At some point you will undoubtedly encounter problems with dew forming on the lens of your camera or objective of your telescope. When the temperature of any object drops below the dew point of the air, moisture will form on it. If it is cold enough you will get frost instead of dew. The temperature of the glass in your lens or scope can fall below the dew point of the air because it radiates heat to the open sky. When you look up at the clear night sky, you are looking into space. The temperature of outer space is only a few degrees above absolute zero. Heat will radiate from your telescope, camera and lenses toward this cold. Contrary to popular belief, dew does not "fall" from the sky. It condenses out of the air. I have seen dew form on the bottom of a table that I use next to the telescope at night. The first line of defense against dew is to use a dewshield. This is a tube that extends out from the telescope's objective lens. Many refractors come with a dewshield built in, so be sure to use it. Schmidt-Cassegrains are particularly susceptible to dew because the large, thin corrector plate is at the very top end of the scope's tube. Since it is thin and doesn't have much mass, it doesn't hold much heat to start with and will radiate it quickly. Most Schmidt-Cassegrains do not come with a dewshield, so you need to make one, or buy one if you use an SCT and don't want to go home early. You can make a very effective dewshield from a foam ground pad sold in a camping store. Cut it to the proper length and glue or tape the ends together. Its length should be about 1.5 times the width of the telescope's aperture. A dewshield will also act as a lens shade and keep any stray light from shining into your telescope. The dewshield shields the objective from most of the sky except for the exact area it is pointing at. A dewshield will slow down radiational cooling, but it won't completely stop it. If you stay out long enough on a clear night you may still encounter dew, even if you use a dewshield. At this point you will need some type of active system to prevent dew from forming. Active Anti-Dewers The best way to prevent dew is to use a heating element wrapped around the camera lens or telescope objective to apply a very gentle amount of heat. This will keep the glass just a couple of degrees warmer than the dew point and dew will not form on it. You can buy commercially made anti-dewers or you can make one yourself if you are knowledgeable about electricity and handy with a soldering iron. Sky and Telescope has an article describing how to make an anti-dewer out of simple resistors. It is best to simply turn your anti-dewer on at the start of the night instead of waiting for dew to form and then trying to remove it. Once it forms, it will have probably already ruined several of your images, and it will take much longer to remove. The faint stuff in images can be also affected well before you can actually see dew on the lens. Several manufacturers make anti-dewers. I use several different sizes - one for my main telescope and one for any camera lenses that I may be shooting with. Some people even use them on their finders and eyepieces if they are observing in locations with a lot of humidity.
You can also buy a controller for your anti-dewers. It will regulate the amount of heat that they put out. On nights when the dew is not that bad, you can turn down the heat and make your battery last longer. Turning down the heat will also lessen local seeing effects at the scope from the heat. Most anti-dewers come with a standard RCA plug connector and run off of a 12-volt power supply. These normally plug into a controller. The controller's power cord has a standard cigarette lighter plug on it. If you are going to use your anti-dewer without a controller, you will have to find a way to adapt the RCA plug to a connector to hook it up to your 12-volt battery. Moisture and Storage Moisture will condense on any object that is brought from the cold into a warm location. That is why it is important to think about this even when you are finished at the end of a night of observing or astrophotography. Moisture can be trapped in any kind of scope or lens if you cap it tightly after being outside at night when there is humidity. If you then warm up the scope, moisture will condense inside of it. Moisture may even be trapped in the tube of a refractor, or in the space between the refractor's objective and dust cap. I have had moisture condense on the inside of my refractor's objective lens. Farpoint makes a desiccant plugs that hold silica gel. Silica gel is a desiccant that absorbs moisture. It turns from a blue/purple color when dry to to clear/pink when saturated. Then you simply replace the silica gel pack. It plugs into the 2-inch focuser of your telescope and absorbs moisture inside the tube while the scope is in storage. I have also taped silica gel packets to the inside of the scopes lens cap, but you have to be sure you have enough space so that it will not touch the glass. Do not cap a cold scope or camera or lens and then bring it into a warm house. Moisture will condense on it, and possibly even inside the camera with its delicate electronics. You can put a camera and lens into a large plastic Zip-Loc bag with some silica gel and seal it before bringing it inside. If observing at a remote location, don't put cold equipment into a car and then run the heater full blast on the way home. Put it in the trunk or covered storage area in the back if you have a pickup truck. If you don't have any choice except putting it in the car where you will undoubtedly run the heater (I know I do), then put the scope into its case with some silica gel. For a camera or lens, put them into a zip-loc bag with some silica gel and then put them into a camera bag. Then put your winter coat on top of the scope case and camera bag and try to insulate it as much as possible from the heat in the car. When you get to advanced astrophotography you may end up using a computer to run your telescope and to acquire images with your camera. You can even use a beach umbrella over the table with your equipment to shield it from the sky and keep dew off of it. Again, not because dew "falls", but because you are slowing down the radiational cooling of your equipment by shielding it from the cold of the clear open night sky. Hair dryers can be used to warm a surface to prevent dew, or remove it after it has formed, but are not as effective, because you have to keep doing it every couple of minutes. It is also too much hot air at one time, which can hurt the local seeing. Dew and Camera Lenses Dew will form very quickly on a camera lens. It doesn't have much mass to hold heat and with wide-angle lenses, there isn't any way to effectively shield them from the sky. A dewshield long enough to work would vignette the image. For a camera lens, an anti-dewer is essential. You can get a 2-inch anti-dewer that will fit most camera lenses for about $20 from Dew-Not. It can be a little bit of a logistical problem to use an anti-dewer on a camera lens. If you wrap it around the barrel of the lens and the focus ring is under it, you run the risk of accidentally throwing it out of focus. You should tape the focus ring down after you have focused the lens on a star, and before you wrap the anti-dewer around it. The focus ring on some inexpensive lenses, such as the Canon 18-55mm f/3.5-5.6 IS zoom, and the Canon 50mm f/1.8 fixed focal length lens, is at the very front of the lens, and is incredibly sensitive. It is almost impossible to tape this focus ring down without moving it and throwing the focus off. Just touching this focus ring seems to shift the focus. For these lenses you can try putting the camera on back button focus (see the manual for how to do this), and then manually focusing with Live View and switching the lens to autofocus. When the lens is on autofocus, the manual focus ring is locked, more or less with these lenses. If you touch it, it will still move. Once you have autofocus activated with the back button, the camera and lens will not try to autofocus when you press the shutter button to start the picture. The bottom line with these types of lenses is that you just have to be extremely careful not to touch the focus ring after you have focused. The good thing is that you can put the anti-dewer strap on the lens before you focus it. Some lenses these days, particularly the most inexpensive ones, are made almost entirely out of plastic except for the glass lens elements. The problem with this is that the plastic insulates the glass. A zoom lens may have a plastic outer barrel which you would wrap the anti-dewer around, and then another plastic assembly inside the lens that holds the glass elements. This is a lot of plastic for the heat to go through to reach the glass. A solution is to use a metal filter and metal lens shade. Then wrap the anti-dewer around the lens shade, which will transfer heat to the filter's glass and keep the dew off. The problem with this method is that with the inexpensive lenses the filter mounts onto the focusing ring, and it will be very difficult to stop the focus ring from moving with the anti-dewer on it. In this case, you may have to wrap the anti-dewer around the lens barrel and run the anti-dewer at a high heat. You will need a 12-volt battery to power the anti-dewers. For more sophisticated operation, you can also get an anti-dewer controller. This device will let you adjust the amount of heat that is applied. For nights with high humidity, you might want to run them at a higher power than on a night of low humidity. Some also cycle on and off to save power.
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