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stormblooper

How critical is it to avoid rain?

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I'm a new owner of a Dob, and the couple of times I've had chance to take it out, there has been patchy clouds and occasional spots of rain. I wondered how catastrophic it is if you get any rain drops on your primary mirror? Is it something to absolutely avoid (and therefore not take your scope out if there's any chance of even a few spots), or is it OK just to promptly get your scope indoors if it starts?

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Good question - have wondered this myself so thanks for asking. Especially with the pollutants/chemicals that may be present and perhaps harmful to mirrors? Tho I guess that may take a bucketful!

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From memory, rain drops form around tiny dust particles in the atmosphere so where there are raindrops there is particulate matter! Worse though is the fact that existing particulate matter may bond to the mirror's surface through dissolving in the water.

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I may regret it one Day, but the last time I cleaned my mirror I used 4 in 1 Windolene.? The Skywatcher mirror isn't the best in the World and is silicone dioxide coated to add protection to the aluminium surface of the primary mirror

Until that over-coating perishes, I don't care too much at present. I know and I'm aware of the 'accepted' ways of mirror cleaning, I just feel that there is too much over concern about touching/cleaning mirrors. They can get extremely dirty and still work, but they do look best when sparkly clean.

If your final image looks good, then leave the mirror alone.  Mine was out again today, just to look at the adjusters, possible replacements pending?

A good coating of a well known fizzy drink left to dry on the mirror will no doubt add some colour to your final image and vastly reduce the contrast, but a few drops of rain wont hurt. The air is full of particulates, anything and everything gets in there, even it appears when its capped, as air can still blow past the primary , as there is still a gap there!

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rain will affect the view from your mirror but not as much as clouds, I would therefore recommend observing on clear nights

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when I had my Dob I decided to flock it..when I took the mirrors off they were filthy! I was amazed I could see anything with them but I got really clear views with them...anyway I washed them in soapy water (I bet a few folk have choked on their biscuits reading that one!) I dried them with coffee filters and put them back in..i even dropped a big screw on it and scratched it but it never made any difference to the viewing..also never needed collimated either!

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rain will affect the view from your mirror but not as much as clouds, I would therefore recommend observing on clear nights

Thanks, that's very insightful and helpful.

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Thanks, that's very insightful and helpful.

apologies I forgot the smileys  there will be dust ion the mirrors after rain and it should be avoided if possible, but most mirrprs are coated so i doubt they will suddenly degrade if you dry it quickly enough. so care is advisable but its not something to get paranoid over a few spots won't destroy it but do not leave it in the rain

Edited by rowan46

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I've left my scope outside imaging a few times only to return to find its raining. On at least one occasion the scope was fairly close to vertical and still no rain had got to the primary mirror. After this I realised that as long as the scope isn't aimed at the zenith or as long as it's not caught in a heavy downpour the primary mirror is probably safe. My scope has been flocked though so this probably helps stop raindrops splashing onto the primary.

I'm now more worried about my precious new mount, camera or laptop getting rained on than my telescope tube.

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the last time I cleaned my mirror I used 4 in 1 Windolene

I'm not convinced this is a good idea and certainly one I wouldn't recommend to anyone. Why take such a risk on optical gear when the accepted practices and precautions are sound, cheap and effective?

Something like Windolene appears to use acetic acid (vinegar, for example) as one of its principle components and it's safe to say one should NEVER use such chemicals when it comes to cleaning optics. Furthermore, the product probably contains unlisted impurities which will leave deposits on the mirror and will damage it.

Regardless of who made the coatings for the reflective mirror, in essence these coatings are a very delicate film, molecule thick and gently coated over the glass surface. Using something like acetic acid is probably the single most damaging action you could do to this coating, short of scratching it up.

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The answer to the original question is that it is not critical at all. A Dob is a very simple device and all parts of it can be cleaned effectively. Washing the primary of a Dob using the traditionally accepted method is a one hour job. Everything else will dry in the Sun or a warm room.

Getting water in a Refractor or an SCT would be a different story as they are not self ventilating.

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Cleaning mirrors is not rocket science and a lot of posts have been written concerning this subject.

Rain will do little to damage as long as you dry it promptly after.

Have a shower cap in your kit that you can use in a hurry.

If you have some distilled water in a spray bottle you can rinse off the rain if you're worried about dust being stuck. A touch of surgical grade isopropyl alcohol mixed in will also help to dissolve crud already on the surface. Up to 50 percent is fine.

Allow to dry naturally in a warm spot.

No hair dryers. ....;)

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apologies I forgot the smileys  there will be dust ion the mirrors after rain and it should be avoided if possible, but most mirrprs are coated so i doubt they will suddenly degrade if you dry it quickly enough. so care is advisable but its not something to get paranoid over a few spots won't destroy it but do not leave it in the rain

No worries, sorry for being a bit sarcy ;-) Thx for the info.

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hello Qualia, I've noted your point of interest, but Like I said, this method wont be for everyone, and is NOT my recommended choice to everyone, for simply not knowing the end results. But  Its what I chose to do as an experiment, and yes, it's not the  standard, accepted way to clean a mirror, but why do folk continually wash them under running water when moisture is the number one enemy of a telescopes optics whilst in storage?


The ingredients to Windolene are Aqua, Butoxypropanol, Methoxyisopropanol, Propylene Glycol, Alkyl Polyglucoside, Acetic Acid, Sodium Laureth Sulfate, Sodium lauryl Sulfate &  Colorant, In decreasing order of percentages.


The product  sure does contain acetic acid ( diluted - in small quantity ) and their data sheet does specify Cleaning Mirrors (doesn't specify Telescope mirror) but does reference the streak free cleaning results for spectacle wearers! do spectacles not have expensive coatings? 


It cost's so little time for me to quickly clean the mirror upon any removal, and looks great, streak free when back in the OTA. If the Mirror does become unusable for me over time,  then at less than 77p a day (to-date) to run this telescope ( cheaper than buying a daily newspaper ) I`ll quite easily afford a new one next time? That said, I've read that mirrors (cleaned or not) kept in perfect  dry conditions still don't last forever before some issues arise ( maybe I'm contributing to some of these early issues, time will only tell).


Also, how many folk have removed/replaced/re-centred their Primary mirror spot using Nail varnish remover, I know folk do it! but that makes me cringe, in the same manner you have  for using vinegar?  Nail polish remover lifts the adhesive, and leaves a smear (which will need cleaning - or not as this will be hidden from view by the secondary - but  the mirror is still dirty to some ? ) The remover contains Citric Acid, stronger than Acetic acid.


The general consensus is to use running tap water to clean the mirror surface  with a  non-abrasive cleaning agent of your choice ( ie just cotton wall balls, dragged across the surface, Washing up liquid, Dreft? ) and to use distilled or de-mineralized water as your rinsing agent, leave to dry, re-assemble - collimate. 


Choose whatever method you desire, the consequences will be your own.

Edited by Charic

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I've heard stories, and seen pictures of (though not seen in real life) dobs being emptied like dustbins after sudden rainstorms - apparently it didn't bother them.

Edit: Not sure I'd fancy it, though. Scope cover for me...

Edited by AndyWB

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....surely, they should leak around the primary mirror cell, shouldn't they?

I'd have thought (and hoped) so! Like I say, I didn't see it; it may have been an exaggeration - that they were just shaking droplets off.  Still, I'd still sooner use  a scope cover.

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I've read twice now about swim /shower caps. I may get one for the base of my telescope, further dust protection?

Its doesn't help looking down through the aperture with a light as everything is magnified by the mirror, no wonder folk want to clean all the time?

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