TheSkySearchers

My 72mn is kept in outdoor ambient storage.
The last time I had it out it was quite cold and got colder as the evening progressed...eventually just at frosting temp for the first time this season. I noticed my Objective fogged. When I looked with a flash light I discovered it had fogged on the inside surface of the Obj Cell.
I took it inside and removed the diagonal so the inside was open and after about 20 minutes the fog had cleared.
It looks fine now.
I have never had any of my larger fracs do that.
Anyone else have this experience?

Fog was not between the elements but on the back side of the cell.

Thank you

I guess I wouldn't be super surprised, depending on weather conditions, but I've not seen it. Maybe b/c I keep a diagonal in my scopes most of the time so a lot of external air doesn't get in the tube?

pakarinen wrote: ↑Wed Nov 10, 2021 3:26 pm I guess I wouldn't be super surprised, depending on weather conditions, but I've not seen it. Maybe b/c I keep a diagonal in my scopes most of the time so a lot of external air doesn't get in the tube?

I have stored it it with the plug cover in the focuser..

It must be that there was enough humidity in warmer temps from the use before that when I closed it up it trapoed that bit in there. When it got cold enough that little bit condensed on that cell. I will put a warmer on it the next time.

I had the same issue couple of times with my 150ST while observing at the desert. As you, I have removed diagonal and let it dry until fog disappeared. The scope is as good as new.

Hey Mark,

It should be fine. Yup, remove the diagonal; I've never had a problem that I could see with my scopes. I remember on Jan 21, 2019 I was out observing the lunar eclipse with my 80mm f/11. The temperature was -40, that's where Celsius and Fahrenheit are the same. The frost on both sides of the objective... I'm surprised it didn't crack

Almost three years later, much of the same since, never a problem

Hope you have a clear night!

All the best buddy,

Removing the plug to insert an eyepiece into the diagonal is all it takes for warmer air to get inside.

Thank you all for your comments here.

The scope seems quite fine now. Just one of those things I had not experienced before.

Thank you again and clear skies to you all.

A sachet or two of desiccant might help, left snug inside the draw tube after the eyepiece/camera removed / in or out a suitable container?

Altocumulus wrote: ↑Sat Nov 13, 2021 10:49 am A sachet or two of desiccant might help, left snug inside the draw tube after the eyepiece/camera removed / in or out a suitable container?

Thank you Altocumulus.
Yes.
This is a good plan.
Thank you.

John Donne wrote: ↑Wed Nov 10, 2021 2:28 pm...eventually just at frosting temp for the first time this season. I noticed my Objective fogged. ... I took it inside and removed the diagonal so the inside was open and after about 20 minutes the fog had cleared.

It is usually warm here, certainly by midwest USA standards, but I do watch the dew-point in the weather forecasts and even so, my backyard seems to get condensation sooner than predicted. So, I have taken to drying everything out when I bring them in. I open the telescope by removing the diagonal. I set the oculars aside, longwise. I also have lots of those little gel packs you get in pharmacy bottles and put them everywhere, like inside the box for my Nagler, I have two.