SCT Focus problem

[cosmos]

Hello everyone,

My uncle has the meade 8" LX90 SCT we seem to have a problem focusing as in it takes at least 4 turns of the focus knob before it engages the mirror, can anyone tell me if this is normal as this is the first SCT i have played with.

many thanks for reading all............

marc

[Merlin66]

That would not be normal.

Check that the little grub screw in the focussing knob hasn't come loose.

[Milamber]

Yup, what he said. There are two grub screws on the focus knob, about 1/8" up from the scope body, at 180 degrees to each other.

Arthur

[cosmos]

thanks for that guys i have checked that and they are tight its weird its at least 4 turns before you see any focus change ....

[Merlin66]

Something's not right!

Are the four turns from focussing clockwise to the focussing anti-clockwise, or re-focussing in the same direction????

The focus mechanism is pretty basic... the knob turns a threaded screw which has a notch at the end which in turn just sits into a slot on the mirror support.

Worst case - remove the knob and see if there's any play in the screw; the small plate below the knob on the rear cell can be removed with the three small screws ( make sure the OTA is horizontal!) and tilting the screw slightly will dis-engage it from the mirror support - have a look and see if the thrust bearings and or the screw are worn or damaged.

Hopefully just removing the knob, checking and refitting will do the trick.

[Milamber]

Actually, the knob *doesn't* turn the threaded screw which has a notch at the end which in turn just sits into a slot on the mirror support... if it did then it wouldn't work. The knob is threaded internally and as you turn it the screw inside moves up and down the thread. The only other place where play can encroach is where the screw end fits into the mirror... do you have mirror flop (as in, point the tube up, wait a bit, then point it down... if you hear a light thunk then it is the mirror moving when it shouldn't - flop)?

Arthur

[Merlin66]

I stand corrected! ( that's what I MEANT to say!)

What if the knob is mounted too far back from the thrust washer??

[cosmos]

thanks all when i move the tube up or down i hcan hear the mirror flop as it clunks about, should it not do that?

[Milamber]

No - it shouldn't - but they do. Does the focus knob move in an out as well when the mirror flops? If it does then we have a solution maybe.

[Milamber]

Point the OTA up.

*Tighten* the focus, i.e.turn it clockwise looking at it. This will put the mirror at the bottom of it's movement.

Loosen the focus knob with the two grubscrews.

Again, rotate the knob clockwise until it sits snug against the bearing.

Re-tighten the grubscrews.

Voila.

If it is not now sorted, then you have an issue with the hook on the end or the bearing washers are missing.

Arthur

[cosmos]

many thanks will try that now let you know what happens