Primary condensation

[Mr Fiery Jack]

When I brought my scope in form the shed yesterday (for a little TLC) I noticed the primary had a milky quality about it. It quickly turned into full on condensation, which I put down to the usual change in temperature issues we all get. However when I had another look at the primary this morning after a night in the shed and it is covered in condensation. The shed is wooden and not a place I considered to be damp, plaster or cement never goes off and nothing ever goes moldy. The OTA cap is the usual old rubbish that OO seem to think is suitable to cap of an expensive telescope. I have ordered a AstroZap number which is allegedly breathable.

The question is have I damaged the primary? There hasn't exactly been many opportunities to observe this year has there? So I have to assume that the primary has been covered in condensation for weeks on end. Its in the shed as the only place I can keep it otherwise is an up stairs back bedroom. Our little cottage was built in 1690 so getting what to me is a big unit up and down stairs is a nightmare. I used to have a 4.8 OO Europa dob in the past which wasn't such a problem, this one certainly is. do we think even with a breathable OTA cap I would still have an issue?

Refractors it has to be said were much less hassle, I got to use them more too!

                        

[mdstuart]

My old 14 inch was fine in the shed until I left the cap off for a week and then it developed marks...I think mirrors cool slower than their environment and so dew can form on them even when the cap is on. The dew is pure water so not as bad as rain which can be slightly acidic but water and mirrors is not a great mix over the longer term. I have heard it said that if you can get more ventilation going in a shed it can help....and use a bag of silica gel in the tube to soak up damp...also it is possible to get mirrors coated again so it's not completely the end if the mirror degrades...

Also many mirrors are overcoated which makes them more resilient than one might think.

Not sure if that helps!

Mark

[spaceboy]

The dewpoint temperature is the temperature at which the air can no longer "hold" all of the water vapour which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

In the case of storing telescopes out doors to reduce cool down times you often find that the mirror will actually cool below the dew point and condensation will form. This often happens at some point during a long observation session even if you store your scope indoors as secondary mirrors or objective lens will eventually cool below the dew point and condensation forms requiring dew heaters or some other method of warmth to continue observing.

It's sort of a catch 22 as you want to avoid long waits for scopes to cool but then when you choose to store scopes outside you run the risk of the scope being too cool.

[Mr Fiery Jack]

1 hour ago, spaceboy said:

The dewpoint temperature is the temperature at which the air can no longer "hold" all of the water vapour which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

In the case of storing telescopes out doors to reduce cool down times you often find that the mirror will actually cool below the dew point and condensation will form. This often happens at some point during a long observation session even if you store your scope indoors as secondary mirrors or objective lens will eventually cool below the dew point and condensation forms requiring dew heaters or some other method of warmth to continue observing.

It's sort of a catch 22 as you want to avoid long waits for scopes to cool but then when you choose to store scopes outside you run the risk of the scope being too cool.

Thanks for that. The mirror looks fine so no damage done. I think I will suffer the cool down time and the perilous journey downstairs, Its not worth the stress leaving it in the shed.