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Centering A Focuser (tutorial with focuser)

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I have just flocked my scope, meaning everything is off it, i just have a metal tube currently flocked, therefore a couple of things 'may' look different but only because I'm rebuilding while writing this. There are arguments for an against precise focuser centering but after reading a fair amount of information and also working with the collimating by video camera thread with doc, I am 100% convinced that a centered focuser produces better collimation.

Tools You Will Need

Either a vernier gauge or a good steel rule


Screwdrivers to remove focuser

a laser collimater OR cheshire

a strip of thin card

The thin card size varies depending on scope diameter, but for a 200p a 1" wide strip cut from the long side of an A4 will do.

Firstly, remove the focuser AND your secondary mirror and measure the diameter of your tubes focuser hole, you must do this with the steel rule/vernier ALONG the axis of the scope, not across it, a round hole in a curved surface measures smaller if you measure along the axis of the curve.

Take the measurement and half it, mine measures 95mm so halved is 47.5mm. Write the figure down

Next, measure the distance between the edge of your hole and the edge of your scope, bear in mind on my photos, the end trim isn't fitted due to flocking, you dont need to remove yours but then obviously you measure to the very front INCLUDING front trim. Mine came to 41.5mm

Add this figure to the one you wrote down, so, 41.5 + 47.5 = 89.

This gives the distance from front of scope to center of focuser hole.



Edited by TopHouse
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Next, using your piece of card, square it up to the bottom edge of the focuser hole and make sure it is in contact with the tube all the way round, it's useful to have a partner for this.

Mark with the pencil the point where the card edge ends in your tube.


The card shown is one I've used previously, note the cutout, this was to accomodate the screws for the spotter scope mount, you may need to do similar.

Do exacly the same but from the opposite side of the focuser hole and again make a mark where the card edge is. This will give you two pencil marks in your tube with a space in between.

pencil used for illustration only, pencils don't work on flock!



Edited by TopHouse
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Next, measure the distance between the two pencil marks in your tube, i used my card again for this, holding the edge of the card to the marks in the tube and just transfering them to the card.

Half the measurement giving you a measurement to center and mark this in your tube (use the card again to transfer marks)

OK, this mark is now the focuser hole center 'across' the axis of the scope.

All you need to do now is to measure from the front endge of your scope 89mm (in my case and mark the tube with a pencil again 'across' the center mark you just made, this now gives you the center of your focuser hole, you can now either just leave the pencil 'cross' or put a dab of tippex on it or a paper self adhesive reinforcing ring.

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I'm not going to give specifics for actually centering the focuser as different focusers have different methods, ie skywatcher crayfords have 3 screws like a secondary mirror, moonlites have 4 I believe. BUT, all you now have to do is re-fit your focuser and using either a laser collimater or the crosshair in a cheshire, adjust it onto the mark you have made. If it needs a lot of adjustment, use washers under the focuser to shim it in the necessary direction.

Refit your secondary and center it up using your prefered method and recollimate your scope.

Done! I think? Let me know if I've missed anything obvious.

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Pencil DOES work on flock, here's mine showing the card marks and you can even see the little cross under my ring binder reinforcer which I coloured blue with a marker pen (the shop had no red ones which was my preferred colour)


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paper isn't strong enough, it needs to be thin card really, as long as it goes somewhere near or even a little over half the inside circumference it'll work, because even if the lines from the card overlap, the middle of the two lines will still always be the center. So no equation really, just 'near enough' to the inside circumerence of your scope minus half the diameter of the focuser hole, that's the beauty of it, no presice equations lol

Just putting my spider back in now and then the camcorders out lol, I've 'stealthed' my spider and secondary too! ;)

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  • 2 weeks later...

Very slight addition.

If you do use a ring binder reinforcement ring as I did above, give it a dab of superglue or similar. Otherwise when you've used the scope in mist conditions a time or two you may find your primary has two center spots!

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  • 11 months later...
  • 4 years later...

Bit of a thread revival...

I have a Europa 200 and I just upgraded the focuser to a Moonlight CR2. I did my best to centre the focuser (by eye only) as there was some play in the bolt holes due to the bolts for the new focuser being smaller than the old.

I am just starting out with this and plan to do some imaging shortly. Do I need to put more effort into getting the focuser perfectly centered with respect to the secondary mirror? At this point I am going to assume the primary and secondary are fine, I accept this is probably a flawed theory with a 20 year old scope.

If better centering is required what equipment would you recommend? I see sight tubes being mentioned quite frequently. I'm not overly fussed in going completely mad with this collimation right now but I probably will go all out at some point in the future.

Thanks in advance,


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