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Slight messy mirror, how fatal?

Astro-Nova

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Astro-Nova

Astro-Nova

Posted
Posted

I took my dob out this afternoon to try my webcam and align the finder scope

I noticed on the main mirror a slight trace of dust, fabric and hair. Would this be a major problem and is it advisable to leave it as it is or to try and get it off? As there is a minor amount on the mirror Iam a bit worried I might get more, which obviously i would try to prevent.

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robbieince

robbieince

Posted
Posted

A few specs of dust, fibres and hairs will not make the slightest difference to your views. you can leave well alone. You can always use a blower brush to move the ones that irritate you but it really won't make any difference.

Remember you probably have 30% blockage already caused by the shadow of the secondary and you never see that (in focus).

Regards

Rob

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bombassa

bombassa

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Posted

I disagree.

This mirror is completely dead. You have to inform the wife you now need a new and much larger scope.

No seriously - don't worry. If it ever gets real dirty you can look up threads on how to clean it safely.

Enjoy

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Geoffteeth

Geoffteeth

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I disagree.

This mirror is completely dead. You have to inform the wife you now need a new and much larger scope.

No seriously - don't worry. If it ever gets real dirty you can look up threads on how to clean it safely.

Enjoy

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Steve Ward

Steve Ward

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Leave well alone !

Cleaned mine once and the stress associated with the operation far out-weighed any benefits.

However , I do sometimes wonder why we all get so [removed word] about details of kit and set-up and continue to put up with at least grimy Primary optics ,

I admit there was no difference in the view after my clean , but it must make a difference imaging-wise surely ????

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sologuitarist61

sologuitarist61

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A good puff with a blower brush, once you have been to a few star party's you will see just how dirty SOME mirrors get and still perform.

Yes a good 'hurricane blower' from any camera shop will usually get rid of all the loose stuff - leave the rest where it is.

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Hals

Hals

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I'd leave alone unless its got really cruddy ie: mouse droppings, bird poop or other such rubbish on it.

I (needlessly) removed and cleaned the dusty primary from my then 18 yr old dob only for it to fall on it's shiny side whilst drying - result a nasty inch long scratch.

Still usable but...

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jerryTheC

jerryTheC

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I admit there was no difference in the view after my clean , but it must make a difference imaging-wise surely ????

Not a lot - you have to double the amount of light to make a one f stop difference.

However, front surface mirror coatings can be surprisingly fragile, and unless you know what you're doing it's very easy to end up with a mirror covered in fine scratches - which would certainly make a difference :) just not a good one :) which is probably why even people who do know what they're doing avoid cleaning them until it becomes absolutely necessary :)

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Moonshane

Moonshane

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I'd set your scope up and look at the moon or anything reasonably bright. while observing put your hand over the end of the scope. still think a little dust will matter? the image with your hand over will hardly change I'd bet.

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umadog

umadog

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It can be scary to clean a mirror the first time but by the second or third time you feel a lot more comfortable with everything. I agree that you should use the blower. Don't use the compressed air can stuff because the propellant may damage the coating. I think it's worth trying to get rid of dust because if the primary dews over the stuff that's already on the mirror can dissolve and become harder to remove. This may also exacerbate coating damage. You don't need to get obsessive about cleaning, though, a dirty mirror will perform indistinguishably from a clean one. It has to get very dirty before it's a problem optically.

Cleaning can be as simple as removing the mirror and hosing it down with distilled water then leaving it to air-dry. If your Dob is a truss then you may able to clean the mirror in situ (that's what I do, anyway). Generally speaking you won't notice any difference following cleaning. I've read reports by people who have cleaned dirty optics and noticed an improvement. The improvement wasn't in image brightness--as Moonshane says, you need to cover up a lot of mirror to make a difference--but in contrast. So darker sky background and less light scatter around bright objects and better contrast for things such as planetary detail. I haven't had this experience myself (I hose down my mirrors two or three times a year) but I can see how it's possible.

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paul schofield

paul schofield

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I was getting a bit stressed with the same problem. Although my primary only has a couple of dots of dust on it. I was thinking of cleaning the mirror & asked on here about it. I was told to leave well alone & to tell you you truth I am glad I did as this does not affect performance at all. I think mirror cleaning should only be done if your mirror becomes unusable ie really dirty or if you drop your coffee down the tube. I think it is impossible to keep the primary pristine, although we try.

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