I got caught out a few months ago when I spied something oily looking on the primary mirror of my 10" dob..
I tried to see if it was just dust...wouldn't blow off..indavertanly sneezed!!
...had to wash the mirror..oily stuff still there..washed and rinsed about 6 times.
.got fed up and reinstalled the primary...
..thinking the worst I called Bintel...the "oily stuff" is just oxidization between the aluminium and overcoating...and it hasn't affected the image quality!!
...how did I find the "oily stuff"?..Yup..shined a torch on the mirror!!..So if I think it's dirty..........
Many thanks again Roger
