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Sirius Bizness

Buying a second hand scope

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Hi all,

I hope I have posted this in the right section. I am interested in buying a second hand scope (specifically an ED apochromatic refractor). I am fairly new to astronomy and only just starting to get my head around the finer details. What I want to know is what should I be looking for when going to see a second hand telescope and how should I go about inspecting it.

I might suspect the only true way is to test it out by looking at a star but with the longer nights it is most likely it will be in daylight.

My other interest is cars which I know my way around well so I have a long mental list of all of the things I will check such as brakes, suspension, oil/coolant condition, engine running sounds, oil leaks, service history, paperwork etc.

What I want to know is do you more experienced telescope owners, who have bought second hand, have a similar list of things to check.

Also, because I am still learning my way around these telescopes, it might be useful to descibe (or better still show with a picture) what is bad/not ideal.

I appreciate any advice that can be offered.



Edited by Sirius Bizness

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Take a torch with you and inspect the lens, that's the most important thing. Allow for specs of dust, but obviously not spider webs, scratches or cracks!

No dents in the scope tube, chipped paintwork is ok but nothing that suggests it's been dropped or walloped.

Twiddle the focuser in and out, check it's smooth and does'nt slip.

If it comes with a finder, check the optics on that with your torch same as the scope lens.

Maybe check the tube rings too, they release ok and are firmly attached to the dovetail bar. If they have an underside of felt check that too.

have a look on AB&S as you are largely buying from the astro community and on the whole decent folk as well as sellers.....

Oh and you will no doubt get better and more experienced responses than mine, good hunting!

Edited by Woking

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As above. Also, dried dew marks are generally nothing to worry about but anything mold or fungus-like is a deal breaker.

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check out this link


tbh if it's your first scope I'd buy new for two reasosn - 1. nice apos hold their value pretty well so you don't gain too much by buying second hand (or lose too much if you sell it); 2. if there is something amiss you might never know as you don't have much to compare it to.

ps not all ed scopes (ie with ed glass) are apochromatic; some claim to be but aren't; some don't claim to be. likewise not all apos use ed glass.

if you know what you want to use the scope for (ie imaging or visual) that would clarify the choice a lot, eg the ubiquitous ed80s are great for imaging but pretty limited for visual.

good luck:)

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You can make an artificial star for star testing very easily. (I did this and set it up on a motorway service station car park, which was my rendez-vous point with the vendor.)

Glue a small ballbearing onto a piece of matt black card. Shine a bright light on it, a decent cycle headlight, for instance. You can do this from an angle off to one side, it doesn't matter. If it is a bright day put the whole lot at the bottom of a big cardboard box to make a sunshade.

Now retreat to about 30 metres and telescopically observe the BB which will be sending light your way only from a tiny point on the ball. Use about 100x magnification and test it on and off axis.

In terms of what to look for, Google Star Testing Astronomical Telescopes.

Most budget apos have Crayford focusers, very prone to slipping. Make sure the draw tube will rack fully in and fully out. (Some William Optics scopes I've seen can't even manage that.) Then pull on the drawtube to see if it slips without the lockring tight. Take a little set of Allen Keys with you to see if it is a simple adjustment. Sometimes the roller gets bent by over enthusiastic tightening.

I'm not sure about the torch test. Almost all optics look dreadful if you shine a light at them. Try the test on a good optic to give yourself a benchmark or you might reject every scope you see! Just looking down the tube in good daylight would be better, in my view. The 'fashlight test' is notoriously unhelpful on reflectors but I've never tried on on a refractor.


Edited by ollypenrice

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Very good advice from all of the above.

A phone chat with the seller beforehand can be a big help to find out what sort of use the scope has had.

For example I once bought a small refractor without the pre-purchase chat. I never thought to ask if the scope was a drawing room ornament or put away after use. Yes it was extremely dusty - inside and out. OK in this case not a big effort to dismantle and clean. But it might have been different.

If you can take something else along for performance comparison, that is good. But not always possible, depending on the circumstances. I'm thinking back here to when I had a Skywatcher refactor that I thought gave OK views and I was generally happy with it. When compared to the William Optics on horizon objects the contrast was like foggy vs clear day. Yes big price difference here, but I think you get the idea.

Hope this helps.

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Thank you for all the excellent advise so far! I am sure this is going to be invaluable.

I am going to use the torch method and visual inspection to look for webs, mould, scratches, cracks, etc. This must be the first thing to do.

Great advise on how to check a focuser.

I agree that communication goes a long way so will definitely be speaking to the owner first. Now I know what to ask!

Olly - thanks for letting me know how to do an artificial star test. Now trying to quickly work out where to source the materials. Perhaps this is one of the things to discuss prior to turning up and building some contraption in the street!

Buying new isn't feasible for me as my interest is AP and the total cost of all bits needed is getting insane.

Feeling much more confident now. Will post up my findings/outcome once put into practice.

Thanks, Martin

Edited by Sirius Bizness

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