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Need some help with Meade LX200 14" focusing

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#1 aeajr

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Posted 11 February 2019 - 12:54 AM

I own two 80 mm F5 refractors, a 127 mm F15 Mak and an 8" F5.9 Dob with a two-speed focuser.   

 

I also have access to and use of a Meade LX200 GPS 14" at an observatory.  That is the subject of this post. 

 

The LX200 is not my scope, so I am not going to do any modifications or changes to it.  But I need to better understand how to focus it.   This is a process and procedure question. 

 

Manual can be found here.  I have read it several times. 

https://www.meade.co...al-size-1-5-mb/

 

There are two knobs on the back of the scope, on the right side.   One is a lock knob, top, and below that is the course focus knob.

 

The scope has an electric micro focuser that is operated from the handset and does work.

 

The question relates to the two knobs.   

 

1) Should that lock knob be loose?  No matter what I do with it, it doesn't tighten.  Now that may be OK, but it seems that if I turn this thing it should be locking the mirror in some fashion.  It seems to be spinning loose, unconnected to anything.

 

2) The lower knob is the course focus knob.  When I use this to do course focus, the image moves around in the eyepiece.  I may get it close to focus but then it seems to jump way out of focus again with only a slight movement.  This does not behave like any focuser I have used before.

 

The micro focuser works fine.

 

I spoke to a couple of people at the observatory. They said this is all normal, but somehow that does not make sense to me. 

 

 

I have experience with a two-speed focuser on my Dob, but this seems to be a very different animal.

 

I need some education/advice.   I am having problems focusing the scope for visual use.  I wonder if I am doing something wrong.

 

Suggestions to buy some different kind of focuser are not helpful as this is not my scope.

 

 

 

Appreciate any information/recommendations.



#2 Taosmath

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Posted 11 February 2019 - 01:49 AM

Ed,

 

unfortunately the image shift in an SCT is very common.  It's called Mirror flop.

 

Unlike a Dob or a refractor where, when you focus you move the eyepiece and keep the objective (mirror or lens) steady, in an SCT you keep the eyepiece steady and move the primary mirror backwards and forwards.  Because this movement is done with a threaded rod and a lever or fork to push the mirror backwards and forwards it suffers from backlash and other mechanical imperfections.  This means that you turn the focuser knob and nothing happens and then it starts move.  When you stop focussing and reverse the direction you're turning the knob, it take a while to start moving in that direction too.  This coupled with the lever arrangement and the other mechanical imperfections mean that in just about every SCT I've ever used , as you change direction of the focusing knob, the star you had carefully centered, now moves off center, and at high magnifications can even move out of the field of view.  Usually if you then recenter, the star will stay in the middle of the FOV as long as you keep turning the focuser in the same direction.  If you reverse the focuser knob, it will shift again.   There are a very few scopes which fortuitously (or by superior Quality Control) have almost no backlash, where this effect is negligible. In all others it happens, and a shift of 10 to 20 arc minutes is not uncommon. (They are after all moving a 8"-16" diameter mirror back and forth and to get no image shift at all requires  extremely high precision engineering.

 

The way to live with this is to get a rough mechanical focus using the knob, then use the external electric fine focus device.  This device  uses a motor to move the eyepiece draw tube back and forth (like a dob focuser) and so won't cause image shift.  They usually have only a small total range of movement so unless you have parfocal eyepieces (or close to it) you will have to go through the whole mechanical mirror flop rigamarole very time you change eyepieces.

 

Welcome to the wonderful world of SCT's.

 

Hope that helps.

Colin


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#3 HeathM98

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Posted 11 February 2019 - 02:00 AM

Sounds like the mirror lock is broken. It’s supposed to tighten.


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#4 aeajr

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Posted 11 February 2019 - 06:19 AM

Thanks for the comments guys. Colin, your comments seem to match what I see. As this is a Goto scope, the tracking is not an issue.

Will consider parfocal rings.

Heathm98, I think you are right.

#5 PAdennis812

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Posted 11 February 2019 - 07:23 AM

Ed,

There are several threads here on CN about the lock issue.

Here's one which shows the mechanism: https://www.cloudyni...t/#entry3457823

 

Mirror flop is indeed a common problem with this focus system which is why I switched to a refractor!!!

 

One thing about the electronic focus is to run the focus all the way in or out.  You'll hear the motor whir when it reaches the end.  Then, looking at the focus run it all the way in the opposite direction until it again stops.  You'll note the total travel length.  At this point, move the draw tube to the middle point of travel.  Focus, as usual, with the main, coarse knob to your best image.  Then fine tune with the micro in either direction.

 

Hope you got to see Comet Iwamoto!  Saturday night was spectacular here in the Lehigh Valley and I spent several hours viewing it. 

That thing was cruising fast at 23 miles/second!  Now, back to a rising moon and 15 days of cloudy, snowy weather.....


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#6 aeajr

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Posted 11 February 2019 - 02:58 PM

Thanks, everyone.   I was not aware of this focusing system.  

 

I have a Meade ETX 125, a Mak.  Does it use a similar system?  I have never observed this behavior in the Mak.

 

My experience with SCTs is limited to looking through the eyepiece of friend's scopes, usually Celestron 6 or 8 SEs.  I have not noticed this issue with their scopes but my focus efforts have been momentary and very small in scope.

 

I gained privileges to use this observatory scope, which had not seen a lot of use, a few months back.  I have only used it a few times.   The AutoStar works just like my ETX scopes.  The images this 14" puts up are amazing.   But getting it into focus has been a bear.

 

I have the electronic microfocuser working and it seems fine.  It just seemed the corse adjustment was weird.  Now I know why.

 

Does Celestron use a similar focus system on their SCTs?



#7 HeathM98

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Posted 11 February 2019 - 03:22 PM

The much smaller mirror on the ETX allows the focus system to hold it in place much better. So mirror image shift isn’t really a problem. But with the much heavier mirrors in large SCT’s like the 14” it can move a bit with changes in angle, hence the need for a mirror lock. But, if you lock the mirror and then use the coarse focus, it can screw up the mirror lock and cause it to no longer work. In a scope that allows members of the public to use it, I’d say that’s what has happened. The lock definitely shouldn’t turn freely without tightening.

 

The scope will still work. It just won’t hold focus well as it moves around. So you’ll have to refocus it as you change subjects and it will not be good for taking photos.

 

Maybe someone at the observatory can work on it and get it fixed?



#8 Peterson Engineering

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Posted 11 February 2019 - 09:52 PM

Hi Ed,

 

Although you don't want to muss with this scope, fixes for the focuser backlash and the inoperable mirror lock are not complex.  In an "open observatory" situation it's best that the mirror lock not function.  Having casual scope users

cranking on the focuser with the mirror locked will break stuff, so it's best left as is.  And the backlash won't be enough of a bother to an infrequent user to effect the repair.

 

 

The 14" is a great scope.  Enjoy the experience.

 

Pete


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#9 carolinaskies

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Posted 11 February 2019 - 10:09 PM

The mirror lock should never be engaged for visual observing.  It is meant for imaging only.  'Mirror flop' is not a drastic condition visually since the actual amount of movement is typically quite small and the eye can adjust whereas a camera cannot.  

An ETX and other small Mak's typically have extremely thin mirrors compared to the big scopes so weight is negligable and thus they tend to hold focus extremely well.  

As advised above, center the external focuser and use the coarse adjustment knob to roughly tune in the object. Then adjust with the external focuser to fine-tune.  If you are doing outreach and find you are having issues with people adjusting the coarse focus knob I suggest putting a 'safety' cap over the coarse focuser which keep unwanted adjustments from happening.  You will find that the external focuser has sufficient range to adjust for different people.  You'll also find that certain eyepieces are better for those who wear eyeglasses so they can use them and not have to remove them which most often necessitates the need to refocus. 

Since you are only borrowing time on the telescope, I would suggest speaking to the owners/maintainers about adjusting the locking mechanism before attempting to repair it.  


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#10 gnowellsct

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Posted 11 February 2019 - 10:43 PM

Image shift is a focusing issue and is really different from mirror flop.  Image shift is movement of the object while focusing.  Mirror flop is the tendency of a focused object to go out of focus when, for example, the mirror position changes as the scope goes past the meridian.

 

If you get to a focus point and then the scope goes out of focus, you probably need to rethink the process.  The best way to focus is to have the final movement of the mirror be up and away from your head, that way any slop in the system is taken up and gravity works for you.  If your final focus is in the other direction the mirror may not follow as quickly as you would like.  It will appear to be focused and continue moving--even past the focus point, giving you the sensation of a scope that doesn't want to be focused.     

 

Remember you're moving the mirror in these systems.  The mirror lock is a LOCK.  It is designed to frustrate the tendency of the mirror to move.  When you focus, you WANT the mirror to move.   When you try to focus a LOCKED mirror you're in essence trying to break the focuser.   For visual observers the mirror lock is a thing you don't use and don't engage until and unless you are breaking the scope down for transport.

 

All that said: this is not a problem that I have had in any of my celestron SCTs.  Image shift is minimal, as in not really noticeable in a field of view ten arc minutes across.  I've never done photography so I don't know how bad the mirror flop would be crossing the meridian.  I don't think it is significant, however, because my scopes hold collimation all sky which is an indicator that things are working well.

 

On a fork mount in alt az position, mirror flop should be much less of an issue than on an equatorial.  On an alt az fork you just go up down up down the angle doesn't change side to side as with a German equatorial.  

 

Mirror locks can have an additional function in photography, once you've got the focus dialed in for your imaging run, you lock the mirror/focus in place and leave well enough alone.    In this case I'll say my advice is free and worth every penny, my experience with Meades has been limited not been very favorable, though I read they are turning out some dynamite 10" you-can't-believe-how-good SCTs these days.

 

Greg N


Edited by gnowellsct, 11 February 2019 - 10:49 PM.


#11 gnowellsct

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Posted 11 February 2019 - 10:54 PM

I guess I would add that it might be illuminating to stand in front of this SCT (or any amateur class SCT for that matter) and with the locks NOT ENGAGED, rack it all the way in and out and watch it move on the center baffle.  See how it gets pushed UP by the focuser going in one direction and how it follows, with a gravity induced slide, in the other.   Sometimes image shift can be reduced by regreasing the center baffle, or even just running the mirror up and down a few times to spread out the grease that is there.   That cure shouldn't be work from what I've read here, but it seems to work, and even if the effect is placebo I'll take it.  GN


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#12 aeajr

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Posted 11 February 2019 - 11:47 PM

I guess I would add that it might be illuminating to stand in front of this SCT (or any amateur class SCT for that matter) and with the locks NOT ENGAGED, rack it all the way in and out and watch it move on the center baffle.  See how it gets pushed UP by the focuser going in one direction and how it follows, with a gravity induced slide, in the other.   Sometimes image shift can be reduced by regreasing the center baffle, or even just running the mirror up and down a few times to spread out the grease that is there.   That cure shouldn't be work from what I've read here, but it seems to work, and even if the effect is placebo I'll take it.  GN

A very good idea!


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#13 aeajr

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Posted 14 February 2019 - 06:35 PM

I had an opportunity to try an LX200 16" last night.   Same basic set-up.   The focuser on that one is in much better shape.  The course focus worked much as I expected it to.   So I think the one on the 14", the subject of this discussion, must be a little messed up.

 

It is workable so I will use the scope, but now I know it is not something that I am doing wrong.

 

Thanks for the help guys!




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