I have recently read a number of posts moaning that the expensive Celestron 9 x 50 RACI Finderscope "wobbles loose" or otherwise isn't fit for purpose. I disagree as I find it to be a fantastic bit of kit, yet can be frustrating if you don't know the knack of adjusting it, but will agree that the supplied instructions are hopeless (as usual). So, I thought I might post some helpful guidance.
Fitting the Finderscope is easy. Just replace the original Star Pointer using the same screws and OTA holes. But the knack to success is knowing how to use the barrel screw adjuster that my screwdriver is pointing at (see image below). Confusingly, it has what appears to be two grooves cut into the barrel (as if for a slot screwdriver). You do NOT turn this shiny barrel unless the barrel unit has itself become loose. That simply locks the screw barrel adjuster assembly into the body of the unit, but it confuses everybody. Instead....
Look inside the shiny metal barrel and rotating inside there is a grub screw (and spring) with a STAR headed screwdriver slot. This is what you adjust, but it confuses most people as it isn't obvious that the adjustment takes place inside the shiny barrel.
Firstly, make sure that the rubber ring is seated properly in the forward ring. This creates a pivot so you can then adjust your aim using the rear ring. Start by aiming your OTA at a distant static object (distant telegraph mast by day or Polaris at night) and get that absolutely centre using a cross hair reticle eyepiece, doughnut technique or whatever you favour.
You then insert the STAR headed screwdriver down into the barrel and turn the grub screw until your view through the Finderscope is slightly below and left of the object.
You then TIGHTEN the other two thumb screws on that same concentric circle until you pull the target centre, checking that both it and OTA are centred.
If these thumbscrews remain too loose, tighten the grub screw another turn, then TIGHTEN the two thumbscrews again. If you have to loosen the thumbscrews to centre the object then the grub screw is probably too loose (and hence the "wobble"). Eventually you will get all three screws snug tight and you should then not need any further adjustment for a considerable length of time. Where folk fail with this is that they turn the shiny barrel assembly tight not realising that the crucial adjustment is the STAR headed grub screw inside the barrel.