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cleaning the secondary mirror of 10" Dob


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I have had this Flextube 10" about 3 mouths and the views are very good. The scope was second hand but the Flextube design is only quite recent so I assume that at most the scope is 18/24 mouths old.

Whereas the primary mirror looks reasonable clean the secondary seems to have a 'film' covering its surface - a bit like a dirty windscreen from inside the car!! The question is should I attempt to clean it? Views would be appreciated.

I assume that if I unscrew the central bolt the assembly will come away - mirror plus holder. I will make sure the tube is at an angle so it does not drop onto the primary!!

If I attempt to clean the secondary I should use the same procedures/products as outlined by Mick (Doc) when he did his 16" Dob.

Anyway any views out there.

Mark

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The general rule is don't clean unless it's really dirty. Like yours mine had a film over the mirror caused by alot of dryed dew deposits etc.

So I thought why not and went ahead.

I don't know how your mirrors are dismantled but once off the scope just wash in warm water with a little washing up liquid. Let it soak for a while and then with surgical cotton wool just pull the wool very gently accross the surface discarding with each stroke.

Once that is done wash with warm water then stand on end and pour IPA over the mirror and watch water evaporate.

I then dried the little water that was around the clips off with a hairdryer.

You then assemble and collimate.

Dead easy really and I had the scope out last night and the objects do seem a little brighter.

Hope this helps Mark

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Thanks Mick. I am little worried about the secondary because if I soak it for too long it might ruin the adhesive that connects it to the holder. I have seen a number of articles about cleaning the primary mirror but none on the secondary. I am convinced that although the image is still pretty good now if I clean the film off the secondary this will improve the contrast.

Bytheway I had a good look last night for the Stephan's Quintet even using James's photo as a guide but could not see it. Have you had a success with the 16" Dob?

Mark

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I had a look to for it using the photo by James but couldn't see it either. I got to the right position looking at the stars in the photo but nowt was there.

Check out my report...

http://stargazerslounge.com/observing-reports/87312-meade-lightbridge-observing-report-12-9-2009-a.html

As for you mirror I doubt to much soaking in warm water would loosen your secondary mirror away from the stalk.

If you worried wash it in cold water.

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  • 1 month later...
Thanks Mick again. Great viewing report from the yesterday - we clearly have the same agenda on DSOs. I certainly intend having another go at NGC891 and SQ.

I think I will remove the secondary and follow your cleaning suggestions.

Regards

Mark

Mark,

Sorry to dig up an older post.

Did you clean the secondary, if so how did you do it and did it all work out ok.

Thinking about doing mine ;)

Dave

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I assume that if I unscrew the central bolt the assembly will come away - mirror plus holder. I will make sure the tube is at an angle so it does not drop onto the primary!!

Correct, though your mirror is probably not worth cleaning - a film of dust and dew-spotting is normal and won't affect views.

But if you do remove it I'd be interested to know how you get on. The glued secondary is a feature of my 12" flextube that I don't like. It'll be fun dealing with it whenever it needs recoating.

I've viewed Stephan's Quintet with an 8-inch and with the 12":

http://stargazerslounge.com/observing-reports/87567-stephans-quintet-other-12-inch-targets.html

I couldn't see all five with the 8-inch but managed with the 12-inch.

For a detailed map of the area try JR's TriAtlas.

JR's website on Deep Sky Astronomy

Andrew

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Thanks Andrew I am a bit worried about the adhesive so I rather not clean it unless its really necessary.

Thanks also for the info on Stephan's Quintet - I used James' photo that he produced a month or so ago to home into SQ but I must admit I could only see 'ghosting'. My skies are pretty good so I will have another go.

Finally I did enjoy your 'targets' list which I read when you first produced it - very good report.

Regards

Mark

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Cheers Mark.

A little follow-up on cleaning, for future reference: I just discovered this page, which describes a method using stuff called Collodion that you stick on and peel off:

Adventures in Mirror Cleaning

The writer found it useful for cleaning glued-on mirrors. So maybe, whenever the times comes, I'll give it a go on my flextube secondary.

Particularly instructive is the dirtiness of the mirrors being cleaned!

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  • 2 years later...

Thanks Moonshane, I do have a DIY dew shield but I've never had a problem with dew (until last night) so never put it on. I dried the scope out in the house and the secondary mirror looks fine, a bit smudged but no visible permanent damage.

I did manage to take it back out in the early hours and saw Neptune for the first time which was fantastic.

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Unless it's on fire, taken up a part time job as a pimp or falling out of it's housing while screaming obscenities loud enough for the people of the south of France to hear, you shouldn't touch the mirrors in any way unless they are filthy. Never wipe them if they are covered in dew - your cloth could be covered in anything even micro fragments that could scratch the mirrors.

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Now that I've managed to filth it up while trying to clean it how do I go about actually cleaning it?

Well, if you are absolutely sure it needs a clean, then the procedure for cleaning the secondary is the same as for the primary.

Remove it ( still attached to the mount ) soak in clean tepid water with a very small amount of detergent (plain detergent not posh stuff with hand softening ) use very soft cotton wool to wipe underwater using no pressure, then rinse with distilled water or RO water (reverse osmosis) from a good aquarium shop, allow to dry.

Don't fuss (like I used to) if you don't achieve perfection, if it looks ok, quit while your ahead :laugh: refit & collimate.

HTH, Ed.

Edited by NGC 1502
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  • 2 weeks later...

this will make you all gasp and cringe. lol

i have the 10" 250p dobsonian auto-tracker.

i cleaned my secondry mirror with spit and and a spectical cleaning rag after coughing on it while looking through the finderscope, and its fine. i then decided to rub the primary with the same cloth because it was covered in dew. seems ok but i do get smeg marks when it steams up. im scared to give it a good rub after reading this lol. anyway that said it still seems to work fine. i think lol

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As I started this thread 3 years ago I think I can update the information on the cleaning of this scope.

I have completely stripped this scope a few times and cleaned the primary and more importantly the secondary at least twice. No problem with the adhesive holding the secondary. I followed the same cleaning procedures with both mirrors and used RO water to rinse both mirrors off. At the moment the mirror looks like a new mirror.

Looking back I think it was worried I might do something wrong but clearly I worried over nothing.

Mark

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  • 3 years later...

Not sure who looks at old threads, but, like the original poster, I bought a 2nd hand Skywatcher gogto dob.

It had quite a film of dust on both mirrors. The Co I bought it from said 'don't clean you will not see the difference'.

They were wrong. After cleaning I was able to see faint galaxies that were just not possible before.

At the time I was worried about collimation as well as the possibility of scratching the mirrors so was happy to find an American site advising cleaning in situ.

The primary was easy. Placed the OTA upright and poured de-mineralised water with a little raw detergent on mirror and left for 15 mins. Tipped water off and it took most of the dust with it. Then used a squeeze bottle to flood with more de-min water. Used 12v dryer to chase water droplets from mirror and dry rear of OTA. No rubbing with cotton wool. The mirror was now almost perfectly clean but I suppose there was still some dust remaining because there were occasional white deposits. Just did the whole thing again. The results were well worth it and it made very little difference to the collimation.

The secondary was more difficult as it was not possible to keep the mirror flooded. But I used a similar process and certainly ended with it much cleaner than before.

As I say I was very pleased with the results.

However my secondary, although clear of dust, does have a slight yellowish haze and I may try a more vigorous process next time.

How much is a new secondary?

What is IPA (India Pale Ale?).

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  • 11 months later...
  • 1 year later...

OK - some great info to look through here! I have had my Skywatcher 250px 10" dob for  a few years ago and recently noticed two highly visible smudges on the primary mirror, each about the size of a 50p piece close to the middle of the mirror. Not sure what it is but suspect some kind of organic matter that may have been a bit damp and somehow got into the tube. When I first looked I panicked and thought it was bird [removed word] (and what a lucky shot for the bird) but the best description is it looks like large,  irregular fingerprints (which of course they are not!). Should I be thinking about stripping out the primary and cleaning as per proper protocol for mirrors? If so does anyone have a guide for doing so on the 250px? Thanks.

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