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Home made tool for secondary mirror centering.

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#1 Javier1978

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Posted 14 September 2015 - 10:34 PM

I use homemade tools to perform the focuser and primary axial collimation in my reflector, and I have always wanted a tool to center the secondary under the focuser with some precision, so I end up with this tool. Not fancy at all, but I think it does a decent job. The grid is centered using the spot that I made in the tube on the opposite side of the focuser. It has been useful too to evaluate the position of the secondary holder.

 

Maybe someone find this useful.

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Edited by Javier1978, 14 September 2015 - 10:45 PM.

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#2 x935418

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Posted 14 September 2015 - 11:02 PM

i don't understand 

if spot is on opposite side of focuser, so it is behind of secondary

and how you can see it ?

 

can you explain a little more (or may be with a picture) ?



#3 x935418

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Posted 14 September 2015 - 11:04 PM

OH i understand it !

 

great !


Edited by x935418, 15 September 2015 - 02:37 AM.


#4 howard929

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Posted 15 September 2015 - 07:48 AM

In my normal state of confusion with no memory, I ask.

 

With the new model of secondary placement, isn't the secondary shifted towards the primary when it "looks" like it's centered under the focuser and using that template renders the old model (physically centered)? Would it even matter?



#5 Javier1978

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Posted 15 September 2015 - 08:14 AM

i don't understand 

if spot is on opposite side of focuser, so it is behind of secondary

and how you can see it ?

 

can you explain a little more (or may be with a picture)

 

Yes, my explanation was kind of short. I centered the central hole of the grid with the marking that once made in the tube to square the focuser, but this isn´t needed. I can just use the collimation cap to get an acurate eye placement and use the lines of the grid to carefully center it under the focuser.

 

As said, it´s not intended to be a precision tool but it´s an improvement from concentric circles reference.


Edited by Javier1978, 15 September 2015 - 08:20 AM.


#6 Javier1978

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Posted 15 September 2015 - 08:17 AM

In my normal state of confusion with no memory, I ask.

 

With the new model of secondary placement, isn't the secondary shifted towards the primary when it "looks" like it's centered under the focuser and using that template renders the old model (physically centered)? Would it even matter?

 

My scope uses traditional offset, but it is my understanding that the secondary should appear rounded under the focuser even with the new model and that the position of the spider vanes was the only thing that changed in the view from the focuser... lets wait to a collimation guru to clear this up.


Edited by Javier1978, 15 September 2015 - 08:17 AM.


#7 Vic Menard

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Posted 15 September 2015 - 09:23 AM

Centered under the focuser = offset (new model or classic).

#8 Kipper-Feet

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Posted 15 September 2015 - 09:42 AM

Clever indeed...It's like having a holographic grid without a laser.

 

Still, I'm thinking that it will be difficult to get away from the 'concentric circles' idea considering those very obvious concentric circles drawn on your template.

 

Even so, I like the template as it could be a useful tool to understand how the secondary is centred and rounded under the focuser irrespective of the offset configuration.  See Vic's post #7 above.



#9 Nils Olof Carlin

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Posted 15 September 2015 - 01:21 PM

As said, it´s not intended to be a precision tool but it´s an improvement from concentric circles reference.


I'm sure it works. But if you make your own tools, try make a sight tube with its length / inner diameter equal to the f/ ratio of your instrument. Put a piece of paper on the mirror box (if one, or otherwise a strip of paper or cardboard piece below the secondary), then move the tube in/out till you see it just frame the secondary with the tiniest line of black between the mirror and its reflection in the inside of the tube. Then adjust the rotation and placement as needed.
I think it is an improvement over even your method - if for nothing else, it uses the true focuser axis. This done, remove the paper, push the sight tube in enough to frame only the reflected primary, and adjust the tilt of the secondary to center it (the reflected primary). This could be done with even greater precision than with the ubiquitous crosshairs, I am sure.
But as with any method, start by tilting the secondary roughly right. The only difficult bit may be finding a piece of tube that fits snugly in your focuser - but once you do, you can easily make a sight tube for each of your Newtonians. To center the peephole, it might be easiest to drill a clean hole (~1.5 mm) in some suitable material, then measure where the outer edge of the tube should be glued.

Nils Olof
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#10 Javier1978

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Posted 15 September 2015 - 06:13 PM

Clever indeed...It's like having a holographic grid without a laser.

 

Still, I'm thinking that it will be difficult to get away from the 'concentric circles' idea considering those very obvious concentric circles drawn on your template.

 

Even so, I like the template as it could be a useful tool to understand how the secondary is centred and rounded under the focuser irrespective of the offset configuration.  See Vic's post #7 above.

 

I get what you are saying, I made the concentric rings in the template to get a rough aproximation, and I worked with the lines then. Some further work is needed to perfectly center the secondary with this last reference, but it´s good enough for this alignment in my scope. I was sourprised that after this, I use the tilt rotate adjustments to center the primary reflection into the secondary and then found that the FAC was almost perfect. 


Edited by Javier1978, 15 September 2015 - 06:14 PM.


#11 Javier1978

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Posted 15 September 2015 - 06:13 PM

 

As said, it´s not intended to be a precision tool but it´s an improvement from concentric circles reference.


I'm sure it works. But if you make your own tools, try make a sight tube with its length / inner diameter equal to the f/ ratio of your instrument. Put a piece of paper on the mirror box (if one, or otherwise a strip of paper or cardboard piece below the secondary), then move the tube in/out till you see it just frame the secondary with the tiniest line of black between the mirror and its reflection in the inside of the tube. Then adjust the rotation and placement as needed.
I think it is an improvement over even your method - if for nothing else, it uses the true focuser axis. This done, remove the paper, push the sight tube in enough to frame only the reflected primary, and adjust the tilt of the secondary to center it (the reflected primary). This could be done with even greater precision than with the ubiquitous crosshairs, I am sure.
But as with any method, start by tilting the secondary roughly right. The only difficult bit may be finding a piece of tube that fits snugly in your focuser - but once you do, you can easily make a sight tube for each of your Newtonians. To center the peephole, it might be easiest to drill a clean hole (~1.5 mm) in some suitable material, then measure where the outer edge of the tube should be glued.

Nils Olof

 

 

Thank you, great tip!




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