Collimation

[GHMikeB]

First of all I would like to say hello to everybody as I have just joined the forum today.

Although I have been interested in astronomy for just about all my life (I'm 60), I have only just bought my first real telescope, although I have been using a pair of 15×60 binoculars for about 25 years.

Anyway I have bought a SkyWatcher 200 mm. on the HEQ5 Pro mount with SkyScan.

My question is - how do I stop the secondary mirror rotating while collimating. I used a sight tube and got the secondary mirror centred in front of the eyepiece tube and in the telescope tube and nicely aligned to show the entire primary mirror with all three clips just showing, as per the instructions.

I then inserted a laser collimator and tweaked the secondary until the beam was dead centre in the ring on the primary and then adjusted the primary until the beam disappeared back from whence it came.

On looking through the sight tube again the primary mirror was no longer anything like centred. With a lot of starting again I have now got it so it looks pretty close, but not quite right. It seems that as I tighten the allen screws on the secondary it rotates slightly.

I thought that once the first part (centring the flat) was done it would stay that way during the second and third stages, but it seems this is no so!

Nowhere seems to mention this added complication which is driving me nuts!

[GHMikeB]

After nearly going mad trying to collimate my SkyWatcher 200 mm., I have at last found the problem.

What the instructions that come with it fail to mention is that the eyepiece focus tube is adjustable and that if this is not correctly aligned you will never get it right.

After carefully marking the tube directly opposite the centre of the eyepiece, I centered the laser collimator spot on it (it was ¼" out), then replaced the secondary mirror and a few minor tweaks to get the spot in the centre of the primary and then adjusting the primary to get it centered back at the eyepiece end and all seems perfect.

Previously every time I centred the laser in the middle of the primary a good chunk of the primary disappeared from the collimation eyepiece. Now everything appears concentric.

It is a great pity the manufacturers don't mention this in their instructions! Many thanks to http://www.astro-tom.com/telescopes/newtonian.htm that put me right!

[redshifter]

Great link! Thanks.