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collimation cap view different horiz. vs vertical

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#1 Quest

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Posted 08 January 2013 - 10:21 PM

I'm hoping someone can explain this. I have an Orion XT8 and I'm checking the collimation. I only have a collimation cap so I collimate by placing the OTA tube horizontal with a white piece of paper in the OTA opposite the focuser and make the adjustments. All looked good but when I tilted the OTA in the vertical position, I just happened to glance through the collimation cap and it looked out of alignment. I kept my eye looking through the cap and rotated back to horizontal. As I went back to the horizontal position, I could see the center circle on the primary go back into alignment. I did this several times. What gives? It looks properly aligned in the horizontal position but increasingly out of alignment as I tilt to the vertical position??

I placed my iphone directly up to the collimation cap in the horizontal position and then moved to the vertical position and took these photos. You can see the brown walls as a background in the horizontal position and my white ceiling in the vertical.

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#2 MikePDX

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Posted 08 January 2013 - 11:04 PM

My .02 is that you are seeing a combination of your original XT8 springs being undersized - spring upgrades are a typical mod - and one or more of the springs being under tensioned. Your mirror is shifting as the load shifts across the springs. A quick even tightening of all three will probably help. The better solution is a spring upgrade. Finally, collimating is best done at 45 degrees to get the best compromise across the altitude range.

#3 panhard

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Posted 08 January 2013 - 11:05 PM

It's probably one of two things.
1 Not enough tension on the springs of the primary.

2 Movement in the area of the spider. Causing the secondary to shift.
Are the legs of the spider tight?

#4 Quest

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Posted 08 January 2013 - 11:13 PM

I apologize. Should have included the "i" as I actually have an XT8i. I don't have springs. I adjusted the tension nobs pretty tight. Two of the spider legs were slightly loose and that helped a bit bit it still appears in alignment in the horizontal position and still appears to move off center in the vertical but not as much as before I tightened down the spider legs. Still it's strange that it doesn't appear to stay in alignment through that movement.

#5 rlmxracer

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Posted 08 January 2013 - 11:17 PM

They are refering to the primary mirror springs not the alt tension springs found on the XT line.

#6 pdfermat

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Posted 08 January 2013 - 11:22 PM

The springs they're talking about are the mirror cell springs (which you do have on the xt8i). They are notoriously weak. I replaced mine on my xt12i with much stronger ones from Ace Hardware, and they definitely help. I also second the advice to make sure your spider vanes are tight enough.

#7 Quest

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Posted 08 January 2013 - 11:29 PM

Oh, "primary mirror springs". Got it. I didn't know that upgrading those springs are a common upgrade. Based on that, I really tightened them down the best I could and used a screw driver on the lock screws rather than thumb tightening. That helped quite a bit. The primary mirror center still appears to move off center by about half as much before. I lined the primary up with the OTA at a 45 degree angle and obviously that improves the movement at both extremes.

Off to look for primary spring upgrades...

#8 MikePDX

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Posted 08 January 2013 - 11:51 PM

One other thing you might be seeing, especially if you are truly horizontal, is that the primary is coming forward slightly and resting against one of the retention clips. This is not a bad thing since you don't want those clips tightened down - there should be a "business card" gap to avoid pinching the mirror. As soon as you begin to tip the OTA up, the mirror comes back to rest properly in the cell and out of collimation since you aligned things without the mirror seated. Ace is the place...where I got upgrade springs as well. Good luck.

#9 sopticals

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Posted 09 January 2013 - 03:22 AM

collimating is best done at 45 degrees to get the best compromise across the altitude range.


+1. :cool:






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