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Mirror centre?

Gottzi

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laowho

laowho

Posted
Posted

Get a piece of tissue paper, cut to proper diameter, fold twice and snip the pointy end (center), unfold and drop it down on the mirror, position correctly, and lower a Sharpie (fat-tipped marker) to the tissue paper center-hole????

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Gottzi

Gottzi

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Cheers guys I knew there must be another way to do it.Collimation is scary enough without removing mirrors as well. Looking forward to November 1st when we have our street lights turned off in Colchester from midnight to 5 am. :grin:

Clear skies

David

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John

John

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I have the feeling that the approach that laowho describes will work much better with the mirror removed from the tube though ?

Surely the secondary mirror and supports will get in the way of i) getting the circle of tissue paper accurately centered on the mirror and ii) lowering the marker pen precisely onto the hole in the tissue paper ?

The approach that laowho descibes is normally carried out with the mirror out of the tube as far as I'm aware.

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John

John

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Sounds like you really needed a truss type tube :wink2:

It can be awkward even then. Taking the mirror out is pretty easy:

- Mark the tube and the mirror cell so you can put them back with the same orientation.

- Stand the scope on it's nose.

- Undo the 4 screws (it's usually 4) that hold the mirror cell into the tube.

- Gently lift the mirror cell complete with the primary mirror in it up and away from the tube.

There is no need to take the primary mirror out of the mirror cell to centre mark it. 

Carefully reverse the above procedure to get the mirror / mirror cell back on the end of the tube making sure the marks you put on it line up and the tube seam sits in the slot cut into the mirror cell for it.

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laser_jock99

laser_jock99

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Also slightly off topic- it's important in optics to understand that the 'optical centre' of mirror may not necessarily be in the same position as the mechanical centre. I.E. it's possible for the optical axis of the curve generated in the glass mirror blank to be in different position to the physical centre. In optics we call this variation 'centration error'. So in theory it's possible to mark the dead centre of the mirror and still be 'off axis'. 

In practice short focal length optics are more prone to this problem but I sometimes wonder if some of the collimation difficulties people experience are down to the centre spot being in the physical centre but not on the optical centre?

More info on centration here:

http://www.edmundoptics.com/technical-resources-center/optics/understanding-optical-specifications/

https://www.cvimellesgriot.com/products/Documents/TechnicalGuide/Centration.pdf

(centration error applies equally to mirrors as well lens optics)

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stargazerslivingroom

stargazerslivingroom

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Cheers guys I knew there must be another way to do it.Collimation is scary enough without removing mirrors as well. Looking forward to November 1st when we have our street lights turned off in Colchester from midnight to 5 am. :grin:

Clear skies

David

Do you have any information as to how switching the lights off has come about? There doesn't seem to be any consistency across councils.

Steve

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