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tyler_jrb

Need help with focuser.

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Have been trying to collimate my scope (skywatcher 150p dobsonian) for the past two days now and i have tried everything. The focuser on my scope for some reason doesn't sit square in its holder and is pointing too far down. There is a small gap of a few mm on one side of the focuser and it wont rack all the way in. Ive tried everything, adjusting the spider veins, adjusting the screws which adjust a small plastic plate inside the focuser but it does only a small amount. When it does appear square with the secondary it has a huge gap and the focuser is too stiff and wont move. The other annoying thing is that because it is pointing too far down then the secondary is quite tilted to make it central and circular under the cheshire. one side of the mirror is around 4mm tilt and the other side is tilted 7-8mm down. when looking down the focuser tube you can clearly see the tilt.

Does anyone have any idea how to square this focuser or does the secondary normally have that sort of tilt. I have never really noticed it untill now.

Thanks, tyler.

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I am never bery good with descriptions (and even photos) of peoples collimations. it sounds like you mihgt need to move the secondary down the tube a little. to do this (carefully to make sure you don't drop anything) sequentially loosen the central secondary bolt and tighten the three adjusters. this will move the secondary towards the primary and hopefully it will then be in line with the focuser axis. I'd recommend you check how much thread you have 'spare' to ensure no droppages.

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I am never bery good with descriptions (and even photos) of peoples collimations. it sounds like you mihgt need to move the secondary down the tube a little. to do this (carefully to make sure you don't drop anything) sequentially loosen the central secondary bolt and tighten the three adjusters. this will move the secondary towards the primary and hopefully it will then be in line with the focuser axis. I'd recommend you check how much thread you have 'spare' to ensure no droppages.

I would add, make sure the telescope tube is horizontal just in case you loosen to far. The last thing you need is the secondary rushing down the tube to kiss the primary "hello."

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Absolutely. If at all possible these things are best done with the tube horizontal and if you really can't then there's often the possibility of just adding a bit of string, cotton or even an elastic band tied off somewhere as a "safety line".

James

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Absolutely. If at all possible these things are best done with the tube horizontal and if you really can't then there's often the possibility of just adding a bit of string, cotton or even an elastic band tied off somewhere as a "safety line".

James

I would add, make sure the telescope tube is horizontal just in case you loosen to far. The last thing you need is the secondary rushing down the tube to kiss the primary "hello."

I am never bery good with descriptions (and even photos) of peoples collimations. it sounds like you mihgt need to move the secondary down the tube a little. to do this (carefully to make sure you don't drop anything) sequentially loosen the central secondary bolt and tighten the three adjusters. this will move the secondary towards the primary and hopefully it will then be in line with the focuser axis. I'd recommend you check how much thread you have 'spare' to ensure no droppages.

I would just like to add the focuser is pointing too far towards the floor e.g if the scope was horizontal. not too far towards the primary. i think making the secondary move more towards the primary will only make it worse because i have centered the secondary in the cheshire with as close to a circle as possible. The secondary is in the centre of the scope i have adjusted the spider viens to ensure this its just when the scope is horizontal because the focuser is pointing too far towards the floor it is tilted towards the floor also, e.g. if you look through the focuser tube you can see the secodary housing tilted down. surely if i were to push it anymore down towards the primary it would not be centeral in the cheshire

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From my experience when I collimate I always start with the focuser and ensure its square to the OTA so that all the steps that follow line up correctly. Obviously once this step is done once it shouldn't need to be done again.

I would remove the secondary and spider and then place a laser or cheshire into the focuser and mark the spot it makes on the inside of the OTA opposite to the focuser. This spot needs to be exactly opposite the focuser so use something like a sheet of card with a measure scale on it and measure around the inside of the OTA, each way, from both sides of the focuser hole - if needed, shim the focuser fixing screws with some washers until the spot measures exactly opposite the focuser. Once this is done replace the spider and centre it up - I good tip to do this is to use a long bolt through the secondary centre hole, and using a laser in the focuser adjust the spider knobs to get the dot on the bolt - you could use a ruler to line it up in the other plane. Then its just a case of centering the secondary mirror under the focuser using a cheshire and getting its rotation and tilt correct, etc, and then adjust the primary. Hope that helps.

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