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Repositioning Newtonian Secondary Mirror


sophiecentaur

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I washed my mirrors. They are better now but not perfect and I can accept that I have done some good. I have come across a bit of a dilemma. Putting the secondary back and doing a collation (laser collimator) I realise that the three collimating screws will give a 'good result' for a range of angles for the rotation of the secondary on its mount. I am trying to get the geometry right in my head but I would like to get the secondary orientated optimally. 

I am hoping someone will reassure me that, as long as I can get the laser spot to disappear into the hole in the centre of the target, the actual combination I use doesn't matter much. If it does matter then how can I be sure that I start off the three screw adjustment with the secondary in the best orientation? Is it just a matter of eyeballing down the focuser tube? I have watched videos and read several links from Google but they don't seem to make that bit clear enough (for me, at least).

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I've not collimated my scope in years so I can't give any advice.

However, I saw a video recently where a guy used a webcam to check this, basically so the secondary looked circular rather than elliptical, when viewed through the focuser. He demonstrates the ability to correct the dot with secondary adjustment screws given a secondary rotational error that you're mentioning, and but also a way to correct that rotational error with a webcam.

https://www.youtube.com/watch?v=8ROvNH5uwDo

To what extent it matters and what the consequences are, I've no idea. But this might be one way to check and correct.

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You need to rotate the secondary mirror such that you see the reflection of the primary centred in it when looking through the empty focuser. You then attach the collimator and adjust the secondary screws until the red dot appears central to the primary. That's it for the secondary. Next, if necessary, adjust the primary until the red dot reflected from the primary diappears through the hole in the collimator.

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Hi. On my Newtonian, there's only one place where it's correct and it aligns with the -in my case- Cheshire cross hairs. To get close, look down the centre of the focuser. A cap with a small hole in the centre helps.. Position the secondary as you see in the snap. Then adjust the primary by looking at an out of focus star or your laser. HTH.

pn208-focuser.thumb.jpg.d0ab7f0ad5940d37029236ee2c6ae297.jpg

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Best to use a collicap for this procedure. Place it in the focuser and undo the centre screw slightly on your spider. Move the secondary around until all three primary mirror clips are visible and the image of the secondary looks as close to circular as you can get, then tighten centre screw. Then run through the rest of the collimation process using your laser.

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