What does a scratched mirror mean?

[drmorbius (Randall)]

Hi everybody,

I suppose this is a YANNQ (Yet Another Naive Newbie Question), but here goes...

I recently saw a 2nd hand 12" Dob for sale that had some "minor scratches" on the mirror. While I wasn't really interested in purchasing it, I was curious as to what that actually meant.

So, my question to you good folk is... how do scratches on the mirror affect the "viewing experience"?

cheers & thanks...

[mickoking]

Scratches will reduce contrast and scatter light. Best left well alone unless the scratches are minor.

[janoskiss (Steve H)]

I'm guessing you mean the one on Andrews' used gear page. It's been there for a while, and I might be mistaken, but it's probably one that was was returned by another IIS member.

Unless you can see it for yourself before you buy, give it a miss. But in any case, I'd want a more substantial discount than what is being offered on the website listing.

[dugnsuz (Doug)]

Agree,
Scratches = increased risk of corrosion.
Not worth the risk I reckon.
Cheers and choose wisely
You've got to be happy weeks/months/years after your purchase.
Doug

[Blue Skies (Jacquie)]

Despite what the others say, you probably wouldn't even notice them in use. I maintain a club telescope with quite a deep scratch in it and you'd never know it was there while you're using the scope. What matters most is how much area they affect. But it's a bit difficult to gauge the damage from a distance. I would consider buying a scratched mirror if I was able to see it first to see just how 'minor' they were. If you are short on funds it's a cheap way of getting a good scope!

[drmorbius (Randall)]

Originally Posted by janoskiss

I'm guessing you mean the one on Andrews' used gear page. It's been there for a while, and I might be mistaken, but it's probably one that was returned by another IIS member.

Yes janokiss... that's the one. Hmmm... sounds like there might be an interesting story here?

Anyway, I think I'll stick with new... unless one of you fine folk are planning on dumping their 12" Dob to make way for a 16".

[Starkler (Geoff)]

One deep scratch is of much less concern than a mirror covered with fine scratches. Each scratch is an anomolous surface scattering light in directions that you dont want.

[Merlin66 (Ken)]

I've been building, testing and using telescopes for 40 years ( God I hate saying that, "Help!!I'm only a 30 year old trapped in an aging body" ( anyone interested in getting some T shirts printed!)

The point with scratched mirrors:
1. If you're really lucky and they are small scuff marks, it maybe just the coating and when you re-aluminise they come good ( believe in fairies syndrome)

2. The glass IS damaged; it won't "grow" better, it's always there and will affect the performance to some degree or other ( If this wasn't the case why would I spend SOOOOOO much time re-polishing during the final manufacture) ( " It's only a little scratch! ...B@#$%)

3. It's like a pimple on the end of the nose; you see it first and distracts you all the time. (Never perfect syndrome); and will get blamed for every other "fault" you find with the scope. (If it wasn't for the scratch syndrome.)

4. If and when you try to sell, guess what the FIRST thing they say/ see/ comment on! (The used car syndrome.) Geee I can buy a new one for THAT price.

5. Life's to short at the best of times, if you've already got one; use it enjoy it and accept it, JFDI.

I've seen some REAL crook mirrors in my time everything from partly polished, crappy figuring, crappy star images, scratched, dinged, chipped you name it, and everyone of them had a "happy" owner ( at the time!) It's a bit like kids..... they're never 100% perfect; but they're yours.

My 2c worth.

[JohnG (John)]

All you got to do is look at the story of the MacDonald Observatory telescope in the US where a p'ed off employee shot a 45 pistol at it, they still use it.

http://www.daviddarling.info/encyclo...DonaldObs.html

JohnG

[matt]

[mickoking]

Maybe his colleagues were hogging the eyepiece

[Merlin66 (Ken)]

The story I have about McDonald was the lovers tiff "hell hath no fury....."
On the same subject I did my Honours Degree ( many years ago!) on The Isaac Newton Telescope and found out MANY things about telescope design and problems with large mirrors.........

If you 're worried about a couple of minor scratches you'll REALLY want to know the truth about the "reject" 100" mirror, the " full of striae" ( cracks between the layers of glass as it's poured) on the 200"; Epoxy glue being used to fill "gaps" in the 98" INT and believe me there is MUCH more.

I had to sign a "not for publication" agreement for my thesis but I think after 30 years they won't send me to jail for tell the truth...... welllllll maybe!.

[Adrian-H]

omg thats so cold, shooting a obserbatory grade telescope!? ive never heard of such terror.

[Blue Skies (Jacquie)]

Originally Posted by JohnG

All you got to do is look at the story of the MacDonald Observatory telescope in the US where a p'ed off employee shot a 45 pistol at it, they still use it.

YES!!