dust on primary mirror

[EzyStyles]

Hi,

I was wondering if I can use a hairdryer to blow the dust off the primary mirror? will it damage it in anyway? thanks.

[RAJAH235]

Ezy, If you use the hairdryer, just be careful not to direct it straight onto the surface ie; 90 degs. Never know what might get picked up in the H/D & blown onto the mirror. Better off with a 'puffer brush' from the camera shop.
How bad is it? If it looks really bad, it may need a wash! Seeing as you have just recently purchased it, I would suggest using it for a while & if poss, take it to an open night or local club night for others to inspect.... A mirror has to be 'really dirty/dusty' to have any great effect on the quality of image. The contrast may be down a little, but you will not notice it.
My mirror has been 'washed' 2 x in the 12 yrs I have owned it.
I hope this helps a bit. :D L.

[Astroman]

HI Ezy,

Rajah bought up some good points, avoid doing anything to the mirrors unless it is absolutely necessary (besides collimation). Surface dust on primary mirrors is a fact of life, you cannot get away from it, it's always going to be there.

I have seen closed tube SCT's with dust on them without degrading their performance (well not the eye can pick up) A friend of mine has an 8" f7 scope that hasnt had its mirror washed since he got it over 10 years ago. If you cover the tube and keep the telescope in a clean space you shouldnt have a problem.

If you really do need to clean the mirrors, goto your local Astronomical society and ask around for some expert help, learn from them how to do it yourself in the future.

[matt]

Yeah Ezy

Like Astro and Raj say, unless it's a really serious case of dust, don't worry about it.

My newt mirror's got what appears to be a fair amount of dust on it but it works fine. It's as bright and clear through the EP as it was when I first got it. At least to my eye, the difference is negligible.

But as Astro says, if you're really worried about it take it along to your local astro club and let them have a look and show you how to do it correctly... and safely:scared:

Better a thin layer of harmless dust than a network of scratches!

[iceman]

What the guys above say is true, however I also believe that a dirty primary mirror just has to have a negative impact on what you're viewing, and when i'm doing high-res planetary imaging, I don't want light scatter or lack of contrast or decreased resolution because of a dirty mirror.

I clean my mirror fairly regularly (2-4 times a year if it's needed), and I think most people have an irrational fear that cleaning their mirror will scratch it - therefore they conclude that a dirty mirror is better than a scratched mirror. And that's true, but if you take the proper precautions then there's no reason you should scratch it, and there's no reason you shouldn't clean it if it needs it.

There's a thread here which discusses many mirror cleaning options and alternatives - http://www.iceinspace.com.au/forum/showthread.php?t=4496 - my post is #17 but there are also other options in there.

I also agree with above though, that you need to determine if it's dirty enough to *need* cleaning. I don't clean mine at the first sign of dust.. but after a very dewey session at Lostock over the weekend, my primary mirror had water condensing right on the mirror, and so now it's left many residue spots so it still looks like it's wet.

Yes you can still view through it, but I plan to clean it again before I do any imaging with it.

[matt]

Well ... if we're talking hi-res imaging....:confuse3:

[EzyStyles]

many thanks for your help guys. ok, i bought the telescope secondhand and got no idea how old it is so im going to give it a go at washing it from the link iceman posted. Do i have to rinse it off in the end with disstill water? or can i just use ordinary water?

[iceman]

Definitely distilled water.. it doesn't leave any residue. Any spots that don't run off, just grab a paper towel and use the corner of it to dab on the spot and soak it up.

[Hitchhiker]

Hi EzyStyles, if you are just visually observing planets and DSOs I wouldn't be terribly concerned about a bit of dust on your primary mirror - the decrease in contrast caused by a little dust is negligible compared to other factors such as poor collimation, insufficient scope cool down, or bad seeing.

If you are doing hi-res planetary imaging then you need to have everything perfect and cleaning your mirror every 3 months is just part of the fun and games :)

If you do want to clean your mirror, I don't think a hairdryer is the way to do it - blowing tiny abrasive particles across the surface of your mirror may cause fine scratches. Follow the link that iceman has provided to learn how to clean your mirror properly.

OK, just saw your last post - good luck with the washing. Yes, do use distilled water so there is no residue after the water dries.

[barees63]

This is the best guide I have found:

http://www.wwnorton.com/astro21/sandt/optics.html

[Robert_T]

I'm one of those guys that has an irrational fear of cleaning mirrors for fear of scratching them... no matter how careful I've been I always do, but them I'm not exactly a neat and clean sort of guy and my house is like a shrine to dust, so I suspect that if you are of the neat, clean and organised variety like Mike says you should have no trouble.

Having said that I'm also amazed at how little impact dust and even fine dew has on views and imaging. Some of my best planetary images have been taken with fine dew spotting up the corrector plate on my SCT and what looks like a desert of dust on my DK cassegrain mirror. What I'm getting at is it really isn't such a big deal, so see how you go as others advise. It's probably something that takes you from 97% to 99% performance.

cheers,

[barees63]

I like this quote (from the URL I posted above):

Dirt happens, and in moderate amounts it has remarkably little effect on performance. In his book Star Testing Astronomical Telescopes, Harold Richard Suiter analyzes the effects of dirty optics mathematically. His conclusion? "The maximum amount of dirt [that a perfectionist] should tolerate on the optics is about 1/1000 of the surface area, [which is] the size of a single obstruction about 1/30 of the diameter." In other words, on a 10-inch mirror this is the equivalent of a completely opaque dirt blot 1/3 inch across. That's quite a pile of crud before there's any detectable effect on contrast.
"Don't decide to clean mirrors on the basis of shining a light down the tube at night," advises Suiter. "All mirrors fail such a harsh inspection." After you've done what you can to prevent dust, ignore it.

[matt]

Rob

re: your comments about mirror scratches and your house being a shrine to dust etc etc

I'm going to check birth records to see if we weren't twins separated at birth:lol: :lol:

But seriously, I'm only recently starting to appreciate comments along the line of "don't look at the dust on your mirror, look at the view through the eyepiece".

[davidpretorius]

Hey Ezy,

its easy, i am bringing a 20litre drum of distilled water to the snake valley camp (near ballarat) in early march. Come along, enjoy the company of us grumpy bums and we will go through washing mirrors like we did last time.

There is a thread for the camp, or even if you just come out for saturday during the day for a drive. Dave47tuc is organising the camp, so pm him for any extra details that you can't find in the thread.

By the way, for the next few weeks i will shamelessly plugging the camp for all VIC people!

[EzyStyles]

relief.. just cleaned my primary and secondary mirrors. Wasn't that hard actually.

1) take primary/secondary mirrors out.

2) clean sink

3) fill sink with warm water with detergent

4) blast both mirrors with warm water without touching it with fingers

5) dip both mirrors in sink with detergent

6) leave mirrors in sink for 5-10 minutes

7) rinse both mirrors with warm water under tap

8) rinse again with disstill water

9) sit mirrors on the side and let it dry. Use paper towel to dap large water drops

10) once dry, put mirrors back together

One thing though, i left the primary mirror rear holder/case as it is when placing it into the sink as i don't want to remove the primary mirror from it. I also adjusted the screws holding the mirror (not too tight).

Many thanks for all your help all. Much appreciated.

[janoskiss]

You did take the mirror out of the mirror cell before soaking it ... :scared2: ... right? :scared:

Uh, sorry, just read your post properly. I don't think soaking the mirror cell and all is a very good idea. Now what's done is done. Just make sure it all dries properly, and, as they say, don't do it again. :scared: :)

I would still take the mirror out of its cell and thoroughly dry everything before reassembly. You could end up with rust, or worse even, mold.

[janoskiss]

... and please don't tell me for the secondary you soaked the OTA, spider, focuser (finder?) and all.. :prey:

[EzyStyles]

OH NOOOO!!!!!!!!!!!!!!!!!!:scared: :scared: :scared: :scared: :scared:

errr yep. I soaked it with the mirror cell.. what happens now? will it stuff it up?
with the secondary i didnt dip it with the spider. took "just" mirror/holder out but primary as seem in pic i dip the whole thing in.


:help: :help:

[davidpretorius]

just make sure it is dry.

take the 3 sets of 2 screws out carefully that are on those clips holding the mirror. lift up clips and move to the side. gently lift mirror by the edges and place in safe area.

now get hammer and smash the crap out of it!!!

(just joking for the hammer)

make sure the supports ie cork is dry etc.

re-assemble, but make sure that the screws are only done up just finger tight. If they are too tight, then they can warp the mirror.

[janoskiss]

Best to take the mirror out of the cell. Moisture could remain between the cork pads and the glass for a long time, and there is a good chance of mold taking hold. You might want to disassemble the rest of the mirror cell and dry each component separately with a towel and then let dry completely. You don't want your collimation screws and springs going rusty.

The other problem with doing this is that there is usually a lot more dirt and dust on the mirror cell and housing than on the mirror itself which will wash into the water, so you end up soaking the mirror in more dirty water than you otherwise would have.

Same for the secondary. If it has a holder where a clamp is stopping it sliding out, then disassemble and thoroughly dry everything. In the GS scopes there is usually a clump of foam that could soak up plenty of water behind the secondary. It's good to do that anyway to make sure the mirror is not being pinched. Just make sure everything is dry before it all goes back together and it will be fine. :)

Well that's one way to learn all about your scope. ;)

[EzyStyles]

im too lazy to take the mirrors out again as everything is set and fully collimated.. gonna have a break and see how i feel after an hour or two :)

[janoskiss]

Don't worry about it! Just throw it in the bin and go to Bintel and buy a new one. :lol: They have a ripper new deal on 8" Dobs with mirror fan + 2" Crayford. :P

[EzyStyles]

lol, if i recall at Bintel, it was $495 for the 8" and around $995 for 12" dob.

[slice of heaven]

All my mirrors are siliconed in, no clips. I wash the mirror cell with the mirror .



Yep

[RAJAH235]

Mine 2, but on 'particle board'. Gotta be really careful not to get it wet..... :D L.

[jackenau]

:confuse3: Like Ezy, I was also concerned about the amount of dust buildup on my primary mirror. Despite having plastic caps fitted over both ends of the scope 99% of the time (12" dob), the mirror still seamed to have an accumulation of crud-dust that I thought must have some impact on my viewing.

(Maybe this is more applicable to dob owners simpy due to the increased size of the equipment beng able to accumulate more crud.)

Maybe its not such a big deal so I chose to ignore it.

Procrastination set in as to do I don't I and what if I stuff up the........ or scratch the ...., anyhow, seeing as I am on holidays, I thought lets give it a go. Read the threads on IIS and decided to give it a go.

By the way I have had my scope and have never collminated it. Bought a cheshire and found after 14 months still to be pretty well collminated. Boy I give this scope a working. Up steps on a trolly and down steps on a trolly, left outside most nights (under cover, but still exposed to the elements). Definately no complaint about the views previously, only the what if's.

Anyhow, decided I must check the optics and if required clean the primary and secondary mirrors.

I was amazed at the buildup of dust-crud that can accumulate on the mirrors, particularly the primary. I have a faithfull observing companion, buddy the labrador and I was amazed at how many of his hairs had accumulated on the primary. (Looking in from the end of the scope I never noticed this).

So, following the steps that Ezy has detailed (and are available on a thread somewhere on IIS) AND, as well as fully removing the primary mirror I gave it a go.

The hardest part of the exercise was to collminate the mirrors once fully re-assembled. (is really simple once you get the hang of it).

Perhaps I should have done this sooner as I always anticipated that as I move along in this hobby, I would get better at viewing and what I was viewing is as good as it gets. Maybe I was putting poor viewing down to selection of eyepieces, or poor viewing conditions, (or maybe even worse failing eyesight, I am getting older).

The viewing is so much more crystal clear that I have noticed before and sharper. i have only had a couple of nights to compare over what I have previously viewed, but is definate improvement.

Point is. Was a simple exercise really, to both clean the optics and collminate. It was worth the exercise and in my opinion so far has been justified in my increased sharpness and clearness of viewing.

It was a matter of following thew steps as highlighted in the various threads.

Nothing went wrong, only increased satisfaction of viewing.

Thanks to the guys that posted the threads on cleaning-collminating your optics.

:) Ken M

[matt]

Onya Ken

I'm glad it all worked out OK and that's encouraging for anyone who's a little shaky at the thought of doing what you've done.:)

The big lesson for me was exactly the same as you've discovered: you've just gotta have a go. Fully informed and well researched with all the handy info freely available on IIS, of course!

[Astroman]

Welldone Ken, takling a BIG mirror such as the 12". Good to see your still alive.

[EzyStyles]

I also noticed viewing is clearer Ken and well worth the time and effort to give it a go. From reading other threads on how others were shaken from scratching the primary mirror, you just have to be really gentle with it. If a 1 week newbie such as me can do it, anyone can. :)

For Newtonian owners, I find it is easier to take the entire tube off the EQ mount. Put the lid on and let it stand upright. Unscrew the screws holding the primary mirror case and gently lifting the primary mirror holder.

[barees63]

One piece of advice for GSO dob owners would be to mark the tube and mirror-cell holder with tape or something so you put it back in the same position (rotation).. after much wailing and knashing of teeth I found my OTA and/or mirror assembly is not perfectly round and it would only fit back properly in one position (ie I had to keep rotating it until I found the position in which it would slide properly onto the bottom of the OTA. a bit frustrating at the time).

[iceman]

Interesting Bruce, my GS mirror cell has a notch cut out of it, which indicates exactly which way it should slide back onto the OTA, because the OTA has a raised section where the tube is pressed together.. so it can only go back in 1 way.

Ken, an excellent story and advice worth reading for those that have been scared to clean the mirror. and COLLIMATE! It's great to hear that you've tackled collimation and won. It's something that all newt owners just HAVE to learn, and you're robbing yourself of crisp views if you "assume" your collimation is ok.

[barees63]

That makes more sense, mine doesn't have a notch so the OTA has been forced slightly out of round.. oh well, just went and checked again and noticed more rust appearing.. grrr!

[wavelandscott]

There is an excellent step by step article in the latest issue of Nightsky about mirror cleaning/washing...

If people are still not comfortable the article makes it look/sound dead simple...

Cheers!

[RAJAH235]

FWIW, before dis-assembly, simply put a permanent marker 'spot' on the adjacent sections. Or you can do what Meade do & use a 'coloured stick on spot' on the mirror cell & tube near one of the screws.... :D L.

[circumpolar]

It's a good article but I don't like the idea of "swishing the surface with cotton" after you have wet it. To much risk of a scratch. Better to blow off dirt then rinse and dry. And I do mean DRY!

[Hammerman]

Everybody has their own way of cleaning a mirror. I try a dry method first, but you have to follow the instructions as far as using a 100% camel hair brush, and VERY light strokes. This only works for dust, not stuck on crud. You can see exactly how I do this on one of the pages at my web site, which is as follows if you are interested in trying this method at all: http://home.comcast.net/~astrohammer/tips.htm (http://home.comcast.net/%7Eastrohammer/tips.htm)

[matt]

wavelandscott

Which month's edition of Nightsky is that mirror cleaning article in?

Couldn't find it in my local newsagent when I went looking today.

Thanks

[jackenau]

Its in the Nightsky January/February 2006 Magazine

Ken M