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Focusing hekp

cassegrain
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#1 Jeri Rocks

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Posted 06 December 2018 - 10:17 AM

I have a LX 90 10" Meade that is about 10 years old. Recently, I noticed that at times the image does not stay in focus. I would focus the deep space object and take a second look and the image is well out of focus. I spoke to Meade and they said they haven't heard of such a strange thing. It occurs with different eyepiece. Could the diagonal be shifting on me?  Suggestions welcome.



#2 Gary Z

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Posted 06 December 2018 - 11:58 AM

While I do not own a Meade SCT, I did a google search essentially "LX90 10 focusing issue".  I came up with numerous results that may prove useful to your situation.  However, since you mention that the focus issue is when you change eyepieces, this isn't a problem unless you are sure they are NOT parfocal eyepieces.  If your eyepieces are parfocal, meaning when you swap out different magnitude eyepieces (these are specifically designed as a set so when you swap them out, focus isn't necessary). 

 

As far as any issue with your diagonal, loosen up your diagonal and rotate the diagonal while viewing.  I don't know about Meade optics and I'm assuming this is an SCT, but you could be experiencing mirror flop which is common among SCTs. 

I don't know if your particular SCT features a mirror lock. 

 

Meade tech support may not be aware of the situation, but there's certainly history in the forums.  Sometimes it's the explaining of the situation that can throw them off. 

 

Gary



#3 carolinaskies

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Posted 06 December 2018 - 12:26 PM

There could be a few things happening here.  

1. Your optics aren't cooled down well so you're getting the focusing variations from this. 

2.  Something isn't secured tight.  The eyepiece in the diagonal?  The diagonal to the telescope. Even a small slip is enough to put the system out of focus.  This is especially true this time of year as the temps drop rapidly once the sun goes down so metal contracts quickly as it loses heat. 

3.  Focuser lubrication.  If you haven't cycled your focuser lately it's possible you have a bit of stiction issue where either the focus knob or the central tube of the sct is sticking a bit when you focus and then releases.  It is adviseable to occassionally cycle the focus very far in and very far out which helps to keep the light great from hardening and causing this problem.  As you mention the scope is about 10 years old I would bet this is the problem.  


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#4 Jeri Rocks

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Posted 06 December 2018 - 07:52 PM

Thank you for your comments and suggestions. I know that jumping from one eyepiece to another requires an adjustment on focusing, but occasionally I have to try the focus know several rotations to get it in focus. I will try your suggestions and hopefully I can get it resolved. The scope is housed in my observatory all of time and yes, this problem seemed to have started when the weather got colder here in Pennsylvania. Thanks again.

#5 Jeri Rocks

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Posted 06 December 2018 - 07:55 PM

Thanks for the suggestions and comments. The scope is permanently in my observatory. The focusing issue seemed to have cropped up with the colder weather here in Pennsylvania. I will try your suggestions both with the scope, diagonal and eyepiece.

#6 Jeri Rocks

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Posted 06 December 2018 - 08:01 PM

I never touched the plastic cap next to the focus know where I think the mirror lock is found. Shoukd I have done something with this when I got the scope? I don't understand the mirror lock.  Shoukd the mirror be locked or opposite when it use?



#7 carolinaskies

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Posted 06 December 2018 - 08:58 PM

The mirror lock should be backed out, usually it only needs to be locked for AP.  



#8 Peterson Engineering

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Posted 07 December 2018 - 01:21 PM

Sounds like mirror flop.  Simply do your final focus turning the knob in a counter-clockwise direction.

 

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#9 jgraham

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Posted 07 December 2018 - 10:44 PM

+1

 

Always set the focus turning the knob counter-clockwise. In this direction you are lifting the mirror with gravity helping to take up any slack in the system. If you set the focus turning the knob clockwise the mirror may settle a bit, causing the focus to shift as you are describing.




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