How to adjust focuser tilt for Sky-Watcher 130PDS?

[BrendanC]

Hi all,

I've just realised how to correct something that's been bugging me for a while - I have a small amount of tilt on my focuser tube. I can see it when using my Cheshire, and fix it by inserting a sheet of paper on one side between the Cheshire and the drawtube.

So my question is: how do I correct this? I've heard tell that 'the grub screws at the base of the focuser' are for this, but then that raises two more questions: there are some at the base of the focuser, and some on the part of the focuser attached to the tube, so which grubs do this (A or B in the photo)?

And if it's the ones on the base, then given that at least one of them is totally obscured by the focuser knob, how on earth would I access it (arrow and question mark)?

Thanks, Brendan

[John]

I think the 3 pairs (1 large + 1 small) of screws around the flange of the focuser (you have labelled one pair "B" in your 1st photo) act as "push-pull" pairs to adjust the tilt of the of the focuser.

Which pushes and which pulls, I'm not sure.

This is from a Skywatcher manual:

 

[symmetal]

As John says it will be the ones around the base. The large screw A in the diagram above is the 'pull' screw and clamps the focuser to the body like any fixing screw. The grub screw B next to A pushes the focuser away from the body at that point.

You start off with no tilt adjustment by having all 3 A screws tight and all 3 grub screws B loose or lightly tight, so B screws do nothing. That just clamps the focuser 'flat' against the body.

If you want to apply tilt then initially with all A screws tight screw in the B screws until they are also tight. Don't overdo it though. Then for the location of the focuser you want to apply some tilt to, loosen off the corresponding A screw and screw in the B grub screw by the amount of tilt to apply. Say a quarter or half a turn to start with. This pushes the focuser away at that point. Then tighten up the A screw to pull the focuser tight again but it's still pushed away by the grub screw B from lying flush against the body.

To save working out which screw you need to adjust by analysing the images, just give each tilt screw pair a large adjustment, say a full turn of B and see what effect it has on the image. Reset for no tilt, and then repeat with the other pairs of screws. This gives an idea of what each does to the image.

As you say some screws are inaccessible which seems to be a common feature of basic tilt adjusters. Those push/pull adjusters you can buy to put in the image train often end up with the adjustment screws too close to the filter wheel to allow adjustment without some disassembly. Zwo cameras with tilt plates on the front can be impossible to reach if the camera is fixed to the filter wheel by 3 screws rather than the T2 or 48mm thread etc. Hopefully the adjustment you want is using the accessible screws. 🙂

Alan

[BrendanC]

Excellent answers! Thank you so much. I did look for a 130PDS manual but obviously there's nothing. Didn't occur to me to look at their Quattro range.

[symmetal]

For the inaccessible screws you could cut a spare allen key down so that the short side will fit under the focuser shaft body, assuming the allen key hex cross section is still present at the cut down end. 🙂

Alan