If the humidity level at your observing location is high enough and the air temperature is close to the dew point temperature, the glass correcting lens on the front of your SCT can collect dew and become fogged,
even though the rest of the SCT does not become damp with dew. For a technical explanation of why this occurs, go to the Why Do Corrector Lenses Collect Dew? section lower on this page.
There are two methods to prevent this dew formation.
If the humidity level is not too high you can use a Dew Shield which is a passive but relatively inexpensive accessory. But if the humidity level is quite high (e.g. in the Deep South in the U.S.) you may need to use a Dew Heater which is an active but more expensive dew prevention accessory. I canít tell you which one you will need, both for the obvious reason that it depends on your location and also because I live in the arid southwest in the U.S. and fortunately have never needed one (although I do use dew shields to shield a scope from stray light and to protect the corrector lens when the Fastar lens is in place, as I note on the Astrophotography Accessories page). The best way for you to determine what dew prevention you need is to talk to other amateur astronomers in your area by contacting your local astronomy club.
A Dew Shield is a hollow plastic, plastic/foam, or aluminum cylinder that
slides onto the front of your scope (the Orion version is the black cylinder on the front of the scope in the photo on the right). It works by reducing
the area of sky that the corrector lens sees, which reduces the correctorís rate of radiative heat loss (see the explanation below). It also has the
advantage of blocking stray light (e.g. from your neighborís porch light) and adding weight to the front of the scope which helps balance heavy items
mounted on the scopeís rear cell. Simple foam dew shields are offered by Orion (about $17 for an 8Ē SCT) as well as Celestron and Meade; Astrozap and Just-Chaney make ones with insulation and felt linings, that are claimed to work better than simple foam shields. These
range in price (for an 8Ē SCT) from about $35 for a flexible foam shield, up to $125 for a rigid aluminum shield.
If the humidity level in your area is very high you will likely need a dew
heater. This is a strip that wraps around the front of your OTA and contains a heating element (the orange strip in the photo on the right). It
warms up slightly and is intended to keep the corrector lens just above the dew point temperature. The advantage to dew heaters over dew shields is
that there is no dew condition they cannot prevent; the disadvantage is that unless they are carefully controlled to only heat enough to prevent dew,
they will create convection currents inside the OTA and degrade the optical image. The other disadvantage is that they require a lot of power, which is
fine if you are observing next to a building or vehicle with power available, but a pain if you are observing at a remote site - you will need to bring a 12-volt battery back which,
being a lead-acid battery, is heavy. (They draw too much current for flashlight batteries, which they would quickly drain.)
Orion makes a dew heater that plugs into a vehicle cigarette-lighter receptacle for about $45 (for an 8Ē SCT) but it doesnít include a controller to regulate the heat output, which is important as discussed
above. A widely-used dew heater system is the one made by Kendrick Astro Instruments. It has a
variable controller to keep the heater at a low level for observing, or at a high level for a while to remove dew that has already formed. The controller can control four heaters, and Kendrick offers a variety of
heating elements that can heat anything but your dinner: the corrector lens, eyepieces, a Telrad or Rigel finder, and yes, your posterior. (Under high enough humidity conditions an eyepiece or finder can
collect dew; hard to say about your derriere.) Of course, this can add up to well over $100 and if you do
run a lot of heaters you will need a fairly large battery pack as well. But if your local conditions demand it, a system like this may be the only thing that allows you to use your scope at all.
Although Iíve never personally needed a dew heater, the experienced observers who
do need them note that there is an important consideration for a dew heater controller. A controller with a closed-loop-regulated power supply, such as Ron Keatingís DewBuster Controller, shown here on the right (or Don Clementís do-it
-yourself circuit mentioned below), has a better ability to keep the corrector as cool as possible yet above the dew point and use less power, than a pulse-modulated
controller like the Kendrick controller (power use is particularly important if youíre observing at a remote site and depending on a battery power supply, since heaters
drain a battery faster than any other scope accessory). However, Ron Keating only sells the dew heater controller - you need to procure the heater strips elsewhere.
So your best option may be to get the Kendrick heater strips and the Dewbuster controller.
If you are handy with soldering electrical components you can try making your own dew heater system - it isnít all that complicated. Jeff Polston has directions for making one on his web site, as do Mark Kaye and Don Clement, and you can find others by searching the Web.
Observers who use dew heaters on SCTs note that it is best to position the heater strip on the scopeís tube behind the thick metal corrector ring (as shown in the photo above). If you put it directly on the
corrector ring the heater will draw much more power for the same result, and may not have enough power to work at all if placed on the corrector ring of large-aperture SCTs.
Why Do Corrector Lenses Collect Dew?
As you may already know, heat can be transferred from one location to another through three
processes: conduction, convection, and radiation. People are usually the least aware of radiation heat transfer because our eyes cannot detect it - it occurs at infrared wavelengths to which our eyes are not
sensitive (although night vision glasses do convert these wavelengths to visible light and allow us to see objects that are radiating in the infrared, like people or animals). We humans do lose heat by all three
of the heat transfer processes and the radiation heat transfer component can be significant. For example, most people notice that at night when itís very cold outside, even though the air temperature in
a room is comfortable they feel cold when standing next to a large glass window or sliding patio door especially if the window or door is not double-pane or low-e glass. Whatís happening here is that you
are radiating heat to the glass window and it is radiating heat back to you, but itís radiating much less heat back to you than you are to it, so basically it is sucking heat out of you. (This also happens in the
case of exterior walls in old houses that donít have insulation in those exterior walls - you feel cold even though the room air temperature is set at a comfortable level.)
This process also occurs outside - at night the ground radiates heat to the sky and the sky radiates heat back but the sky itself doesnít radiate much heat back so the ground cools down at night. If the
ground temperature drops below the dew point temperature the ground will condense water vapor out of the air and dew (or frost, in the winter) will form. Note that dew tends to form on horizontal surfaces but
not much on vertical surfaces. Vertical surfaces are not looking at the cold sky, but rather at the warmer objects on the surface of the earth.
Similarly, when you aim your SCT at the sky the corrector lens is looking at deep space which has a very cold radiative temperature and the corrector loses heat. Glass doesnít conduct heat very well so
the corrector doesnít keep warm by grabbing heat from the rest of the scope (which isnít cooling down as much because the rest of the scope isnít looking exclusively at the cold sky). If the correctorís
temperature drops below the dew point temperature it collects dew.
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