What to check? - buying used skywatcher 8" dob

[Oat]

Hi All

Was going to buy a new dob when i had money, however a friend of my mums neighbour is selling one. It all came out in conversation and when i found out it was a skywatcher dob my eyes lit up!

The price seems ok but i have no idea how to check the telescope before i hand over some folded. Can you point me in the right direction.

Thanks

Oat

[knobby]

where do you start ?

I'm no expert but if its a friends neighbour could you try it first at there place, have a look at the mirrors, look for smears (indicating possible bad cleaning) a bit of dust is fine. Take the E.P. out to have a look at the small secondary mirror for the same.

Look at the screws / thumbscrews, do they look mangled.

Check the base for smooth movement, but don't worry too much theirs loads of threads on here re: modding / sorting Dob bases.

I think the most important thing really is the mirrors, anything else can be sorted out later, collimation, base etc.

If it looks well cared for ... hopefully it is !

I'm sure you'll get more hepful replies as well

Good luck

(and I think you are probably from Oxfordshire, wheres Oxforshire )

[damnut]

Hi Oat,

Best way to test is to try it out. point at something relatively distant and see if you can focus ok and the image looks right to you. Then try out at night, at least if close by you can return if it is no good!!.

Also have a look down the tube in daylight to make sure the mirror is not covered in grot and grime or scratched!..

Cheers

Damian

[umadog]

The best thing you could do is remove the primary mirror cell and take a close-up look at the mirror surface. Dust is ok. Small bits of gunk (e.g. tree sap) are also ok. Scratches that look like a cleaning attempt are not ok. The scope may still perform just fine but it will be worth less as there may be damage to the optical surface. Just peering down the tube won't reveal fine scratches. You could also look at the secondary, but they're cheaper so whether you want to remove it or not is up to you. Make sure there are no cracks in the optics. Small chips in the coating don't mean the views will be compromised but they will mean the scope is worth less (e.g. half what it would normally be worth second hand).

Unless you're already quite experienced it will be hard to evaluate the optical quality. e.g. you'd need to be experienced with the star test or know well what a good planetary image should look like. Even if you are experienced, you'd need a night of good seeing to really put the scope through it's paces. Otherwise, poor optics can hide behind bad seeing. It goes without saying that if the scope isn't collimated you can't infer anything about it's optics. Nonetheless, if you can point the scope at a bright star and check that you can get a tight focused image, that would be good. There should be a fixable explanation for any asymmetry (like a flare shooting off to the side of a bright star). Looking at objects during the day won't tell you much: the critical assessment is with tight focused star images, the star test, and good views of low contrast objects (e.g. details on Saturn). Some eyepieces, even expensive ones, produce unappealing day time views, such as obvious pin-cushion distortion, but look just fine at night.

[StarryBob]

I think with it being someone you know then you can probably try before you buy or at least possibly throw it back if you find a fault soon after getting it. Check the obvious and if you can then check collimation. If the scope looks ok and is not too old it should be fine, there is not much to go wrong that can't be fixed other than mirrors really.

[CFC]

Mine even has a big dent in the side but is no cause for concern - if they haven't been mucked about with they're as tough as old boots really, it seems.

[Oat]

Thanks all. I'll go take a look. It's a friend of my mums neighbour, so nobody i know or have real links with - so if i buy it i guess it will be to buy it there and then or not at all.

Fingers crossed....