#1  
Old 13-07-2020, 08:47 PM
bluesilver (Peter)
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Dobsonian primary mirror cleaning

Hi, I had a strange issue tonight when observing the planets with my 16" Skywatcher dobsonian with goto.

Rolled the scope out about and hour and a half or so before nightfall, around 4 pm here, to get it acclimatised to the air as it cools.

As the night went on, i found that right in the centre of the eyepieces i was getting what looked like a slight fog issue, kind of like when your glasses fog up.
But it was right dead centre of the eye piece, Only noticed this when viewing the bright planets.

I thought i might have some issues with the eye piece internally fogging up, So swapped it out for another and same issue.
Got me thinking a bit, so i got a torch and had a look at the main mirror only to see that there was condensation / fog right around the centre of the mirror and then also right around the outside of the mirror,
The rest of the mirror appeared to be fine, forgot to take a look at the secondary.

The night was very cold and there was condensation all over the outside tube.

So i am thinking, do i need to clean my primary as i was getting that odd due/ fog on the primary but only around the outside and centre.
And if so, what is the best way to clean them?

Or is it just one of these things that happen?
I have fans for it, but in my error, didn't have them on.

Any advice is appreciated.
Thanks.
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  #2  
Old 13-07-2020, 11:56 PM
Saturnine (Jeff)
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Hi Peter

The moisture on the primary may be promoted by excessive dust on the mirror but 1st, it is an open tube system, so were you using a shroud at all to help keep the dew from reaching the mirror too easily. Also you say it was a damp night, open tubes and cold damp nights aren't a great mix. Having the fans on may have helped for a while but in such an environment it would have fogged up eventually, especially if pointing at high altitudes.
Personally, unless you are using the scope at high magnifications I wouldn't be too fussed about putting it outside to cool to ambient on damp nights. At low and moderate mags. the seeing isn't so critical that the mirror has to be equalised to the ambient temperature unless there is a large differential from the temperature it was brought out from.
In daylight after the mirror has dried off tomorrow, does it look particularly dusty or still have a noticeable ring of discolouration or spot in the middle, a slight staining from the moisture. If not , I wouldn't worry about having to clean the mirror, it will be fine.
No telescope enjoys damp , humid conditions
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Old 14-07-2020, 07:28 AM
Startrek (Martin)
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I have a Skywatcher 12Ē Goto dob and set it up with dew control knowing that it could be an issue in the future which can ruin your night
I bought a Skywatcher black elastic shroud to fit over the truss section which eliminates stray light and held prevent dew forming on the primary mirror. Then bought a Kendrick secondary heater band which fitted around the stalk of the secondary and finally Kendrick heater straps to fit around the finder scope and a spare for eye pieces if needed.
Iíve have the scope for 2 years and never had to end a nights observing because of dew. There have been nights of fairly heavy dew particularly in the early hours and it hasnít ended my night
I donít know about a 16Ē though , thatís a big dob , hopefully a shroud and some dew control may prevent your issue
PS: Iíve only cleaned my mirrors once in 2 years and Iím located 250m from the beach.Ive been advised not to clean your mirrors to often unless they have built up salt, mildew or mould evident
Good luck !!
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Old 14-07-2020, 10:18 AM
Quopaz (Nick)
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Dew ended my night last night! I couldn't work out what was going on. When the sun came up there was a low layer of heavy fog, then it all made sense. Everything got really wet, primary mirror fogged up. Mine's a solid tube dob as well. I recorded 0.7mm of ... fog. Apart from that it would have been good seeing. Not real hard to take out the primary mirror and clean it, I've done it twice. I make sure it goes back in the same place (marked it with stickers). Then re colimate it.
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Old 14-07-2020, 04:03 PM
bluesilver (Peter)
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Thanks for the replies,
Sorry i left a lot of the details out, sorry about that.
I am using a Astrozap shroud over it, it stays on all the time, only undo the bottom of the shroud to take off the plastic tube cover each time before i take it out and then it goes back on as soon as i pack up for the night.
There is a little dust on the primary mirror as it lives in a workshop at home.
The night was cool, around 0 degrees or maybe just hitting -1. ( it gets cold quick here)
Just had a look, There is no real marks or anything on the mirror now, mainly just light dust.

The only thing i didn't check was the secondary last night.

But thinking it could be good to clean the primary, easy enough to get out once you get the tube off the mount and upside down.

What is the best way of cleaning them, just warm water. clean hands and lightly rubbing the mirror, then rinse of with distilled water?
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Old 14-07-2020, 05:10 PM
Startrek (Martin)
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My procedure for cleaning mirrors

Donít touch mirror with fingers or hands !!!

Run warm water from tap at about half and hold mirror over sink to let water wash off surface dust ( a minute or 2 )
Then apply a small drop of plain dishwashing liquid ( not lemon or fragrance but plain ) on a wet cotton ball and start at the centre of the mirror ( not on the centre donut mark ) with light finger pressure and wipe in a circular motion from centre of mirror to outside whilst tap is running drops of warm water
Once done, wash off mirror in warm water again from running tap and inspect mirror under good light or daylight near a window to check surface is clean. If not, repeat once more only
Then pour a decent amount of distilled water ( 1 litre ) with the mirror tilted at a 45 degree angle and leave tilted at 45 degrees to allow drips to run off
Any remaining drip marks on mirror can be removed using a Q tip or cotton tip by carefully touching the drip ( but not touch the mirror if possible ) You may need a few Q tips as they will only remove drips if new or dry , not reused

Leave mirror to completely dry indoors ( 1 hour or more depending on conditions) and re install in Scope

Hope the above is helpful
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Old 14-07-2020, 05:51 PM
Saturnine (Jeff)
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Hi Again

From the sounds of it the mirror is clean enough and it was just a very damp, cool night causing the mirror to fog up. If you have similar conditions fairly often , it may be worth considering dew heaters for the secondary and maybe the primary as well, but sometimes even though the sky is clear other conditions will spoil the night.
If you really must clean the mirror, there are plenty of tutorials online and even written articles in the Beginners section of Projects and Articles, up in the top left of the home page.
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