The AT8RC has something of a reputation of being a bear to collimate. I think a lot of people get in trouble because they try to collimate this like a regular SCT. That will lead to nothing but trouble.
Fundamentally, a couple things need to be understood. This is a quick summary of ideas contained here:
http://www.deepskyin...ure_Ver_1.0.pdf
1) Adjustments of the primary are used to adjust the shape (coma) of stars at the center of the field.
2) Adjustments of the secondary are used to make off axis stars symmetric across the field.
What often happens instead is that people use the secondary to try and adjust star shapes at the center. This is completely wrong.
My steps are basically as follows:
1) I use a Cheshire to adjust the secondary (see manual that came with AT8RC for how this should look).
2) With my camera in place, I defocus somewhat. I then use a star near (preferably at) the center of the field and adjust the primary until coma is eliminated. I then defocus it less than before and repeat. As you get closer and closer to in focus, any coma left becomes more obvious. When just a little out of focus, what you will notice if the primary is not collimated is that one side of the star will be significantly brighter than the other side. You want to move the shadow of the secondary closer to the side that is not as bright. Repeat until you can't tell if you are improving things or not even when almost completely focused. (If you use Metaguide, it will tell you the direction of adjustment even when completely in focus).
Hint: I put blue painters tape on the back of my scope near the collimation screws to indicate the direction changes will occur. I use only two of the 3 screws.
3) Observe the 4 corners. Are any aberrations balanced symmetrically? If not, adjust the secondary so that the corners are balanced. If you totally mess up this step, you can put the Cheshire back on and adjust it to the starting position.
Where people mess up using a camera instead of their eyeball is they defocus too much. If you defocus enough, even a telescope that badly needs collimating will look like the shadow of the secondary is centered.
Another hint: If you have the secondary setup with the Cheshire, you can get the primary near where it needs to be by getting the vignetting of the field symmetrical (centered). It probably still need adjusting but you will at least be in the ballpark. If everything was made perfectly (mechanical axis of both mirrors was the same as the optical axis, the mechanical axis of both mirrors was actually centered with the focuser center, there was no tilt, etc., then perfect adjusting of the secondary with the Cheshire, and then perfect centering of the vignetting with the primary would collimate the scope. However, those conditions are probably not true. And they are probably not true enough that collimation is still a must to get good performance from the scope).
Fundamentally, a couple things need to be understood. This is a quick summary of ideas contained here:
http://www.deepskyin...ure_Ver_1.0.pdf
1) Adjustments of the primary are used to adjust the shape (coma) of stars at the center of the field.
2) Adjustments of the secondary are used to make off axis stars symmetric across the field.
What often happens instead is that people use the secondary to try and adjust star shapes at the center. This is completely wrong.
My steps are basically as follows:
1) I use a Cheshire to adjust the secondary (see manual that came with AT8RC for how this should look).
2) With my camera in place, I defocus somewhat. I then use a star near (preferably at) the center of the field and adjust the primary until coma is eliminated. I then defocus it less than before and repeat. As you get closer and closer to in focus, any coma left becomes more obvious. When just a little out of focus, what you will notice if the primary is not collimated is that one side of the star will be significantly brighter than the other side. You want to move the shadow of the secondary closer to the side that is not as bright. Repeat until you can't tell if you are improving things or not even when almost completely focused. (If you use Metaguide, it will tell you the direction of adjustment even when completely in focus).
Hint: I put blue painters tape on the back of my scope near the collimation screws to indicate the direction changes will occur. I use only two of the 3 screws.
3) Observe the 4 corners. Are any aberrations balanced symmetrically? If not, adjust the secondary so that the corners are balanced. If you totally mess up this step, you can put the Cheshire back on and adjust it to the starting position.
Where people mess up using a camera instead of their eyeball is they defocus too much. If you defocus enough, even a telescope that badly needs collimating will look like the shadow of the secondary is centered.
Another hint: If you have the secondary setup with the Cheshire, you can get the primary near where it needs to be by getting the vignetting of the field symmetrical (centered). It probably still need adjusting but you will at least be in the ballpark. If everything was made perfectly (mechanical axis of both mirrors was the same as the optical axis, the mechanical axis of both mirrors was actually centered with the focuser center, there was no tilt, etc., then perfect adjusting of the secondary with the Cheshire, and then perfect centering of the vignetting with the primary would collimate the scope. However, those conditions are probably not true. And they are probably not true enough that collimation is still a must to get good performance from the scope).