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Help with Meade 8”LX200 GPS image train

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#1 mklosterman1

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Posted 25 October 2021 - 09:34 PM

Hi,

I’ve got a Meade 8”Lx200 GPS (non ACF) with a .63x focal reducer/corrector (the Chinese one) and a zwo asi294MC pro. I am trying to figure out the order and spacing of all components to include micro focuser, reducer, spacers (do I need a t-adapter?), IR Cut filter, and camera. Also I may want to use diagonal if image train won’t clear the fork mount.

 

thanks

 

Mike


Edited by mklosterman1, 25 October 2021 - 09:35 PM.


#2 michael8554

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Posted 26 October 2021 - 04:06 AM

First item to attach to the OTA is the focuser.

 

Then the FR. This has SCT threads, so may need adapters to match your brand of focuser, and to match the next item:

 

The camera. Your ZWO ASI294MC Pro has female M42 x0.75 threads, the sensor is 17.5mm inside the front of the M42 adapter.

 

I don't know what size of IR filter you have, but the ZWO site shows a 2" Filter will screw onto the ZWO.

 

Distance from the end of the FR to the sensor should be 105mm.

 

If that lot won't clear the mount then you'll need focuser, then FR then diagonal (or diagonal then FR), then camera.

 

Though a diagonal is a visual device, the less glass the better for imaging.



#3 dcaponeii

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Posted 26 October 2021 - 09:03 AM

For planetary imaging I'd wonder why you'd want the reducer in the first place.



#4 carolinaskies

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Posted 26 October 2021 - 10:36 AM

Be aware when using an external focuser and placing the reducer outboard from there the actual focal length being reduced will be longer than when the reducer is mounted directly to the telescope. 
The most basic setup is

  • FR>(OAG/Filter(wheel))>Camera - In this setup the .63 reducer will make your system ~1260mm 
  • FR>Focuser>(OAG/Filter(wheel))>Camera - in this setup there are 2 scenarios 
  1.   Scenario A - FR does not move with the focuser tube - the .63 reducer spacing to camera is variable changing the effective focal reduction depending on the distance change.  The added spacing of the focuser + accessories will require the coarse focuser of the OTA to need to move the primary closer to the secondary to achieve focus. The reduction effect will vary by how far the distance changes from the back of the FR to the sensor of the camera
     
  2. Scenario B - FR DOES move with the focuser tube - the .63 reducer spacing to camera sensor remains constant at .63x but to achieve focus the baseline focal length may be greater than the native 2000mm.  This will effect the final FOV size depending on how much the external focuser is racked out. Also, the focuser takes up part of the backspacing of the FR which may not allow a filterwheel mechanism and require close attention to the total spacing​
  • Focuser>FR>(OAG/Filter(wheel))>Camera - in this setup it's similar to scenario B except spacing between FR and Sensor(and any accessories in between) is not affected by the focuser.  
     

There isn't really a serious issue with placing the FR after the focuser except as you note the possibility of clearance issues between a fork system.  If you use this method all the time, the minor loss of FOV from the increased focal length is not significant.  


 


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#5 mklosterman1

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Posted 27 October 2021 - 12:22 PM

Thanks! Just to clarify...I am not using a filter wheel or OAG. I have a ZWO ASI294MC Pro one shot color and only shoot DSO (not planetary). I really don't understand scenario B if the FR is first (i.e. screwed onto the SC threads on back of scope how it would be able to move with the Meade microfocuser....

 

So will probably first try : Focuser>FR>spacers with 2"UV/IR Cut filter>Camera  then I will try it FR>Focuser>Spacers with 2"UV/IR Cut filter>Camera

 

I may need to get a filter drawer if I can't adapt the filter in between spacers since the camera side is T-2

 

Mike



#6 dcaponeii

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Posted 27 October 2021 - 01:02 PM

Thanks! Just to clarify...I am not using a filter wheel or OAG. I have a ZWO ASI294MC Pro one shot color and only shoot DSO (not planetary). I really don't understand scenario B if the FR is first (i.e. screwed onto the SC threads on back of scope how it would be able to move with the Meade microfocuser....

 

So will probably first try : Focuser>FR>spacers with 2"UV/IR Cut filter>Camera  then I will try it FR>Focuser>Spacers with 2"UV/IR Cut filter>Camera

 

I may need to get a filter drawer if I can't adapt the filter in between spacers since the camera side is T-2

 

Mike

I didn't understand you were not trying to do planetary.  I found for the case of my 0.62x Lepus reducer that I could adjust the position of the main mirror such that the back focus distance was correct with my JMI NGF-S focuser mounted on the Lepus reducer.  The small focus adjustments made with the microfocuser were not large enough to change the back focus such that the image degraded appreciably.  Note:  I use a 4:3 chip so it's possible you would see an effect for an APS-C or full-sized chip but I had good results.  I could NOT reach focus with the reducer mounted ON the focuser.



#7 carolinaskies

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Posted 27 October 2021 - 06:42 PM

Thanks! Just to clarify...I am not using a filter wheel or OAG. I have a ZWO ASI294MC Pro one shot color and only shoot DSO (not planetary). I really don't understand scenario B if the FR is first (i.e. screwed onto the SC threads on back of scope how it would be able to move with the Meade microfocuser....

 

So will probably first try : Focuser>FR>spacers with 2"UV/IR Cut filter>Camera  then I will try it FR>Focuser>Spacers with 2"UV/IR Cut filter>Camera

 

I may need to get a filter drawer if I can't adapt the filter in between spacers since the camera side is T-2

 

Mike

Option B is available with some external focusers, Moonlight makes a version like this. The Meade external focuser doesn't have the option. 




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