Which screw(s) tighten the focuser?

[blinky]

I think I have a bit of focuser slop in my WO ZS73 - I get elongated stars in a couple of corners that move when I rotate the camera, so I believe this indicates is not the camera but the scope/focuser?  Im wondering if its possible to tighten the focuser on the ZS73 and if so what screws?  I have an electronic focuser connoted to the scope currently, and had to remove the focus lock screw for this to fit, I also cant feel any slop in the focuser but I assume that doesn't mean there is none as it only needs to be a fraction of a mm anyway

 

 

[blinky]

[symmetal]

The two recessed screws on top apply some downward friction push on the focuser to remove any up/down sloppyness. Just tighten them until there is some resistance. Not too tight.

However when the scope is pointing in the direction of the meridian these screws are now on the side of the scope and so are not so effective at preventing slop in the current up/down direction. I had this problem in my FLT98 where tilt was evident when the scope was on its side. Increasing the pressure on the two screws mentioned just jammed the focuser preventing autofocus.

My solution was to insert long enough nylon grub screws in the unused mounting holes for the finder mount either side of the top friction screws, until they touched the focus tube, applying some force in the up/down direction when the scope is on its side and it was enough to stop the sloppiness without causing too much friction to affect autofocus. I think I used 10mm M4 nylon grub screws though 8mm would be fine. The screws were a nice snug fit in the threads so don't move on their own once set. 🙂

Alan

Edited August 18, 2021 by symmetal

[blinky]

Cheers, will order some and see how they go, could you feel the slop in the focuser? Mine to be honest feels rock solid!

[symmetal]

I could convince myself that I could feel movement but it would be hard to measure. Best to check your finder mount holes are M4 before ordering. They are on my FLT98 and ZS61 so probably are on yours. The ZS61 threads were a bit looser than the FLT98 so a dab of thread lock on them may be needed. The finder holes on one side are above the engraved scale on the focuser so it's best to use the finder holes on the other side to avoid the screws catching on the scale.

Alan

[blinky]

And just to check, the finder screws are the ones that are not countersunk in the image above? The ones on the side

[symmetal]

1 hour ago, blinky said:

And just to check, the finder screws are the ones that are not countersunk in the image above? The ones on the side

Yes that's right. Along with probably a similar pair on the other side. I assumed they are mainly used for attaching finder scopes which is why I called them that, but they can of course be used for mounting anything. 🙂

As they're open holes and the focuser tube is visible through them they are useful for helping reduce focuser flop. For that it would be better if they were placed at 90 degrees away from the countersunk main tension screws but using them saves having to drill a hole in the side of the scope. 😁

Alan 

[blinky]

I'm really wondering if I need the focuser lock screw back in, I removed it when attaching the motor focuser and forgot to put it back in. To refit would mean removing the motor again as the bracket goes under the focuser and won't let me get the screw in. That might be today's job! One of the advantages of work from home.....

[symmetal]

If you have a motor focuser then you don't need the focuser lock screw. Locking the focuser would stop the motor focuser from working, and the holding torque if using a continuously powered stepper motor would stop the focuser from slipping. In most cases the detent torque from an unpowered stepper motor is enough to hold the focuser in position.

Alan