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nuffsed81

Secondary mirror so far our and how to center do concaved mirror

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I have just got a second hand Astromaster 130EQ. I want to say that i have collimated scopes before without the use of a laser but in this case i have 2 questions as this is so far out i cant see what i am looking at half the time. So hopefully you can help, thank you.

Info about the first problem:
The secondary mirror fixing screw that hold the mirror and the collimation allen key bolts were so loose the secondary mirror had rolled facing the bottom of the OTE. In every other case the holding screw never needed touching and all i had to do were make small adjustments with an allen key, no screwdriver was needed. So this case is a whole different scale then any other collimation i have ever needed to do. You probably all know that i need to get the center of the eyepiece in line with the center of the mirror before can move on with this step. 

Question 1
What is the best way to do this, shall i just eye it for now? (i have seen people measuring to the center of the eyepiece and then do the same with mirror but then i still dont know if the mirror is facing me directly.)


Info about the second problem
Every other time i have collimated a scope the mirror and scope were very low quality and just used for practice basically. The old scopes were used to see if i wanted to go further into this hobby which i definitely do. Any how the old scopes had a flat mirror and i could find the center by placing the mirror on paper and tracing a circle template. The thing is this mirror is concaved so any template iswill have to be pushed into the vurve (i think).

Question 2
So how can i center the mirror without placing it face down on a piece of paper which cant be a good thing?

Thank you. I hope i have not gone on to much, thank you for your patience in reading a long winded post.

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26 minutes ago, nuffsed81 said:

What is the best way to do this, shall i just eye it for now? (i have seen people measuring to the center of the eyepiece and then do the same with mirror but then i still dont know if the mirror is facing me directly.)

You would not measure to centre the secondary under the focuser. You should put your cheshire/sight tube in the focuser and adjust the focuser position so that the circle formed by the bottom of the sight tube is only a tiny bit larger than the size that the secondary appears. You can now easily see when the mirror is at the correct height and orientation. 

29 minutes ago, nuffsed81 said:

Any how the old scopes had a flat mirror and i could find the center by placing the mirror on paper and tracing a circle template. The thing is this mirror is concaved so any template iswill have to be pushed into the vurve (i think).

No, they didn't. A flat mirror would not focus light to a point. They may have had less obvious curves but they still have them. You still mark the centres in the same way and the curve of the mirror should not be a problem. 

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When you sai eyepiece I assume you’re referring to the focuser tube? That is where you want to center the secondary to. You’ll need at least a collimation cap to achieve that to a decent degree of accuracy. If you have a 1.25” dust cap you can carefully drill a tiny hole in the center of the cap which works for now. Once you place the cap in the focuser you’ll want to look through it and center the image of the secondary so it look round (it is ova yes)

but you want it to appear round and, centered within the outer edges of the focuser tube whilst your focuser is racked down al the way.

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3 minutes ago, Ricochet said:

You would not measure to centre the secondary under the focuser. You should put your cheshire/sight tube in the focuser and adjust the focuser position so that the circle formed by the bottom of the sight tube is only a tiny bit larger than the size that the secondary appears. You can now easily see when the mirror is at the correct height and orientation. 

No, they didn't. A flat mirror would not focus light to a point. They may have had less obvious curves but they still have them. You still mark the centres in the same way and the curve of the mirror should not be a problem. 

The probel is the old mirror was small so i could put a piece of paper directly on it and mark the center, with the larger mirror (still only 130mm) i can see i will have to push the paper into the curve.  That means the paper will be creased and the template not of any use.  Is there any other way.  You would think celestron would have a central mark already but it doesnt unfortunately.

The main problem is the first step and gettin the secondary sorted. How can i do this?  Do i just do it by eye? Approximate it it?

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5 minutes ago, Sunshine said:

When you sai eyepiece I assume you’re referring to the focuser tube? That is where you want to center the secondary to. You’ll need at least a collimation cap to achieve that to a decent degree of accuracy. If you have a 1.25” dust cap you can carefully drill a tiny hole in the center of the cap which works for now. Once you place the cap in the focuser you’ll want to look through it and center the image of the secondary so it look round (it is ova yes)

but you want it to appear round and, centered within the outer edges of the focuser tube whilst your focuser is racked down al the way.

I believe i have the secondary sorted as good as can be for now. I made a  coli cap and the circles are all concentric.  Have you any ideas how i can center dot the primary, i have watched many YT videos  but is there anything you can think of where i dont have to trace the mirror.  I dont have acompass and cant get into town to get one sorry.

 

Thank you

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19 minutes ago, nuffsed81 said:

The probel is the old mirror was small so i could put a piece of paper directly on it and mark the center, with the larger mirror (still only 130mm) i can see i will have to push the paper into the curve.  That means the paper will be creased and the template not of any use.  Is there any other way.  You would think celestron would have a central mark already but it doesnt unfortunately.

The main problem is the first step and gettin the secondary sorted. How can i do this?  Do i just do it by eye? Approximate it it?

The height difference is only 3mm. The paper will be fine. 

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1 hour ago, nuffsed81 said:

I believe i have the secondary sorted as good as can be for now.

sorry, i hope you were able to understand my post absolutely full of spelling errors lol.

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3 minutes ago, Sunshine said:

sorry, i hope you were able to understand my post absolutely full of spelling errors lol.

Yeh its fine.  Thanks for help.  i have sorted the secondary i just need to center the primary now. Thanks

 

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2 hours ago, Ricochet said:

You would not measure to centre the secondary under the focuser. You should put your cheshire/sight tube in the focuser and adjust the focuser position.

That is part of the problem i dont have a cheshire.

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don't sweat it, a collimation cap, even a home made one will get you close to where you want to be, this is what you want to see down your focuser tube, just get the secondary looking round, and also entered within the focus tube edge, then work on adjusting the screws so the primary reveals itself in the secondary.

image.png.9826d7b5e0bb77562c1ca5ec3e09ce97.png 

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I had a very similar problem with my new 130EQ some years ago when I first got back into astronomy. I bought a cheshire but found the collimation cap worked just as well and was simpler so I wouldn't bother with one.The secondary adjustment was a pain not helped by the fact that the focuser tube was not running true because a couple of the teflon guides were out of place. I pretty much had to start from scratch, like yourself, getting the mirror position correct and then fine adjusting it. but it got sorted in the end, just taking it a step at a time. Once the secondary was good the primary is easy. I centre spotted the primary OK using the paper method. Fortunately once the collimation is sorted the 130EQ holds it pretty well, just the occasional tweak on the primary.

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