Author Topic: Squaring the focuser to the optics?  (Read 887 times)


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Squaring the focuser to the optics?
« on: December 29, 2016, 08:22:20 AM »

I've received a request for a link with info on how to square a focuser to the optics when installing a new focuser.  Anyone have a recommendation?  I've always just used a sight tube.




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Re: Squaring the focuser to the optics?
« Reply #1 on: December 30, 2016, 01:38:07 AM »
Sight tube is the tool, agree.

On my I have this link on my links page that explains by S&T what I will elaborate on below:

The goal is you want the focuser to point right at the secondary mirror and should not be distracted by the image in the secondary so much except to ensure the secondary is basically rotated to face the sight tube as best as you can estimate and it's angle is very close to 45 degrees or 'normal' for most dobs.  Only after secondary mirror is centered in the axis of the focuser using the sight tube do you worry much about precise tilt and rotation of secondary to see primary nicely and that is stage 2 of collimation with stage 3 being primary mirror adjustments.

Your goal is to adjust the sight tube so that the view of the actual secondary is seen as a full circle and just fits inside the sight tube as you look in the sight tube.  This will make it easier for next steps.  So a proper sight tube diameter for your scope is one where you can back it in and out to have the image of the actual secondary fit just inside the end of the sight tube view.   One trick is put a piece of white paper on other side of the secondary cage so you can make out where the edge of the secondary is.   If your secondary is in a holder you can put whiteout on the perhaps black holder edges so you can make out there the edge of the holder is but of course none on the mirror itself.   

So once you basically see the full outside rim of the secondary and you feel the focuser is very perpendicular to the wall of the secondary cage you wish to adjust secondary distance from primary so top and bottom of secondary is centered in the sight tube view.  (This is with reference to scope pointing straight up)

Next you should be able to adjust focuser so right and left side of secondary are both same distance from edges of sight tube (the farthest part of the sight tube you see should be just larger than secondary image to judge best).

Once you are roughly set you may wish to use laser to center laser dot in primary by adjustments to the secondary then verify the sight tube view still sees the full secondary nicely centered in sight tube view.

 With optics that are way off all these things become iterative so revisit this process if you had to move secondary angles to verify secondary is still centered in sight tube.

There is some discussion of this in 'The Backyard Astronomers Guide' which is big book but loaded with lots of other stuff.   

Let me roam the deep skies and I'll be content.


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Re: Squaring the focuser to the optics?
« Reply #2 on: December 30, 2016, 10:09:48 AM »
Thanks, Marko.  I sent a link to this thread to the person inquiring.  Very helpful.