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Should I clean my primary mirror?


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So I have had my scope for 10 months and it has had a fair amount of use. It is stored in my garage, tube (collapsed) covered with a plastic bag with the tube cap on and the mirror at the bottom. When I put the scope away a few nights ago I noticed that the primary mirror had quite a lot of dust on it.

I have not tried to clean the mirror.

So two questions;

Should I clean the primary mirror, if so should I use an air spray like i use to clean my camera, or not.

Secondly should I try and store the scope with the tube "upside down" with the mirror at the top?

Any advice from those more experienced than I appreciated.

Andrew

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No on all three of your questions,you would be surprised how dirty a mirror can be to have any impact on your views,the spray can throw out droplets of moisture and i would neve keep a scope upside down,you could keep it horizontal though.

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I've had my 10" for almost two years. In that time, it has probably had more than 200 sessions. It has traveled over 10,000 km and has been all over Spain, and parts of France, Italy and Portugal. It has been out in desert dust storms, snow, wind and gales, rain and scorching sun. It has suffered temperatures of over 45ºC and many below 0ºC. It has been camping for weeks on end, climbed rocky mountains and bumped over arrid desert lands. It has mellowed in the middle of grassy poppy fields, freezed in snow and has chilled by the mellow Mediterranean sea.

Sure, the mirror is dirty, my girlfriend says "it's filthy and even looks like tiny bugs have decomposed on its surface" :p but that word 'filthy' is quite a relative term. I was thinking that perhaps I should clean the mirror, so before doing so had a little session with objects I'd seen before, you know, to make a mental comparison. To play the game in the mirror's favour, I compared my evening sights to those I had made in the Black Triangle in France.

Sure, the Black Triangle had better skies in terms of LP, but comparing sketches of the actual objects there wasn't any notable difference or none which my eye could discern.  Of course, there could be other reasons for this, but none which can persuade me to clean the mirror. I'll probably give the scope another 150 to 200 sessions, probably wait another 10,000 km and see what happens.

My advice would be for you to do something similar.

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I know the traditional response to this question is not to clean the mirror unless *absolutely* necessary, but I'm inclined to disagree with this blanket "rule". There was a study a while back by an observatory and they showed that regular cleaning prolonged coating lifetime. I can't find the link now, though. So a correct cleaning procedure does not damage the coatings. Even simply hosing down the mirror with DI water will clean it to some degree and this is 100% safe on the coatings. On the other hand, if you leave it dirty then stuff that's on the mirror gets cemented to it by dew/drying cycles, etc. This stuff can eat away at coatings and promotes more dew formation as droplets tend to nucleate on it. The longer you leave it, the harder it is to clean off. 

The concern with mirror cleaning is that it opens up the possibility of dropping it, etc. If you have a good protocol for accessing the mirror and you're confident, then I see no reason not clean it three times a year. If you aren't happy doing this, then just don't clean it at all and have it re-coated more often.

I know that a dirty mirror still functions remarkably well, but that doesn't mean the views haven't been impaired due to increased scatter and decreased reflectance. If the mirror is getting dirty slowly then you won't notice the gradual reduction in the image quality. So looking though a dirty scope and seeing perfectly acceptable images doesn't tell you the whole story.

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What is the science behind the don't bother cleaning rule? To the uninitiated, it would be easy to assume that dirt in the optical path is going to impede/scatter/reflect the light and cause a loss of photons making their merry way to your eyeballs. What gives?


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I think some confusion arises with the cleanliness of mirrors due to the human mind trying to equate a telescope mirror to a (for example) bathroom mirror.

When you look in a bathroom mirror you see an image of yourself. If you look at your left eye, the mirror is reflecting the particles of light in that region of the mirror alone - the rest of the mirror is redundant. If you were to place black card over the rest of the mirror (except your eye) you would still see the reflection of your eye at the same intensity as if the card weren't there. Conversely if you were to block the mirror with black card in the position that you can see your eye - you wouldn't be able to see your eye!

Now consider a curved mirror in a telescope. Photons of light from a point in space (a star say) don't hit just one part of the mirror, they hit the entire mirror. That is to say that a photon of light emanating from a star hitting the left side of the mirror will be reflected towards one point of the image plane (which the eyepiece helps focus), and a photon of light emanating from the same star hitting the right side of the mirror is also reflected to exactly the same point of the image plane due to the different angle of incidence with which the photon hits the mirror. This is true for all photons of light hitting all points of the mirror from that same point source of light.

Therefore lots of dust on the mirror will theoretically affect some of the light entering the mirror, but there are always lots more photons from other parts of the mirror making it to the eyepiece.

This is a fairly complex subject which combined with my lack of understanding makes it difficult for me to explain, but I hope this does make at least some sense...

Dave.

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What is the science behind the don't bother cleaning rule?

I think this thread actively participates in the spirit of 'the science' :grin: We've got observations, an example or two of testing hypotheses, an honest pursuit of trying to discover information about the natural world, and so on. Perhaps what is missing is a Dust Law or a Dust Principle, or at least something like an organised body of Dust Information, but I think we're getting somewhere :grin: There's an article from S&T which does talk a little about dust and mirrors and mentions a 1/1000 of dust limit on the entire surface area. So I guess on a 10" mirror that's about 0.25mm of surface dust.

Umadog, I don't know if I've missed your posts, or you haven't posted in a while but either way, it's great to see you on the boards. The Black Triangle in France I visited was in the Causses du Quercy Region which is rather pretty and picturesque. I imagine other areas of France have equally good skies and I know Spain does as well. With regards to France, I stayed at a great campsite which doesn't permit loud noises or anyone under 18 to enter. The place was therefore very peaceful, and full of mild spoken, educated Brits who turned in at around 10pm each night. I left the scope up all day and night and didn't have any fear of theft or damage. So as not to throw the thread out any further, if you want any more information just drop me a line  :grin:

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I've had my 10" for almost two years. In that time, it has probably had more than 200 sessions. It has traveled over 10,000 km and has been all over Spain, and parts of France, Italy and Portugal. It has been out in desert dust storms, snow, wind and gales, rain and scorching sun. It has suffered temperatures of over 45ºC and many below 0ºC. It has been camping for weeks on end, climbed rocky mountains and bumped over arrid desert lands. It has mellowed in the middle of grassy poppy fields, freezed in snow and has chilled by the mellow Mediterranean sea.

Sure, the mirror is dirty, my girlfriend says "it's filthy and even looks like tiny bugs have decomposed on its surface" :p but that word 'filthy' is quite a relative term. I was thinking that perhaps I should clean the mirror, so before doing so had a little session with objects I'd seen before, you know, to make a mental comparison. To play the game in the mirror's favour, I compared my evening sights to those I had made in the Black Triangle in France.

Sure, the Black Triangle had better skies in terms of LP, but comparing sketches of the actual objects there wasn't any notable difference or none which my eye could discern.  Of course, there could be other reasons for this, but none which can persuade me to clean the mirror. I'll probably give the scope another 150 to 200 sessions, probably wait another 10,000 km and see what happens.

My advice would be for you to do something similar.

That really puts mirror cleaning in to context 

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What is the science behind the don't bother cleaning rule? To the uninitiated, it would be easy to assume that dirt in the optical path is going to impede/scatter/reflect the light and cause a loss of photons making their merry way to your eyeballs. What gives?

Dave's post answers this nicely. The key thing is that each location in the image is created by light coming from the entire mirror. The location that really matters in terms of seeing bad stuff in your image (like dust spots) is the focal plane. The optical element nearest the focal plane will be a lens element in the eyepiece. Which element  it is will depend on the eyepiece design, but it's likely one of the innermost ones. Get dust on there and you will see it. The optical term for this is a "conjugate plane". The primary mirror focal point is conjugate with your retina.  These planes are used to good effect in microscopy to provide uniform and dimmable illumination. See this: http://en.wikipedia.org/wiki/K%C3%B6hler_illumination I reckon the same sort of thing is probably done with an SLR iris, but I'm not sure. If you want a graphic depiction of how little effect damaging the objective has, you should see this: http://kurtmunger.com/dirty_lens_articleid35.html Amazing, huh?

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I'm more worried that the mirrors/corrector plates on my scopes will degrade over time whilst being stored away rather then being used often (due to rubbish weather). They are stored in their boxes inside the house away from any radiators etc. 

I'm probably just to much of a worry wart.

p.s.~~~no need to clean mirrors that dont often get used (in my case).

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  • 2 weeks later...

I have not cleaned my mirror once in the 4 years it's been since I got my scope based pretty much on the advice you don't really need to. I look at it now and it has a few cobwebs on the mirror, a good layer of dust, looks a tad dull and no doubt some tobacco smoke residue which is probably the most concerning.

Does it need to be cleaned?

I don't really know, I don't think it affects my views a great deal but to be honest it rarely gets used for observational work and mainly for imaging and I do wonder if the same "don't clean it until it's really grubby" holds up when imaging is concerned.

I don't know and can't speak for people but I find it a hard sell that the best imagers are using dirty mirrors even if it doesn't really matter.

I'm still mulling it over on whether I should clean mine or not and the more I think about it the more I feel it's worth giving it a shot at least once.

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I refer you to my earlier post!... However once the seeds of doubt have been sewn, and to be frank you should remove cobwebs. Do it safely and follow the guides on here. Use a very good quality blower brush first as you may find that this will be all you need.

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Forecast is threatening clouds for the next few days, could even last a week or so :BangHead: If that turns out to be true, I might give the mirror a clean, just to see what happens. Or again, might be better just to enjoy the dull weather and have another  :coffee2: 

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