Collimating with collimate cap and laser give different results

[skysurfer]

Last week I got a 40cm f/4.5 Dobson with a GSO mirror.
I collimated it by a laser and adjusted teh secondary first and then the primary.
Then I checked it with the collimation cap and saw the peephole completele out of the center ring ... and could not even see the entire primary:-(
So I recollimated it with the collimation cap (first adjusting the secondary until the primary is completely into view through the peephole and then adjusting the primary until the image of the peephole is in the center of the center ring of the primary.
That looked better.
I checked rotating the laser in the eyepiece input but the laser spot did virtually not move so the focuser or miscollimated laser is not the problem

Who is right , the collimation cap or the laser ?

[alistairsam]

Hi,

I'm guessing you're referring to a cheshire collimator as well as the laser.

with the laser, you may not notice the laser moving when rotating unless you point it to something at a reasonable distance. so I'd suggest making a simple "V" jig and point it across the room on a wall and mark the initial spot, then rotate and watch if it moves away when rotating.

I'd go with the cheshire more than the laser.

Other simple test is after you've collimated with the cheshire, remove it and try and look at the reflection of your eye through the focuser. it should be roughly centered with the primary centre spot. make sure your eye is centered in the focuser. one way to do this is to use the base of the drawtube as a reference.

its just a rough test. but it'll give you an idea of which one has errors. so try the eye after the laser and then try it after the cheshire. one of them will definitely be close and that'll give you a baseline.

have you centered the secondary in the focuser as well?

Cheers
Alistair

[barx1963]

Skysurfer
This is a common issue. The laser is only useful for a final "tweak" of the tilt of the mirrors, so it is no surprise that you got this result.
A collimation cap is quite useful for an intial setup, but a good cheshire will enable you to be more precise.
A good guide can be found here http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide .htm which takes the process right through without laser.
I find that with a solid tube I only needed to use the laser on a nighty basis to tweak it and do a full check every couple of months. With the truss scope I use the cheshire each setup and try to make setup identical each time. I number the struts and always tighten knobs in the same order to reproduce setup as close as possible.

Hope this helps

Malcolm