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Shipping scratches on primary is recoating necessa

Started by dj123, 04/15/2009 10:54AM
Posted 04/15/2009 10:54AM Opening Post
I shipped my mirror off to be fit into a custom made truss system and it came back with light scratches around the outer edge. It looks like the mirror shifted and rubbed against the tissue paper cover. I can only see the scratches when the light is at an angle. The mirror coating is about 4 yrs old. I have tried it a few times and I do see some softness but that is not unusual for Western Washington this time of year. Are these scratches bad enough to require a recoating? I don't want to wait for good weather to confirm that I have to send it out for recoating, I also don't want to spend the money if recoating is not necessary. I don't have a lot of experience with this so, I am asking for opinions.

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Posted 04/15/2009 11:09AM #1
David:

I wouldn't worry about it. Shouldn't effect the image at all. Certainly not the ability to effect image sharpness. Technically, the scratches might scatter some light when viewing something bright like the moon but I doubt it would be noticable at the eyepiece.

Those scratches are going to bother you a lot more than they will the image. Kinda like dust on a mirror.

Clear, sunny skies.
Tom Masterson
Coronado-Lunt-Daystar Solar Filters forum Moderator
Posted 04/15/2009 11:47AM #2
Yours don't sound to bad. I shipped a 12" once that I went through great lengths to pack correctly. When the coater shipped it back, they trough my packaging away and left the mirror face down in the box on some soft paper. The mirror was not tight in the box and it must have rubbed back and forth with the motion of the truck. The paper was full of sparkly flakes of coating. The mirror under a tester showed a shadow rubbed permanently in the glass where it rubbed. The coater said sorry about that but all we can do is re-coat it for you. I had to get it re-figured. It is very important to have the mirror tight in the shipping box.
Posted 06/14/2009 11:47AM #3
Actually you would be better off if you just made a circular mask for the last 1/4 inch and put it all the way around the mirror. One of the chief sources of "noise" in the Newtonian mirror is turned down edge which is nigh on impossible to eradicate from the parabolizing process. This is actually a more important source of light scatter than your scratches.

But I agree that the mirror should have been packed in such a way as to avoid this.

regards
Greg N

"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin