Collimating Newton focuser

[Bender]

After receiving my new dob I went out to order the cats eye kit and then took the ota apart to instal the cats eye dot on the primary.

While I was on it I thought I might as well flock the upper and lower part of the tube.

I then started the collimating process, making sure the 4 spider vanes are equally long (tick), the focuser is perpendicular to focal axis (holding a ruler in front of the Cheshire and on the opposite side of the tube, tick).

Then I inserted the secondary screw on its own and checked if the cross in the Cheshire lines up with the screw to see if the focuser points towards the primary axis.

Unfortunately it just touched the outside of the screw..

Turning the Cheshire, the focuser and finally the focuser base by 180 degrees didn't change anything so it seems ota is slightly oblated?

2 washer on one side between focuser base and ota brought the Cheshire in the focuser in line with the screw from the secondary.

I can't find much that other people had similar problems, might there be anything I am missing ?

[Charic]

This is new,  aligning the focuser assembly to the  bolt ? 

Are you saying you look through the Cheshire  at the side of the  bolt as it exits the secondary mount, no mirror in place? I don't think anyone else collimates in this fashion ( or am I missing something here?)

What is important is the quality of the final image, the secondary mirror can be slightly offset and still have a perfect image. The centre of the secondary mirror may not be  precisely aligned to the centre bolt!  Unless you have exhausted your collimation skills, and you still  think something is still awry, then maybe start adding some spacers

[Bender]

I followed the instruction that came with the cats eye set. I think flo has the book as well (new perspectives on Newtonian collimating)

I only started as I couldn't properly centre the secondary in the sight tube.

It was offset to the right so it kinda made sense to lift the focuser slightly in the right side.

[Moonshane]

Collimation of the focuser with no secondary in place is a standard aspect of collimation but I never bother

Is your scope an orion optics uk unit? I find they are usually slightly oval so two vanes opposite each other are the same but not necessarily all four.

[Moonshane]

Ps I never bother as there's enough adjustment in other components. If you want to then use the collimation grub screws on the focuser if it has them or use shims to raise one or more corners.

[Bender]

Thanks Shane, yes the washers did the job. Was a bit worried they might leave marks on the tube but what can you do.

Yes it is a oo UK scope. The ring on top of the ota is round so I took it from there

[Moonshane]

If you have a bit of felt or flocking maybe put this tube side

[Bender]

Good idea, I try this, is not much I need.

Just thought after investing in the oo scope and collimation tools I try to get it as good as I can

[Charic]

Moonshane, accepted now! I thought fitting a focuser came under the description of just, 'Squaring the focuser' hence my comment, but makes sense now. Cheers.