2nd hand Dob - what should I check?

[Alan_B]

I've got an itch to buy more aperture and I'm considering buying a second hand 12" Dob. I just wondered if there is any way of checking this type of scope out in the daytime to ensure a trouble free purchase.

All suggestions welcome

[perrin6]

I don't think so as the sky is probably always too turbulent to judge how good the optics are, you really need a night with reasonable seeing conditions to do a 'star test'.

[spaceboy]

Other than obvious.....checking the primary for scratches, chips etc you can also check the coatings by pointing at the sun and seeing if there is any visible light coming through the back of the primary.

[swamp thing]

Other than obvious.....checking the primary for scratches, chips etc you can also check the coatings by pointing at the sun and seeing if there is any visible light coming through the back of the primary.

Many owners of large aperture scopes may not like you pointing there scope at the sun. I certainly wouldn't let any potential buyer point my scopes at it. They're more than welcome to use a powerful torch to check coatings, but not the sun.

You should never point a large aperture scope at the sun with no protection on the primary. The heat that a large primary can produce can seriously damage the upper end of the scope.

Regards Steve

Sent from my iPad using Tapatalk

[spaceboy]

I agree with what you are saying Steve but is this not the case for solar projection which is considered the safest way of observing the sun ???

[Moonshane]

safest for the eyes but not necessarily the other parts of a scope train?

assuming the price is right, the optics look OK (shiny with no obvious failing or bad scratches - a bit of dust is not a problem), the focuser is smooth and the mount itself relatively smooth and works well in both axes, then there's not a lot to go wrong with a dob.

if you can take a couple of eyepieces and give it a trial run (after checking collimation of course) then all the better. good luck

[todd8137]

if the optics look god and every thing is tight and no bits falling of,take a cheshire ,and just knock the scope of a little and try and get it back perfect if the thing will not col its a prob,then go for it if the guy as had it for a while its a good sign,Christmas is coming scopes are for sale all over the place second hand

[spaceboy]

safest for the eyes but not necessarily the other parts of a scope train?

MY MISTAKE!!! you quoted holding the mirror up to the sun not pointing the scope at the sun. http://stargazerslounge.com/beginners-help-advice/122782-mirror-aluminizing-buy-new.html

Not possible to remove mirrors when it's someone else's scope I agree but what if the secondary was covered ???

[Moonshane]

I suppose you could put a sock over the secondary but if I were selling I might not be too happy about that either. depends on the seller really. all that said, what are the chances of the sun being out in this lovely country of ours!?

[Alan_B]

Thanks for the reponses guys - I think its confirmed what i suspected ie that there's not a lot to go wrong as long as all optics are scratch free.

So let the search begin

[E621Keith]

I don't know how to check a dob's optical quality but don't point it into the sun whether you are viewing it or not.

Mike Lynch's "Exploding"Telescope - News Blog - SkyandTelescope.com

[perrin6]

Also with a reflector out in the daytime, beware of the sun even if the mirror isn't pointing at it, especially with open truss reflectors, as the focal point can lay a long way to the side if sunlight shines on the mirror from the side.

[Alan_B]

I don't know how to check a dob's optical quality but don't point it into the sun whether you are viewing it or not.

Mike Lynch's "Exploding"Telescope - News Blog - SkyandTelescope.com

Well at least Mike's insurance company are paying out. Salutory lesson for all of us with scopes I guess.

I must admit though I would have thought he'd have put the mirror cap/cover on if only to protect the optics from wind blown debris and such. (As well as his decking from going up in flames)