Can't seem to hold focus ...

[Paul-F]

I am getting a wierd issue that I noticed recently where I can't hold focus with my 13mm Hyperion in my scope - lower power seems not too bad. If I focus by turning the focuser counter clockwise it gets blurry after a few seconds (I assume the mirror is settling) ... if I focus clockwise it stays sharp for longer but still drifts a bit off focus .. this is not what I am used to. With the good seeing last night this was a pain and I was constantly fiddling with the knob ... it was windy which affected the scope but just tried again on the moon and the same thing ... are my eyes playing tricks on me! I was initially observing at 6.5 fl as the seeing was brilliant when I noticed this so tried 13mm on its own. The amount of knob movement needed to re-focus seems to be quite a bit for just off-focus!

Had the LX90 for 5 years now and have only been aware of this since getting the Baader Hyperion eyepieces last week - max power I used to use on a regular basis was 190x ...

[Peter Drew]

Hello Paul.

When used in very cold conditions the mirrors on a SCT can be a bit sluggisg when using the primary mirror focussing system. Try always turning the focus knob anticlockwise to focus, this will raise the mirror under its own weight and will stop there when you let go the focuser. If you overshoot just refocus and try again.

[ollypenrice]

Telescope House used to do a gizmo which I still have and works well. It helps in the same way as Peter's good advice above. You thread a length of threaded bar in through the back of the scope to the transit lock bolt hole. This bar then has a spring over it which pulls back steadily and lightly on the mirror drawing it to the rear and aiding gravity.

I don't know if they are still around or if the LX90 still has the same transit lock bolt holes etc but TH would tell you.

Olly