To those who have tried collimating a Schmidt-Newtonian or a Maksutov-Newtonian:
I also feel your pain.
I just got through collimating mn Meade SN-8 for the 5th time a few days ago. It is closer to perfect collimation than ever before......BUT
I looked in Weasner's website and he had a few tips to collimate the Schmidt-Newtonian that I never considered. I MIGHT be tempted to try them, if anybody else has seen a better result than by going by the Meade LXD75 owner manual (I know it isn't much, but it's what I started with). I now sort of have my own personal way of doing it since a laser collimator and a peep cap are my main alignment tools.
I used a laser collimator to get my diagonal rotated square with the focuser. If the diagonal is straight in axis with the optical tube and the focuser is square with the optical tube, then the laser dot will hit the mirror in a line parallel to the focuser axis and hopefully in the center of the primary. The dot will move left or right of center if the diagonal mount is rotated counterclockwise or clockwise respectively. You can rotate the diagonal mount from the front without removing the corrector cell. Weasner uses a T shaped template that fits snugly in the focuser tube. I guess that works as well, but I would rather have laser light than a cardboard T template on my mirror, personally.
He also removes the primary to get more light in the tube and places white paper behind the diagonal in order to get the diagonal directly under the focuser. I had a hard time seeing everything when I aligned the diagonal with the focuser tube, and I aimed my OTA at white curtains that were flooded by direct sunlight in order to get enough light into the tube without removing the primary.
One thing he did that I wish that I had done is remove the front cell retaining ring and see if the Schmidt corrector plate was EXACTLY in the center of the cell. If it is off, then the correct mirror offset will not be right when the diagonal is centered under the focuser and/or the secondary will sit to the left or right when the diagonal axis is parallel to the optical tube axis.
I made sure my diagonal was in line with the optical tube axis. The diagonal can thus be square to the focuser and you can pull it forward or backward to get the correct down and back offset. The penalty is a bit of light loss if the diagonal is not exactly centered under the focuser. Light falloff is not nearly as severe at f/4 as it is at f/8 if the mirror is not exactly centered under the focuser and is more of a photographic consideration that a visual consideration.
The method of doing the final alignment by centering the reflections and shadows on the corrector plate by using a Cheshire sight tube and a bright light were new to me. I'll have to get a Cheshire sight tube in order to try that one.
As it is, it sits pretty good as far as a star test goes. I was getting pretty good results even with a less than perfect star test result, but my main use of the scope is deep sky, not planetary or lunar work. The lack of chromatic aberration relative to my Stellarvue Nighthawk achromat refractor seems to have made up for the lack perfect collimation, and my Nighthawk is perfectly collimated, which is standard with just about any Stellarvue scope that hasn't been abused.
Measured at the back of my primary cell, the mirror tilt is within 1/32" from high to low reading measured from the rear of the primary holder to the tube mounting ring. I suspect from that measurement that things must be getting close to excellent collimation. The factory alignment was atrocious with a very definite egg shaped star test.....and the scope still gave up acceptable deep sky views.
I suspect that most Meade Schmidt-Newtonians are poorly collimated. I suspect that those who have halfway decent collimation are rarely seeing anything bad enough to really get into getting theirs perfectly collimated.
As you move the scope around with a heavy binoviewer in the focuser (I use a JMI NGF55), you will see a slight change in the star test due to the bending of the focuser and the weight of the diagonal/corrector plate assembly bending a bit. I collimate my scope with the optical tube pointing straight up and some weight on the focuser. You can then see the laser dot move off center as you move the optical tube all around. I maintain that prefect collimation is thus a bit of a myth, and I try to split the differences and just keep the laser dot in the primary mirror center dot as I move the scope around.
Weasener's method gives him a perfect alignment with the scope tube horizontal........but the funny thing with me is that I rarely use my SN-8 in that orientation.
YMMV
Phil Canard
I also feel your pain.
I just got through collimating mn Meade SN-8 for the 5th time a few days ago. It is closer to perfect collimation than ever before......BUT
I looked in Weasner's website and he had a few tips to collimate the Schmidt-Newtonian that I never considered. I MIGHT be tempted to try them, if anybody else has seen a better result than by going by the Meade LXD75 owner manual (I know it isn't much, but it's what I started with). I now sort of have my own personal way of doing it since a laser collimator and a peep cap are my main alignment tools.
I used a laser collimator to get my diagonal rotated square with the focuser. If the diagonal is straight in axis with the optical tube and the focuser is square with the optical tube, then the laser dot will hit the mirror in a line parallel to the focuser axis and hopefully in the center of the primary. The dot will move left or right of center if the diagonal mount is rotated counterclockwise or clockwise respectively. You can rotate the diagonal mount from the front without removing the corrector cell. Weasner uses a T shaped template that fits snugly in the focuser tube. I guess that works as well, but I would rather have laser light than a cardboard T template on my mirror, personally.
He also removes the primary to get more light in the tube and places white paper behind the diagonal in order to get the diagonal directly under the focuser. I had a hard time seeing everything when I aligned the diagonal with the focuser tube, and I aimed my OTA at white curtains that were flooded by direct sunlight in order to get enough light into the tube without removing the primary.
One thing he did that I wish that I had done is remove the front cell retaining ring and see if the Schmidt corrector plate was EXACTLY in the center of the cell. If it is off, then the correct mirror offset will not be right when the diagonal is centered under the focuser and/or the secondary will sit to the left or right when the diagonal axis is parallel to the optical tube axis.
I made sure my diagonal was in line with the optical tube axis. The diagonal can thus be square to the focuser and you can pull it forward or backward to get the correct down and back offset. The penalty is a bit of light loss if the diagonal is not exactly centered under the focuser. Light falloff is not nearly as severe at f/4 as it is at f/8 if the mirror is not exactly centered under the focuser and is more of a photographic consideration that a visual consideration.
The method of doing the final alignment by centering the reflections and shadows on the corrector plate by using a Cheshire sight tube and a bright light were new to me. I'll have to get a Cheshire sight tube in order to try that one.
As it is, it sits pretty good as far as a star test goes. I was getting pretty good results even with a less than perfect star test result, but my main use of the scope is deep sky, not planetary or lunar work. The lack of chromatic aberration relative to my Stellarvue Nighthawk achromat refractor seems to have made up for the lack perfect collimation, and my Nighthawk is perfectly collimated, which is standard with just about any Stellarvue scope that hasn't been abused.
Measured at the back of my primary cell, the mirror tilt is within 1/32" from high to low reading measured from the rear of the primary holder to the tube mounting ring. I suspect from that measurement that things must be getting close to excellent collimation. The factory alignment was atrocious with a very definite egg shaped star test.....and the scope still gave up acceptable deep sky views.
I suspect that most Meade Schmidt-Newtonians are poorly collimated. I suspect that those who have halfway decent collimation are rarely seeing anything bad enough to really get into getting theirs perfectly collimated.
As you move the scope around with a heavy binoviewer in the focuser (I use a JMI NGF55), you will see a slight change in the star test due to the bending of the focuser and the weight of the diagonal/corrector plate assembly bending a bit. I collimate my scope with the optical tube pointing straight up and some weight on the focuser. You can then see the laser dot move off center as you move the optical tube all around. I maintain that prefect collimation is thus a bit of a myth, and I try to split the differences and just keep the laser dot in the primary mirror center dot as I move the scope around.
Weasener's method gives him a perfect alignment with the scope tube horizontal........but the funny thing with me is that I rarely use my SN-8 in that orientation.
YMMV
Phil Canard
