Does the age of a telescope make a big difference advice needed

[Jemmo (Adrian)]

Hey guys I'm looking at buying a dobsonian scope I can either afford a new 8" for around $500 but a 2nd hand 10" has popped up for $350 I spoke to the gentleman who is selling it and he said it is ten years old and only thing that needs done to it is a mirror clean.
I know the 10" is sooooo much better but my question is does age matter um quite new to this so just after a bit of help.

[multiweb (Marc)]

Originally Posted by Jemmo

Hey guys I'm looking at buying a dobsonian scope I can either afford a new 8" for around $500 but a 2nd hand 10" has popped up for $350 I spoke to the gentleman who is selling it and he said it is ten years old and only thing that needs done to it is a mirror clean.
I know the 10" is sooooo much better but my question is does age matter um quite new to this so just after a bit of help.

It's like everything else. If it has been looked after then age doesn't matter. In the case of the mirror cleaning I'd ask him to clean the mirror then bring an experienced person with you to have a look through the scope and see if everything looks ok. 10yrs without recoating is a long time. Recoating a 10" could set you up for a couple hundred dollars or more.

[Merlin66 (Ken)]

A good mirror or objective, well looked after will last indefinitely!
My Genesis 4" is over 25 yo and still as good as new.
The 200" mirror at Palomar was finished in the early 1940's and still working well.

[Jemmo (Adrian)]

It's going to be a 3 hour drive so I can't take and expert he is sending photos like I said his mid 80s and was just going trade it in on other items but if I want it he won't. He is selling because he struggles to move the base. What would it cost to get the mirror cleaned as I don't know how to do it.
I'm kinda taking the guys word for it he said it all works well but mirror might need a clean and he didn't want to do it himself in case he scratched it.
So in the end do you guys reckon I'm better off getting an older 10" for a bargain instead of paying over $500 for a new 8" dob?

[barx1963 (Malcolm)]

My 20" mirror was 20 years old but was fine. Ended up recoating simply because after 20 years it is going to have some issues sooner rather than later.
With the Palomar they recoat the 200" regularly (there are videos on Youtube showing the process!)

Malcolm

[astroron (Ron)]

If the telescope is in good condition and all that is required is to clean the mirror then there is no problem,just depends what he means by "cleaning"?
Ask him to send the photo's and publish them on here to give us some idea as to what the state of the mirror is.
If the telescope has had very little use and the mirror has no mold then I see nothing wrong in buying it at that price.
A 10" mirror could cost upto about $400:00 to recoat depending who you get to do it?
Cleaning is very easy and instructions are available on this site.
Cheers

[doppler (Rick)]

The best way to check a telescope mirror is to point it at a light source and look up through the bottom of the scope (back of the mirror), any scratches or thin spots will be readily visible. A good coating will still be slightly transparent , you should be able to see the fillament on a incandesant light globe. A few scratches or blemishes won't affect viewing but can be a good bargaining point to get a better deal. 10 years is not that old for a mirror and 10" is a lot better than 8".

Rick

[Jemmo (Adrian)]

Thanks for all the help guys he's 84 the guy selling it so hes going to try get his neighbour to help email me some pics if he does I'll put them up thank you guys

[Regulus (Trevor)]

There are some excellent videos on Youtube showing the mirror cleaning process, and it's not difficult.
You need to also look at videos and info on collimating a Dob mirror, of which there is enough around.
You need a collimating tool to help with this, and what it's called has slipped my mind, but someone here can tell you what it is, and where to get it.
The photos of the mirror may make it look worse than it is if he uses a flash, because the dust on it will appear like large white spots covering it. Very hard to tell the condition from a photo down the tube.

The 10" is on the 8" base but is noticeably heavier and more awkward to carry. Some use a dolly cart to move theirs. Unless you are particularly strong, it is a two person job if it has to be moved 20~30 yards each way.
Frankly, having used an 8" and a 10" I would be happy to go back to an 8 because it's lighter, and I can't say I really noticed that much more detail in nebulas. Of course that may well be my optically challenged vision too.
Either way, you will love you Dob whichever you end up with. They are great scopes.

Trev

[Jemmo (Adrian)]

Cheers Trevor I've got a collimator laser and have collimated my newtonian so I've got an idea on doing it

[Jemmo (Adrian)]

So this is the 10" um looking at buying for $350 the Te. Year old one these are the only photos I've got so far looks in good nick though

[Jemmo (Adrian)]

Another photo

[mental4astro (Alexander)]

How's the primary mirror looking? A sqizz down the tube will show a lot. Also, have a look at the rear end of the scope so you can see the backside of the primary. Then have a bright light shine down the tube onto the primary - can you see any pin-pricks of light shining through the primary. This will tell you a lot about the condition of the primary - no pin-pricks of light is ideal. Few pin-pricks, yeah ok but not great. Many pin-pricks (or worse, big holes in coating) scope still usable (just and not great by a long shot) and not worth very much.

[mental4astro (Alexander)]

Originally Posted by Merlin66

The 200" mirror at Palomar was finished in the early 1940's and still working well.

Yes and no - I'll explain.

All professional scopes are recoated about every two years, the 200", the scopes at Siding Springs, etc. These mirrors are recoated on site with facilities built into each observatory for the largest of instruments. These have just the naked aluminium and are not overcoated as this hinders the recoating process.

While overcoated mirrors can be recoated, if the substate IS Pyrex, this is relatively easy. Any other substrate requires the coatings to be ground off as the substrate will bedamaged by the chemicals that would be used with Pyrex as Pyrex is inert, other glass types will be dissolved. BK7 & other types are NOT Pyrex.

[Jemmo (Adrian)]

I haven't had a chance to go see it yet and check the mirrors as it's 3 hours from me. I will def shine a light down the scope those is a iPhone torch good enough?

[mental4astro (Alexander)]

Should be just fine. Bright sky or white wall too.

[Regulus (Trevor)]

It looks like a nice Dob, and the right-angle finder is a bonus.
It looks clean so I am hopeful it will all work out well.
Good luck - Trev

[Jemmo (Adrian)]

Thanks trev. And cheers guys for all the info with torches on the mirrors etc. I put a deposit on it and will make the drive next week. If it's not what I want then I leave with my deposit. Sounds great though. I don't get lenses with it but I have some the same size from my $200 Newtonian but I'm tipping they aren't the best quality. But they will do the job till I can afford better ones

[ralph1]

If you're very adventurous you could try stripping the coating and silvering it yourself. I haven't done it so I can't comment on its effectiveness but my understanding is that it takes multiple attempts to get good at and it involves lots of nasty chemicals. The reading I've done seems to indicate that if you make a mistake you can get silver nitride which has a tendancy to explodeIf you're thinking of silvering, read up on it yourself to weigh up the risks.