conference lectures I gave on this topic, the last one was a joint work with Dr. John B. Hayes adjunct research professor at Wyant college of optical science, university of Arizona. [1] Gaston Baudat, “Low-cost wavefront sensing using artificial intelligence (AI) with synthetic data”, Proc. SPIE 11354, Photonics Europe, Strasbourg, 4-1-2020 [2] Gaston Baudat, John B. Hayes, “A star-test wavefront sensor using neural network analysis”, Proc. SPIE 11490, Optical Engineering + Applications, San-Diego, 8-21-2020 [3] Paul Hickson & Greg Burley, “Single-image wavefront curvature sensing”, SPIE Adaptive Optics Astronomy, vol. 2201, 549-553 (1994)ohn John is an active member of astrobin, here is a link to some of his experiences with SKW: https://www.astrobin.com/txk760/H/?q=john%20hayes Following Roddier and Roddier's work on curvature sensing (CS) used with success in the context of AO (1993), Hickson & Burley (1994) (see [3]) have shown, using the same irradiance transfer equation, that a single defocused star image is enough to retrieve the wavefront phase without ambiguity with proper defocus and care. SKW levearges this work and others, it is designed for telescope alignment in this context, but not only. It uses long exposures under the Kolomogorov's turbulence model for retrieving the wavefront, hence the telescope aberrations, either native or misalignment induced, the main task for the SKW-Collimator version. The AIWFS technology it self can be used for optical metrology, in the lab. Experiments (see [2]) done by John comparing with a Twyman-Green interferometer (PhaseCam 6000 from 4D) have shown very good agreement in an optical bench. Sensitivity around 1/1000 of the wave, accuracy around 1/100 of a wave at the time. The latest tests done on the lab comparing with a Shack-Hartmann sensor (SH) brings now the accuracy with our new mathematical models (Neural Networks) around few 1/1000 of a wave, below are some results:  Of course one does not need such performance for telescope alignment. One should remember that a Strehl ratio of 80% is about 0.075 wave rms. Therefore accuracy in the range of few 1/100 of a wave is enough, especially under seeing limited conditions at which the telescope operates. Below a test done on the sky with an actual star between SKW and a SH sensor on the same telescope, at the same time, at Patrick space force base (Florida) in November 2021:  The methods are in very good agreement, despite a below average seeing (FWHM was around 2.3", Fried's parameter r0 = 4.5cm) at the time. The mathematical model was trained with seeing limited synthetic (simulated) images (long exposures). Obviously this telescope needs some alignment. As a matter of fact the model reports/outputs the seeing, acting as a seeing monitor too. One interesting aspect of the AIWFS technology is that one does not need any dedicated wavefront sensor hardware, a simple camera and a mean to defocus is enough. Also when using an imaging camera one can, at once, access several stars which offers the unique opportunity to measure field dependent wavefronts and aberrations (on and off axis at once) with a single frame and sensor, no need of several wavefront sensors or any mechanical scanning. In order to leverage this capability we are actively working with the DAG team in Switzerland and in Turkey for the implementation of our AIWFS in the context of active optics (for keeping the telescope optics aligned and the primary mirror M1 figure on specifications, M1 is a deformable mirror pretty much like the VLT). This is a 4m meters telescope for the East Anatolia Observatory: https://www.innovationnewsnetwork.com/dag-turkeys-21st-century-grade-international-astronomical-observatory/17050/ We are still accepting some beta testers for SKW Pro, anybody interested please feel free to contact me directly. One of the goal is to implement the ESO technique applied to the VLT active optics using field dependent astigmatism from AIWFS for retrieving mirrors misalignment values at once. We are in contact with Stéphane Guisard (ESA/ESO) who is helping us for providing the amateur astronomers with a similar technology and methodology. For information here is the basic paper on the topic: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.507.8063&rep=rep1&type=pdf |
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2/10/2022
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Using wavefront for telescope alignment (collimation) provides an actual optical quantitative way for measuring a telescope performance, such as its Strehl ratio (SR), MTF, and more. This means we know when the collimation is over, or good enough for a given local seeing at which the scope operates. A very common occurrence we have seeing many times over during the beta tests of SKW is the central obstruction (CO) shadow offset effect. This can be traced back to mechanical offsets, usually in the range of few millimeters, of the secondary (M2) mount itself. It is only a tiny percent of the scope aperture (~1% or so), therefore hard to spot without measuring it. It does not have any effect in term of telescope optical perform at best focus (in focus) however it can be easily spotted on a focused star pattern. Telescope makers care about the telescope optical performance in focus not out of focus. In this context the mechanical alignment of the M2 mount's shadow is not very critical (unlike the mirrors optical axes alignment), as a consequence there is no need for spending energy, time and money to build a more precise mechanic for making the CO better centered. Assuming that a defocused star should exhibit a symmetrical diffraction pattern with a well centered CO when doing qualitative telescope alignment (basically the goal) using a classical star test (and human inspection) can lead to surprises and sub-optimal performances, lower SR and below average MTF. Below an example, quite representative, of a 14" SCT under a 1.8" FWHM seeing condition. Its M2's mount is offset few millimeter, from factory, resulting to an asymmetrical defocused star pattern (left side of the figure below), including the CO shadow offset. The wavefront analysis (here with SKW) shows a SR of 90% with some spherical a little trefoil aberrations, hence a DL telescope, there is no need for any collimation adjustment, in focus this scope operates at its diffraction limit indeed. On the right side of this figure are the results after a standard qualitative test alignment (by human inspection) for the same telescope. Now the defocused star pattern is much more symmetric and the CO is centered, however the wavefront exhibits horizontal coma and so the telescope PSF. In essence one traded coma for a nice symmetric star pattern. Without any optical quantitative measurement it will be hard to spot this, beside below average in focus performance and likely more than normal off-axis aberrations (here the star was on axis during the alignment process). This situation is very common. During the course of SKW beta testing I would guess this has been the case for at least 70% of the telescopes involved, across many different types and sizes, up to 24", so far.  |
 2/11/2022
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Gaston Baudat: Hi Gaston, After reading your description I got a bit „confused“. I understand your point that a symmetric defocused star image may leave residual optical aberrations when in focus (I observed this myself in some optics). However, I would claim that with more experience, an astronomer would know that the final judgment on alignment is done in focus. Of course, this requires a good seeing condition. Nevertheless, one would see if the optics is misaligned, as in your example to the right, if one has a bit of experience in star testing a telescope. One should say of course, that many instruction manuals which come with OTAs are stopping short of illustrating a well aligned optics. Usually it ends with having the donut symmetrical. This might explain why „at least 70%“ of scopes are looking like in your example . CS, Björn |
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2/11/2022
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Björn Arnold:Gaston Baudat: Hi Björn, All valid points. I concur that when doing a star test it makes sense to check the collimation result in-focus (on and off axis) as well, however it may be challenging to judge because seeing. On the right example with a near perfectly symmetrical "donut" one may easily come to the conclusion that the scope is aligned and the job is done, especially under seeing conditions. Scintillation makes it hard to detect a subtle intensity gradient in the diffraction pattern due to coma. I guess my point was that even under good conditions one should often expect that a well aligned telescope (reflector) should exhibit a asymmetrical defocused star. Aiming for symmetric one, like suggested in most telescope user manual collimation procedure, may often lead to a sub-optimal scope alignment, a failed collimation, while having a sense of successes. I have not doubt that with experience one can eventually overcomes this, but it is not straight forward for most. The long term (mean) wavefront is a plane-wave, seeing averages out (seeing is a phase noise with a zero mean). There wavefront senors, SH or SKW, allow measurement with accuracy in the order of 1/100 of wave even under below average seeing conditions. Having access to the wavefront, hence quantitative optical collimation, removes any guess from the equation, IMO. |
2/11/2022
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Gaston Baudat: Gaston, I fully agree with your statement. For the classical procedure to work one needs experience and sky conditions which permit a fine-tuning of the optical alignment. That's a reason for me to pursue some different techniques to align or simply confirm alignment of my optics. I guess, the ultimate access to the wavefront would be an interferometer in auto collimation but that's a bit too extreme for most. Björn |
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2/11/2022
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A double pass test on the lab is nice because you do not care about the weather. However a large flat mirror with a good flatness is quite expensive. When I was at the Patrick's space force base (FL) last November with our SH and SKW we did some tests on their lab in double pass to compare SH versus SKW. We used a huge flat which costs about a half millions, and counting... SH or pyramidal wavefront sensors are used on the sky for AO on many professional telescopes with either an actual or artificial star(s). They are also used for active optics (aO), keeping the telescope aligned and controlling the M1's figure for those having a deformable primary mirror, such as the VLT and the DAG 4 meters telescope (Turkey) under development for which our AIWFS will be used in the aO context. It is obviously not practical to use a double pass and an interferometer for most telescopes at their sites, a wavefront sensor is the way to go. In the context of aO the wavefront is averaged over time to filter the turbulence such only the telescope optical contribution is left in the wavefront. An AO on the other hand goes fast (ms) to compensate for the turbulence in real time, but both use the wavefront (WF) from a stellar source (or a an artificial one from the sodium layer of the Earth upper atmosphere), either a short time WF to freeze the turbulence for AO (faster than the seeing coherence time) or a long time WF for aO (the time averaged wavefront of an actual star is a planewave). With proper acquisition and care wavefront sensors compete with interferometers. Since SKW's engine (AIWFS) is a wavefront sensor it can be used instead of a SH, including under seeing limited conditions. The results match the interferometer without the need of one, nor any special hardware. For years I have used a SH for telescope alignment on the sky, this is a nice tool but expensive (about ~$5K). With SKW you can do the same at a tiny fraction of the SH solution, or interferometer cost. Amateurs have this way the possibility to use the same kind of tool than the professionals have been using since a long time. The AIWFS can be run at video rates, therefore aO and AO can benefice of such technique which has an inherent very large FOV, unlike a SH sensor. Below is a link to a 10 seconds video showing the WF short term fluctuations due to the seeing (image credit: Dr. John B. Hayes) on a CDK20". I removed the telescope aberrations by subtracting the long term wavefront such we only see the seeing contribution. This video is made of 50 frames of 5 seconds displayed at 5 frame per second rate. Of course the raw rate was only 12 frames per minute, but this is not a limitation of the method. The WF were computed using SKW, the video shows side by side , the star in focus and the short term wavefront phase error in the form of a 2D heat plot (red are peaks and blue valleys of the WF phase error): https://www.innovationsforesight.com/Wavefront/AIWFS_SKW_Turbulent_WaveFront.mp4 |
