Trouble collimating secondary mirror on a Newtonian

[Screwdriverone (Chris)]

Hi Chris,

I too have recently bought an 8 inch Skywatcher reflector and had the same problem with the mirror reflections not lining up like Jason D's avatar pic says they should look.

After having three different reflectors and not having this problem before, (the secondary circle was not centred inside the primary circle) I found the problem was the PRIMARY. It was too high in the tube, what I did was back the locking screws right off and then screw the collimation screws right down so the primary moved towards the bottom of the tube. I had spent over an hour fiddling with the secondary adjusters as well as squaring up the focuser (mine had an extra washer on one screw which skewed it off to one side), until I thought to try the primary screws....

Problem solved, the secondary then lined up concentrically with the primary mirror and all LOOKED fine, then all I had to do is laser collimate slightly to finesse it a bit.

I think the dudes in the factory wind it up and away from the bottom of the tube to avoid vibration damage or chipping during transit. Once you wind it ALL the way down to the bottom, start all over again with the secondary and you should find it will easily all line up and look like Jason's avatar pic like it should.

Hope this helps.

Cheers

Chris

[Jason D (Jason)]

Originally Posted by Riv39

Jason, I aligned the secondary edge with the primary using a collimation cap. I assume it was correct as I could just see all three primary clips. The problem is that when I then use the laser to complete the process only one clip can then be seen when aligned.

It is important to maintain the eye axis along the focuser axis when assessing the alignment of the secondary mirror. It is possible to look slightly off-center to align the secondary mirror and primary mirror reflection edges but that is improper. A sight-tube can help to maintain the eye axis along the focuser axis by referencing the sight-tube edge in addition to the secondary edge and the primary mirror reflection edge.
It helps to ensure the focuser is squared and the secondary mirror is centered.
Finally, ensure the laser collimator is aligned by monitoring the laser dot on the primary mirror while rotating it in the focuser.

Here is a useful link – just the first few pages.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1

Jason

[floyd_2 (Dean)]

Hi Chris,
there is a book called New Perspectives on Newtonian Collimation (5th Edition) that's a must have for your book collection. I have the 3rd edition and it's an excellent resource that taught me how to collimate a newtonian telescope to perfection. The 5th edition also has discussions on using laser collimators, as well as the older sight tube / cheshire / auto collimator that I use.

Dean

[Riv39 (Chris)]

Originally Posted by Screwdriverone

Hi Chris,

I too have recently bought an 8 inch Skywatcher reflector and had the same problem with the mirror reflections not lining up like Jason D's avatar pic says they should look.

After having three different reflectors and not having this problem before, (the secondary circle was not centred inside the primary circle) I found the problem was the PRIMARY. It was too high in the tube, what I did was back the locking screws right off and then screw the collimation screws right down so the primary moved towards the bottom of the tube. I had spent over an hour fiddling with the secondary adjusters as well as squaring up the focuser (mine had an extra washer on one screw which skewed it off to one side), until I thought to try the primary screws....

Problem solved, the secondary then lined up concentrically with the primary mirror and all LOOKED fine, then all I had to do is laser collimate slightly to finesse it a bit.

I think the dudes in the factory wind it up and away from the bottom of the tube to avoid vibration damage or chipping during transit. Once you wind it ALL the way down to the bottom, start all over again with the secondary and you should find it will easily all line up and look like Jason's avatar pic like it should.

Hope this helps.

Cheers

Chris

Thanks Chris, I will follow your suggestion and try again! Glad you suggested this as I would never have worked this one out.

[Riv39 (Chris)]

Originally Posted by floyd_2

Hi Chris,
there is a book called New Perspectives on Newtonian Collimation (5th Edition) that's a must have for your book collection. I have the 3rd edition and it's an excellent resource that taught me how to collimate a newtonian telescope to perfection. The 5th edition also has discussions on using laser collimators, as well as the older sight tube / cheshire / auto collimator that I use.

Dean

Thanks Dean, I will track a copy down and have a look.

[Jason D (Jason)]

Originally Posted by floyd_2

Hi Chris,
there is a book called New Perspectives on Newtonian Collimation (5th Edition) that's a must have for your book collection. I have the 3rd edition and it's an excellent resource that taught me how to collimate a newtonian telescope to perfection. The 5th edition also has discussions on using laser collimators, as well as the older sight tube / cheshire / auto collimator that I use.

Dean

I 2nd Dean's book recommendation. It was written by Vic Menard who is an expert on collimation.
Besides, I have contributed some of the graphics in that book
Jason

[floyd_2 (Dean)]

I thought your avatar looked familiar Jason