Focusing problems on Meade LX10 8" SCT

[ralphtoman]

A while back i bought a 2nd hand Meade LX10 8" SCT as i was upgrading from my first scope a 6" newtonian. i have taken the scope out in numerous nights allowing it the proper cool down time and continue to have problems getting it to focus clearly. when looking at say jupiter it appears to have a sort of halo around it. the same can be said for stars. it seems that as the focus zeros in on the correct allignment it nearly gets there and then goes past it and out of focus again. its driving me crazy and im wondering is there anything i can do or have i just bought a dodgy scope ???

everything looks fine while looking through it during the day and i have collimated it with a laser collimator on a couple of occasions. i've tried looking on the net but have grown tired trawling through countless blogs/forums. any help would be much appreciated.

thanks in advance

ralph.

[Peter Drew]

Hello Ralph.

The effects that you describe would usually be down to slight dewing of the optics or a film on the optics from egassing of the internal paintwork which can happen over time, however I would have thought you would have been aware of this. The other, but unlikely cause, would be poor optical correction, it would be worth defocusing a star inside and outside of focus and comparing the extrafocal rings and reporting back your findings

[ralphtoman]

many thanks for your reply , yes im aware of dewing and have left the scope out for 1 or more hours on a few occasions to equalize the scopes temp with the outside and have still experienced some issues with it . i'm starting to wonder that maybe there is some moisture trapped inside the tube ??? i was having a closer look at it today and was using it alongside a 90mm refractor during the day just to compare the focusing , now im aware that the refractor maybe slightly sharper but even still the difference was noticeable. the view is ok but it just lacks that sharpness. i shall try your second suggestion tonight weather permitting and see how that goes , if all else fails il take a hammer to it

[ralphtoman]

i meant to say that i was comparing the focusing on the two scopes on the top of a telegraph pole a few fields over from my house , on initial viewing it looks ok but when you try and focus in close on a small object on the pole it just cant seem to get it clearly.

[Merlin66]

Just check that the screws holding the front retaining ring on the corrector plate haven't been over tightened.

I had this problem on a s/h Lx5 I bought and the images were "rough"

These screws should be just nipped up.....if you do find they have been too tight - loosen them off and leave the scope over night to "relax" then gently nip them up.

[earth titan]

I had a similar scope to yours, and what it needed was a collimation through star testing. I did it with a webcam, so it could be done on a nice big screen, on a defocussed star. This fixed it.