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How bad of a scratch is this??


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Hello all!

Well, happy to get my "new" used 10" Meade Lightbridge and thinking the mark was dust, but after cleaning it (very appropriately without touching), what I thought was dust is a definite scratch. :)

Bought it used, guy said he never did anything to it except remove dust (yiks, another bad point), but it really didn't look bad from my noob perspective.

Do I return it? Or is this acceptable? What do you guys think?

Lots of input appreciated! I'm feeling a bit desperate at this point.

http://img818.imageshack.us/img818/6678/mirorscratch.jpg

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Hello EasyFlier.

Nothing to worry about there. Think about the 4 spider vanes supporting the secondary, they effectively operate as 4 "scratches" and nobody gets upset about that. (Except the refractor enthusiasts!) :).

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The 24" Newtonian at Keele University Observatory has a chunk the size of my fist missing out of the edge of it. There must be some light loss but not enough to make any perceptible difference either imaging or observing.

The scratch on yours is really insignificant and won't spoil it's use at all :)

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Thanks for all the feedback guys, it makes me feel better, BUT

For some reason, trying on Saturn now, I am completely UNABLE to get a crisp image. It seems that when I have it dead center and focused the right side of the image is blurred. When I move my eye around, i get it sharp. I collimated it before bringing it out, and it is dead on both settings.

My other scope is a 114/900 and I always have a sharp and crisp image, no matter what.

Is it my mind messing with me, or is it bad luck with the spot at the wrong location?

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I don't think the scratch is the root of your issues with the views you are getting.

I don't think you would ever be able to perceive the effects of that scratch, any effect would be so small.

Mike.

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Can I just ask how long it had cooled down for? It doesn't sound as if this is the problem with it not being 'uniform' in nature but it's very difficult to assess possible faults until the mirror temp is reasonably close to the ambient.

Also, as you rack in the focusser, does the 'soft image' get sharp at any point in the travel?

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Thanks guys, the mirror has been sitting out for an hour or so.

Turning the EP doesn't spin the blur with it ;-) (good thinking though).

The "soft image" does get sharp, and the image gets focused when I view the image at a certain angle, but when I offset the image with my eye, it gets sharp, like I'm used to.

Will go over collimating it once more...

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Personally wouldn't bother, but i can kinda get the frustration you are feeling. If it's a light scratch you could get it re-coated, although you'd be hard pushed to notice a difference.. cost would be around the 120 Euro area...is that worth it?

Just to add, check the collimation of the collimator you are using (F8 can be very forgiving) as for the 10" being an F5<?> you would more than likely see a dodgy view if it was out.

HTH

Glen

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Bingo! :)

What a relief!

This is what happened: after remounting the mirror on its mount and collimating the secondary, I noticed the laser (which is projected on the target) was in the center.

Well, it wasn't. The light was diffused on the target in the same way with my other scope when it is just right. The collimating screws were also very tight as they weren't aligned, so I took everything apart and set it straight. I also noticed one of my lasers wasn't exactly straight compared to a higher quality Baader laser.

It is amazing how much you can get used to one scope, know how to set it up perfectly, and then carry that "feel" over to the new unit...

Lessoned learned (with much stress thinking the scratch had something to do with it).

The moon is out, yet I can see so many stars so sharply it is outstanding! Never seen anything like that with my first 114/900.

Lots of learning to do. :)

I cannot thank all of you enough for the immediate support. You guys are amazing and brought some objective feedback.

How can I give back? MP me with your emails, hopefully I can send a little something to say thanks.

Cheers!

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Honestly wouldn't know, if i may leave it open to others with a better understanding*

Good luck :)

Glen

*I'd be under it like whippet looking at bearings and stuff..but i'm a hands on type.

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EasyFlier - if you pop over to the "Dob Users" group in the "Communities" section there's all sorts of tips for axis smoothing, setting circles, levelling, balancing, etc. Look for the "Dob Mods" thread - loads of good stuff in there.

(Click the link in my signature to get there) :)

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As others have said, I wouldn't worry about that scratch. It'll make zero difference the views. Just make sure everything's collimated and the mirror clips aren't touching the mirror's surface. The latter can cause astigmatism, but this reversible if the clips are loosened off.

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After cleaning the mirror well (still no touching), it looks sharper.

I did shine a laser on the back side of it and measured the different spots. Totaled up, it equals a loss of approximately 0.0118% of light compared to the whole surface ;)

At that point, it was just mental torture looking at it.

The weather is bad here, got nothing else to do... :BangHead:

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  • 2 months later...

For Information - that mirror has been replaced with one just a tad shorter focal length. It is pure Newtonian - not Cassegrain perforated. The reason is that almost all modern large CCDs are optimised (probably by accident) for focal lengths about 2500mm or so. Its true that the old mirror lost a few percent of light gathering power, but as we had the funding we went ahead with a new mirror. What is really tough is the environment which plays hell with mirror coatings...

Regards, JAmes Albinson, SUpport Group Keele University Observatory

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