Collminating a Newtonian focuser

[Tandum]

Does anyone have an easy way to collminate a focuser on a newt?

I've just put a moonlight on a 10" dob and basically used a set of calipers to estimate the focuser center, removed the secondary and pointed the focuser at the estimated spot on the other side of the tube. It was out quite a bit.

Is there a better/more accurate way to do it?

[multiweb]

You need to find the exact projection of the focuser axis onto the other side of the tube and mark it with a dot. I used a paper rolled over the tube then measured it flat and marked it. Then with a cross tube you can shim your focuser and square it into position. When you're done you can put your secondary back into the tube and center it, then start the collimation routine.

[Tandum]

Can you explain this better, I don't under stand.

I've used digital calipers to get the distance down the tube to the center of the focuser and transfered that measurement to the other side of the tube. However, that measurement would change when the focuser is moved.

Moonlight focusers have collimination screws, no shims.

[multiweb]

You sticky tape a couple of A4 sheets together so you can make a ring around your tube (tight fit). Then you mark the center of the hole where you're going to fit your focuser in. Then remove the paper, fold it over the dot you did and on the other fold you make another dot. Then place the paper ring over your tube again and center the first dot into your focuser hole again. The second dot marks the intersection of the tube and the focuser axis. Drill a small hole in the tube so you can see it.

[Tandum]

Ahh, I see but that supposes that the focuser will be centered in the hole.
I know that this focuser is not exactly centered on the original hole.

[multiweb]

That's where the shimming comes into action. The focuser axis needs to intersect the tube axis. So if you're not centered you need to compensate by the same opposite amount looking at the hole you made with a cross tube.

[Tandum]

Shims won't move the focuser laterally. I think I might just take the focuser off and poke a pointed stick through the hole with white out on one end and a square on the other. I'm not drilling holes in it.

[Ian Robinson]

I did that .... I used my AstroSystems Laserbeam to get the spot dead on the centreline of the focus .... I also noted that the 1.25" adapter that came with focusor moves that point a fraction of mm too ... which I I thought was too much so I used the laser in 2" focusor mode.

I also checked the spot didn't move when I rotated the tube .... would indicate slop in the focusor or loose bolts holding in to the tube.

I also checked the spot didn't move when racked the focusor in and out.

I also checked , when I had my tube on a dead flat surface (by spirit level) that the tube was square and parallel with the flat surface at the place I had attached the focusor .... I'm certain the focusor is on a normal to the centreline of the tube.

Then it became a bit complicated .... I installed the secondary and did my best to make sure it was centred on the centre of my focusor by racking the focusor out and looking in through it to see if the inner surface of the focusor tube looked concentric with the outer edge of the secondary and I could see the secondary as a concentric circle and not an ellipse. I don't know if this is the proper method , but it makes sense to me.

I then checked this by looking into the OTA with my AstroSystems Sighttube gadget (too bad it's the 1.25" model .... add the 2" model to the to get list) to check the secondary was centred in the tube .... apparently from I've been told this a good idea.

I will probably draw up a template of the secondary on a piece of soft plastic (Word Drawing has circles and ellipses and gives the centrelines or minor and major axes) so this should be easy to do on printer at the exact right size and then I'll trace it out with marker . This way I can sort out my required secondary offsets too when finalise my collimation.

I've also ordered some CatsEye (Laser) Spots for my mirror and a template. These came highly recommended by the good folks in the ATM subforum at CN and Randy at AstroSystems recommended them to me too when asked for some tips on collimating using one of his laserbeams.

[Ian Robinson]

I did that trick to get the spots to drill the 4 holes for the spider and 3 holes for the mirror cell , and to check I'd cut my tube ends square and straight (needed some tweeking ... a job for the Dremel and its cutting disk).

[Ian Robinson]

What if the ends of the tube aren't square or perfectly straight ....?