by David J. Watkins
Astrophotography Aquiring Images - Dew
One major problem that you will eventually encounter is dew formation. It will quickly put an end to your imaging session. As the air cools at night, all exposed surfaces pointing up toward the sky cool off too. Those surfaces exposed to the sky actually cool to a temperature slightly lower than the ambient temperature of the air around them. When that surface reaches the dew point, dew forms. The dew point is the temperature at which there is 100% condensation or humidity, and dew forms. As dew forms on your lens, corrector plate, or mirror, it will fog up the surface so you can no longer see clearly through it, and neither can your camera! Dew will affect all of your equipment, in addition to your telescope, it will cover your finderscope, your eyepieces, and even your telrad viewer.
I never even thought about dew until my first night out with my Celestron EdgeHD. Dew formed on the corrector plate within the first half hour after sunset and I could see nothing out of my scope! Lucky for me somebody had a 12VDC hair dryer and a large battery for it. They warmed the corrector plate enough to get rid of the dew. I quickly made a temporary dew shield out of cardboard and duct tape. I was able to view, but not image, for a couple of hours with the temporary dew shield. There was still enough fogging of the corrector plate to not allow any imaging. Eventually the cardboard got wet from dew forming on it and it collapsed and that ended my viewing session.
You will at least need a dew shield for your catadioptric telescope or lens. For reflector telescopes, the tube already acts as a dew shield. For large open Dobsonian telescopes you will want a shroud, though you probably won't be imaging with one of these types of scopes anyway. If you are using a lens with a lens hood, the hood will keep the dew off for a little while. Eventually the lens will cool to the dewpoint. And with only a dew shield, eventually your corrector plate will cool off to the dewpoint. A dew shield only slows down the heat loss of the surface below, it does not prevent heat loss. Reflector scopes with long tubes may get away without dewing up because heat loss slows down with the length of the shield or tube. If you use a telrad viewer you will want the dew shield for that as well.
One way to prevent your lens, mirror, or corrector plate from cooling off to the dewpoint is to use a dew heater. There are dew heaters made to fit just about every telescope. A dew heater is typically a long band with a heating element inside and velcro to secure the ends. They typically run off the same 12 volt supply that your telescope mount powers from. For lenses and catadioptric scopes, you place them around the lens or corrector plate, on top of the dew shield. For reflector type scopes they do sell heaters for the mirror. For catadioptric scopes, you can buy a dew shield with a heater built in. You should consider a dew heater for your eyepiece and for your finderscope as well. Another thing a heater works well for is if you have a mounted laser pointer and you are out in the cold, a heater will keep your laser pointer working.
Dew heater controller:
Connecting the dew heater directly to 12 volts will cause the heater to output it's maximum amount of heat and draw a lot of current from your power supply. The full amount of heat is not really necessary to warm the surface above the dewpoint. A better solution is to use a dew heater controller. A controller typically has four RCA style recepticles that will accept four heaters. Most heaters have an RCA style plug on the end of the cable for power. Controllers typically have two channes that can be adjusted independantly for low to high output. Each channel can typically control two heaters.
Where to find sheilds and heaters:
So where do you find dew shields, heaters, heaters built into shields, and controllers? I purchased a dew shield with heater built in, a controller along with some other heaters from AstroZap (www.astrozap.com) They sell shields, heaters, shield/heater combos designed for just about every telescope on the market. They also sell heaters for eyepieces and finder scopes too. Just about all the items are made in north-eastern Ohio, only their scopes and mounts are made elsewhere. Shipping is fast too. The only heater AstroZap does not carry is for the laser pointer. You can get one of those through ScopeStuff (www.scopestuff.com) ScopeStuff carries AstroZap heaters too.