Collimation issue with a refitted secondary

[Grunthos]

As a mini project today I fitted a plastic washer between the secondary mirror and it's holder (attached to the spider vanes).

Pretty straightforward job. I carefully unscrewed the secondary without touchng the tilt adjustment screws. Placed the washer then screwed the secondary mirror back in place.

Trouble is that when viewed though my collimation cap the secondary mirror is no longer concentric under the focuser tube.

Sorry I don't have a photo so will try to describe it.

The mirror appears as a circle under the focuser tube (i.e. I've rotated it so it appears as a circle not an oval.

The alignment of the mirror appears to be OK in the axis up and down the telescope tube (i.e. from the holder to the primary).

The mirror does appear to be an equal distance perpendicular to this i.e it now looks a lot "lower" in that axis.

I can't work out why. Surely I shouldn't need to adjust the spider vanes to move the secondary as I didn't touch them during my fitting of the plastic washer? If so then what have I done "wrong" i.e. what has happened between before and after fitting this thin plastic washer).

I'm happy enough to start playing around with the spider vanes but thought I'd post this as to check first.

Does all that make sense?

Many thanks for any advice/comments.

[Skybrowser]

Check it's central maybe? Vanes out a bit?

[Peter Drew]

If you only removed the secondary mirror rather than the complete spider assembly then the change in secondary mirror position should only be the thickness of the new washer. That amount should be possible to adjust back by backing off the three adjustment screws and then tightening the central screw wich will pull the diagonal up to its original position.

[jimmyjamjoejoe]

Best bet is not to wonder why, just fix it by doing a full collimation.

Make sure the vanes are central first of all, then do the "paper in the tube" trick to try and get the secondary round and central. If like before you can get it round, and central along the primary/objective axis, but not the other way, then you probably need to adjust your focuser, as it may not be square. Fortunately, this is simple, i had to do it a few weeks ago.

[Grunthos]

Best bet is not to wonder why, just fix it by doing a full collimation.

Yes, I thought that might be the answer.

Trouble is I'm confused about why it should be out in the first place (I never touched the vanes).