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AI Telescope Collimation: Review of SkyWave-Collimator

Wavefront sensing using AI to measure aberrations to determine perfect telescope collimation. Credit: Innovations Foresight.

 I recently received a 14-inch reflecting telescope I’ll be using for high-resolution deep sky imaging. While most telescopes are collimated by the manufacturer, they are often knocked out of perfect alignment during shipping.

I installed the new collimation software from Innovations Foresight, SkyWave-Collimator, developed by Dr. Gaston Baudat to ensure my collimation was perfect before I begin my long exposure imaging projects. I will share my experiences here with some samples and results.

Collimation

Collimation is the process of aligning the optics at their design positions to achieve optimal performance with minimal aberrations (distortions).

Even slight displacement will cause aberrations such as coma, astigmatism, or spherical aberration. A miscollimated telescope will perform poorly, with odd-shaped stars and a blurry image.

Schematic of a two-mirror Cassegrain type reflector. The mirrors need to be perfectly aligned to work as designed. Credit: Rouzbeh Bidshahri

Classical Collimation Methods

Collimation tools like Cheshire eyepieces, collimation scopes, lasers, etc. are useful but they do not achieve true precision collimation. These methods depend on mechanical centering rather than true optical collimation, but there is no guarantee that the physical center of a mirror is exactly the same as the optical center of the polished mirror curve.

My holographic laser showed perfect alignment, failing to detect a slight tilt in my optical alignment.  Credit: Rouzbeh Bidshahri

The defocused star test has been used for years and will get you very close but, again, there is no guarantee that the defocused “doughnut” represents the true optical axis. Baffles and secondary mirrors are often slightly offset from perfect optical center.

The defocused star below is not concentric, with the dark middle ring slightly thinner on the upper left compared to the bottom right, despite the telescope being in its best alignment positions as verified by various methods.

The image of a defocused star is not concentric despite the telescope being at its optimum alignment as verified by various methods. Credit: Rouzbeh Bidshahri

SkyWave Wavefront Analyzer

SkyWave-Collimator is unique in using artificial intelligence wavefront sensing to analyze a single defocused star image to identify aberrations due to collimation errors in the optics.

All you do is point the telescope at a star, defocus the telescope, and take a single image. The method can even tolerate quite a bit of poor seeing.

You then feed that image file to the software, which analyzes it by comparing it with images in its large database to work out the differences from perfect collimation.

Software Setup

Because the software uses a model specific to your telescope, there’s a process to go through setting it up. You first install the free, limited version of the software and then purchase the mathematical model for your telescope type. You then send the company information on your scope – aperture, focal length, central obstruction, etc. – and receive the tailor-made model the software will use to analyze your stars. There are purchase options for pay-per-use and unlimited use.

Collimation Process

The collimation process is surprisingly simple. What I describe here is for reflectors, but SkyWave-Collimator can be used with refractors as well. Reflectors with spherical secondary mirrors such as SCTs, CDKs, and IDKs are the easiest to collimate as they don’t exhibit astigmatism from misalignment.

Tilt alignment

Tilt misalignment of the optics induces on-axis coma. SkyWave-Collimator checks this using a defocused star image. The correction is usually made by adjusting the secondary mirror.

The telescope model you receive from the company specifies the requirements for the defocused star image to within narrow tolerances. The parameters include how well the star is centered, how much it is defocused, and the exposure length, along with the star’s brightness.

The software determines if your image meets the requirements. If not, it indicates the changes needed for a new image. Repeat the procedure until the software has what it needs for analysis.

The software then displays the bullseye chart below showing the telescope’s collimation. If it’s off, you adjust the secondary tilt, take another image, and check it again, repeating the procedure until it’s perfect. A supplied PDF guides you in determining the direction you need to turn the secondary tilt adjustment screws.

Adjusting the collimation knobs until the bullseye is perfectly centered. Note the slightly non-concentric defocused star image (lower left).   Credit: Rouzbeh Bidshahri

Mirror Spacing

Incorrect spacing between the mirrors will induce spherical aberrations, which can’t be tested using the classical defocused star method with most telescope designs. While CDKs can be tested with special spacers and a Ronchi eyepiece, SkyWave-Collimator will do it without the need for additional equipment and procedures.

Here too, Skywave-Collimator will indicate if the spacing is too close or too far and you can adjust accordingly.

Using the same defocused star image as above, SkyWave-Collimator indicated that the mirror spacing was nearly perfect. If it wasn’t, the plus or minus sign would have indicated the direction of the error (too far apart or too close). Credit: Rouzbeh Bidshahri

RC telescopes collimation is more complicated than what I describe here as it involves tilting both the primary and secondary to correct on axis coma and then off axis astigmatism. My telescope is a CDK, but the company outlines the steps required to collimate an RC using Skywave-Collimator as well.

Results and Verification

SkyWave-Collimator indicated that my collimation was nearly perfect once I completed the procedure, but I wanted to double check its results.

First, I took an exposure with a small-pixeled full-frame camera, a type that is notorious for picking up any minor aberrations, especially in the corners.

The image below shows the extreme corners at full resolution with tiny 3.76-micron pixels and a focal length of 2565mm, yielding 0.3 arcseconds/pixel. The image shows perfectly round and symmetrical stars, a very good sign that the optics are well collimated.

Mosaic of only the extreme edges and corners of a full frame image. The stars are perfectly round and free of the aberrations that would be present if the collimation (or optics) wasn’t perfect. Credit: Rouzbeh Bidshahri

I then replaced the imaging train with a planetary camera and a 5x Barlow. This increased the focal length to 12,800mm, allowing me to capture the Airy disc of a focused star using lucky imaging. This is a lengthy process, but it is very sensitive to on-axis coma, which would indicate any tilt misalignment (astigmatism and spherical aberration need other methods to verify).

An almost perfectly symmetrical Airy disc imaged with the CDK14 at 12,800mm. The symmetrical Airy disc, absent on-axis coma, is a good indicator of near perfect tilt collimation. Credit: Rouzbeh Bidshahri.

SkyWave-Collimator indicated my mirror spacing was nearly perfect. Mirror spacing does induce spherical aberration, and it also changes the focal length compared to the design focal length.

When I took an image and plate-solved it to calculate my actual focal length, it was exactly what the telescope specifications stated, 2565.3mm. This meant the spacing was indeed correct and agreed with SkyWave-Collimator’s analysis.

 Final Thoughts

Getting started and setup for SkyWave-Collimator requires a bit of work to install and register. The website and its diagrams are a bit confusing and difficult to follow at times. The unlimited license is relatively expensive, but the pay-per-use license costs less which I use. I was concerned about needing to buy more credits, but it didn’t require too many attempts.

Collimating with SkyWave-Collimator is very straightforward, though. You need only point your telescope at a star, defocus, and take a single exposure. The software will indicate how far your collimation is off and in which direction, making it easy to make adjustments.

Big benefits include not needing auxiliary tools and not having to remove your camera and imaging train. That can be a complex job, and the system may also need to be rebalanced when the equipment is reinstalled.

More importantly, collimation with SkyWave-Collimator is more accurate than simply visually inspecting a defocused star for concentricity.

I found the collimation process to be quick using SkyWave-Collimator, allowing me to get on to other aspects of imaging. SkyWave-Collimator can be a game changer for collimation, the mere mention of which sends shivers down the spine of most reflector users!

 

MSRP: Various plans including pay-per-use and unlimited use. See the website for details.

Website: https://www.innovationsforesight.com/

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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