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Now I know little bit more about telescopes, I thought I'd take the mirror cell out of my old Konus 114mm reflector.  The primary is filthy and could do with a proper clean when I can get hold of some deionized water. I was amazed just how much force was required to remove/replace it though - had to actually tap it with a hammer to force it off.  The end of the steel tube is bent inwards to accommodate a mirror cell that is clearly slightly too small for the diameter of the tube.

 

Surely this is not normal?!

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On 21/02/2021 at 16:02, Orange Smartie said:

Now I know little bit more about telescopes, I thought I'd take the mirror cell out of my old Konus 114mm reflector.  The primary is filthy and could do with a proper clean when I can get hold of some deionized water. I was amazed just how much force was required to remove/replace it though - had to actually tap it with a hammer to force it off.  The end of the steel tube is bent inwards to accommodate a mirror cell that is clearly slightly too small for the diameter of the tube.

 

Surely this is not normal?!

How filthy is your mirror? the rule of thumb is to not clean it when the dirt is less than 10% of the mirror's surface.
Cleaning a mirror is a tricky job and has to be done with great care.
Use distilled or even double distilled water, do not use de-ionized water. De-ionized water is water that lacks ions coming from sodium, calcium etc.  It still may contain other organic stuff.  Distilled water is purer. Double distilled water is super pure and also used for medical purposes. Last flush can be done with Acetone. It dissolves the remaining waterdrops and evaporates very fast and without residue.

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For optical mirror rinsing another suitable option is reverse osmosis water used for tropical fish. Take a clean container to a reliable aquarium shop and ask...

 

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4 minutes ago, Waldemar said:

And please be more than careful with a hammer close to optics...

Ha ha, sure!  It's a really old telescope, recently superseded by a 6" dob, so I wouldn't be too upset if I couldn't get it clean.  It's filthy, after about 25 years of accumulated muck so it really does need a clean.  And I'm not kidding, I really did have to ease it out by tapping on the mirror cell with a hammer and screwdriver...... 

Thanks for the tip re distilled vs deionised water.  I'll make sure I get distilled.  

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13 minutes ago, Waldemar said:

Use distilled or even double distilled water, do not use de-ionized water. De-ionized water is water that lacks ions coming from sodium, calcium etc.  It still may contain other organic stuff. 

+1

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9 minutes ago, NGC 1502 said:


For optical mirror rinsing another suitable option is reverse osmosis water used for tropical fish. Take a clean container to a reliable aquarium shop and ask...

 

Given the choice then, RO or distilled?  I fancy RO might be cheaper to get hold of.

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Can you not get distilled water in 5 litre containers from the supermarket (for Ironing purposes) cheap as chips. Well over here it is, As normal tap water here high limestone content (hard water is it in England) Being from the North (UK)our water was soft and never had problems with kettles irons etc etc.

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8 minutes ago, Waldemar said:

The thing with RO water is that the pH is quite low and thus is acidic. Acidity can damage the metal coatings of your mirror
Distilled water is pH neutral so it can not damage the coatings on your mirror. 
This: What Is The Difference Between Reverse Osmosis Water And Distilled Water? | Drinking Water Base will tell the story.

Interesting! I did some research into this to understand why having read your comment, and it seems it is pH neutral at the time of filtering, but fairly quickly (over the course of an hour or so) goes to around oH 5.5 due to absorption of CO2.

https://www.123filter.com/ac/why-reverse-osmosis-water-is-acidic
 

Thing is, exactly the same happens to distilled water too, down to 5.8 I think, so it’s something to bear in mind with both products.

https://sciencing.com/ph-distilled-water-4623914.html
 

Looks like the best option with either is to use fresh product unopened until used and then discard and re-buy. Does that sound sensible? RO water is cheap at least.

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11 hours ago, fozzybear said:

Can you not get distilled water in 5 litre containers from the supermarket (for Ironing purposes) cheap as chips. Well over here it is, As normal tap water here high limestone content (hard water is it in England) Being from the North (UK)our water was soft and never had problems with kettles irons etc etc.

It's very hard water here, but I can honestly say I've never noticed distilled water in shops. Perhaps I haven't been looking hard enough.  

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I would not get too hung up on the pH of water that has absorbed CO2. Even if you buy new each time, by the time it has dried it will reduce in pH on the mirror. Personally, I would clean in normal tap water then rinse in distilled at the end.

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