...I received my chesire/sightube a few days ago, and I certainly don't like it. My main concern is that is long and heavy and won´t go deep into the focuser. I put some tape to prevent play, but its weight produces flexure, I can tell. This make it quite inaccurate. Besides, it won´t be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I can´t see the primary clips. I guess this is the wrong lenght for my scope.
The optimal length of the sight tube approximates the focal ratio of the primary mirror. You can find more here
(scroll down about half way). When the sight tube is too long, you encounter this scenario:
1.) When the sight tube is pushed in to far, you can't see the edge of the primary mirror
2.) When the sight tube is pulled out to see the edge of the primary mirror, the apparent size of the secondary mirror is reduced effectively occulting the edge of the primary mirror.
The solution is a shorter sight tube (perhaps a bit less than the primary mirror focal ratio).
FYI, the sight tube focal ratio is equal to the distance from the pupil (top) to the bottom of the sight tube divided by the aperture at the bottom of the sight tube.
That said, the registration problem (sloppy fit) you've described indicates a problem with the sight tube, 2- to 1.25-inch adapter, and/or the focuser drawtube.
I also have a hard time when focusing the crosshair to perform the FAA and I don´t like the reflection that produces to perform the primary axial collimation, every thing is so tiny and confusing!
Some people find that if they pull their eye away from the sight tube pupil, the sight tube cross hairs become more distinct relative to the primary mirror center spot. With regards to "tiny and confusing"--everything's the same size as it is in your collimation cap, except the field of view is narrower
. There's certainly more going on with the sight tube cross hairs overlapping the field of view, but the fact that you can "see" errors that aren't obvious in a collimation cap is the reason why sight tube collimation is important.
Anyway, while waiting it to arrive I made some last tweeks to the mechanicalls alignments and I improved my collimation cap with a shiny surface. Now is much easier to collimate the primary.
This is how my final collimation looks.
Why did you cover the primary in the second image?