My 3D printed telescope has a weird problem: it's infected by ghosts. When I look at Saturn, I see a second ghost saturn. It happens for all other objects, too, and stretches Jupiter's moons into lines. I've been assuming my mount is wobbling too fast for the eye to see, so I've gone through 3 revisions of stiffer and stiffer mounts.

I finished adding extra front bracings to mount v3, and went out observing, hoping it would be stiff enough to see the ring nebula.

Once I aim the telescope, it takes 5s or so to settle down. But once it's settled, I noticed one star looked like it was split into two stars. Here's what a star (which should be one point) looked like to my eye:

When I looked at Saturn, I still saw a double saturn. Picture also above!

I checked a double star and saw a fun sight: two stars, each split into two points. Picture also above! (It was eta lyrae, the "double double". While each star in that image is actually a double star, they're definitely not double stars in the same direction)

But this time, I finally noticed the direction of that double image effect: it was in the direction of altitude. I looked at something on the horizon - the second image was straight up and down from the first one. I look at something 45 degrees up - the double image was tilted 45 degrees. Interesting.

I had assumed the double image was coming from side-to-side wobble. But if I look at something on the horizon and see up/down motion, then it's clearly not side-to-side wobble. "It must be up/down wobble!" I thought, writing this post.

But then I remembered a test for astigmatism. I unscrewed, the assembly with the primary mirror and put it back on rotated 120 degrees... and the "wobble" was in a different direction. Here's what I drew while out observing:

So if the "wobble" changes direction when I change the mirror... Double Saturn was never a wobble in the mount in the first place. My mirror is just bad, and focuses a point of light to two points barely next to each other. My mount was great all along!

I talked about this in a discord and someone offered to send me a new primary mirror for $20. I'll be installing that once it arrives.