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Scratched / Cracked Primary Mirror?


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Hey Guys,

I didn't really want this kind of thing to be my first post, but I'm seeming to have a bit of bad luck recently.

So, I bought my Skywatcher 200p brand new about two weeks ago, and with the nice weather this weekend, I finally took this as an opportunity to set it up in my living room with a view to 'first light' yesterday evening.

Once it was all set up, on the tripod and balanced, I noticed a bit of a scratch on the rear of the primary mirror. I cant quite work out if its a deep scratch or not - either way it certainly shouldn't be there out of the box.

My question to you guys is, would you think this is something worth worrying about? Are these mirrors quite thick? My concern is with change in temperature, this is going to get worse, and leave me with a large crack in the mirror.

I cant see any damage to the reflective surface.

c6cncq

http://flic.kr/p/c6cncq

c6cnso

http://flic.kr/p/c6cnso

Thanks in advance :(

Will

Edited by absy
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It looks like a deep scratch to me, if you look at the 2nd picture in my post, you can see the refraction going on due to its depth. My worry is how deep it goes is it going to get stressed and get worse :((((

I just want to look at the stars! :)

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It looks like a scratch on the pic and if it is then I can't see it mattering.

It's t'other side that matters. The glass is only there to hold the aluminium in place!

Olly

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I agree with Olly it should not have any impact. Indeed a disgruntled employee fired a hand gun at the shiny side of the 107" primary of the Harland J Smith telescope at the McDonald Observatory. The opticians smoothed the edges painted the craters black and it is still in use today!

Andrew

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If you are worried, then nothing we say will make you feel better, do what you feel is right for you :(

How about emailing the photos to the supplier to ask them what they think?

At least then you have some record if in the future it does crack ...

Good luck mate :thumbup:

Sent from my HD2Droid

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I'd get onto your supplier. Remember that in winter on a clear night temperature will be well below zero. There are stresses and strains in a thick mirror without the additional possibilty of damage.

Nick.

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Thanks for the advice guys,

I've emailed a copy of the pictures to the shop and will follow up with them tomorrow. I think thee bottom line is that it shouldn't have come with this kind of mark on the mirror, front or back. If it was just the tube, or a scratch on the tripod, I Wouldn't have thought twice about it.

They're in the process of trying to sell my dad an LX200, so I would think that they'd like to prove how good their after-sales support is :(

I'll keep you guys posted!

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Just to put the scratch (if that is what it is) into perspective, I have had 3 special order optics from Orion Optics UK for a 200mm F5.4 1/16 lambda and two similar spec 300mm optics one in Zerodur. All has the focal ratio and surface accuracy engraved (scratched) on to the back along with the name of the optician. I have never noticed any issues with them and they performed/perform flawlessly. Regards Andrew

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If it was me I wouldn't be asking them their opinion of the damage, I would be telling them I was returning it and to expect it in the next couple of days. You did not pay for a damaged scope, so don't accept one. Again if it was me I would be fretting about it for as long as I owned it. Another thing to consider, what if you up-grade, the betting is you will, how will you sell it on?

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If it was me I wouldn't be asking them their opinion of the damage, I would be telling them I was returning it and to expect it in the next couple of days. You did not pay for a damaged scope, so don't accept one. Again if it was me I would be fretting about it for as long as I owned it. Another thing to consider, what if you up-grade, the betting is you will, how will you sell it on?

This is a very good point. If I came to sell it, Joe Bloggs isn't going to take my word for the fact that it's superficial, and I wouldn't expect him to.

I've just had a little read of my statutory rights, and it seems that I have the right to a product that is of 'satisfactory quality' including a good appearance and finish and freedom from minor blemishes. So, I have some backup in case they get difficult! :(

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Interesting. My 300p flextube had a similar thing. I've now sold it on and I guess I did knock off a bit when deciding the price because of the flaw. The scratch/flaw/crack/whatever-it-was never got any worse despite plenty use over 3 years in varying conditions. When I removed the mirror to clean it, I felt a lot better because it's such a big hunk of glass the scratch/cut looked minor. I checked the figure of the mirror with a Ronchi eyepiece and it seemed very good to me. When I got the scope, I wondered about making a fuss and sending it back, but decided I couldn't be fagged! I find it really interesting that mine wasn't the only example. I have no idea how these features come about.

7_Back.JPG

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A small scratch shouldn't be too much of an issue. Indeed when I figured mirrors at OO a few came to me with what at first seemed like surface scratches but when looking from the side of the mirror turned out to be light cracks and air pockets deep inside the mirror. I was told that they did not affect the mirror in any way as long as they did not propagate all of the way through and weren't on the business side.

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I've just had a little read of my statutory rights, and it seems that I have the right to a product that is of 'satisfactory quality' including a good appearance and finish and freedom from minor blemishes. So, I have some backup in case they get difficult! :hello2:

Little known fact - if you buy anything over t'internet, with the exception of food, underwear and a few other things, you legally have 7 days to return an item, regardless of the reason. It's called a 'cooling off' period, and the supplier was obliged to inform you of this before taking your money. No doubt they did in some piddly small print!

Interestingly, during this 7 day period, you don't even have to have a reason, although if you return rather than replace, you may be liable for postage.

Source - selling a gazillion items on eBay!

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Interesting. My 300p flextube had a similar thing. I've now sold it on and I guess I did knock off a bit when deciding the price because of the flaw. The scratch/flaw/crack/whatever-it-was never got any worse despite plenty use over 3 years in varying conditions. When I removed the mirror to clean it, I felt a lot better because it's such a big hunk of glass the scratch/cut looked minor. I checked the figure of the mirror with a Ronchi eyepiece and it seemed very good to me. When I got the scope, I wondered about making a fuss and sending it back, but decided I couldn't be fagged! I find it really interesting that mine wasn't the only example. I have no idea how these features come about.

7_Back.JPG

Honestly, I'm dead certain that this mark is totally trivial. I'd expect to find such marks on any mirror. The back of our 20 inch looks as if it did service in the Battle of the Somme!

Olly

Edited by ollypenrice
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when I made my first mirror many years ago annealing marks (like a deep groove) were common in pyrex and didn't cause any problems on the back face. The pyrex blanks were about 2.5" thick but I don't know how thick yours is.

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when I made my first mirror many years ago annealing marks (like a deep groove) were common in pyrex and didn't cause any problems on the back face. The pyrex blanks were about 2.5" thick but I don't know how thick yours is.

It's about the same, or just a bit less.

Olly

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I actually have a bit of a scratch on the front of the primary for my dob - makees no difference whatsoever, as far as I can tell. I'm sure in a very minor way it does affect the image, and would probably harm any photos a little, but it hasn't damaged my visual experiences at all

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