Primary Mirror : to Clean or Not to Clean!?

[coxy322]

Recently watched some videos on the net about this. I am confused with the conflicting advice; the videos suggest it's easy (one dude even using his hands to wipe the mirror under water & with soap) whereas the official word is to leave well alone!

Hand up - who does it, and who stays away? Also if you do it, did it really make a difference?

[Moonshane]

I don't wash mirrors generally. sometimes if selling I do to make it 'look nice' but I doubt there's any discernible difference to the actual human eye.

I feel there's more to lose than to gain even though it is a straight forward , low risk process.

personally, I'd sooner buy a dusty mirror than one that's damaged which can happen. all three of my scopes have dust and minor marks on from condensation but I won't be washing them any time soon.

[boabsta]

I've done it on a 130 mirror. It was thick with dust as he'd been left for a few years with cap off and pointing skyward as an ornament. Not by me. Blew most dust off 1st but was still covered so I had a quick read up on how to do it and went for it. I got the scope for cheap anyway so wasn't overly bothered if I made a mess of it. I boiled a kettle and let it cool so it was warm and then poured it over the mirror which I held at an angle in the sink. Thankfully there was very little beading and nothing needed scrubbed. Came up rather well I have to admit.

Once again I want to say I only done it as I had very little choice.

Did you see the state of the keilder mirror? You can have a lot of everyday muck and still get good performance so only clean if it is essential.

[swamp thing]

I do clean mine, but hardly frequently, and they have to be in a right old state before I even think about it.

I use a method similar to he guy using his hands. I prefer to touch the surface with a known object than leave it to cotton wool (surgical) for some reason I just don't trust cotton wool. A good tip he gives is anything that doesn't come off with the slightest of touches LEAVE ALONE.

Does it make a difference hmmm! it does if you've let it get to the point where it actually needs a clean, and trust me this is dirty.

Otherwise no, dusty mirrors work Perfecly well.

Regards Steve

[Mr Flibble]

I did mine a couple of years ago. I had some flecks of some sort of plasticky black material on the primary and secondary. I bought my scope second hand from a shop so just wanted to check that it wasn't anything nasty. I put the mirror in a bowl of warm water with a bit of washing up liquid, let it soak for 10 mins, emptied the bowl and added fresh water, dragged some hospital quality cotton wool (under its own weight) across the surface using a fresh piece each time, rinsed off with distilled water and left it to air dry. Looked so shiny when it was dry but by the time I put it back in the OTA it was already collecting dust again!

[coxy322]

So we purchase these very expensive telescopes and the main cost is for the mirror. The mirror is expensive because of, amongst other things, the coatings used to ensure maximum reflection and therefore performance.

If there is loads of visible dust and a few small bits of fabric/hairs then this surely goes against the gain of having reflective surfaces and therefore one would think that performance is compromised?

So does it provide a noticable improvement if you clean the mirror say once a year? My scope lives in the shed and has no cover outside the OTA/rearside of primary to protect air currents from the mirror. It's not hampered any attempts for observing or imaging (but then how would I know!?) but she does look filthy.

[mdstuart]

I have cleaned a few using the warm soapy water approach with no bother. You should spray with distilled water at the end otherwise it leaves water stain marks..

I agree with the best leave it alone unless its really dusty approach but maybe you are like me and just like clean things and cannot resist!

I do agree that if the dust / hairs do not come up under running water or a swish of surgical cotton wool then leave well alone!

Mark

[cotterless45]

Very last resort is cleaning. It has to be really filthy to affect viewing.I've seen one tube with an active spider's web in. Try and keep the tube closed over including the focusser when not in use, even observing.Bats seem to like getting their poo down tubes.

I caught a thunder fly on his way to look at the mirror last set up.

[Skybrowser]

If it's really really bad, then clean it like this....

[Islander]

I've done it a few times when necessary. Tepid water with lux flakes (lux is pure soap - no perfumes or other ingredients) dissolved to soak and wash it - tipping the mirror to get the water across the surface, clean tap water to rinse the soap off and a final rinse with distilled water to remove any remaining deposits. It's not that difficult or risky really.

[rfdesigner]

I've cleaned but very infrequently. I also use extreme caution, so I don't get my sticky acid ridden fingers near the glass, I also use the best water I can get for washing off.

If you have dust all it's doing is shutting out a very small percentage of light, it won't hit resolution. It will reflect a small degree of light in the wrong direction but as it's genally optically absorbtive it really isn't much of a problem.

If you damage your mirror while cleaning then it can hit resultion a bit, more importantly it can cause general scattering causing your background to 'glow'.

Much better is to make sure your optics are kept clean and dry (mould can eat into glass but it can't grow without water, so I keep camera lenses in an airtight box with dessicant)

Another solution I've seen is used by 'dreamtelescopes' who advise using their mirror cooling fans 24/7 as they also use air filtration on their scopes so all dust is removed and never gets near the optics (spiders don't like the air movement either)

just my 2 penneth.

Derek

[basher0010]

when i eventually clean my primary i think i will be taking the opertunity to do the flocking and eyepiece assy in black while its in bits might as well kill 2 birds with one stone so to speak..

[Frank the Troll]

I've cleaned my, not a biggie IMO, just rinse off with distilled water when done, and leave to air dry.