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Last summer I bought, second-hand, an Orion Optics UK 12" Dobsonian.  Unable to spend the summer in France, as I have been able to for the last 12 years, I needed a telescope in UK as a cheap substitute for the kit I keep in France.  Gumtree gave me the opportunity to keep my eye in.

The 'scope from Gumtree was clearly of an age that was reflected in the price but working on the principle that something that basic should be usable in spite of its years I took a chance.  The secondary was badly misaligned to a degree that made it impossible to test the 'scope but a squint down the tube at the main mirror was fundamentally reassuring.  I parted with the cash, took it home, upgraded the museum piece pretending to be a finder and even under the heavily-light-polluted skies near Gatwick Airport managed to get some decent views of Saturn, Jupiter, Mars and the moon.  However, light-pollution could not be entirely blamed for less than perfect observing.  A closer look at the main mirror showed an accumulation of about 20 years of dust and dried condensation.  I don't have space to work with anything that big in England so brought the 'scope down a couple of weeks ago to France where it will take up permanent residence.  Another look at the main mirror yesterday reminded me that the 'scope had been stored in an upright position in a shed where the owner had kept a variety of small animals in cages.  The 'scope had not been used for some years as the elderly owner's eyes were no longer able to afford any pleasure from observing.  I guess he had decided he needed the space taken up to make room for more ferrets.

So, today I set about cleaning the main mirror.  I was stunned by the difference and the pictures might give some idea.  The mirror is pictured more or less in the same position before and after, reflecting the beams and floor boards in the ceiling above.  They are barely visible in the Before shot.  However this lengthy preamble brings me to my question.

In the After picture there are spots on the mirror showing as highlights.  I had followed standard procedure and let the mirror soak in slightly soapy water for about half an hour giving it an occasional swirl.  After disposing of the first wash I used rather warmer water, again with a few drops of washing up liquid, for about fifteen minutes while wiping cotton pads under water across the mirror with no pressure other than the weight of the sodden pads.  I then removed the mirror and using fresh pads soaked in the water gently passed them across the surface, discarding each pad after every pass.  Job completed I rinsed the mirror in de-ionised water, then stood it on end and rinsed again.  Any drops that failed to run off were soaked up with a corned of kitchen towel.  Only after completing the task did I then notice that some spots, presumably deposited as the end product of the digestive processes of long-dead flies or spiders, had failed to clear.  So, any advice please on how to remove these deposits without resorting to a Brillo pad?  The mirror is back in the 'scope now (amazingly without losing collimation) but I'm happy to start over.

Before.jpg

After.jpg

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Relpet

Better to use distilled water rather than tap water

Tap water has a lot of chemicals including fluoride, chlorine, salts and rust from pipes

A while ago, had a club 10" collapsible dob come back off loan, and the mirror was covered in cockroach muck, and spiders web in the tube

Club member who had it, said he stored it in his garden shed

I used distilled water, with mild detergent, using handful of cotton wool soaked in distilled water

Unlike tap water, distilled water leave no residue on the mirror

Others will disagree with me using cotton

John

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You might get some useful tips here:  https://www.youtube.com/watch?v=8TObET2F-EA .

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12 hours ago, Merlin said:

You might get some useful tips here: https://www.youtube.com/watch?v=8TObET2F-EA .

Thanks.  That was the exact advice I followed.  Unfortunately I think his mirror wasn't in the same state as mine with no advice on how to remove stubborn muck without rubbing.  Got a clear view of M42 tonight and the difference was remarkable.  Next tiem I might try cotton buds with stronger detergent just on some isolated spots and see what happens..

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I’d probably play the long game with the stubborn marks. They are unlikely to be affecting the visual performance of the scope by any measurable amount. 

So, rather than risk damaging the mirror and risk performance I’d just clean it in the recommended manner but maybe more often - every 6-12 months and see if they slowly disperse that way. 

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Well done on the cleaning. If it were my mirror, I would ignore the spots for now. As the previous post, you won't notice them in the view.

Last time I tackled an abused or neglected mirror, I used warm water, liquid soap, time and gentle agitation.

The leftover marks had a gentle wipe using (once only) cotton wool lumps.

After a rinse with deionised water (to remove the soap) I left the mirror on a draining board plate rack.
I was going to be left with droplets, so gave it a quick spray using Baader wonder fluid (eyepiece & lens cleaner).
This reduced surface tension and the remaining water film just drained off.

Hope this is useful. David.

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Excellent job Peter. I probably wouldn't worry too much about the spots. Better to leave and not risk scratches perhaps? Otherwise I guess a longer soak might do it. In the past I have let my fingers soften in the water for a little while and then used them underwater very gently just to encourage stuff off the surface.

Looking at the comparison this must have made a major difference!

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5 hours ago, Carbon Brush said:

I was going to be left with droplets, so gave it a quick spray using Baader wonder fluid (eyepiece & lens cleaner).
This reduced surface tension and the remaining water film just drained off.

Hope this is useful. David.

Thanks, David.  On leaving school in 1956 I worked in a lab cleaning glassware, amongst other things.  We always rinsed out with industrial methylated spirits to clear water droplets and leave the apparatus clinically clean.  I was reminded of this when I was faced with droplets all over the mirror but the closest thing to IMS in the drinks cupboard was gin - and I could think of a better use.  The Baader wonder fluid looks like a good tip.

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4 hours ago, Stu said:

 In the past I have let my fingers soften in the water for a little while and then used them underwater very gently just to encourage stuff off the surface.

Cheers, Stu.  Every time I get down to France there's so much brush clearance, weeding, digging etc I'm quite the horny-handed son of the soil.  My fingers might take a month to soften but, as you suspected, the seeing tonight was a revelation. and no indication of the quite minor bemishes still on the mirror.

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Leave the spots alone!!!!  They will make absolutely no difference whatsoever.

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4 hours ago, kirkster501 said:

Leave the spots alone!!!!  They will make absolutely no difference whatsoever.

Right you are, guv'nor.  When I saw details of the moon last night in crispness I had never seen before I guessed it was either the particular angle of the sun emphasising the relief that night, exceptional seeing conditions, or the mirror was -at last - performing to expectation.  (Or maybe all three!).

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I think it might have been 10 or more years ago when I was invited to attend a lecture at the University of Newcastle 's  observatory which was then located at  Close House , Wylam Northumberland.  They had a large  reflecting telescope . perhaps of the order of 14 imches or more  which they were proud to show to us visitors.  The mirror, I thought,  was very grubby indeed  and I  mentioned this to our host who told me that the performance of the  mirror was not un duly affected  by this and that they would only clean it as a last resort..........he didn't explain where this last resort lay.  I think that I have heard elsewhere that  primaries can look a lot worse than they would  appear to be.

Pete

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It's surprising how well a dirty mirror can still perform. Unfortunately, poor cleaning methods can worsen rather than  improve  optical quality.

Sensible folk will always seek advice first, before applying a Brillo Pad Scrubber to the surface?.   An exaggeration I know, but there are similar destructive ways that might seem benign, but can be damaging. If in doubt, refrain from doing anything, until a safe procedure is found.

Ron.

Edited by barkis

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I was apprehensive about cleaning my 12" dob mirror but took advice from here, got a gallon of distilled water and I think medical grade cotton wool and have no regrets and was very pleased with the results.

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This will make you laugh (or scream in horror) but I recently cleaned a bit of dust off my 12 inch Dob  mirror by sticking the vacuum cleaner hose down the tube, and while holding it really tightly sucked the dust off without getting it any closer than 2 inches from the mirror. Worked a treat and left the mirror spotless! ?

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The residual spots may well be where the coating has oxidised. If so, no amount of soaking or rubbing is going to make an improvement. The test is to look through the back of the mirror towards a bright light. From the look of the "after" image I would think the mirror coating has a few years left.   ?

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1 hour ago, Geoff Barnes said:

This will make you laugh (or scream in horror) but I recently cleaned a bit of dust off my 12 inch Dob  mirror by sticking the vacuum cleaner hose down the tube, and while holding it really tightly sucked the dust off without getting it any closer than 2 inches from the mirror. Worked a treat and left the mirror spotless! ?

I tried that too with my 10" ? I've never admitted it before!

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3 hours ago, abberation said:

I was apprehensive about cleaning my 12" dob mirror but took advice from here, got a gallon of distilled water and I think medical grade cotton wool and have no regrets and was very pleased with the results.

I also used this method on a friends telescope with great success. The mirror was extremely dirty though.

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2 hours ago, Peter Drew said:

The residual spots may well be where the coating has oxidised. If so, no amount of soaking or rubbing is going to make an improvement.

Thanks, Peter. I am convinced the spots are biological.  Some of the spots made streaks when I passed the cotton pads across but I've followed Kirkster501s advice and left alone.  Lunar observation has been simply phenomenal since the clean.

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Hi ,use R/O water better than standard distilled water,available in garden center/aquarium  section ,it is commonly used in fish tanks,

HTH

Mike

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22 hours ago, Demon Barber said:

Hi ,use R/O water better than standard distilled water,available in garden center/aquarium  section ,it is commonly used in fish tanks,

HTH

Mike

Thanks, Mike.  De-ionised water is the most readily available here but I'll check the pet shops next time.

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