Lots of dew

[george7378]

Tonight's observing session was very good with some nice seeing, but the scope gradually dewed up on the primary and secondary so that I could see lines and streaks of mist on the primary when I looked at it even without a bright light, and the secondary was completely covered. The whole setup was very wet when I got it inside, and although I know that the dew won't have caused any damage, I am a little concerned about just capping the scope up and putting it away - what should I do if the primary has streaks/a film of condensation on it?

Thanks.

[jonathan]

From what I read on here the general advice is to just let it dry out naturally, tip the tube down so that any condensation can just roll out and leave the caps off.

Just about to head out myself, got the telescope cooling down now with the dew heater/shield on (if that makes sense!) Hopefully these clear skies will last until 11pm at least.

[perrin6]

I just use a hairdrier to dry it off (during the observing session & after) as I have read a wet mirror will degrade the coating, especially if you live in an industrial area as the dew might be acidic. Once the mirrors have warmed up to the room temperature no more dew will appear. Whatever you do don't wipe it off as you may scratch the mirror.

[jonathan]

I just use a hairdrier to dry it off (during the observing session & after) as I have read a wet mirror will degrade the coating, especially if you live in an industrial area as the dew might be acidic. Once the mirrors have warmed up to the room temperature no more dew will appear. Whatever you do don't wipe it off as you may scratch the mirror.

If using a hair dryer I have also read that it should not be pointed directly at the telescope, just wave it around to heat the air around the telescope otherwise too much sudden heat might damage the mirror or other delicate parts.

[jonathan]

DAMN! Spent all that time getting out there, setting up the telescope, dressed up for a Russian winter, only to find the eyepiece completely fogged up with dew. The telescope was fine, probably nice and comfy in it's dew shield and dew heater, but my only eyepiece (and the red dot finder) was just a no-hoper.

What a lot of dew out there tonight though - I could even see it hanging in the air while my security light was on.

Still, got some rather nice views of the moon through binos as wispy clouds passed by, at one point it looked like it was sitting on a bed of fluffy pillows, so not a complete disaster.

Must confess that in my frustration I *may* have wiped the eyepiece outer glass with a tissue, and the red dot finder... hope this won't do any damage to the glass! I did notice while I was inspecting the glass with a torch that the main telescope glass (SCT) is covered in what looks like water droplet stains, will this affect the view at all? Is there anything I could do to remove them? I didn't touch this glass (never have) but have noticed it fog up when I brought the telescope in from the cold.

Now, where can I buy an eyepiece dew heater tape...

[perrin6]

If using a hair dryer I have also read that it should not be pointed directly at the telescope, just wave it around to heat the air around the telescope otherwise too much sudden heat might damage the mirror or other delicate parts.

Yes - it seems to be in my character to do silly things and learn from my mistakes after the damage is done ! I do however wave it around rather than hold it close to the surface, but still - I don't really know how careful one should be - any sudden change in temperature at the mirror surface is probably best avoided.

[perrin6]

DAMN! Spent all that time getting out there, setting up the telescope, dressed up for a Russian winter, only to find the eyepiece completely fogged up with dew. The telescope was fine, probably nice and comfy in it's dew shield and dew heater, but my only eyepiece (and the red dot finder) was just a no-hoper.

What a lot of dew out there tonight though - I could even see it hanging in the air while my security light was on.

Still, got some rather nice views of the moon as wispy clouds passed by, at one point it looked like it was sitting on a bed of fluffy pillows, so not a complete disaster.

Must confess that in my frustration I *may* have wiped the eyepiece outer glass with a tissue, and the red dot finder... hope this won't do any damage to the glass! I did notice while I was inspecting the glass with a torch that the mai n telescope glass (SCT) is covered in what looks like water droplet stains, will this affect the view at all? Is there anything I could do to remove them? I didn't touch this glass (never have) but have noticed it fog up when I brought the telescope in from the cold.

Now, where can I buy an eyepiece dew heater tape...

I don't tend to leave my eyepieces in the scope - bring them in each time I go to look on the computer for my next object, I keep them in the case and one in my hand so they get cold more slowly then, plus I usually turn away from the scope to breathe so I minimize likelyhood of dewing up the secondary mirror and eyepiece - astronomy can be so complicated ! I am keeping a hair dryer handy and give them a blast when one dews up (from a safe distance so as not to heat them too suddenly).