Condensation!

[DevonSkies]

I managed to get an unexpected hour or so of observing in last night during breaks in the clouds. When I brought the scope (SW 250PX Dob) back into the house, both mirrors rapidly fogged up and took a few hours to completely clear. During this time I left the scope angled downwards with the caps off and a room heater on. Now the mirrors are dry, I notice there is a little 'dew spotting' on the primary and a little haze on one edge of the secondary.

I'm wondering whether I should have done anything differently? Or is this just a fact of life? As you can probably tell, I'm new to telescope ownership. Should I leave the caps off or on when I bring the scope in from the cold? I don't want to end up having to clean haze or spots off my mirrors, so is there anything I can do to help prevent dew deposits when bringing a cold scope back into the house?

Thanks!

Ed

[Tinker1947]

Leave it in a shed if you have one.....

[ChrisLX200]

If you bring a cold scope into a warm house it will mist over, and if there is any dust or other fine particulates in the air (and there will be) then some of that will stick to the misted surface and remain after the water evaporates. So it's better to keep the scope covered until it reaches room temp and the condensation is gone, then uncover it to allow air to circulate and remove the moist air from the tube. It's no guarantee you still won't get dusty spots deposited but they should be less severe. It's a fact of life and eventually you will need to wash the mirror off, but it needs an awful lot of dust to adversely affect performance so don't get anxious looking at the mirrors all the time! Washing the mirrors every couple of years or so is a normal operation if your scope gets regular use. If you can store your scope in a cood _dry_ environment (low humidity) then that would be better, but needs must and if the house is all you have then live with it, it won't do any harm.

ChrisH

[Ruud]

You could put the caps on before you bring the scope in and remove them for a while after the scope has reached ambient temperature. That  would prevent the dewing over.

[brantuk]

I would just let it dry naturally in the garage or shed (if you have one). Don't put it too near heaters cos rapid changes of  temperature can crack the mirror. Pointing down is good so any condensation drips on the floor rather than mirrors.

If there are any blobs of dew on the mirror just wick them off with a tip of soft, rolled tissue paper (e.g. loo roll or kitchen roll). Odd small marks left by condensation spots won't spoil the mirror's performance.

[DevonSkies]

Thanks for the replies. I do have a garage (in fact that's where the scope is normally stored), so next time I will bring the scope straight into the garage with the caps still on until the temperature has equalised.