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wasteland

Collimation Fail on a Orion xt8i

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As the title suggests, I am in dire need of help with the collimation of my Orion 8'' dobsonian. I suspect that the problem might be a displacement of the primary mirror along the axis of the telescope, but I'd like to hear a more competent diagnosis from someone far more experienced than I am.

 

So, here's what happened:

 

1. For a number of months the telescope worked alright, until last month I caught a certain softness in the way the stars were displayed at medium to high power (let's say from 120x upwards, with hints un un-pointlikeness visible with some effort already at 60x). 

2. Suspecting a decollimation of the telescope, I proceeded to adjust the primary slightly. I noticed how one of the three screws was somewhat hard, and applied some force to it with my hands, without succeeding in unlocking it. I eventually made it during step 7.

3. Noticing how the reflection of the primary was a tad more than slightly off the center of the shadow of the secondary mirror, I tried to adjust the secondary. 

4. Moved by an unhealthy dose of healthy curiosity, I removed the focuser and then put it back where it was. Twice.

5. I reached a satisfactory collimation image through the hole-y collimation eyepiece, then went to have a routine look at the moon. I found myself unable to put it to focus in any of my ES eyepieces. 

6. After a month of forced inactivity I tried to collimate again from scratch today, with no visible improvement. Interestingly enough, I found I was able to focus on a far away chimney using my Celestron 15mm plossl, which is not parfocal with any of the other eyepieces I own.

7. I tried to adjust the mirrors again, but nothing changed.

 

In a staggering inverse variation of the eyepiece curse, the sky is completely clear and the transparency looks good today. It would be a great night for observing the winter sky through a telescope for the first time (yes, I very much a newbie, if the rest of the post wasn't convincing enough). Can any of you help me save the daynight? smile.gif

 

Thank you all in advance!

 

Marco

 

 

 

(also posted on CN.com)

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Hello Marco,

I would suggest starting by removing the primary mirror cell and checking the primary is centered within the cell. I would also back off the three adjusting screws to the same mid point of their travel. This should give you a mirror that is roughly parallel to the secondary and gives you plenty of movement to adjust it backwards or fowards if you cannot achieve focus. If the primary is not center marked then this is a good time to do it.

Once you have replaced the mirror cell go to the secondary and collimate it to the primary in the normal way and then adjust the primary,

Hope that helps.

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When you collimated your scope you moved the mirror up or down in relation to where it was when it focused, this will push the focal point out or pull it in depending on which way you went. If you can remember which way you turned the screws just re collimate, adjusting the screws opposite to what you did.

Why did you remove the focuser?

VG advice from dweller25...

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Ok, so I have now resolved that somehow the primary mirror has traveled about 0.25-0.5cm towards the front of the tube, as I needed to pull the eyepiece around 1cm out of the focuser for the image to snap into focus. How do you think I might reverse the mirror walk?

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Ok, so I have now resolved that somehow the primary mirror has traveled about 0.25-0.5cm towards the front of the tube, as I needed to pull the eyepiece around 1cm out of the focuser for the image to snap into focus. How do you think I might reverse the mirror walk?

The best way to get to know your scope is to practise collimation and look through the EP to see the different effects. You can tighten the collimation screws so that the mirror is at the top of its cell travel and check the focus position, then back the screws off lowering the mirror and see what happens there too.

I use this to get many different EP's to focus  that have widely different field stop locations ie 30mmES 82 vs the 21E.

If your cell has "snubber" screws I would leave them just touching, kind of loose. The mirror is supposed to float in the cell.

As dweller says putting the mirror in the middle of its travel is a great place to start and then re collimate.

Edited by jetstream

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