How often should the mirror on a reflector be cleaned?

[Jerry Barnes]

I was wondering if I should attempt to clean the mirror in my 200 PDS while the nights are still too light to do anything.  Depending on the angle of the light reflecting on it its either perfectly clean and really sharp or has a very light layer of dust.  I guess that means its a little dusty but not bad enough to make a signicicant difference to an image.

Is it the right thing to leave well alone until I notice a problem?

Cheers

Jerry

 

[Peter Drew]

Unless you have an OCD about dust then it's best left well alone.  Apart from any potential accidental damage, removing the mirror for cleaning can open a raft of collimation issues.   🙂  

[AstroKeith]

As Peter says, if its just a little dust - leave it.

With dust and dirt build up you won't 'notice a problem". It will just very gradually get dimmer and less contrast. Its a judgement call, unless you are going after Sirius B, or a faint solar system moon, etc., when clean optics are a must.

Insect 'debris' should be removed as it can attack the coating. My 18" gets an annual wash, but then it spends its life going to star parties and picks up a lot of stuff. 

[bomberbaz]

I'm with the others, leave it until such a point it is undeniably dirty. That said, I used the 16" fork mounted newt at galloway AC and the secondary was truly dirty, the primary too but it still gave super sharp images of DSO's. Equal to my then spotless 14" dob. TBF, it would have benefited from a clean though.

[pipnina]

The only times I have cleaned my mirrors is when:

My visual 250mm dob was sat in my room for so long that it had actual big tufts of dust and the surface appeared quite matted. I remoevd the cell and ran it under the cold tap to remove the worst in the most gentle way imagineable. Then I used demineralised waterand cotton swaps to gently swipe off what remained, using single strokes per side of cotton ball, all going in one direction until I'd gone from one side of the mirror to the other. Then, as the cotton was not as clean as the water I used, I used a dry cotton ball to dab the drops of water up to prevent them leaving deposits behind as they evaporate.

I also did it to my 130P-DS when I was trying to sell it (sure it didn't NEED it, but if you're trying to move something on it doesn't hurt to make it look its best!)

[bosun21]

I have only had to clean my primary mirror once and that was due to a pollen build up. I place mine in a basin with tepid water with a drop of soap and let it soak to loosen the pollen. I then use my fingers to run over the surface to feel for anything that has adhered to the mirrors surface. Then wipe from the centre spot out with a clean cotton ball each wipe. I then rinse the mirror with distilled water and blow the excess off with a hairdryer on cool. Stand to dry.

[ONIKKINEN]

Have cleaned my mirror twice now, the first time because there was some oily looking rainbow coloured residue on it (not sure where from, possibly from my car exhaust as the scope was nearby) and the second because i had my mirror out of the scope for other reasons. The second cleaning wasnt really necessary, but might as well since the mirror was already out. Generally speaking i think its best to not clean it unless there is something on the mirror that could damage the coating and if you're unsure then id say dont clean it.

[Guest]

Mine get a cleaning every couple years weather they need it or not.  

[Jerry Barnes]

it makes sense, dont mess if its not needed.

thanks

Jerry

[Spile]

I clean mine every 12-18 months using the fingertip method https://astro.catshill.com/mirror-cleaning/

[The Lapwing]

On 09/07/2023 at 10:07, Peter Drew said:

Unless you have an OCD about dust then it's best left well alone.  Apart from any potential accidental damage, removing the mirror for cleaning can open a raft of collimation issues.   🙂  

Absolutely

[The Lapwing]

23 hours ago, Mike Q said:

Mine get a cleaning every couple years weather they need it or not.  

I reckon 3 or 4 years with average use is plenty. I think most of the urge to clean is psychological 

[Guest]

15 minutes ago, The Lapwing said:

I reckon 3 or 4 years with average use is plenty. I think most of the urge to clean is psychological 

Mine lives in a unheated uninsulated garage, which is also where my woodshop is.  So getting them cleaned every couple of years is almost always necessary. 

[AstralFields]

There are some studies out there which show that reflectivity drops every month of continued use. By how much it depends on the amount of use/environment etc.

1997PASP..109..303M Page 304 (harvard.edu)

I cleaned mine after about 20-30 nights of going out during a one year period (it was a busy year). Did it for the first time and made a video about it to show how easy it is, so people are not afraid to do it.

I am not so concerned about reflectiveness as such but a fine layer of dust causes also light scatter. Not to mention insect c*** can react with dew and damage the coatings. Especially if the scope is brought indoors where condensation may form on the mirror. That is bad enough already but add a lot of dust mixing with it and that can't be any better than washing the mirror twice per year.

Just make sure to do it right! Lots of bad advice (cotton swabs) out there on YouTube. That can end in a disaster real easy. Another reason I made my video.

 

 

Edited July 24, 2023 by AstralFields

[johnturley]

27 minutes ago, AstralFields said:

There are some studies out there which show that reflectivity drops every month of continued use. By how much it depends on the amount of use/environment etc.

1997PASP..109..303M Page 304 (harvard.edu)

I cleaned mine after about 20-30 nights of going out during a one year period (it was a busy year). Did it for the first time and made a video about it to show how easy it is, so people are not afraid to do it.

I am not so concerned about reflectiveness as such but a fine layer of dust causes also light scatter. Not to mention insect c*** can react with dew and damage the coatings. Especially if the scope is brought indoors where condensation may form on the mirror. That is bad enough already but add a lot of dust mixing with it and that can't be any better than washing the mirror twice per year.

Just make sure to do it right! Lots of bad advice (cotton swabs) out there on YouTube. That can end in a disaster real easy. Another reason I made my video.

 

 

Not so sure about using fingers rather than cotton wool swabs.

John 

[AstroKeith]

8 minutes ago, johnturley said:

Not so sure about using fingers rather than cotton wool swabs.

John 

The idea is that your finger tips are very sensitive and will tell you if you are dragging dirt/grit across the surface, whereas cottonwool etc will just go ahead and do the damage.

For eyepieces I use a reverse rolling technique with cottonwool, which lifts dirt away from the 'wipe' but means you need a new swab every 50mm or so of sweep. This isn't practical for a mirror, so fingertips for me.

The coating is actually quite hard, but cannot resist scratching from grit. Wet finger tips won't harm it.

Edited July 24, 2023 by AstroKeith

[AstralFields]

5 minutes ago, johnturley said:

Not so sure about using fingers rather than cotton wool swabs.

John 

I came across a big grain of dust that felt like a stone so I stopped. With a cotton swab this would not have been possible to feel. That moment made it to the video.

Edited July 24, 2023 by AstralFields

[johnturley]

42 minutes ago, AstroKeith said:

The idea is that your finger tips are very sensitive and will tell you if you are dragging dirt/grit across the surface, whereas cottonwool etc will just go ahead and do the damage.

For eyepieces I use a reverse rolling technique with cottonwool, which lifts dirt away from the 'wipe' but means you need a new swab every 50mm or so of sweep. This isn't practical for a mirror, so fingertips for me.

The coating is actually quite hard, but cannot resist scratching from grit. Wet finger tips won't harm it.

I actually prefer to immerse my mirror in a bowl of lukewarm soapy water first, and leave it to soak for about 30 minutes to loosen any grit, and then rinse before swabbing.

John 

[AstroKeith]

51 minutes ago, johnturley said:

I actually prefer to immerse my mirror in a bowl of lukewarm soapy water first, and leave it to soak for about 30 minutes to loosen any grit, and then rinse before swabbing.

John 

Ah, soaking and soap are two more curate's eggs! Much debate might ensue!!!

A good coating will withstand both. A weak, old or one with pinholes can be damaged, depending on the length of soak and strength/type of soap. The usually recommended three drops of pure soap, isn't going to clean anything, but rather allows the water to wet the surface better and hence dissolve solids. But this leaves organic residue largely untouched.

Soaking can be OK, but I wouldnt recommend it as a general method - in case it is used on a weak coating.

[bosun21]

 I use the finger method on the entire mirror. Then I finish off with cotton balls and rinse with distilled water.

[AstralFields]

8 minutes ago, AstroKeith said:

Ah, soaking and soap are two more curate's eggs! Much debate might ensue!!!

A good coating will withstand both. A weak, old or one with pinholes can be damaged, depending on the length of soak and strength/type of soap. The usually recommended three drops of pure soap, isn't going to clean anything, but rather allows the water to wet the surface better and hence dissolve solids. But this leaves organic residue largely untouched.

Soaking can be OK, but I wouldnt recommend it as a general method - in case it is used on a weak coating.

This is why I didn’t soak my mirror.