Buying Old Telescope

[littlegreenman]

Hi All,

I am new here and this is my first post.
I've been visiting this forum for many months now.

I have a question about buying a telescope (dobsonian).
Are older dobsonian telescopes as good as the new ones?
I've been watching the fleebay and gumtree for used telescopes.
Some are around 10 years old (skywatcher, celestron, meade etc)
They look very similar to the new ones.

My plan is to get 8" - 12" dobsonian.
I only want to use it on my backyard for occasional stargazing.
I've seen the view on 8", 10" and 12" (new ones).

Thank you.

[Allan_L]

The main thing to watch is the condition of the primary mirror.
Depending on how well it has been cared for.
The mirror coating may be deteriorating or damaged in some way.
I believe it is not uncommon for mirrors to need re-coating after about 10 years.
Another killer can be mold if allowed to remain damp, or kept in a damp environment.
Ask questions like how and where it has been stored.
Has the mirror been cleaned. If so, how?
Check other glass surfaces similarly (secondary mirror; finder scope; eyepieces too).
Make sure rocker box is in good condition. (MDF and moisture don't mix well). Assess the effect of any DIY mods.

A preloved scope can be a bargain, with extras, accessories, and beneficial mods already made, but remember, you (generally) don't get any warranty, and you need to feel comfortable that this risk is covered by any cost saving.

BTW Welcome to IIS :welcome:

[barx1963]

I have seen some awful dobs. Some of the early ones were really bad. The Celestron ones had cardboard tubes which in theory should be OK but are really susceptible to moisture.
As Allan said the mirror and the mount are the main items.

Just make sure you get to inspect first and particularly avoid any ads with pics lifted from dealers websites rather than the actual scope.

Keep an eye on the classifieds here as good bargains come up from time to time.

Malcolm

[littlegreenman]

Thank you for the advise.

Maybe not such a good idea if the primary mirror usually need to be re-coated after 10 years.
I'll keep an eye for a used telescope over here too.

Generally speaking, with popular brands like skywatcher, meade etc, do they make better quality mirrors and lenses now than say 10 years ago?

[Wavytone]

Not really, no. The technology has improved - things like adding encoders to do "push to" (dobs), even motorised GOTO and so-on, but:

1. the optics are largely governed by the laws of physics, the materials used, and the quality of the manufacturing.

Not much has changed in 40 years - there are lots of scopes with rather "average" optics, perhaps 10% with exceptionally good optics, and perhaps 20% with substandard optics that are not what most would expect as "diffraction limited".

The mirror coating technology has have not changed in 30 years. Some mirrors have cheap aluminium-only coatings - these have high reflectivity but are soft, easily scratched and oxidise (pinholes develop and grow with time) and need recoating after 2-3 years. Some have exotic metals added to last longer (Beryllium). Others have a quartz or SiO2 overcoat which will last 10+ years, but eventually they too need to be stripped and recoated.

2. The cheaper mounts and tube assemblies - especially the ones sold for budget dobsonians by Skywatcher, Orion and meade - are built down to a minimum price. They will work well enough at first for casual observer, but after a while moisture will get to the MDF or chipboard and they start to go soft or fall apart. The tube assemblies are also very flimsy.

Eventually if you learn a bit more about what is going on you will realise the mechanicals are junk, then strip it to re-use the optics and some of the hardware to build a new dob yourself to fix these issues.


3. The optics in the commercial schmidt-cassegrains - Meade / Celestron - in recent years have been a lot better than they were 10/20/30 years ago. Especially the ACF (Meade) or EdgeHD (Celestron). Prior to these the ordinary SCT's could be a bit of a gamble - there were some good ones among a lot of inferior ones.

4. Maksutovs - the Meade 7" maks had consistently good optics, if you are happy with an f/15 scope. Likewise the Orion/Skywatcher cheap chinese maks in recent years seem to be optically quite good but not always as perfect as they could be. The maks from Intes Micro had very good optics but are hard to find.

5. Then there is Questar, in a class all by itself - perfect optics in the only small telescopes built to last a lifetime - and continue to have no issues with obsolescence after 50 years. But you pay a hefty premium for one of these.

[littlegreenman]

Thank you very much for the information.

I think I will look for one that is not too old unless I can find the ones you mentioned.
I'll keep looking and maybe just buy a new one when they are on a discounted price.

:thanx:

[Val]

Could I ask if there's a place one goes to to get a dob mirror recoated in Australia? Cheers.

[littlegreenman]

I did a quick search and found this
http://www.astro-optical.com.au/mirror.php

[AstralTraveller]

See

http://www.iceinspace.com.au/forum/showthread.php?t=96859
http://www.iceinspace.com.au/forum/showpost.php?p=896285&postcount=11
http://www.iceinspace.com.au/forum/showthread.php?t=86474
http://www.iceinspace.com.au/forum/showthread.php?t=103089

[littlegreenman]

Is this a good quality telescope?
http://www.gumtree.com.au/s-ad/ellenbrook/other-home-garden/12-5-inch-meade-dobsonian-telescope/1022194679

[Allan_L]

It is a good brand.
It looks like a good clean scope.
It appears to be a good price.
Subject to inspection, and a few relevant questions.
I'd give it a chance if I were closer.

But it is a big scope and may be difficult to move around.
Pick it up during your inspection and assess if it will be OK.
If you can't move it, you won't use it.
Then it is not such a good bargain.