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RE-COATING OF REFLECTOR MIRRORS.

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RE-COATING OF REFLECTOR MIRRORS. was created by COMET12

Just a word of warning about sending your reflector mirrors off to a certain company for recoating folks. About 4 years ago I decided to embark on another Dobsonian reflector project. I obtained the mirror and flat from a friend who had bought them but had lost interest in completing the scope build. The set was a 10 mirror and flat bought by him in 1992. Part of the mirror coating was starting to break down due to poor storage by him of the said optics albeit being well protected from accidental damage. Having already started the Dob build and of course checked the Focal length of the mirror I sent the set off to Orion Optics in England for re-coating. I completed the build less than six months after this about three years ago. At first I was delighted with the perfomance of the scope and obtained great planetary, galaxy, double star and views of the Moon. The scope c/w optics was stored in a decent out-house/observatory specially made for this. One year and nine months into the optics use the mirror coatings started to separate from the glass over a period of some three months. The mirror is now unusable and has been since sent back to Orion for , or so I thought,a free recoat. NOT SO, say Orion Optics. They will not stand over recoating because they maintain that the coatings bear the symtoms of a mirror that was not taken care of properly, even though all the conditions for the storage and use were followed to the letter by myself. Strange thing is though that the flat mirror is un blemised and still has a perfect surface even though it was recoated at the same time as the main mirror. Funny that,eh? My word of warning is that even though my mirror seems to have had a bad coating at their factory this company refuses to stand over or guarantee their coating, for ANY lenght of time. Watch out. Regards. COMET12.
FORRENFOR
17 years 5 months ago #15160

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Replied by gnason on topic Re: RE-COATING OF REFLECTOR MIRRORS.

The scope c/w optics was stored in a decent out-house/observatory specially made for this. One year and nine months into the optics use the mirror coatings started to separate from the glass over a period of some three months. The mirror is now unusable and has been since sent back to Orion for , or so I thought,a free recoat. NOT SO, say Orion Optics. They will not stand over recoating because they maintain that the coatings bear the symtoms of a mirror that was not taken care of properly, even though all the conditions for the storage and use were followed to the letter by myself. Strange thing is though that the flat mirror is un blemised and still has a perfect surface even though it was recoated at the same time as the main mirror. Funny that,eh? My word of warning is that even though my mirror seems to have had a bad coating at their factory this company refuses to stand over or guarantee their coating, for ANY lenght of time.


You may have cause for complaint but as you don't state the conditions you stored the scope under or precautions taken after use, it's a bit difficult to comment. Perhaps the mirror was not recoated under optimal conditions but on the other hand, Orion Optics may be correct in what it says. It's a very experienced company with regard to mirrors.

Is the shed totally unheated as it sounds like damp may have got to the mirror and damaged the coatings? We live in a damp climate so precautions must be taken. For mirrors stored in an outside shed or observatory, you should leave a 100W bulb on to guard against damp. Alternatively, if you observe by the sea and dew formed on the mirror after use, salt air deterioration could degrade the mirror if not cleaned off afterwards. Flats are not as exposed as main mirrors so I shouldn't think it particularly unusual for it to appear undamaged.

Gordon
Gordon

SDAS

Stargazer am I
It seems that I was born
to chart the evening sky

Mark Knopfler - Sailing to Philadelphia
17 years 5 months ago #15161

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Replied by Seanie_Morris on topic Re: RE-COATING OF REFLECTOR MIRRORS.

I bought a mirror set from Orion Optics in 1996, and (through fault of my own) only had to get it recoated last year. The coating is fine as far as I can tell. Perhaps Gordon's suggestions caused your apparent bad luck Comet, or perhaps Orion just had a bad day the day they did your mirror.

PS. Welcome to the boards!

Seanie.
Midlands Astronomy Club.
Radio Presenter (Midlands 103), Space Enthusiast, Astronomy Outreach Co-ordinator.
Former IFAS Chairperson and Secretary.
17 years 5 months ago #15163

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Replied by eansbro on topic Re: RE-COATING OF REFLECTOR MIRRORS.

I used to be involved many years ago in thin film technology. From your description and not knowing the circumstances, I can make some comments for eg. not using a dehumidifier in your observatory, deterioration of coatings in Ireland is a quicker process than other lower relative humidity areas in other dryer countries.

It is also possible, as the two mirrors were coated at the same time that the primary mirror was not cleaned properly, ie. leaving grease on the surface. Even using special gloves, the sweat can still get through to the surface, leaving scuff marks. Cleaning glass surfaces before coating is a whole procedure.

The surface condition of the blank may have unnoticeable stains, blemishes, not seen by the eye before the mirror enters the high vacuum chamber. This prevents a neccessary tenatious bonding for the aluminium and silicon overlayer. Instead of the normal visicules on the aluminium surface showing up ie. black dots showing up, you see irregular flaking only after some weeks. A good test is to clean the mirror, and you will find that the coating comes off very easily, which is not meant to happen.
This is a sign of bad cleaning at the pre coating stage.

The reflectance of aluminium is dependent on the procedure, and a very high vacuum coupled with very fast evaporation of aluminium, followed by a coat of silicon monoxide, are essential conditions to prevent the growth of an absorbing oxide film.

These shared comments does not mean that this was the actual real circumstances.

Eamonn A
17 years 5 months ago #15167

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Replied by COMET12 on topic Re: RE-COATING OF REFLECTOR MIRRORS.

Gordon, Seanie and Eamonn, Thanks a million for all your quick responses to my problem. Gordon with respect I'd like to state without reservation that the mirror was stored correctly in the optimin conditions as recommended by 'Orion Optics'. I have been building and been involved with reflector telescope making for some 15 years now. After removal from the mirror cell the coating was found to have come away a good deal. I'd like to state that I think a lot of Orion and have had good dealings with them in the past. It's just that in this case they don't seem to want to meet me any part of the way. I pose the question. Considering that the scope/mirror was well looked after in that time what is the viewpoint then? Note. I have a 6 ins Dob that is stored in the same conditions. The coatings on same are 19 years old! The telescope can still be used for observation. Greatest Regards. COMET12.
FORRENFOR
17 years 5 months ago #15181

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Replied by gnason on topic Re: RE-COATING OF REFLECTOR MIRRORS.

Gordon, Seanie and Eamonn, Thanks a million for all your quick responses to my problem. Gordon with respect I'd like to state without reservation that the mirror was stored correctly in the optimin conditions as recommended by 'Orion Optics'.


I don't know what storage arrangements Orion suggest (can't find on its website) but assuming this includes the prevention of damp by heating (or humidifier as Eamonn suggested) if telescopes are kept in an outside shed and you followed this, then it does seem strange that the coating degraded in such a way unless it was defects in the process as Eamonn said. Unfortunately, Orion Optics seem well covered legally. Although the company provides a 12 month guarantee, it is conditional and anyway, your mirror was recoated far longer than 12 months ago.
Guarantee: We guarantee your satisfaction. All new products are guaranteed for a period of 12 months from purchase date. In the case of coating or re-coating customer's mirrors, we suggest strongly that you
insure your mirror(s) as we cannot be responsible for damage to mirror(s) during the coating process. Very occasionally mirrors sustain damage during the coating process and also during the pre-cleaning process of removing old aluminium or similar coatings and, the removal of any holders which may be affixed to the mirror(s) The greatest care is
extended to customer's mirrors but unfortunately, due to several factors, age of glass (substrate), semi etched surface(s) due to atmospheric corrosion, glued on holders for mirrors etc., we cannot guarantee a successful coating or zero damage to the mirror(s) or their holders.

All Orion Optics products carry a one year guarantee against faulty materials or workmanship unless otherwise stated in any literature. The exceptions are aluminised mirror surfaces which have been cleaned by the customer or aluminised surfaces which show obvious signs of abrasion.

Would degradation due to a defect in the cleaning or coating process not manifest itself much sooner than the time period mentioned by you (i.e three to four years including one year nine months observing) as I've read reports of mirrors degrading in a matter of weeks in such circumstances? There is one other possibility: could whatever the mirror was wrapped in between recoating and installation (a year and a half or so judging by your first post) have leeched acid into the coating, especially if the material became damp?

Gordon
Gordon

SDAS

Stargazer am I
It seems that I was born
to chart the evening sky

Mark Knopfler - Sailing to Philadelphia
17 years 5 months ago #15187

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Replied by COMET12 on topic 10 inch mirror coating.

Gordon, I constucted my first 10 inch Dob in 1992. That mirror, before construction of the telescope,was stored, as my last 10 inch scope, in the same careful conditions until completion and is still serving the current owner well as their club telescope. 'The East Antrim Astronomical Society'. That scope is still performing very well after 13 years of service. I know how to look after reflector mirrors is the point I'm making. Greatest Regards, COMET12.
FORRENFOR
17 years 5 months ago #15250

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