Collimation problem

[Lerxst]

As reported in the Hotech SCA Collimator thread, I got a new Hotech laser collimator yesterday and collimated the secondary mirror as it was off.

When extending the focuser with the collimator still inside and on, I noticed that the laser dot on the primary's doughnut moved with about a centimeter. I then could see (which I have never seen or looked for before) that the (rack and pinion) focuser drawtube wobbled as I extended it.

Worried I then went on to collimate the primary, and sadly the end result is that everything is horribly out of collimation.

Now I realise what I probably need to do is buy a better focuser, but since I spent a great deal extra already on eyepieces, motors, a laser collimator (hah!) and stuff, a new focuser is a bit more in the distant future than next week, so I need to get things back into collimation hopefully without any expenses.

What is the best practice? Collimation with the drawtube fully retracted? Since I can easily shift the position of the drawtube when it's extended to about the EP length, I don't trust that that's where I should position it when collimating. Is there anything else I can do? Grease the drawtube? Should all screws be tight or can they be loosened like when collimating the mirrors to ensure a steady extension of the drawtube?

[ZOG]

I would fully retract the drawtube if there is no wobble when it is wound fully in.

Also look underneath the focuser, there may be a small screw or thumbscrew which locks the drawtube.

[Lerxst]

Yes, the problem was that the thumbscrew was too tight! Even though the drawtube wobble, it's only when I rack it in/out fast - much faster than you would with an EP. With careful movements, it was actually pretty perfectly dead on, so I got it collimated.

[albedo0.39]

Well done.. but don't forget to do a Star Test when you have the chance. Find Polaris. Wind out the focuser until the image is well out of focus and ensure the black spot in the middle (the shadow of the secondary mirror) is exactly in the centre.

Then, you'll know it's right.

all the best

S