I have a Dall Kirkham iDK. Its specified focal length is 2128mm yet my plate solved images currently come out to 2123mm. Does Skywave base its spacing adjustment "+/-" indications on the specified 2128mm or does it compute what the best spacing should be to minimize spherical aberration solely from the wavefront analysis? In other words, can Skywave's recommended spacing be different than the manufacturers spec?
A very important consideration for me is that the collimation will result not only in optical congruence of both mirrors but also that the primary mirror M1 is optically congruent with the focuser tube. I wish for this to be the case so that no sensor tilt is introduced when the focuser is rotated. Can Skywave be used to first optically align the M1 reference to the telescope backplate/focuser/camera? iDKs do not have a fixed primary mirror and mine may be out of alignment with respect to the focuser tube. Any suggestions on how to do this part of the alignment?
I downloaded SkyWave and played with it a bit. I created an instrument and connected to it, and loaded a raw inage. It starts off with a Trial Mode and the center of the target blocked but then if I Simulate the blue center block goes away and a point is plotted on the target. If I simulate again, a different point appears. I'm confused as to what is going on here. If I'm in Trial mode why is the center (results above 6) becoming unblocked and why does re-simulating produce a different result?
I also requested a Trial model for my particular telescope about a week ago and have not received anything. How long does the Trial model for my scope take to receive?
thanks
Dave
All optical surfaces are subject to tolerances relative to the specifications, which is inevitable. Telescope makers pair M1 and M2 mirrors to optimize results for a specific telescope. Consequently, the actual focal length of any telescope rarely matches the specified one, meaning there are no two identical telescopes.
A few percent difference in the actual focal length is common and should be expected. For a nominal value of 2,128mm, this tolerance can result in a difference of a few inches. During telescope assembly, the spacing between both mirrors is adjusted to minimize spherical aberration using metrology tools. This spacing is influenced by the M1 and M2 pair, as mentioned earlier in relation to tolerance.
Additionally, the telescope's focal plane relative to M1 varies from one telescope to another. Manufacturers typically set the M1 mechanical position to maintain a constant back working distance from the telescope's visual back. This distance is where the camera sensor plane should be located.
SKW's engine is a wavefront sensor, providing aberration data and functioning as a metrology tool. SKW models are based on nominal (specified) focal lengths, apertures, and central obstruction values. However, users can freely adjust parameters like wavelength and pixel size. Tolerances (typically a few percent) on these values have minimal impact on wavefront results, generally causing differences of just a few nanometers. When using SKW, one should use the telescope's specified optical parameters.
A word of caution regarding measuring the focal length with plate solving: Given that the difference is usually small (in your case, only -0.25%), one may question whether the inherent measurement errors in plate solving are as significant as the actual focal length difference we are looking for. Also attempting to adjust the M1 to M2 spacing based on this information to match the specified focal length is not recommended since the actual focal length is unknown, and the spacing was likely adjusted accordingly at the factory. Changing it may worsen the situation. The only reliable way to adjust the spacing is by measuring the optical effect, spherical aberrations, using optical metrology tools like the Ronchi test or wavefront sensing (e.g., SKW).
The tilt of the optical train relative to the telescope's optical axis results in a defocus gradient across the field of view. Since SKW measures aberrations (not FWHM values, which combine everything), you can independently align both mirrors (congruence) despite such tilt because you are primarily addressing coma rather than defocus. In the Zernike decomposition, these two balanced aberrations are orthogonal (independent) of each other. However, this isn't true if you use FWHM or HFD values, as they combine everything and make it challenging to separate collimation and mechanical image tilt.
It's important to note that an uncollimated scope (from a mirror congruence standpoint) will have a tilted optical axis and, consequently, a tilted focal plane relative to the sensor plane, even if both become congruent after collimation. In short, seeing tilt (defocus gradient) in the image for an uncollimated scope is normal, and attempting to correct tilt before achieving scope collimation (congruence) is not recommended. This is why FWHM or HFD values are less suitable and challenging to use.
This is also why we have optical metrology tools when doing optics and optical alignment, using the right aberration at the right place and time.
In your case, after addressing on-axis coma, you can use SKW to examine defocus across the field to measure mechanical tilt, if present. SKW Pro offers a direct tool for this, while SKW collimator provides a defocused error (in microns) vertical gauge on the left side of the SKW collimator tool. By observing how this error changes for various stars across the field (with the focus position unchanged), you can calculate the tilt. The goal is to achieve a balanced defocus across the field. Any tilt makes defocus unbalanced relative to the on-axis point.
Since version 5.5 (currently at 5.6), SKW automatically offers a trial model for any SKW instrument settings. There is a built-in generic model, eliminating the need to request a trial model. However, this generic model is limited, and SKW provides collimation feedback only for scores below 6 (coma aberration). Beyond that, you'll need to purchase a model tailored to your telescope.
The SKW simulation function allows you to explore the software without providing actual FITS images. Each time you click the simulation button, SKW generates a new randomly defocused image with some aberrations. You can view this image in the small thumbnail image window next to the collimator tool target (bottom left). Collimator scores obtained using the simulator are based on the simulated random images, so they change each time. This function is designed for software evaluation and training.
For more detailed information on SKW and its functionalities, you can refer to the SKW documentation, which is also accessible within SKW as interactive help:
https://www.innovati...Collimator.html
Edited by Corsica, 02 December 2023 - 09:59 AM.
