How serious is a thumbprint on a mirror ?

[Skipper Billy]

The primary mirror in  my Ritchey Chretien was loose. In the process of fixing it I have accidentally got a fingerprint on the mirror.

Given all the advice about not cleaning mirrors - how serious is this? Will it degrade images? Will it corrode the mirror surface?

Should I clean it? 

[Peter Drew]

I would clean it, acid from your skin might well etch the coating or at least give a base for fungus to grow. I would apply a weak solution of distilled water and  washing up liquid on a cottonwool ball with minimum pressure. Make sure that the area is on bottom dead centre to avoid liquid accessing the rest of the mirror. Once clean do the same with fresh distilled water. Once dry, if necessary, polish the cleaned area with a new dry ball. 

[Skipper Billy]

Thanks Peter - that's what I feared!