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Rain marks on Dobsonian mirror

Rapallokid

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Rapallokid

Rapallokid

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Hi,

I am looking for some advice on some marks on the primary mirror of a secondhand dobsonian that i have bought. It looks like it has been left out in the rain at some point and there are dried rain spots on the mirror!

Will this adversely affect the observing, or do i just stick with it and avoid removing the mirror to clean it.

any advise would be appreciated.

Rapallokid.

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Telrad

Telrad

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Dried rainspots may not look nice but will not affect the view. The mirror on my main scope is very grubby, and I may wash it sometime, but the views are excellent.

I'd say relax and enjoy using it.

Regards, Ed.

I agree with Ed, it will not affect the view tremendously. DO NOT wipe your mirror, this is a sure way to accidentally scratch the coating. A slight amount of dried water will not hurt the views.

The worst thing you can do is shine a torch onto the mirror, it can make even new ones look terrible!

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Hypernova

Hypernova

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Most likely someone has let the mirror dew up with the tube closed, it is always advisable to leave the tube end cap off and point it down if possible after bringing it inside after a session. As everyone says though a bit of dirt or marking on the mirror surface will hardly affect the views and it would be best to leave them rather than risk scratching a mirror during cleaning. I wouldn't say completely avoid cleaning it as there will come a time when it is necessary, but don't go OCD and clean it every time it collects a hair or dust granule ;)

And never shine a light up the tube.... ever!!!

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Rapallokid

Rapallokid

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Hi,

Many thanks for the responses. I will probably take the advise and live with the marks, best left alone seems to be the overwhelming consensus.

Just an additional note. I have added some images to show how "bad" I think it is. But unless I receive other advise I will not be removing it to clean it.

My thoughts before posting were, as I had just bought the scope and it was second hand, get the mirror cleaned and then leave it alone. I will however bow to superior knowledge and experience.

Any additional comments would be appreciated.

Rapallokid.

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NGC 1502

NGC 1502

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It looks like heavy dew has been left to dry on a dusty mirror and given that it appears to cover the whole mirror I'd be tempted to clean it.

Well now I've seen the pics I agree.

If it were mine I'd give it a careful wash using the guides others have posted.

Regards, Ed.

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Rich MedRev

Rich MedRev

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Another vote for a clean, it is actually a fairly simple process. Taking care when removing the mirror, wash the mirror in tepid water with a tiny amount of fairy liquid, rinse well with distilled water then stand to drain, once dry replace in the cell, dont pinch the mirror, take note before removal on how it was held in the cell, and re-collimate. Should take no more than 20 mins from start to finish.

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Hypernova

Hypernova

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Ok after seeing the pics I will stand by my post earlier but I will agree with everyone that the mirror has passed the threshold and probably should be cleaned. Read the tutorials carefully and if you are unsure of anything please ask, I am confident that any one of us will be able to help.

For the record when I cleaned my mirror not long ago I followed these steps:

1: Place mirror in a bowl which has a few inches of warm and slightly soapy water.

2: Whilst the mirror is submerged and using clean surgical cotton, make strokes on the mirror from the centre to edge, letting the cotton roll over as you move it so no debris that is picked up scratches the surface as it is dragged along.

3: Move around the mirror radially using a new piece of cotton for every stroke. Do not be tempted to scrub a stubborn patch as you may inadvertently damage the mirror. Simply let the cotton glide over under light pressure.

4: Rinse the mirror off with a shower head or a jug of water to get the soap off.

5: Do a final rinse of the mirror with distilled/de-ionised water so that no water marks are left when it dries.

6: Do not use a towel to dry the mirror, just prop it up in a safe place so that and water that is left on the surface simply drains off.

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todd8137

todd8137

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sir Patrick moores 12" is like frosted glass i watched a sky at night about 3 months ago,you would have thought there would be no chance,they was looking at the Andromeda galaxy and m57,anda few others faint ones with no probs,i would avoid cleaning them you can send them of and get them done

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JoLo

JoLo

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Tom's step wise procedure is what i have followed a couple of times with various scopes. I do avoid cleaning until necessary, but it is quite easy and nothing to fear, as some do. Lesson learned by me...be very careful not to overtighten the retention screws when reinstalling the mirror....if too tight, the mirror shape is warped and astigmatism is introduced. Just tighten enough to hold the mirror in the cell, no more.

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swamp thing

swamp thing

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Lesson learned by me...be very careful not to overtighten the retention screws when reinstalling the mirror....if too tight, the mirror shape is warped and astigmatism is introduced. Just tighten enough to hold the mirror in the cell, no more.

Good advice here.

Much better to read this than to experience thinking you've messed up your pride and joy, when all you've actually done is over tightened the retention screws.

Regards Steve

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Rapallokid

Rapallokid

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OK an update on the mirror.

I have remover and tried to clean it but what ever it is it is not budgeing.

I have followed all of the care instructions and wiped gently but once the mirror is dry the marks return. Could it be mildew?

what ever it is it it aint movin!!!!.

Any further advise would be appreciated.

Rapallokid

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NGC 1502

NGC 1502

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It's hard to know from a distance why the mirror is not responding to cleaning.

Hopefully you are cleaning it with soapy water and something very soft like good quality cotton wool and no pressure.

You could try giving the mirror 20 mins soak, then change the water and try again with the cotton wool. The soak may help the gentle detergent to lift whatever is on the mirror. If that doesn't work, there are other slightly more agressive methods. This is contraversial I know, and some will be horrified at the thought of using isopropyl alcohol etc. But some have successfully used that as a last resort, and some use it routinely.

But I would try the 20min soak method first. If that doesn't work, then have a good search on the web, ask here etc. You will find lots of very differing advice.

But be gentle whatever you try.

An absolute last resort is a recoat at somewhere like Galvoptics of Basildon Essex.

Hope you sort it, Ed.

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JoLo

JoLo

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Definitely, a 20-30 min soak is in order. Never went the alcohol route, but i have read others who have....didn't hear any shrieks of angst, so must have worked....I would be careful though. Also good advice on the cotton balls....I bought a gigantor bag of them, and use a clean one with each swipe across the mirror. I do vertical swipes across, then horizontal...always use deionized water as the final rinse.

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blinky1983

blinky1983

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I have never soaked my mirror before... I always clean under running water with normal fairy liquid and gently rub the mirror with my fingers just barely touching the mirror then rinse with some distilled/de-ionized water, never had any problems with it..

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