Buying used - What to look-out for?

Hi, Rick in San Diego here. I'm a newbie to this hobby, and I often see reflectors for sale on Craigslist at what seem to be reasonable prices.

Without expensive equipment, how can I evaluate (or can I even tell?) if a used scope is performing well?

I understand that I need to look through known good eyepieces at known stars, and compare to my own experiences, and also to buy from a known entity like an Astronomy club member. But I'm asking here about stuff like how to differentiate bad collimation from a disfigured or dirty primary mirror. I guess this could also apply to new telescopes that seem to perform sub-par to what's expected.

Here's a list of potential issues with used reflectors. Maybe y'all can help me understand how each one affects what I'd see? I'll start with my understanding on the first one: bad collimation. Please add and/or fill-in the rest as possible.

Not Collimated:
You'll get a coma on stars. Star-test rings not concentric. images don't focus correctly.

Dirty primary mirror:

Badly figured primary mirror:
(Maybe a breakdown of common issues: When you'll get coma, What do astigmatism, dips/peaks, etc. do to the image.)

Scratches on primary mirror:

Bad coating or badly aluminized:

Edge issues on primary mirror:

Bad or not good reflectivity:

Thin vs. thick primary mirror:

And lastly, in this internet age, should I just forget about buying used equipment, and save to buy new with a warranty, and be done?

Thanks for the advice!

- Rick

I wouldn't buy used unless I could take the telescope on a test run under the stars. You would look for the obvious--damage to the mirror, smooth operation of the telescope, and so on. If you look at the mirror during daylight, with the light hitting off angle, you will see any damage to the mirror and secondary. On the mirror, dust is no big deal, and neither are minor scratches. Look for deep scratches and larger chips and gouges on the mirrors. Look for obvious damage on the non-optical components. If the telescope contains go-to DSCs and electronics, have the owner show that they work. If you see things, ask questions. As the Russian proverb goes--'trust but verify.'

Since your in san Diego, pay Jon $20 to look over some scopes for you. He knows what to look for.

If the scope is a decent sized scope, the seller should be happy to show it to you at night and give it a run through. If not, then I wouldn't trust the seller or the scope.

Both the used C11 and 18" Obsession sellers gave me a good run through and I even collimated the C11 a bit because it was a bit off.

Both also saved me a ton of money over new.

Get a 250lpi Ronchi screen for 10 bucks. That won't tell you if it is superb or not, but it will tell you if it's a lemon or not...

Even just checking if the star images focus down nice and tight is a good test, assuming you tweak the collimation first. (no flared star images allowed)

The star image should stay round as it expands and contracts during focusing, and should look the same when defocused inward or outward.

It's not a Russian proverb, that 'trust but verify'

Ronald Reagan said that, about the Russian warhead reductions in Salt II.

Both of my scopes were from craigslist. I bought the 6" as a newbie, and the 8" about 2 years later. In both cases I met with the seller to inspect both telescopes before buying them. I basically looked at the overall shape of the scope, took a look through the focuser to inspect the secondary, looked down the tube to inspect the primary, ran the focuser in all the way, and out all the way. Both scopes appeared to be in excellent condition so I bought both of them for significantly lower prices than retail.
Its true that you wont get a warranty on a used scope, but generally its hard to break one unless you really try, or have an unfortunate accident like dropping it. I think the trade off is worth taking the risk if you can find a really nice scope for about half the price of retail.
Good luck in whatever direction you choose, and welcome to Cloudy Nights.

we must "trust but verify" posts on "trust but verify".

http://en.wikipedia....ust,_but_verify

what to look-out for when buying used scopes? If someone invites you into their home to check out a scope, bring a friend! If your buying from someone on a site like this one or the other asto classified place you can ask for info up front, and especially pictures. Also the reputation of the seller is important.

When I bought from Craigslist, The first seller i met, for the 6" Starblast, was in a shopping mall parking lot. They were a very nice retired couple that wanted their telescope to go to a nice home. After chatting with me for a few minutes the wife gave me her blessing to buy her scope.

The second seller (for the 8"Skyview Pro with mount) was at his house. I had reservations about meeting at his house, but the reason he was selling was because after buying the scope he realized it was just too heavy for him to lift and carry around. So, to play it safe I told my wife the address I was going to. I called her just before turning into the driveway, and told her I will call her in 5 minutes to verify all is safe. I said if I don't call, call 911 and direct them to the address.
The second seller was a very friendly and nice guy. He had the scope set up outside for inspection and after about 3 minutes I called my wife up and said I was fine. The transaction went smoothly and I got an excellent deal.
But, just to repeat what Curtis said, if you can, bring a friend, and at the very least do what I did and take some precautions. Always be careful when buying from Craigslist.

if you can star test the scope.

https://www.youtube....WzPj8e6TjWJb_lQ

finding central obstruction shadows noticably different on both sides of focus means the sa error is greater than 1/4~

It's not a Russian proverb, that 'trust but verify'

Ronald Reagan said that, about the Russian warhead reductions in Salt II.

Ronald Reagan did say that. However, he quoted it as a Russian proverb. It just might be. https://en.wikipedia.org/wiki/Trust,_but_verify

if you can star test the scope.
https://www.youtube....WzPj8e6TjWJb_lQ

The star image should stay round as it expands and contracts during focusing, and should look the same when defocused inward or outward.

I basically looked at the overall shape of the scope, took a look through the focuser to inspect the secondary, looked down the tube to inspect the primary, ran the focuser in all the way, and out all the way.

Thank you! This is info I can use while actually inspecting the scope.
... And as for "It's hard to break one" - I'm pretty confident I can spot a broken scope, but I'm more worried about buying one that seems good on first inspection, then discovering after the sale that it has a flaw in the mirror.

Thanks for your help,
- Rick

One thought, ask the seller if you can return it for a full refund. If they say yes, likely there are no hidden issues. If they decline, simply thank them and look elsewhere.

Not 100% accurate, as some simply wish to be rid of something they don't use but I have found it useful when those hairs stand up and something feels kinda odd,

Jack