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Hi Bruce,
Thats an easy 1.
You have just discovered 1 of the major shortcomings of a laser collimator. Fact is, your scope isn't even close to collimated.
If you do a star test you will find that the secondary shadow (the black blob in the centre) is not centered within the diffraction pattern on both sides of focus. Also when you defocus on a bright star you should get a small spot of light "the poison" appear in the centre of the secondary shadow. This should appear at the same point on either side of focus, ie. with the same amount of defocusing in both directions. If it doesn't your not perfectly collimated.
Now that I have explained all that cr_p and probably confused you further let me explain the problem. The laser does not take into account the positioning of the secondary in relation to its distance from the primary. ie how close the secondary sits in relation to the primary. This is compounded by the fact that notwithstanding the secondary positioning is incorrect, you can adjust the tilt of both mirrors to have the laser return beam strike back perfectly on its point of origin, making the scope "appear" to be perfectly collimated when its not.
The only way to correctly position the secondary mirror in regards to its distance from the primary is in 1 of 2 ways:-
With a sight tube, which you have just ordered and the other is to use a laser collimator and center spot the secondary. The latter method however is only applicable with a truss dob where you can see the laser hitting the face of the secondary from underneath it.
IMO a laser is only useful for quick adjustments of the tilt of the primary mirror in the field, once the telescope has been properly collimated in the 1st place by other means.
CS-John B
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