12" MIRROR CLEAN PROBLEM...I'M STUCK!!

[rob7]

I've got my mirror and cell out of the ota of my dob.. I am attempting to remove it from the cell and have so far removed the 3 clips on the mirror faced edge, but its still not coming out. There seems to be 3 triangular plates on the cell which look like they have some sort of sticky/suction pads which stick' to the rear of the mirror. Some are loose, but I'm not sure if I should continue to loosen the rest to free the mirror. I've read the posts for the actual clening part, but can anybody help me with the stage I'm at as desribed above? I was going to leave and wash the mirror in the cell, but with the cooling fan and all, thought it best to do the job with the mirror completely free of the cell.

I'm at a bit of a deadend till someone can advise...

Regards, Rob.

[barkis]

Rob, there has to be three adjusting screws at the rear of the cell, will allows you to collimate the scope.If those adjusters have been deliberately stuck to the three triangular plates underneath the mirror, then the rear adjusters must be controlled by wing nuts. If that is so, then completely removing the nuts should allow you to lift the mirror out altogether. Be careful, as there may be springs beteen the inside of the cell and the back of the mirror. Really, the plates should not have been glued to the back of the mirror, not deliberately anyway.

Could take a picture of the way it looks if you don't want to proceed, it might help to identify what is necessary to get it out.

Ron.

[rob7]

Thanks, Ron for your help. To be honest iit looked like only these sticky pads on the triangular plates were stopping the mirror freeing itself. Some were loose, some were still sticking. What I've done meanwhile is to 'chicken out' and wash the mirror in its cell, like another thread mentioned [Obsession Telescopes cleaning method] and it worked better and not as messy as I thought it would be. The mirror seems to be quite clean now apart from some ,scabby sopts' and is now in the procees of drying. I had to clean it sooner before the advice was given, so it had time to dry and then cool down after being in 25 degree heat all day. I'm hoping on some viewing tonight!! Cheers for you offer of photo's, but I think I'll clean it in the cell next time as well [hopefully a long way in the future].

Regards, Rob.

[The Warthog]

I think you may want to question whether you need to clean your mirror any more. A few scabby spots will have zero effect on your viewing or photography, and the chance of damaging the mirror rises directly with the number of times you clean it. Your mirror can be coated quite thickly with dust, to look at it, and still perform well. It isn't worth the risk of scratching your mirror to get it perfect.

[saturn5]

Agreed WH,mine has a couple of dozen small specks on it and i'm quite sure it looks much worse than the actual detrimental effect(if any)it is having on performance.I'm quite at home having a go at d.i.y projects but the idea of having to clean a primary doesn't come high on my list of "to do" jobs. :nono:

[supernova]

If it aint giving you a big prob from your first light then my advise is the same as WH and Sat 5.

:nono:

[rob7]

Trouble is I just couldn't leave it as it was. From a very early mistake with some Baader wonder fluid which left some smears on it and the fact that theres always been these little scabs and spots I never quite new what I 'might' be missing. Having never seen anybody elses 'imperfect mirror I just thought If I cood clean it to nice standard once, I could put the 'chamois leather' away for some time. Its cleaner now, but the scabs and spots are still there.

From my pictures does any of you know what the scabs and spots are and also if there not bad enough to worry about? There a lot of them throughout the whole mirror.

Rob.

[rob7]

Please stay wth me here.. the pics aren't loading now, got to lessen the file size. Urghhh!!

[rob7]

Here at last is the spotty mirror!!

[supernova]

Hay Rob7 any idea what the fluffy bits are?

[barkis]

Rob, those black bits are not anything that is stuck to the mirror, they are the places where the aluminium coating has lifted off the glass. If you were able to hold the mirror up to a light. (Daylight for example), you will myrriads of spots including the big ones where the coating is missing. The small ones will probably have been there since the mirror got it's original coat of ally, and are no reason to worry. If the large ones increase in size over the next few weeks, or their numbers increase, then it might be worth considering recoating it. But it needs to be quite bad before that step is taken. Aluminium reacts to many chemicals, some very adversely. Washing up liquids have minute quantities of acid in them, which is very harmful to aluminium, especially on a mirror where the thickness is in the order of a few microns.

The old mirror makers of days of yore, mostly guys in the US, advocated a solution called Tepol, I don't know if it is still available, I doubt it.

Maybe Astroman has information on that. Personally, I would use nothing other than tepid water to clean mine, I let is soak on a towel bed in the bottom of the bath. A few soap flakes disolved in the water will help shift any persistent grime. Do not use soap powders. You can use a gentle flow of luke warm water from your shower head to sluice over it, taking any soap residue off, which might leave a stain otherwise. Once you are sure it is clean enough, a final wash with distilled water will finish the job.

Stand the mirror carefully on its edge until all the surplus water has run off, don't let anyone near it for fear off tipping it over.

Don't be tempted to rub the surface with anything, however soft it might feel. It does not take a lot to put loads of minute scratches on that thin ally coat, you might not see them, but they will be there.

This always seems such a big chore, but your mirror is the heart of your instrument, it deserves careful treatment.

I rest my case.

Ron.

PS. As a matter of interest, Sodium Hydroxide is what is used to remove aluminium coatings, prior to re coating.It is a strong acid, and you would be amazed at how quickly the aluminium disolves when this stuff touches it. It used to be called Caustic Soda, and is lethal stuff. Anyone who may be tempted to remove their own mirror coating, shouLd exercise great care. As with most acids, you have to use the rule of the three A's

ALWAYS ADD ACID.to the water, although it will still generate a fair bit of heat, although the other way could cause it to violently to boil up in you face. Rubber gloves Goggles, a deep Belfast sink, no children within 10 miles. Simply. it is a dangerous process, but only if you are careless.

[rob7]

Thanks Ron for the in depth help. I always thought the spots and scabs were something to do with the coatings [hoped they were'nt though]. With the mirror as it looks in the pictures, would it be worthwhile having it recoated or wouldn't I notice any improvement in the optical quality. I don't mind the cost if it isn't silly if it would help my beastie be at its best. I'm trying to buy good quality eyepieces, etc and keep it collimated, so I would not want the mirror to be the 'player that lets the team down'.

Rob.

[barkis]

Rob. In my opinion, looking at the image of your mirror, I would say it needs re aluminizing. It looks like some erosion has taken place over a wide area of the mirror. They look for all the world like sunspot Umbra and Penumbra, for want of a better analogy.

Now you have got it out, you may as well go the whole hog and get it done. I would not attempt removing the coating yourself. The people you select to do it, whoever they may be, will levy a small charge, no more than a fiver I would think, for removal of the old coating.

Your main concern, is getting it transported to the aluminizers safely. That may mean making a box for it. This can be made of plywood easy enough. I was fortunate so when ever I wanted a mirror recoated, I used to drive over to Ryeton near Gateshead to my old pal Dave Sinden, who had his optical workshop over there. He would coat the mirror the same day, and I would have it back in the scope the following day.

It would be handy if you lived near someone who did aluminizing, but I think they are a bit thin on the ground now.

The only one I can think of used to be called Gavoptics, but they changed their name, and I don't remember what to. I know their place is in Basildon, so maybe Rog. knows them. They at a place called Burnt Mills Harvey Road Industrial Estate, somewhere like that anyway.

Cheers.

Ron.

[rob7]

Ron, Are you saying that if it is recoated it might improve the image quality. From my inexperienced eye I've always enjoyed the views from my dob. Thats part of the problem being inexperienced as I don't know exactly what to expect. How might the tarnished coating effect the image?

On the who could I send my mirror to side, I think there may be a chap who possibly could get it done for me. His shop isn't far away from me. I don't know if you've heard anything of him, but his name is David Lukehurst his website is Dobsonians.co.uk. He's supposedly a maker of dobs, so if I contact him he may be able to help me. I hope he don't drive a Bentley!!!!

Thanks for your help, Rob.

[barkis]

Ron, Are you saying that if it is recoated it might improve the image quality. From my inexperienced eye I've always enjoyed the views from my dob. Thats part of the problem being inexperienced as I don't know exactly what to expect. How might the tarnished coating effect the image?

On the who could I send my mirror to side, I think there may be a chap who possibly could get it done for me. His shop isn't far away from me. I don't know if you've heard anything of him, but his name is David Lukehurst his website is Dobsonians.co.uk. He's supposedly a maker of dobs, so if I contact him he may be able to help me. I hope he don't drive a Bentley!!!!

Thanks for your help, Rob.

Yes if you try to Rob, I have heard of David, I don't know him, but if he can do it for you then that's great.

The thing about how much would the improvement be, is difficult to give an assesment of. I think if you try to work out how much of the mirror surface is devoid of the aluminium coat due the erosion, and bearing in mind that the reflective capability of a newly coated mirror. excluding the enhanced coatings I believe are available, is about 88% give or take a bit, I think it's fair to assume that the images returned by your mirror will have a loss of brightness. You have to understand I am only looking at a picture of the mirror, which to me looks quite poor.

If you could get David Luckhurst to look at it, he would probably give you a more definitive diagnosis.

If you were to remove the coating from the glass, and put it back into the scope, you would still get images, only they would be quite a lot dimmer.

Mirror makers of old, used to put their uncoated mirrors into a scope to assess the figure on the surface. This was long before Leon Foucault a French Physicist developed the tester that bears his name, enabled mirrors to be figured, using a fine illuminated slit, and a knife edge.

If you have never seen a paraboloidal shadow pattern on a mirror using this device, it is a sight to behold. Of course there are more sophisticated ways to test them nowadays.

Ron.

[rob7]

Thanks Ron, you know your stuff! The bigger scabs and smaller spots as shown on the section of mirror in the first pic' are repesentitive of the rest of the mirror, so I always thought it can't be helping the image quality. Everybody says dust and alike don't effect an image on a mirror so I didn't know if the stage my mirror is at is one stage to far. [i had the tarnishing as well as the dust]! When I get a chance I'll ring this David Lukehurst and see if he'll have a look at my mirror and what services he offers. I suppose on a posistive note I might have something to look forward to as since I've had the lightbucket I've probably not seen through it at its best..We'll see..I'll let you know what David says and does.

Cheers for all the other replys and a special thanks to Ron for babysitting me.

[barkis]

Rob, if you want to see the condition of the mirror in reallity, stand it on it's edge, and place a strong light behind it, the look through it from the front. If you do this in a darkened room, your mirror face will look like a very dense starfield with loads of bright nebulae. It will also let you see how much of the light hitting your mirror will be passing straight through, and not refected back to your eyepiece.