Collimation problem

[Roger2525]

Hi - I think I have finally lost the plot. To make collimation of my 12" skywatcher dob easier I purchased Bobs knobs screws.

I use a laser which is collomated but every time I centre the red dot in the donut then adjust the primary, when I go back to check the secondary it is slightly out of centre. I repeat the process and then it happens again. I seem to be going round in circles! The screws are tight and I can't see why the red dot keeps going off centre as the secondary mirror seems tight. Does adjusting the primary mirror throw the secondary mirror out? I thought this collimation was easy. I'm now getting paranoid ! Help please.

thnks Roger

[pipnina]

1 minute ago, Roger2525 said:

Hi - I think I have finally lost the plot. To make collimation of my 12" skywatcher dob easier I purchased Bobs knobs screws.

I use a laser which is collomated but every time I centre the red dot in the donut then adjust the primary, when I go back to check the secondary it is slightly out of centre. I repeat the process and then it happens again. I seem to be going round in circles! The screws are tight and I can't see why the red dot keeps going off centre as the secondary mirror seems tight. Does adjusting the primary mirror throw the secondary mirror out? I thought this collimation was easy. I'm now getting paranoid ! Help please.

thnks Roger

I know that adjusting the secondary will require changes to the primary. However I don't believe adjusting the primary to be correct to the secondary should set the secondary off.

I don't know anything about laser collimators, but I might suggest using a more low-tech solution for the secondary (like a collimation cap) and then using the laser to collimate the primary.

[Buzzard75]

Adjusting the primary may move the laser a little bit in the center dot, but it shouldn't move that much. It should still be fairly close. I always check the secondary alignment after adjusting the primary and I can probably count on one hand the number of times I've had to adjust the secondary after adjusting the primary. Adjusting the primary should make it move around that central point which should have no effect on the secondary alignment. It almost sounds like your entire primary mirror is shifting around in the cell.

My two cents anyway.

[Charic]

1 hour ago, Roger2525 said:

To make collimation of my 12" skywatcher dob easier I purchased Bobs knobs screws.

I did not buy Bob's Knobs, simply because of the reach! I cant look down the focuser and adjust the knobs, so for me the task would be no easier.

However, print, read,  re-read, maybe dream about the info here  http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/  until  you fully understand the method, and what's involved, then collimating becomes easier.
There are several guides on the web, but I often refer to this one.......

A simple 35mm or telescope dust cap is the simplest tool, then a Cheshire or laser, but if your using a Laser I would suggest you look up how to Barlow the laser, which again makes the process simpler. But understanding and getting the process right in the mind first will help enormously, it did for me.

[Ricochet]

4 hours ago, Roger2525 said:

Hi - I think I have finally lost the plot. To make collimation of my 12" skywatcher dob easier I purchased Bobs knobs screws.

I use a laser which is collomated but every time I centre the red dot in the donut then adjust the primary, when I go back to check the secondary it is slightly out of centre. I repeat the process and then it happens again. I seem to be going round in circles! The screws are tight and I can't see why the red dot keeps going off centre as the secondary mirror seems tight. Does adjusting the primary mirror throw the secondary mirror out? I thought this collimation was easy. I'm now getting paranoid ! Help please.

thnks Roger

Are you moving the telescope angle at all between steps? Is it possible there is some focuser slop allowing the laser angle to move slightly? I don't remember ever having to readjust my secondary after the primary but I guess it is possible you have to perform iterations to get the two to converge.