Condensation

[Voxish]

I have an old dobson scope, a 10 inch Orion Optics number. Not sure which model it is but it's a white tube 6.9fl with a 2 inch crayford and suspect post Europa. I have encountered mirror fogging over the last couple of nights. I keep it in the shed so the the ambient temperature should not be an issue. I still leave it out for an hour or so , or until the clouds have gone or its really dark. When I took the end cap off last night the primary was covered in condensation. Should I uncap it as soon as I set it up instead. I recaped it the other night after use thinking that there would be no condensation as there was no great change in temperature. I has horrified the next day to discover the primary was completely fogged up. The tube appears to be almost hermetically sealed once the end cap is fitted, should the scope be allowed to breathe?

[Stu]

I would give it a go with the end cap off but the tube angled at 45 degrees. This will allow some air circulation but should prevent dew forming

[Voxish]

Ok thanks I will give it a try, is this common?

[Stu]

To be honest, my primary rarely, if ever dews up provided it is allowed to acclimatise properly.

Is your shed dry? If it is slightly damp then I suspect the air inside the tube will be damp too, so leaving it outside with the cap on will only help the condensation to form. Perhaps consider a dehumidifier or at least done dessicant traps in the shed?

[Voxish]

Nah, the shed is as dry as a bone. I have bags of filler opened for months and they never go off. I will just un cap it as soon I take it outside.