Ring around primary mirror

[bosun21]

I noticed a ring that had formed around the outside edge of my primary mirror on my 8” dobsonian. I have no idea what caused it. It was like condensation had formed but it wasn’t that at all. I tried a hairdryer on it but it didn’t do anything. It was really annoying me so I removed the mirror and washed it in the usual manner. Tepid water in the sink with a few drops of fairy liquid and pure cotton wool balls. Rinsed thoroughly with distilled water. I then dried it with the hairdryer, reassembled and recollimated. The ring has now gone but I would like to know what caused it in the first place, as I don’t want to be repeating this every time. Thanks  

   Ian 

[Astronomist]

It could be dirt deposited in condensation, but that wouldn't explain the ring shape. What colour was the ring??  

kind regards 

Harry

[Guest]

Do you run a fan to cool down your scope or while observing? It is possible that air that is being drawn over the scope by the fan is bringing some stuff with it and depositing it on the mirror edge 

Edited November 15, 2022 by Mike Q

[bosun21]

12 minutes ago, Astronomist said:

It could be dirt deposited in condensation, but that wouldn't explain the ring shape. What colour was the ring??  

kind regards 

Harry

The ring was like steam on a mirror only denser. I have no clue 🤔

[bosun21]

5 minutes ago, Mike Q said:

Do you run a fan to cool down your scope or while observing? It is possible that air that is being drawn over the scope by the fan is bringing some stuff with it and depositing it on the mirror edge 

Good thinking, I only run the fan for the hour or so whilst the scope is cooling. On reflection could it have been pollen pulled in with the fan from a session earlier in the year, and that was me just noticing it?

[Astronomist]

pollen does sound like a likely explanation, you may not have noticed it until it caught the light at an angle.

 

[bosun21]

Just now, Astronomist said:

pollen does sound like a likely explanation, you may not have noticed it until it caught the light at an angle.

 

I am leaning toward that cause as well, and the fan would cause it to be deposited around the edges. When I looked straight on it wasn’t noticeable, but at an angle it appeared clearly.

[Astronomist]

yeah that definitely sounds like pollen to me. 

[Guest]

Good grief ... You mean i was actually right for once?  I am writing this down