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  1. #1
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    Default How do you put a center mark on your primary mirror?



    My scope doesn't have one and I'd like to be able to align my secondary mirror correctly so I need a mark in the center of my primary. How do you know where the center is? Do you just measure? How can you measure in all the directions? But most importantly, how do you apply it? The primary mirror is ALL THE WAY down at the bottom of my tube. I can't reach. The last thing I want to do is take out my mirror. Do I have to do this?
    Celestron Astromaster 130eq - 130mm f/5 Newtonian on an equatorial mount. Viewing from New York, USA

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  3. #2
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    Default

    You would have to take it out I'm afraid .

    Easy way to center ( once mirror is out ) , is to get a piece of paper the exact diameter of the mirror, fold it exactly in half , top to bottom , then fold exactly in half again , right to left.

    Once you open the paper up , you have a vertical and horizontal crease/fold line .

    Where those two lines intersect is the exact center of the circle. Some people use those small hole strengtheners ( sp? ) that you get from stationary stores for A4 binder paper,

    Use the paper fold for a template to place the hole strengtheners on you're mirror.

    Replace mirror and collimate .

    Et voila.
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  5. #3
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    Default

    Thanks Pete!

    yesterday I was about to ask the same Q as smokesha. I'm afraid about damaging my pm, so if you have some tips to have in mind on mirror handling it will be appreciated (using gloves, some kind of protector, etc...). Thanks!

    Saludos!
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  6. #4
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    Default

    What I recently did was place the mirror onto a piece of paper and draw around it to get the "perfect circle", then, as said, fold once down the middle, then *that* down the middle... then I cut the tip off which gave me a perfectly centered hole. I made it *just* big enough to fit one of the circular stickers through it.
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  7. #5
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    Default

    Quote Originally Posted by jairsinho View Post
    Thanks Pete!

    yesterday I was about to ask the same Q as smokesha. I'm afraid about damaging my pm, so if you have some tips to have in mind on mirror handling it will be appreciated (using gloves, some kind of protector, etc...). Thanks!

    Saludos!
    Def try not to touch the mirror surface, will leave oily residue from hand/fingers. If using a glove ,don't use latex, believe it or not fngerprints can still be left on surface's even with latex gloves , photographic stores used to ( not sure if they still do because of digital ) sell white gloves for handling photographic paper. Can't mind if they are made of cotton ( I think they were ) or linen .

    Nice clean dust free area to work in . Be methodical , don' rush. Place mirror on clean fabric or rubber mat ( to stop slippage )

    I forgot to say about cutting the tip for the folded paper, thanks for adding that Enig .
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  9. #6
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    Default

    I'll just add .......

    Not to worry too much if you touch the primary in the centre inch or so while you are positioning the marker.

    Light rays coming straight in from the front don't get reflected by the centre inch or so of the mirror - as it is shadowed by the secondary mirror and holder.

    I found the centre of my 10" Dob mirror by simply measuring with a ruler.
    Across one way, mark the centre with a fine-pointed 'sharpie' - then across at 90° to that, and mark again to give a little cross right in the centre.
    The tiny cross doesn't affect the light path when viewing through the scope - and actually HELPS to see tha exact centre of my marker ring, when collimating with a cheshire sight-tube
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  10. #7
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    Default

    just in case anyone wants to know. I did it. taking the primary off was a lot easier than I thought. it was just 3 screws on the bottom and it slid out. I thought I was going to have to go in through the tube. Well, I took it off, cut a piece of paper the size of my mirror, folded it twice, cut the tip (a little too much but I think it was still ok) and placed the sticker. I really hope it's centered within a mm.
    Collimating afterwards was a lot easier than before. tilting the secondary was easier cause now I was centering the center mark instead of trying to center the border of the primary around the sides. then tilting the primary was easier just can I had that mark as a reference. I don't have a laser collimator yet so I gotta wait until the next club meeting so I can borrow someone's. Just wanted to thank everyone who contributed to this thread because it put me one step closer to seeing the heavens.
    Celestron Astromaster 130eq - 130mm f/5 Newtonian on an equatorial mount. Viewing from New York, USA

 

 

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