2.15
...
·
·
1
like
|
|---|
|
With this post I want to draw attention to a problem I found with my GSO 10" Truss RC. During collimation I found that when I align the optical axes of the focusser, secondary and primary mirror, the mechanical alignment as it can be seen in a Cheshire eyepiece is way off, and vice versa. To find out why this is I measured the distance the edge of the secondary mirror has from the edge of its holder as can be seen in the following picture:  Using a vernier caliper I found that the distance varies along the circumference of the mirror with a minimum of 28.4mm and a maximum of 28.9mm. In other words: the secondary mirror is tilted within its holder ! With this tilt, the inconsistent and confusing results of my various previous collimation attempts (and possibly also of some other owners of RCs) suddenly make sense. What I learn from this is that collimation should always be done by aligning the optical axes, not the mechanical components of a telescope. I used the following simple steps for a successful collimation:
The only assumption is that the mark on the center of the secondary is its optical center. If this were not completely true, a final star collimation would have helped to correct this. But it turned out that it was not necessary. Here is a simple image of M13, just one 2min exposure with an L filter, to demonstrate that there are circular stars up to the edges of the full-format sensor:  |
