Dew on the mirror?

[EntropyStar]

Hi,

So I took my Dob out tonight for a first light, all was great until I got in and looked at the mirror, it looks really dirty from condensation and was immaculate before. Must've been out for around 3 hours tonight......are you supposed to leave it or clean it? Been reading around and it seems cleaning it is something you should avoid at all costs.......bit confused...

[Madhatter]

Did you look at the mirror by shining a torch down the tube?? this will make it look really dirty! if its just visualy and it looks dirty i am not too sure, but i would imagine its best to let it dry by itself and not attempt to clean it yet.

i have had my mirror quite dewed up, and just let it dry by itself and it looks immaculate until you shine a torch on it.

Hopefully someone with more exsperiance will give you a better responce.

[Klesk14]

Do you let the dew dry on its own inside or outside by pointing the tube down. In this case a SCT.

[umadog]

Mirrors will begin to look dirty quite quickly. The views aren't affected by the dirt unless it's really bad. Cleaning can simply take the form of hosing the mirror down with distilled water and then leaving it to air dry. This sort of mild cleaning shouldn't affect the coatings (which are fragile) and could potentially be safely done every two or three months. The catch, of course, is that each time you remove the mirror is a new opportunity for dropping it. Some guides recommend gently dragging cotton wool over the damp mirror for cleaning. IMHO, this isn't necessary and best avoided. Semi-regular cleaning with DI water (and maybe ethanol) is non-contact and avoids the need for anything more drastic.

You can dry off dew-sodden mirrors with a hair-dryer. Don't worry about the mirror cracking, the rate of temperature change elicited by the hair dryer is nowhere near the level required to damage the mirror.

Finally, mirrors look dirtier than they are. Particularly if you shine a torch down the tube and peer at it face-on.