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Old 15-04-2020, 03:07 PM
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Zander
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Mirror cleaning advice

So thanks to Covid, I got my 13 year old Bintel 12 inch dobsonian out of the shed the other night after years of neglect

The mirror definitely looked like it needed a good clean, so I followed the instructions given in the manual to clean it.

disassembled and took out the mirror, soaked it in room temperature tap water in a clean sink with a few drops of dishwashing liquid for 2 hours, then lightly brushed across the surface with a new cotton bud each stroke and then rinsed it off with demineralised water. managed to clean it somewhat (got all the insect poop marks off), however, it certainly still doesn't look too great...

https://imgur.com/a/VDc0vnL

Have I just been too gentle with the cleaning? Or should I try a different method? Or could it be that the mirror surface has just oxidised and any cleaning attempts won't be successful?

Any advice would be super appreciated!

Thanks!
Zander
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Old 15-04-2020, 09:21 PM
Startrek (Martin)
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I only use visual and imaging newts and clean my mirrors once a year in a similar way but my scopes are only 2 to 4 years old
I wash the mirror in the kitchen sink with warm water to wash off dust and grit, then a drop of dishwashing liquid and a cotton ball wiping gently in a circular motion for 20 sec under the tap. Then rinse off twice with distilled water
Tilt mirror to let most of the droplets run off then remove and tiny drops carefully with a Q tip. Leave to dry in a cool place for an hour then re install

That mirror of yours looks in poor condition !

Alex ( mentalastro ) a member on IIS has a lot of experience with newts and mirrors etc... maybe pm him for his advice ?
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Old 15-04-2020, 11:57 PM
ausastronomer (John Bambury)
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Hold the front of the mirror in front of a white light source then look at the back of the mirror and see how many "holes" and how much light is coming through the coating substrate itself. What it is reflecting is inversely proportional to what's coming through.

As a general heads up, the coatings on a Newtonian mirror can be in pretty poor shape, yet the telescope will still perform ok. Herschells speculum mirror would have been lucky to have 50% reflectivity, yet he was still able to find a few things with it. While there are a number of small marks on your mirror it doesn't look too bad. My 18" mirror has no coatings left on the outside 2cm of its entire circumference and it still performs fine. It's always set up beside 4 or 5 other 18" Obsessions and you can't really tell any difference. You might if you were pushing it on dim threshold targets, but you can't on the average everyday eye candy. Unfortunately that's what happens to 15 yr old coatings when you live near the ocean.

I would think the scope will be fine to use.

Cheers
John B
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Old 16-04-2020, 07:59 AM
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doppler (Rick)
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Hi, I wouldn't scrub too hard you'll end up damaging the coating, if it doesn't improve with a light clean. I recently picked up a 10 y/o 10" gso for a parts price. The overcoat had gone hazy and no amount of cleaning would improve it.

Rick
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Old 16-04-2020, 09:39 AM
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Zander
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Thanks so much for the replies!

Quote:
Hold the front of the mirror in front of a white light source then look at the back of the mirror and see how many "holes"
I did this and didn't notice too many holes. There were only half a dozen or so, where I think the mirror surface has been etched through/oxidised over time.

Quote:
I wouldn't scrub too hard you'll end up damaging the coating, if it doesn't improve with a light clean. I recently picked up a 10 y/o 10" gso for a parts price. The overcoat had gone hazy and no amount of cleaning would improve it.
I ended up (probably ill-advised ) using a bit of window cleaner and a glasses cleaner pad on the edge of the mirror to test if the haziness could be removed.

Here is the result

https://imgur.com/XPtVWNi

It looks like the haziness has actually been diminished somewhat, however I'm not sure if I would've damaged the coating by doing this, so I'm hesitant to do this over the entire mirror surface.

I didn't have any iso-propyl alcohol (which the manual recommended to add to the sink while soaking), but perhaps if I get some it might help dissolve the haziness with gentle wiping? What do you think?

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Old 23-04-2020, 02:32 PM
appiice (Ed)
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Hello Zander

I took up an offer from www.photoniccleaning.com in the States, they were selling a paint on polymer at a 50% discount earlier in the year due to the fires in Australia.

I was a bit wary of washing the front of my Edge 1100 HD and this product looked like a good way to clean the glass.

I did it once bit the coat was to light, the second cost ( as per the recommendations ) lifted all the marks, dust and left an incredibly clean surface with out marks.

Not sure if the discount is still in place but it was worth the cost.

Hope this helps

Ed
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Old 23-04-2020, 07:10 PM
astro_nutt
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Hi Zander. I had a similar problem with my 12 inch mirror. I set about doing a warm clean. Mostly oil from aircraft passing overhead, diesel smoke, gum tree haze accumulate onto a mirrors surface as well as dust, etc. I dust off the mirror with a blower, then soak it in warm water for about 5 minutes. Rinse off the mirror in warm water, clean and rinse the sink then refill. I have the water temp at around 25 C.then add a tspn of baby shampoo diluted in a glass of water.. (Oil dissolves better in warm water.) I let it soak for a few minutes then use cotton wool. I then raise the mirror out of the water and spray the surface with iso. I then soak it again, drain off the water and refill with warm water, baby shampoo, soak again, cotton wool clean then rinse in warm water and gradually reduce the temp till it's room temp for a few minutes. Final rinse with demineralized water. Then, whatever's left on the mirror, stays there till next time. Hope this helps. Cheers!
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