Hello:
I am having a rather frustrating time getting consistently good image quality out of my 12.5" f/15 classical cassegrain. Twice when I've used it, the image quality was great, and twice it was not so good with lots of astigmatism. I use the same collimating procedure that I use for my Newtonian and I don't see why it shouldn't work. First I made sure that the focuser is centered in the primary mirror hole, then I use a cheshire collimation tool before sunset each night to:
1) tilt the focuser to point at the secondary
2) tilt the secondary to center the reflection of the primary in the secondary - since the focuser has been centered in the primary this should accurately put the reflection of the focuser in the center of the secondary
3) tilt the primary to center the reflection of the spider on the actual spider
4) point to a star, put in a 300x eyepiece out of focus, and tilt the primary to center the hole in the donut
This procedure uses the focuser to secondary axis as the reference for the alignment. I have read procedures that call for very accurately centering the spider in the tube, but it should not be necessary to align the optical axis with the tube axis. Step 2 is meant to align the secondary to the focuser. I could try to get the reflection of the cheshire in the center of the secondary, but since the reflected image of the primary is just a little smaller than the seconday I think it is easier (and thus more accurate) to center the edge of the reflected image of the primary in the secondary mirror.
So, my question is: Is there a better procedure to accurately collimate of the telescope? Is there some obvious thing that I am missing? When I go to the star, is it better to adjust the primary or the secondary? I have been adjusting the primary since the primary collimation screws are easy to adjust from the eyepiece. I am coming to appreciate that this telescope is very sensitive to collimation, and that driving it up mountains and packing it into cars isn't good for the collimation. Any help will be greatly appreciated.
Cheers
Mike Connelley
I am having a rather frustrating time getting consistently good image quality out of my 12.5" f/15 classical cassegrain. Twice when I've used it, the image quality was great, and twice it was not so good with lots of astigmatism. I use the same collimating procedure that I use for my Newtonian and I don't see why it shouldn't work. First I made sure that the focuser is centered in the primary mirror hole, then I use a cheshire collimation tool before sunset each night to:
1) tilt the focuser to point at the secondary
2) tilt the secondary to center the reflection of the primary in the secondary - since the focuser has been centered in the primary this should accurately put the reflection of the focuser in the center of the secondary
3) tilt the primary to center the reflection of the spider on the actual spider
4) point to a star, put in a 300x eyepiece out of focus, and tilt the primary to center the hole in the donut
This procedure uses the focuser to secondary axis as the reference for the alignment. I have read procedures that call for very accurately centering the spider in the tube, but it should not be necessary to align the optical axis with the tube axis. Step 2 is meant to align the secondary to the focuser. I could try to get the reflection of the cheshire in the center of the secondary, but since the reflected image of the primary is just a little smaller than the seconday I think it is easier (and thus more accurate) to center the edge of the reflected image of the primary in the secondary mirror.
So, my question is: Is there a better procedure to accurately collimate of the telescope? Is there some obvious thing that I am missing? When I go to the star, is it better to adjust the primary or the secondary? I have been adjusting the primary since the primary collimation screws are easy to adjust from the eyepiece. I am coming to appreciate that this telescope is very sensitive to collimation, and that driving it up mountains and packing it into cars isn't good for the collimation. Any help will be greatly appreciated.
Cheers
Mike Connelley
