Meade LX90 Focus Knob help

[Reggieo]

Hello everyone, I have a problem with my Meade LX90 focuser!
Can anyone advise please as I am not sure if my focuser is working properly.. the focuser completes 41 revolutions from lock to lock.
I have an object to focus on about 40 meters away and it takes 18 revolutions to focus, then another 3 turns to focus on an electric pylon about 3 or 4 miles away. 
If I go too far past focus, about ¾ turn, I have to reverse about 2½ turns then forward again to get it back in focus.
should I have to turn the focuser that many times to focus on something so close, 
and surely, I was expecting if I go past focus ¾ turn, then it should be back in focus ¾ turn back!!!
There is a slight physical movement of the focuser, if I push or pull the focus knob about 2mm
If it is faulty,  what is best,  repair or replace how easy a job would it be.
Many thanks

[ronin]

I think on Meades that a lot of turns is normal, and there is a lot of play/slack, so ¾ turn past usually results 2½ turns to get back.

They operate by moving the whole mirror assembly and are not the most precise in mechanical terms.

I do not bother to count I just accept that I have to rotate the thing back by some amount.

Not sure if the whole thing can be converted to a more normal focuser, I am sure it would have made more sense when they first made them.

If you get other opinions and decide that it needs attention then I suspect that SCTelescopes are the best option, I think they advertise on the front page of ABSUK

If the whole thing is a pain to adjust - think the adjustment is a small solid knob - then I have the email of someone that makes the flexi-focuser's for some Meades - not 100% sure if the LX90 is one they do - but if wanted them PM and I will give you the email. Cannot give the site as I have lost it, it was on ebay. I have an email as there was a hiccup in the order - sorted immediatly with lots of apologies from the other end.

[rwilkey]

Best focusing on astronomical objects, the further away the object the better. Three or four miles should be OK, however, there is always a lot of focus travel in these type of scopes.  Focuser should not be loose, perhaps tightening up the tension bolt if there is one underneath?

[barkis]

I believe there is a modification to eliminate the free play that plagues the Meade focusing mechanism.

I know there is a device that screws into the mirror cell, which incorporates a spring, but that is to reduce mirror flop when reversing focus, very annoying when a centred object flies out of the field of view 

when focusing back and forth.  This latter device is no help with the problem you describe though, the latter is for small movements when focusing.

That devices screws into the mirror cell via a small opening next to the focus knob, and has a blanking grommet covering the access hole.

  There is a forum here might help you regarding mods to your LX90.

Ron.                     

             http://www.meade4m.com/

And here too.

http://jan.eaglecreekobservatory.org/links.html

You can also join the Yahoo LX90 User Group.

[Astroboffin]

Hi,

The focus knob on these scopes screws onto a brass shaft, and there are tow lock screws at the bottom of the focus knob, if you find that it is going past focus by 1/2 a turn and you are having to turn back 2 1/2 turns, then I think the knob is slipping on the brass shaft, check that the two lock screws are tight.

Turn the focuser clockwise, ( I think it is clockwise you want the mirror all the way forward, look into the tube) all the way and when it stops make sure the focus knob is all the way on, down as close the the scope as possible, then check the lock screws with an Allen key, they should be pretty tight.

If you go past focus by 1/2 turn then you should only go back 1/2 turn to get back to focus, if not there is something wrong.

And also it is normal for the Meade scopes to have approx 40 turns of focus movement, my LX90 has around 44.

Hope that helps

AB

[Astroboffin]

I believe there is a modification to eliminate the free play that plagues the Meade focusing mechanism.

I know there is a device that screws into the mirror cell, which incorporates a spring, but that is to reduce mirror flop when reversing focus, very annoying when a centred object flies out of the field of view 

when focusing back and forth.  This latter device is no help with the problem you describe though, the latter is for small movements when focusing.

That devices screws into the mirror cell via a small opening next to the focus knob, and has a blanking grommet covering the access hole.

  There is a forum here might help you regarding mods to your LX90.

Ron.                     

             http://www.meade4m.com/

And here too.

http://jan.eaglecreekobservatory.org/links.html

You can also join the Yahoo LX90 User Group.

Those focus lock mods are only for the LX200 not the LX90 range

AB

[Peter Drew]

I think the 2mm physical movement is due to the knob having moved on its shaft. If you set the tube vertical and pull out the 2mm then undo the grubscrew that secures the focus knob you should be able to push it upwards closing the 2mm and then relock. This should reduce the focus backlash to a more acceptable level.    

[barkis]

Those focus lock mods are only for the LX200 not the LX90 range

AB

Sorry, you're wrong AB, I had one fitted to my own LX90.

The anti Mirror flop one I'm referring to here of course.

But you are probably correct in the other mod. mentioned

[Astroboffin]

Sorry, you're wrong AB, I had one fitted to my own LX90.

The anti Mirror flop one I'm referring to here of course.

But you are probably correct in the other mod. mentioned

Ok, sorry

I stand corrected then,,it's just on mine there is no hole to put one, but I have heard of people that have drilled one specially so maybe there are company's that do it.

Regards

AB

[Astroboffin]

I think the 2mm physical movement is due to the knob having moved on its shaft. If you set the tube vertical and pull out the 2mm then undo the grubscrew that secures the focus knob you should be able to push it upwards closing the 2mm and then relock. This should reduce the focus backlash to a more acceptable level.  

The focus knob on the LX90 does not push on it screws onto the brass shaft, it would have to be screwed on first then the grub screws tightened, as I have already said in my post above

Regards

AB

[Peter Drew]

Obviously another difference between LX90's and LX200's. It must still be some form of maladjustment. 

[Reggieo]

WOW! many thanks guys, your advice has certainly given me something to try.

I'll keep the topic posted on progress

[michael8554]

Most SCT's have a large back focus that allows you to focus with a long stack of gear hanging off the back.

Unlike most Newtonians that have difficulty focusing onto just a camera.

The disadvantage is that there will be many turns on the focus knob to give that range of adjustment.

I believe that only Meade SCT's bigger than 8"/200mm have the hole in the back for fitting a mirror lock.

If you google for Meade LX focus mods you'll find plenty of diagrams and instructions for adjusting the knob.

Michael