[Stefan Buda]

An interesting new case: The 8" Meade Lx200 ACF that can't be collimated.

The owner showed me a TriBahtinov image with the best possible collimation and it clearly didn't look good enough.

To start, I placed the OTA on my EQ mount, in a horizontal position, and I examined the multiple reflections for concentricity. From a distance of about 7 or 8 meters I could clearly see a good amount of misalignment.

Next I used a well collimated laser to check the retroreflection from the secondary mirror and the beam came back with an offset of about 12mm.
That is a huge error and would not be consistent with the reported performance if it was simply due to a misaligned secondary mirror.

Before going any further I unscrewed the Baader 2" clicklock adapter and measured it to make sure it was not introducing a tilt. The result was less than 10 microns tilt across 52mm - very good.
So I proceeded to set up an artificial star about 30m away to have a look at the collimation. I found that the defocused donut was not too bad - consistent with the reported performance.
Then I reinstalled my laser into the Baader clicklock and adjusted the secondary collimation to get the secondary collimated and then I replaced the laser with an eyepiece to have a look at the artificial star. This time the star donut looked completely eccentric - telling me that the OTA must have a severely tilted primary mirror.

This is where the problems started. When I tried to separate the back assembly from the tube, it turned out that the tube is bonded to the back casing.
Not being able to get to the primary I decided to put up the whole OTA on the lathe and give it a spin to confirm my reasoning about the location of the problem.
I turned a solid 2" internal thread to fit the back end of the OTA and I was able to get the scope attached and rotating nicely on the lathe.
Everything looked reasonably concentric but the primary mirror showed a very serious wobble. I wish I could post the video.

Now it looks like I will have to try and extract the primary through the front end and find out what is giving it the tilt.