Mirror damage

[libraryman]

Several weeks ago an unfortunate accident led to my main mirror being damaged (attached)

fortunatly my insurance are covering the cost.

does the piccy indicate a scrapper or could it be refigured to a higher wave front.

Ray

[skybadger]

Ray - have you access to a ronchi rig or a a foucault tester ? I would like to bet there is no damage other than a scrape to the aluminium surface. I have a mirror that shows no ill effects whatsoever after a similar mishap. foucault test shows no difference.

In the worst case, paint the surface black or get it resilvered, but otherwise expect to behave as a slightly older mirror where more of the mirror surface has been damaged by the atmosphere.

regs

Mike

[libraryman]

Hi Mike, i dont have access to those tools, however when i look at the scratch, as the photo shows, it's gone down to a dark surface,i wondered whether thats too far down.

i'm pleased with your comments though, it gives me some heart, if i can get away with resilvering i can use the money elswhere.

Ray

[Peter Drew]

Hello Libraryman. The damage shown will affect only the mirrors resale value, you won't notice any difference to the performance. Aluminising that has been damaged always makes the exposed glass look black. If just the aluminising has been damaged a recoat will make the mark disappear, if the glass is damaged the mark will print through.

[libraryman]

Cheers Peter, it will be off Orion Optics in a couple of weeks to see if a recoat is feasable

Ray

[derekm]

It may be worth contacting Galvoptics too.

telescope mirror aluminising, silvering, coating, realuminising, resilvering, recoating

[Merlin]

The mark isn't really black. if you hold up the mirror to the light and look through the back, you'll see the light coming through.

Sometimes we can get "black" specks on a mirror. This is called pin-holing, where the aluminium has come away.

I had a friend who thought the specks were muck on his mirror. He almost rubbed his mirror away ( a touch of hyperbole there ) trying to get rid of the specks, until I asked him to look through the back.

The mark on your mirror isn't worth bothering about. Caryy on as normal until the mirror needs a complete recoat.

Consider this. The 82" MacDonald reflector mirror in Texas has three or four bullet pits in it's front surface, inflicted by a disgruntled ex-employee with a revolver. Apparently, the pits made little or no difference to the 'scope's performance, so they left it as it was.

[libraryman]

Thanks Merlin, i looked through the back and i can see the light!! there are several other small marks that let light through so a recoat is definatly needed.

The mirror is a std 1/4 wave so i will take it to Orion Optics they are reasonably close to me, i'll ask them if i can refigure it to 1/6th wave or maybe better hopefully that will completely remove the marks.

is this a practical way forward?

Ray

[vince 66]

If you are going to get your mirror re figured you may as well buy a new one as your mirror becomes nothing more than a blank for them to work with. this is what OO told me when I was thinking of getting my dob mirror upgraded. It would have been a costly excercise so I left it as it was. If the glass is not damaged a re coat will make your mirror like new. I had my mirror re coated by vacuum coatings and was advised that all mirrors get pin holes just some have more than others. I think it has somthing to do with the way the coating takes to the glass.

[Peter Drew]

Refiguring an existing mirror removes so little glass that the original saves considerable work so it is much more than just a blank. The big difference is if an owner wishes to have a mirror reworked to a different focal ratio, in this instance the whole grinding, polishing and figuring process would have to be completed and the original would effectively be a blank. .

[John]

A while back I discussed the prospects of improving the quality of a mirror with Orion Optics and got a similar response to the one Vince66 got. And my mirror was an OO mirror !.

I got the feeling that they were just not that interested in that type of work at that time.

I've heard good reports of the work that Galvoptics do so if it was me I'd probably just get the mirror re-aluminised and overcoated with them.

[libraryman]

I'm going to ask Galvoptics just what they can do? price wise Orion Optics are more expensive (i guess thats due to the investments made over recent years) but when you add in postage etc, for me it isn't much of a difference and i can talk to john with my mirror in hand.

i dont want to start anothe OO vs everybody else debate but I'd like to think that they would be interested in refiguring work, "25years service to astronomy" quote from their website.

OO have a mixed following, but their quality of mirror seems to find favour amongst the more experienced amature, i have found them to be reasonable in my limited dealings although Barry likes to quote salesman like at times.

Thanks for the suggestions an advise all

always apprieciated

Ray