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Newbie: LX200 EMC Classic 12" track lurching?

astrometry beginner cassegrain dslr Meade
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#1 hpcnp

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Posted 21 January 2021 - 11:44 AM

Hello CloudNights.  

 

This is my first post to CN and I really don't know what I'm doing with regards to using an SCT for Astroimaging - so I'll ask for your patience.   I also need to apologize for my questions that come from newness with using an instrument with a focal length as long as 3000mm.   I'm lucky enough that my local astronomy club offers scoes for rent, so I currently have this LX200 EMC Classic 12" with an EQ wedge.    However, most of my experience has been based on refractors (600-1000mm range).   I have read the full manual for the LX200, and done extensive you-tubing, and CloudNights reading.   And finally, I'm sorry this is such a long post.  I am trying to be thorough.

 

I've documented the configuration of what I'm using below to perform these tests in the "Setup" section below.  Further, I've included some images to make these challenges more concrete.  

 

Challenges:
   1) Focus is soft / not sharp
   2) Tracking appears to be lurching

 

Setup

  • LX200 12" Classic EMC with Ultra EQ Wedge
  • Laptop running Win7, APT controlling the camera only (USB3 connected)
  • Camera:   Unmodified Canon 5D mark IV
  • Standard Meade T-ring and extension tube

 

Images
image 1: Screenshot from the laptop of columnation from APT
image 2: A good tracking image - 15sec exposure @3200 ISO
image 3: A bad tracking image - 15sec exposure @3200 ISO (4 minutes after image_2)
image 4: A screenshot of APT (focus tool) FWHW when not touching the scope
image 5: A zoom in on one image showing tracking induced vibration / sticking
Please Note:
    i) I've converted these images to JPG and compressed them just to make them easy to access.  I'm shooting ASTRO images in RAW.
   ii) image_1 & image_4 were near zenith at the time to minimize atmospheric effect

Challenge #1 - Observations
I am comparing the focus I can get with this scope to focus I can get on the same night, same location using my refractor (Orion 120, 600mm).   To some degree, I expect the extra focal length to magnify the unsteady "seeing" - but I don't think it should be this bad - am I wrong?   Image_2 shows the best focus I've been able to get.   Image_4 shows the wild behavior of FWHW when trying to use the focus tool in APT - with nothing touching / changing on the scope the FWHW is this sawtooth pattern varying by as much as 50%.   Also it seems too regular in variation to be the "seeing" conditions.

Challenge #1 - Things I have tried:

  1. Focus Lock check - Since I can move the focus easily - I don't think I've missed a focus lock.   
  2. Columnation - using eyepiece down to 7mm and also using live view and zoom through the camera.   I hoped this would be the issue - but so far, the error in columnation (using the APT columnation tool) has been very slight.   Easily adjusted - no change in focus quality
  3. I have let the scope cool for as long as 4 hours to minimize the expansion / contraction effects
  4. I have waited until as late as 2am to avoid earth heat haze in the atmosphere
  5. APT Focus tool (FWHM) usage - very difficult to use as the variation without touching focus jumps around wildly.   (see image #4)

Challenge #2 observations
The inconsistency of the images (some are streaks, others are not as shown in images 2,3) tells me that there is some kind of issue with the OTA or the camera moving inconsistently.   If my polar alignment was off (and I'm sure it's far from perfect) then the images should be consistently streaked for constant exposure - but clearly they aren't.    Further study shows more than streaks (see image 5) .  In some cases there seem to be vibrations or sticking and slipping where the streak looks like morse code (short bright spots & dashed more faint streaks) introduced, which makes me wonder if the worm drive may need to be cleaned / greased ?  (somehow sticking and then slipping as it rotates the OTA).

 

Challenge #2 - Things I have tried:

  1. Polar alignment + Refined polar alignment
  2. Shutter lock (minimize camera induced vibration)
  3. Remote operation (I operate the camera over my network, so that I'm standing more than 10 meters from the scope) - minimize ground vibration & heat from my body
  4. Balancing the scope with the camera attached.

Things I plan on trying for both challenges (suggestions and advice appreciated)

  • using a Bahtinov mask for focus rather than the FWHW tool in APT
  • adding additional weight to balance the DEC
  • PHD guiding with an Orion TOAG
  • Using the two speed motorized focus instead of touching the scope for focus - Currently non-functional.   I hear the motor running, but the focus doesn't change.   I expect the internal gear isn't meshing.   I'm confident I can fix this.

Thanks for your thoughts
chris



#2 michael8554

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Posted 21 January 2021 - 02:23 PM

Collimation looks okay.

 

Did you Polar Align ?  For unguided imaging it needs to be very good.

 

The LX200 mount will suffer from Periodic Error, gradual increases and decreases to the Tracking Speed due to RA worm and wormwheel not being perfectly circular

 

Most LX200s suffer from Dec Backlash, which means Dec can wobble on its own.

 

Short answer is that unguided imaging at 3000mm FL with this mount will be hit and miss.

 

Bahtinov mask will help focusing.

 

Off Axis Guiding will help but not completely cure imaging at 3000mm FL



#3 hpcnp

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Posted 21 January 2021 - 09:33 PM

Ranger 4 - 

 

Thank you so much for your response and the time you took to look at the images.

 

Yes, I did the two step Polar align and then refined polar alignment as suggested in the manual.

 

I had not considered (obviously) Periodic Error.   Increases and decreases in the tracking speed is exactly what I'm seeing.  

 

Just so I understand - you're recommending guiding, are you recommending guide scope guiding vs. off axis guiding?  Also, I have been considering getting a field flattener for the LX200 which will knock down the FL to ~2000mm.   Generally that's probably a good idea for imaging anyway I expect.   

 

Thank you again



#4 michael8554

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Posted 22 January 2021 - 02:17 PM

Yes my advice concerning OAG is ambiguous.

 

Don't try a separate guidescope even at 2000mm FL, you need an OAG with a sensitive guidecam such as LodeStar.




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