erriegraven these horizontal spikes when nothing is in the lightpath?
BTW, I think those spikes are not perfectly horizontal. I took crop from the image and get this:
The black line is aligned to graphical horizon. Notice that the most horizontal spike is angled from the black line.
From this, I suspect that the spikes are not sensor related.
You can also test for sure by rotating the camera angle and see if the spikes rotates. If they do, the problem is somewhere on the OTA end, otherwise it is on the camera end..
Since the diffraction spikes are nearly horizontal, the imperfection is in the vertical direction (consquence of the 2D Fourier Transform). Look for anything that is not perfectly smooth.
I don't know what the retaining ring of your Askar objective is, but on my smaller Askar 300, the retaining ring has a notch for the lens spanner, and it encroaches dangerously to making the aperture not perfectly circular for a skewed ray. You might check yours and look to see if there is any imperfection (even a shiny scratch) all around the aperture.
Speaking of skewed rays, try the image with the bright star perfectly centered. If the spikes are present with skewed rays but not with centered rays, I would look really closely at the objective lens surroundings including the dew shield (which you can also experiment with by retracting it fully). Your smaller camera does not have to worry as much since the angular FOV is smaller.
Chen
