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Hi Folks, I would like to share a little tutorial with you about the alignment of the secondary mirror vanes. The goal is to create nice and narrow diffraction spikes without splitting towards the ends. This applies to all Newtonians with adjustable vanes. I have performed this simple procedure on my 150PDS from Skywatcher. If you are, like me, completely reinstalling the secondary and the spider, start here: It is helpful to have little friction in the threads of the vanes. So it is easier to apply a good pull on the vane without bending it due to torque. I have put a little MoS2 grease on the threads to achieve this. Bring the center of the spider approximately into the middle of the tube. You can do this with a ruler or a caliper. This first step is geometric and super precision is not needed. This is easier if the secondary is attached to the carrier with the offset already applied. This is the case for the Skywatcher Newtonians. You need to figure out the design offset of your spider if the Newtonian was designed to have a decentered spider. Once your spider is approximately in the center, apply a bit of pull to the vanes to have them secure and stable. Especially the thin Skywatcher PDS vanes behave a little bit like bicycle spokes. They need some tension to do the job and hold collimation properly. Don’t overdo it, the tube of the newton should not be severely deformed. If everything is stable and good to go, make sure you have installed all the parts that can influence the stiffness and shape of the Newtonian near the secondary mirror. e.g. the front ring that covers the edge of the tube. Now do a rough collimation of the secondary and the main mirror to achieve a good star shape in the center of the field. And now the fun part begins. Also for those of you who have a working telescope and just suffer from diverging spikes. Attach your camera and center a bright star in your field of view. The star should be higher than 30° over the horizon to have limited air mass and turbulence in the atmosphere. Higher is better. Defocus the star so much, that you can see the diffraction spikes splitting. It is ideal to have a little gap between the spikes. Do a short exposure to get a high-quality preview of the diffraction (better than your live view). e.g. a 5-10s exposure. Most likely you will see the double spikes to not be perfectly parallel from end to end. You might see something like this:  Having diverging lines means that the two opposite vanes are not parallel. You can now adjust the vanes of one axis by a good turn of the adjustment screws to see in which direction you have to go. Always undo one side and then tighten the other side by the same amount. After you have done this, take another exposure to evaluate the change. Note: the vertical spikes are created by the horizontal vanes and vice versa. You may need some turns on both axes to see the direction you need to adjust. The goal is, of course, to have both pairs of spikes perfectly parallel to each other. The small distance between the two spikes of the defocused star helps you a lot by giving you a huge visual magnification of the effect. You might be happy with a parallel set of spikes like this one here:  (in this picture you can see a third and relatively faint spike in the vertical axis. This is caused by my focuser. It is a little bit too long and reaches into the optical path, creating the additional spike) After you have aligned your two pairs of spikes, you should give the scope another collimation. Most likely, adjusting the secondary will do, unless you had to shift it by a lot. After you have collimated your scope, you may have spikes like these:  The star in the picture is 51 Herculi, a Mag 5.0 hypergiant. Not super bright, but something comparable with the stars I often have in my FOV. Sure, Alnitak will give you more headaches near the horsehead… The picture is a 100% screenshot stacked from 2h of 300s each with ISO400 with my EOS 700Da. The diffraction is wavelength dependent, so the spikes show the light spectrum of the star. You should have nice shapes of your spikes without any notable divergence. One thing is worth mentioning about the spikes on my scope: I have black velours on the vanes. I found that this makes the spikes shorter. But I don’t have a comparative picture of this subjective finding. Best regards and clear skies! Volker |
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The only reason that black velour on your vanes makes the diffraction spikes shorter is because you are making them thicker in the entrance pupil. Making them thicker diffracts more light, but the pattern ends up closer to the source--and brighter. That's just the way Fourier optics works. It has nothing to do with the fact that the velour is black. John |
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I thought it is informative to add this information and point to the subjective character of this. If the spikes get thicker but shorter, it is ok for me. Usually the length of the spikes is what bothers me. But this post was mainly about the defocused alignment method. To get them congruent within a few minutes. |
