Collimation of focuser

[poider]

G'day, I am thinking of making or buying my own collimator, but I am Curious to know how do we know if our focuser is straight, I know with a laser collimator you can ensure that the laser is pointing at the secondary and then the primary and then back through the collimator hole, but how do we know our focuser is straight and square to the telescope tube?
Peter

[poider]

Is there a tool or s formula to use?

[Merlin66]

Peter,
Verifying the squareness of the focuser to the OTA in Newtonians can be problematic.
The general method is to place a straight edge across the end of the focuser parallel to the optical axis and measure the gap between the straight edge and the tube at the extremes....
Any differences can be corrected by adding shims between the focuser body and the OTA.
(It's much easier with refractors!!)
Hope this helps.

[rrussell1962]

It does not really matter if the focuser is exactly square to the tube. The focuser axis will define your optical axis when you collimate, wherever the focuser axis is pointing. The tilt and rotate adjustments of the secondary will align it to the focuser axis. Having said that I did spend a bit of time adjusting my focuser because I found it helped in minimising the adjustments needed to initially get the secondary centred in the sight tube. If you really want to delve into collimation I found Vic Menard's booklet "New Perspectives on Newtonian Collimation" very useful and a wealth of information.

[75BC]

I read somewhere online about this, can’t remember where though.

They said, with secondary removed, to mark a spot inside the OTA, exactly where a laser collimator should hit when placed in the focuser by measuring down from the front edge of the tube and around the inside of tube. Then put a laser in and see if it hits that spot. If not use shims as Ken suggested to adjust.

Never tried it when I had my Newt but it sounded plausible. :)

[knightrider]

I had this problem once with my newt when it was new.

I had issues getting matching/consistent results with both optical and laser collimator (I liked to have both agree with each other).

Turned out the focuser wasn't sitting square, fortunately the focuser had adjustable grub screws to square it. I just did some trial and error between optical and laser collimators to get it right....probably not the right method but I got there in the end.

[poider]

How did you know you were there? ...... in the end?
Peter

[doug mc]

I used a length of 1.25inch aluminum tubing. With the secondary removed i inserted the tube into the focusor till it was half way into the ota. Using a short ruler i mesured how central the end was in relation to the optical tube.

[rrussell1962]

Put a laser collimator or sight tube with cross hairs in the focuser, measure down to the laser dot or centre of the cross hairs. That gives you the vertical distance from the end of the tube.

Remove the secondary holder, measure the centre of the spider from the tube wall along each vane and centre the spider by adjusting the vanes. Put a length of threaded rod in the spider where the secondary holder fits and hold in place with a nut either side of the centre of the spider. Adjust the focuser to centre the crosshairs or laser dot on the threaded rod. Remove threaded rod. You have now found the lateral position opposite the focuser on the tube wall. Vertical and horizontal lines passing through these points define the point opposite the centre of the focuser.

Adjust focuser to align cross hairs or laser dot where the lines intersect. Mark with a small dot for future reference.

[poider]

Thank you all

[knightrider]

For me, I collimated visually then checked with the Laser, or vice versa.
It was weird, because I'd collimate visually and it'd looked great with the chesire, but then the laser would say it was way off, because of the angle of the laser was pointing.

I knew I had it right when both eventually harmonised through trial and error.
Not a very good way to do it but it worked for me.