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# Best way to find the center of a 110mm mirror?

9 replies to this topic

### #1 Jethro777

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Posted 13 August 2019 - 07:53 PM

I have a 110mm reflector, however the mirror is not marked at the center, making it tough to use the tools I usually do to collimate.

What is the best way to find the exact center?

### #2 Stevegeo

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Posted 13 August 2019 - 08:27 PM

Just like finding the center of a circle, only you have a glass circle.

Rather than explaining here... I will suggest  a lesson in geometry, or you tube or a Google search.

Once found you can mark it with a water based marker, or a simple stick on dot.

### #3 Jethro777

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Posted 13 August 2019 - 10:17 PM

I suppose so.

The mirror is in it's cell and I don't intend to remove it.  This complicates things.

I also can't use a pencil to draw lines, nor can I use a compass.

So, rather than a geometry lesson, I just need some practical suggestions as to how to go about it.

### #4 Kunama

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Posted 13 August 2019 - 10:18 PM

I have a 110mm reflector, however the mirror is not marked at the center, making it tough to use the tools I usually do to collimate.

What is the best way to find the exact center?

Take a square piece of paper, fold it in half first in one direction then at 90ยบ, make a pinhole where the creases cross. Measure 55mm along each crease and make a pencil mark.

Line up the piece over the mirror so the 4 marks match mirror edges and using a marker pen make a mark where the pinhole is...

Be careful not to slide the paper along the mirror surface ...

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### #5 Jethro777

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Posted 14 August 2019 - 03:19 AM

So so practical.

### #6 azure1961p

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Posted 14 August 2019 - 09:22 AM

I have a 110mm reflector, however the mirror is not marked at the center, making it tough to use the tools I usually do to collimate.

What is the best way to find the exact center?

Find the middle.

Pete

### #7 Vic Menard

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Posted 14 August 2019 - 09:27 AM

Now that you know how to mark the center, what's the focal length of the mirror?

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### #8 Jon Isaacs

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Posted 14 August 2019 - 09:29 AM

What's the focal ratio?  If it your new Tal 1, it may be slow enough that the mirror is spherical and placement of the center spot is not critical.

Jon

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### #9 Jeff Morgan

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Posted 14 August 2019 - 09:50 AM

### #10 Jethro777

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Posted 17 August 2019 - 08:34 AM

Reporting back - used the square paper method and it was fantastic - so easy.

Added a center spot, collimated wonderfully - very happy.

Amazing Jupiter, Saturn and the moon tonight.

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