Trying to shoot the Milky Way on a very damp night, I got a horizontal line going through my images

[photofun25]

I was shooting the Milky Way on a dark and very damp night.  I used my Canon 5D Mark111 with the 24 to 105 lens. I wiped off my lens many times through out the night.  I could see the dampness on my body of lens and on lens glass.  I shot with same camera and lens that night before it got damp and didn't get the line in it.  I also shot with same camera and lens the next day and didn't get line in images. So the only thing I can think of was the dampness of the night. But no one else had this problem that night that I shot with. I also covered the eye hole so there was not light leak. A week earlier I also shoot the Big Dipper and the Comet and those photos turned out good.  It was not very damp that night.  I would like to know what you think caused this line in my images.  I want to go soon to try to get better shots of the Milky Way.  Why am I getting this line? How do I avoid getting this line!? I appreciate any help you may have. Thank you. 

[jrhoffman75]

Seems like you did a good assessment. It happened on a very damp night. Use a lens warmer of some sort. Some folks just rubber band a hand warmer to the lens. There are also USB powered heater bands.

[photofun25]

To jrhoffman75 - I have not thought of doing a lens warmer. Thanks for information.  photofun75

[TCampbell]

I'm not sure what the green band is ... but it's not dew.  Dew just fogs over the lens and you start loosing contrast until it gets so bad that you can't resolve anythng.

If you are at your home (or some locaction where you have AC power) then grab your hair-dryer ... and every few minutes... warm up the lens (specifically the front most element).  

If you are away from home, then you can get a dew heater.  These would be sold by dealers that sell telescope products ... e.g. HighPoint Scientific or Oceanside Photo & Telescope, etc.    

The system needs two components...

• A Dew Controller - this is the box that provides power to the dew heater straps.  It uses 12v power (which is what most astronomy products use because that's what astronomers use out in the field when they have no AC power available ... and note this is really "12v system" power ... e.g. 13.8v ... but it's pretty tolerant since most batteries decrease voltage as they drain.)  It also has knobs to adjust the power level (more on why in a moment).
• A Dew Heater - this is the "strap" ... it wraps around the lens (or telescope) and sticks to itself with hook & loop (Velcro).  You position this strap on the lens (or telescope) so that it's just behind the front-most lens element ... the goal is to keep that front-most lens element warm enough so that dew wont condense on it.  These come in sizes ... based on the diameter of the lens.  e.g. a 4" strap means it has a circumference long enough to wrap around a 4" lens barrel (that's outside diameter)   The end of the strap will have some elastic so the measurement doesn't have to be perfect -- it'll stretch within reason.

The straps plug into the controller using the RCA "tulip" style jack (much like you'd see on a phonograph).  They are 12v power and pretty basic circuits.  This means it generally doesn't matter which brand controller and which brand straps you use.  For example, I use a DewBuster brand controller ... with Dew-Not brand straps.

The goal is to warm the lens just enough... but not more.  You just want to get the lens a few degrees warmer than the dew-point so that dew wont condense on the lens (and really just the front-most element matters).

If you heat it too much then you can get heat-currents rising off the lens.  If you just above a candle flame at something in the distance, you can see the heat from the flame distorts the optics.  That's the sort of thing you don't want the dew-strap to do.  It wont be able to get hot enough to hurt the lens, but it can get the lens warm enough that heat currents affect the optical quality of your images.

My newest telescope has an integrated dew heater system.  My old telescopes do not ... so I use one of these external controllers and straps.  Most controllers have multiple "channels" so that you can heat the main telescope, heat the finder scope, heat the eyepieces, etc. with different dew heater straps all plugged into the same multi-channel controller.  For photography through a camera lens... just one channel would be enough.

The controller is simply pulsing the 12v power on/off to the strap.  E.g. if you set the dail at 25% ... that means it's on for 1/4 of a second and off for 3/4 of a second ... and it just pulses the power on/off following that pattern.