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Howdy, I recently bought a used Skywatcher Skymax 150 Mak from a fellow SGL member. I had the scope out last night and discovered, once I put the camera in, that there are a lot of marks / dust specs on the diagonal. These are very apparent one the lappy screen, not so while using it for visual.

What is the best way to clean a diagonal? Should it be treated like any other mirror in the OTA?



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Hi. Gary,

The diagonal flat mirror can certainly be treated the same as a main mirror when it comes to the cleaning of.

If you can remove the flat from the holder, so much the better. You can then Immerse it in a washing up bowl in tepid water.

Do not put washing up liquid in the water, just allow the mirror to soak a while to soften any gunge. You can even run water over it from a shower head, but not hot. Luke warm. A final rinse with de ionized water, then prop it on it's edge and allow it to dry on it's own. If any water globules persist on the surface, just use the corner of a soft tissue to soak it of without touching the glass.

Do not be tempted to rub the surface with cotton wool or any other material, the chances are you would leave minute scratches on the aluminium coating and thats the last thing you want. You can repeat the procedure until you are satisfied that it is clean enough.

Good Luck, and remember. No touchy. :D

Ron. :D

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I am afraid I do not know enough about this product, except to say it is apparently produced by ordinary water being passed through a membrane. There are seemingly different compositions of membrane, to achieve a certain result, but there ends my knowledge on the subject.

I would use distilled water myself, but only because I know it works, in as much as it contains nothing that would harm the coating on my mirror. Perhaps someone with more information on the other stuff will impart some details on exactly what it is, and if it is safe to final rinse a mirror with.


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Hi Gary

I'm not sure that RO water is the same as deionised (the salts removed eg Calcium, Magnesium carbonates etc).

However you can buy it easily and cheaply at Halfords etc - it is used to top up lead acid car batteries. Or, you can get it for free from your domestic fridge freezer - just melt the ice that builds up in the freezer section but make sure its clean!


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Well, had a go at cleaning my Diagonal tonight. Took the mirror from the housing and submerged it in luke warm water for a while. I then took the advice of Ron and rinsed the mirror under a warm running tap and then a final rinse with Halfords Battery Topup Water. It looks great now, much better than before.......... :D

I must admit to being a bit apprehensive about this as you read so much about do or do not clean mirrors. I eventually decided that I wouldn't use it like it was so, as it was only a diagonal and not a primary, it would be relatively inexpensive to replace if I made a cods of it.

Thanks for your advice guys and I might even be prepared to tacle a Primary Mirror if I ever have the need...... :hello1:


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