Dobsonian mirror cleaning? (SkyWatcher Heritage 130p)

[CharlyLeyequien]

It has been a few months and I noticed that it already has a layer of dust(I live in a dusty area, there is some construction going on a few blocks away and we get caked with dust here unfortunately)

Is there any guide to cleaning the mirrors? Or any advise you guys can give?

[wookie1965]

https://www.firstlightoptics.com/blog/cleaning-a-newtonian-mirror.html

[ollypenrice]

The first guideline is simply 'Don't.' Even quite a significant amount of dust has no discernible effect whatever.

When, eventually, it becomes necessary you can follow the FLO guidelines above.  My one caveat would be to suggest a first use of cotton wool by dabbing once and discarding. I wouldn't use any wiping action till the mirror was already very clean and I would always wipe both gently and in curved strokes since straight scratches (god forbid) will show at the eyepiece when curved ones won't.  Remove rings etc before starting, or use thin rubber gloves.

The website I normally recommend is no longer active so I took a quick look at the first few U-tubers who popped up in Google. They were not at all good. Stick with the FLO advice.

Olly

[Xilman]

3 hours ago, ollypenrice said:

The first guideline is simply 'Don't.' Even quite a significant amount of dust has no discernible effect whatever.

By and large I agree with your advice, especially for Dobsonians used visually.

However, there are particular fields, in both visual and imaging use, where cleanliness is critical to reduce the amount of scattered light to a minimum and so raise contrast to the maximum. Examples include trying to detect the faintest possible objects, whether star-like or extended sources, and observing not so faint objects against a bright background, which includes things like Sirius B in the glare of Sirius, or objects in bright moonlight and twilight. Sometimes, at occultations for instance, you have no choice but to observe when the event happens regardless of the sky brightness.

What prompted me to post the above is that I need to clean the window on my Dilworth again. It was possible to perform asteroidal photometry near full moon recently but the oblique moonshine scattering off the dust on the window did hit the SNR badly.

[doublevodka]

There's some good advice here - https://www.skyatnightmagazine.com/advice/how-to-clean-a-reflector

However, as Olly said, best advice is don't, cleaning always comes with a level of risk.

For visual observations, a scope can be surprisingly dirty before it affects the views, so try not to worry too much.

A good first start is something like this - https://www.firstlightoptics.com/astronomy-optics-cleaning-protection/jumbo-silicone-hurricane-blower.html to simply blow the surface dust off of the mirror without touching it.

[dweller25]

The FLO advice is good, but as already said, it is important to initially dab the mirror to remove any small grit particles.

Edited March 16 by dweller25

[Spile]

I use the fingertip and distilled water method as per my blog to clean my mirror every 18 months or so when the dust has built up to a degree that I am uncomfortable with.

[Piero]

What @Spile said above.

Regarding the frequency, it really depends on how dirty it gets.

Also note that on this forum there is a widespread phobia about cleaning telescope optics. On Cloudynights, the "finger procedure" is mentioned several times and members are quite comfortable with it. My advice is to learn how to do it, always be gentle making sure the surface is wet, and don't be afraid of doing it when it is needed. A bit of dust won't hurt, a pile of accumulated dirt as I have seen on some photos posted by a few SGL members who said that you should never clean it, in my opinion, does hurt.

Also remember that whilst dust is not necessarily bad and it can also be blown away with a manual blower, air chemicals, pollen, fungi etc are bad for the coatings.

The mirrors of my truss dobsons get cleaned every 2-3 years as our weather does not allow me to use them very frequently. They are also truss telescopes so the mirrors are more exposed.

It can be less frequent in a close tube dobson, but again, a lot depends on how dirty it gets. 

 

 

Edit: if you use FLO guidelines, personally, I would use distilled water for the whole process (no tap water at all).

Edited March 17 by Piero

[AstralFields]

Yes it is really easy and there is almost no downside to cleaning a modern mirror with modern coatings regularly the right way. Some people do it 4 times per year for many years with no side effects, even on big 12" , 16" , 20" telescopes.

I do mine once on average for every 20-30 observing nights.

The claims that dust does not make much of a difference are simply wrong and not backed by any scientific studies, quite the contrary is the truth. I cover that below as well that dust DOES make an impact and it takes little of dust to do so.

Check the video I made to avoid a couple of key bad advice that seems to be everywhere, such as cotton swabs:

 

Edited March 21 by AstralFields

[Jim L]

Here’s FLO’s cleaning methodology explained in a video by Optic Wave Lab:

If you’ve just shod a horse, wash your hands first. Then do the dishes. And the pots and pans. Use plenty of soap and warm water.

By this point your hands are clean and soft. Use a nail brush if necessary to remove every last trace of dirt from your nails and fingers. Now you’re ready.

I’ve not yet had to dab anything off my mirrors as all of the grit and most all of the debris simply washes off under the sink faucet, but you may have to. The fellow in the video dislodges stuck particles with a fingernail, which seems relatively safe and benign. I’ve no experience to collaborate his technique but that’s what I would do also.

The sink wash technique also works wonders on neglected SCT corrector plates:

As has been stated elsewhere, there’s no need to over-do cleaning, nor to be afraid to clean when it’s needed. My own experience tells me it’s quite safe, and intuitively among the safest method, if you take caution and use common sense.

Good luck!

 

[Captain Scarlet]

I‘m another one in the camp that disagrees with the notion that mirrors rarely need cleaning. The most telling thing in the first (quite funny) cleaning video was where you see the part of the mirror that had been under the side-clips, and contrast it with the surface immediately adjoining. A huge difference, that I also see whenever I’m cleaning my mirrors. And looking at my own reflection in such a mirror it’s obvious the contrast is heavily dulled compared to that small bit of reflection from the tiny “under-the-clips” bits.

When I look down the tube of my 12”, having not cleaned it for a number of months, I see a blue-ish haze. With such a mirror I’ve been disappointed by the visual experience under the stars, then comparing a day or so later to a cleaned mirror, the difference is, er, night and day. Massive difference in what I poorly describe as “overall sparkle”.

I’ve been puzzled as to the source or mechanism for this haze. It looks like condensation when illuminated, but it’s not water condensation. It does clean away though. The penny only dropped a few days ago when browsing Kriege & Berry’s dobsonian book. Talking about condensation, they say that a thick mirror (stored in say a garage) rarely dews up in the field, but does dew up in storage, as the area daily heats up and cools down, and the thick mirror’s temperature lags behind. That daily micro-dewing gradually sticks and deposits any very fine dust in the air onto the mirror. It makes sense to me.

So, I’m very much an advocate of a regularly clean mirror, and the visual difference in overall contrast is to my eyes dramatic.

Cheers, Magnus

Edited March 21 by Captain Scarlet