Newtonian Set Up

[xp1965 (Trevor)]

Hi I recently purchased a 200 mm f4 Newtonian scope and have been having trouble culminating it correctly .I have aligned secondary mirror directly under the focuser but when it come to aligning the mirrors i seem to have a problem.I used a pinhole cap and everything looks right ,but when i use laser its says is out of alignment .I have tried different lasers and also different pinhole caps and the problem is the same.I can rotate each laser or each pinhole cap and it remains centered eliminating a misaligned laser or cap. Is this common ? Any help will be greatly appreciated

[glend (Glen)]

Normally fast f ratio newts like yours require a secondary offset. I suggest you read Astrobaby's excellent article on collimation, which is in the articles section here I believe or download it from the net.

[xp1965 (Trevor)]

Thanks Glen I have read that article but i must be doing something wrong I think I have done it correctly but I dont understand as to why it changes when i compare a laser culmination and a pin hole cap culmination I would have thought if its correctly aligned it would show the same result

[glend (Glen)]

When you look into the focused tube with your pin hole (assuming that is centred) do you see the mirror centre mark offset toward the front as per Astrobaby's photos for fast scopes? Did you alter the centre screw on the secondary - that controls offset distance as well as locking function?
Another thing is that not all lasers are properly collimated themselves, the way to tell is to put it in a level cradle (crossed nail supports in a piece of timber will do) and rotate it while aimed at a target at about three or more metres away. If the red spot stays with a tight circle of say 1cm then it is OK. If the laser wanders around it is not collimated and you may have to use the adjustment screws to centre the beam. Where did you get the laser or what brand is it?
BTW if you drop a laser on the ground, table, etc it can get knocked out of collimation.
Do you have a Cheshire tube?

[Robh (Rob)]

You haven't mentioned how far off alignment the laser is indicating.

Lasers are OK for rough collimation but I always prefer a Cheshire collimator for more accuracy, especially with compression ring draw-tubes. Try this test ...

Wind your focuser right in. Hold the laser flush against the focuser body and turn it on. Note where the beam hits the primary. Now do up the fastening screw(s) that hold the laser in the focuser. Does the beam move from where it was on the primary mirror. Invariable, it does. I have used many laser collimators, including the Glatters, they all have this problem. The fastening screws send the beam off axis.

Regards, Rob

[xp1965 (Trevor)]

Thanks Glend the laser seems to not be off centre when you rotate it and I have tried another laser and its the same I have moved teh centre screw to get the mirror central to teh focus tube.
Robh the laser says the alignment is about 5to 8 mm away from where teh pinhole method says it is I have spent a long time today trying to make sure its perfect and i wll give it another go when its dark tonight ...I would have thought it should be close comparing the 2 methods

[Allan]

Laser collimators at the cheaper end of the scale are notorious for introducing more problems than they solve.

If you think you are close to collimated, then I suggest you just get it out side under the sky and do a star test to check.