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Does this look like off collimation, or mirror flop?

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#26 starman876

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Posted 15 July 2019 - 06:27 PM

There is nothing on the back of my 8" Meade ACF other than a focusing knob. So does this mean that this OTA is only useful for planetary?

are you sure you have an ACF model?


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#27 Ballyhoo

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Posted 15 July 2019 - 06:43 PM

are you sure you have an ACF model?

No, I made a mistake, I have the UHTC model. 



#28 rkelley8493

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Posted 15 July 2019 - 06:49 PM

There is nothing on the back of my 8" Meade ACF other than a focusing knob. So does this mean that this OTA is only useful for planetary?

The LX90 models do not have a mirror locking mechanism. I think it's only on the LX200, 600, 850. Not sure about the earlier models. 



#29 Ballyhoo

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Posted 15 July 2019 - 06:50 PM

The LX90 models do not have a mirror locking mechanism. I think it's only on the LX200, 600, 850. Not sure about the earlier models. 

Mine is LXD75


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#30 rkelley8493

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Posted 15 July 2019 - 07:12 PM

Mine is LXD75

Some Meade SCT owners add a different type of focuser to the visual back to get around the problem with mirror flop/image shift. I replaced my single speed focusing knob with a Feather Touch Micro-focuser. It makes it much easier to fine focus, but it still has issues with image shift. It doesn't bother me too much though because I don't do astro-imaging. Maybe someone with more experience with the other type of focuser can add some feedback and elaborate on how it works. 



#31 WadeH237

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Posted 15 July 2019 - 07:14 PM

Do you have a picture looking into the corrector, so that we can read what it says on the corrector ring?

 

In any event, you don't have a mirror lock.  It does not mean that your scope can only be used for planetary.  It does mean that you may have some mirror shift and flop.  If you follow the focusing instructions above for focusing, it will minimize this (the step that clears focus backlash also loads the mechanism, making it less susceptible to shift).  Also, you will probably need to refocus after a meridian flip.



#32 WadeH237

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Posted 15 July 2019 - 07:24 PM

Some Meade SCT owners add a different type of focuser to the visual back to get around the problem with mirror flop/image shift. I replaced my single speed focusing knob with a Feather Touch Micro-focuser. It makes it much easier to fine focus, but it still has issues with image shift. It doesn't bother me too much though because I don't do astro-imaging. Maybe someone with more experience with the other type of focuser can add some feedback and elaborate on how it works. 

Any focuser that uses the primary mirror mechanism (except  the newer scopes with the internal Crayford) works by moving the primary mirror back and forth on a simple threaded rod.  This is even true of the FeatherTouch.  The FT offers two speeds, and it's very smooth, but it's still working on that same threaded rod.  That mechanism inherently has focus shift when you focus.

 

Mirror flop is similar but different.  Mirror shift happens while focusing.  Mirror flop is what happens when gravity has its way with the primary mirror which is riding in the carrier along the baffle tube.  To prevent binding, there has to be some play between the baffle tube and the mirror carrier.  This flop can be sudden and big, or it can happen slowly over time, or both.  If I understand the new Crayford mechanism correctly, it uses roller bearings to ride the baffle tube, which is how it addresses both shift and flop.   The reason that I always recommend clearing focuser backlash and using a counterclockwise turn to finish focus with an SCT, is that it pushes the mirror up the baffle tube against gravity.  When the system is loaded like that, it's hopefully repeatable and minimizes the opportunity for the mirror to move down the baffle tube with gravity.

 

And finally, some people add external focusers, usually Crayfords, on the back of the scope.  These focusers don't use the internal mechanism at all.  They are normal, drawtube focusers like you would use on a refractor.  They don't suffer from shift, but they don't by themselves help with mirror flop.  For scopes that have mirror locks, you can set the primary mirror focuser so that the scope is in focus when the Crayford is in the middle of its travel range.  Then, you can lock the primary mirror and focus exclusively with the external focuser.


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#33 Ballyhoo

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Posted 15 July 2019 - 07:37 PM

Any focuser that uses the primary mirror mechanism (except  the newer scopes with the internal Crayford) works by moving the primary mirror back and forth on a simple threaded rod.  This is even true of the FeatherTouch.  The FT offers two speeds, and it's very smooth, but it's still working on that same threaded rod.  That mechanism inherently has focus shift when you focus.

 

Mirror flop is similar but different.  Mirror shift happens while focusing.  Mirror flop is what happens when gravity has its way with the primary mirror which is riding in the carrier along the baffle tube.  To prevent binding, there has to be some play between the baffle tube and the mirror carrier.  This flop can be sudden and big, or it can happen slowly over time, or both.  If I understand the new Crayford mechanism correctly, it uses roller bearings to ride the baffle tube, which is how it addresses both shift and flop.   The reason that I always recommend clearing focuser backlash and using a counterclockwise turn to finish focus with an SCT, is that it pushes the mirror up the baffle tube against gravity.  When the system is loaded like that, it's hopefully repeatable and minimizes the opportunity for the mirror to move down the baffle tube with gravity.

 

And finally, some people add external focusers, usually Crayfords, on the back of the scope.  These focusers don't use the internal mechanism at all.  They are normal, drawtube focusers like you would use on a refractor.  They don't suffer from shift, but they don't by themselves help with mirror flop.  For scopes that have mirror locks, you can set the primary mirror focuser so that the scope is in focus when the Crayford is in the middle of its travel range.  Then, you can lock the primary mirror and focus exclusively with the external focuser.

personally, I am inclined to upgrade to a nicer OTA than to adding anything to this one. I presume Meade marketed this LX75 as a good goto instrument. But really the mount it came with was God awful. I never knew how bad the LXD75 mount was until I got an AVX. and I never knew how bad an AVX was until I got an IEQ45.

Still, maybe I will buy en Edge or  Rasa at some point, and if I have to re-invent the wheel to do DSO, I will use this OTA for planetary.  

 

Edit, it is impressive how much you know about the architecture of these Wade. Have you ever taken one apart?


Edited by Ballyhoo, 15 July 2019 - 07:39 PM.


#34 cfosterstars

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Posted 15 July 2019 - 08:21 PM

All but the most expensive Meade OTA with mounts are not really designed for AP. They are designed for mass market to get people to buy a telescope. The LX75 is one of there lowest end mounts. This is also true of Celestron. It is only the higher end products from either vendor are really any good for AP. They sell them many of them on fork mounts without wedges in ALT/AZ mode. This is very much for visual and not for AP. I agree that added to this OTA is probably not worth it. I have a Meade 10" F/10 ACF and I have had to put a lot of money into it but now its a great OTA for AP. Its actually not the best, but I would buy a new F/8 OTA at some point, but there are other Manufacturers that I would consider first like ORION optics or Planewave that are significantly better. But you pay a much higher price - but it is good money well spent.



#35 Ballyhoo

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Posted 15 July 2019 - 09:09 PM

Well I think Celeston has Meade beat for AP, with Rasa, and to a lesser extent Edge.



#36 Ballyhoo

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Posted 15 July 2019 - 09:13 PM

Maybe for this scope I will focus on objects like this:

 

https://www.astrobin...?page=6&nc=user



#37 rkelley8493

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Posted 15 July 2019 - 11:05 PM

Maybe for this scope I will focus on objects like this:

 

https://www.astrobin...?page=6&nc=user

You could probably get some pretty good lunar images with this scope. I took these photos of the moon with my phone thru the LX90-10. Not the sharpest images, but I'm sure you could do better with the right equipment.

 

moon1.jpg

moon2.jpg

moon4.jpg



#38 Ballyhoo

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Posted 16 July 2019 - 12:03 AM

You could probably get some pretty good lunar images with this scope. I took these photos of the moon with my phone thru the LX90-10. Not the sharpest images, but I'm sure you could do better with the right equipment.

 

attachicon.gif moon1.jpg

attachicon.gif moon2.jpg

attachicon.gif moon4.jpg

that is very impressive from a phone. Are they stacked?



#39 rkelley8493

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Posted 16 July 2019 - 12:10 AM

that is very impressive from a phone. Are they stacked?

Thanks! No, they aren't stack or anything, just raw images. 



#40 WadeH237

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Posted 16 July 2019 - 06:20 AM

Edit, it is impressive how much you know about the architecture of these Wade. Have you ever taken one apart?

I've owned several 8" SCTs from both Celestron and Meade.  I also have a C14.

 

I've had the correctors and secondary assemblies off of all of them, except my EdgeHD 8 (which I haven't needed to yet).  I've replaced the focus knob on most of them.  I've never removed the tube from the back of the scope, and I've never removed the primary mirror from them.

 

They are really pretty simple devices.  Most of the issues that I've run into have all had to do with the secondary mirrors.




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