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Walking on the Moon

Dovetail Bars on a OTA


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I've read some comments on yahoo groups from people who don't fully tighten both ends of their dovetail bars onto the OTA so as to allow for some expansion and so avoid possible damage to the OTA. No mention was made of the fact that loose rails would then give rise to vibration and poor stability of any accessories mounted on them ?

Before I finalise my setup are any members aware of any significant problems / issues after fitting dovetail rails to a SCT OTA due to possible differential expansion between the Aluminium rails and the carbon fibre OTA ?

Your comments on this would be appreciated.

Thanks, Andy

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Andy, I suspect that the comments come from people in the USA which sees much more significant temperature variations than we do - in the Arizona desert (where my in-laws live) you can drop from 110F at 5pm to 40F before midnight, so the equipment is dealing with temperature changes of 20F per hour for several hours. That's a real sod to deal with for things like focus, and I can well imagine that there are long-term effects from differential expansion if that's happening every day.

That sort of thing never happens in the UK, we have a much more constant climate, so I'd imagine it's not really an issue.

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I've never heard of it being a problem, and I live in a place where temperatures range from -30ºC to +40º and can change by 20º in a day. I think this is a little overcautious. Still, I never tighten a screw more than firmly, as I've learned about stripped threads and things.

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Thanks for the help guys.

Apparently the secondary and corrector plate on my particular scope is not physically bolted to the OTA rather it is retained and is allowed to slide both inwards and outwards on support rods and threaded focus rods which are attatched to the scope base. Any small expansion lengthways in the OTA will therefore not affect the focus.

That is unless somebody knows different ?


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