FREQUENTLY ASKED QUESTIONS
WARRANTY & REPAIR
What is the warranty period?
5 years minimum. Please note that with all warranty repairs you will be responsible for return shipping (shipping is the same as listed on the order form for new controllers).
Can the DewBuster™ Controller be repaired if it breaks after the warranty?
Yes, every DewBuster™ Controller built to date is still repairable. Parts obsolescence is not a problem due to the use of industrial components with very long manufacturing life-spans. When you invest in a DewBuster™ Controller you can count on it to be in service for many, many years to come.
How much does it cost to repair a DewBuster™ Controller after the warranty expires?
Out of warranty repairs are done at a flat rate repair fee which is currently $30 plus return shipping (shipping is the same as listed on the order form for new controllers).
I purchased my DewBuster™ Controller used, does the warranty transfer?
It will be considered "out of warranty" and can be repaired as described above.
Will my warranty be voided by modifying my DewBuster™ Controller's power cord or case?
Unlike competitors, I do not look for reasons to void your warranty. Your DewBuster™ Controller's "no hassle" warranty is unaffected by any modifications you made to make it fit your telescope. So feel free to replace the cigarette plug or remove the back cover to drill holes for mounting screws or a lanyard. And if you need me to repair any components damaged by your modifications, you will only be charged for components damaged by your modification, all other repairs will be covered under warranty. Before making any modifications, I suggest you contact me as I can usually provide you with helpful advice to insure your success.
What else will I need besides the DewBuster™ Controller?
The DewBuster™ Controller includes everything but heating strips and a power source. The heating strips are purchased to fit the telescope and are plugged into the DewBuster™ Controller's heater output jacks. Typically heaters are used on the main lens (refractor) or corrector plate (Schmidt-Cassegrain). Newtonian Reflectors do not need a heater on the primary mirror but do need a heater on the secondary mirror. Heaters are also needed for the other optics such as eyepiece, finderscope, and Telrad. The DewBuster™ Controller is powered by a 12VDC power source. The standard power cord has a cigarette plug but a variety of connectors are available. If you wish to operate your DewBuster™ Controller from 120 VAC house current you can use a 13.8VDC power supply but for your safety you should power it via a ground fault interrupt (GFI) protected power outlet.
Why a DewBuster™ Controller if cheap ones will prevent dew?
Preventing dew is only part of the goal. You bought excellent optics to see sharp highly magnified images of objects in the night sky, yet blindly applying heat to your lens blurs the images preventing your telescope from performing at its best. As humans we have difficulty sensing temperature and we can not rely on "feel" to adjust dew heaters, if it feels warm then it is definitely too hot. The only way to be sure we are not heating the lens any more than absolutely necessary is to accurately measure both the telescope temperature and the air temperature and then constantly fine tune the power going to the heater strip so that the telescope is kept just a little warmer than the air temperature. The DewBuster™ Controller does exactly that and it does it automatically leaving you free to enjoy your telescope.
Why does dew form at night?
During the day, when warmed by the sun, the air is like a sponge absorbing any moisture it comes into contact with. After sunset the dropping air temperature is like a hand squeezing that sponge and most of the moisture absorbed during the day is deposited as dew. The air must rid itself of this moisture so dew will form, it is just a matter of what surfaces it will form on. Objects which radiate heat will cool faster than the air and dew up first, while objects which are warmer than the air will not dew up.
Why does dew form on my lens or corrector plate?
As described above, dew can form on any object which gets colder than the air temperature. Since the corrector plate is thin, it does not have much mass to store heat and since it is exposed to the night sky it quickly radiates that heat and cools below the air temperature. A dew shield will reduce the area of sky that is robbing heat from the corrector plate, but it will eventually cool below the air temperature and if the air is near the dew point temperature then dew will quickly form.
How can my telescope get colder than the air temperature?
When you first bring a warm telescope outside, the air around it immediately starts cooling it off because heat travels from warm objects (scope) to colder objects (air). The greater the temperature difference the faster it cools, so it may cool 3/4 of the way in the first 5 minutes, but take half an hour to cool the rest of the way. After that, radiant cooling by the night sky (cold object) will continue cooling the telescope (warm object) until a balance is reached where the air (warmer than scope) gives up heat to the telescope to replace what is being lost through radiant cooling by the night sky. The telescope tube will also drain heat away from the corrector plate and radiate it to space. A balance is point is reached with the telescope a few degrees cooler than the air temperature. The balance point is affected by wind (helps warm scope) and sky transparency (increases radiant heat loss).
Why not set the DewBuster™ Controller based upon the dew point?
Regardless of how low the dew point may get, your telescope performs best with the optics at the same temperature as the air around it. For this reason the minimum setting of the DewBuster™ Controller is zero degrees (telescope at same temperature as the outside air) so that the telescope will not get colder than the air. For humid conditions the temperature control knob can be set higher to maintain the telescope warmer than the outside air temperature. This is needed because some heat loss occurs between the telescope tube where the heater strip is located and the corrector plate. So unless the telescope tube is kept warmer than the air temperature, the corrector plate may fall below the air temperature.
Doesn't it waste power warming the corrector when the dew point is well below the air temperature?
A DewBuster™ Controller set a few degrees above 0 only applies a small amount of energy to keep the telescope tube at the air temperature, much less than the lowest setting of most controllers. The small amount of energy used is well worth it to keep the corrector from cooling below the air temperature and risking dew formation. In addition, keeping the telescope near ambient air temperature provides much better performance than waiting for the dew point to be reached and then applying large amounts of heat to warm it up. Also, it is not worth the risk of allowing dew to form because it takes much more energy to dry it up than to prevent it from forming in the first place.
Will the DewBuster™ Controller work with non-metal tubes or lens mounts?
Carbon fiber tubes conduct heat well enough to use a DewBuster™ Controller in the normal fashion (heater just behind corrector casting). On economy refractors and finderscopes with plastic lens mounts place the heater strip around the metal tube so that it warms the air within the tube which will in turn warm the lens.
What temperature should the DewBuster™ Controller be set at for my telescope?
Every situation is different so I suggest 5 °F (for 12 inch and larger SCT's 10 °F) as a starting point and if no dew forms try a lower setting the next night. The goal is to run at the lowest temperature that never allows dew to form. The ideal setting is the one that works every time, so if 5 works all the time but 4 works most of the time then just set it to 5 at the beginning of the night and you'll never have to worry about dew. You do not want to keep changing the temperature during the night as this will prevent the telescope from reaching thermal equilibrium.
How can the DewBuster™ Controller apply heat without causing tube currents?
See Telescope Cooldown for a full discussion.
Where would be a good place to mount the DewBuster™ Controller?
One popular location on all SCT's is on the optical tube assembly. The advantage is that the heater wires don't have to flex as the scope moves, so the only cord you have to worry about is the DC power cord. The power cord is 6 feet long to allow flexibility in routing. On fork mounted SCT's a popular choice is to mount the DewBuster™ Controller on the fork arm. If you chose this location, run the heater cables through Split Loom Tubing to keep them organized and out of the way of the moving parts of the telescope.
HEATERS & TEMPERATURE SENSORS
Can I purchase heater strips on this web site?
No, because I prefer to concentrate on building the best controller available. Heater strips are mass produced items and there are many excellent commercial heaters available so I provide links to places where you may purchase them. You may also build your own heater strips following my instructions.
What heaters do I need?
On an SCT or refractor you will need a heater to fit the telescope tube (not the dewshield) to keep the corrector plate or objective dew free (place heater around main tube as close to lens cell as possible). On a Newtonian you will not need a heater on the primary mirror (tube or shroud should be enough, if not add a fan blowing on back of primary mirror) but you will need one on the secondary mirror. On all telescopes you will need a heater for your eyepiece but you only need to but the largest size, a 2" heater will fit a 1.25" eyepiece and the excess will not cause a problem. For SCT's and Refractors you can just put the heater strip on your star diagonal (a 2" heater typically fits a 1.25" diagonal, a 3" heater on a 2" diagonal) and the star diagonal will warm the eyepiece by conduction (makes changing eyepieces much easier). In a similar fashion, if you have a Dob with a Paracorr you can put a heater on it to warm the eyepiece. You will also need heaters for your finderscope objective and eyepiece (usually 0.965") unless you only use it for the initial star alignment and then cap it. If you have a any other finder devices (Telrad, Rigel, etc.) you may also need a heater for those as well.
Do I still need a dew shield with electric heaters?
A dew heater is a supplement to a dew shield, not a replacement. A dew shield helps reduce heat loss to the night sky and can also block stray light to help improve image contrast. To be effective, the dew shield should extend at least one lens diameter in front of the lens and longer is better. Most refractors are already equipped with a dew shield, but if that dew shield does not extend at least one lens diameter past the objective then an extension is recommended. A dew shield does not have to be expensive, some of the best are home made. When a DewBuster™ Controller is being used in combination with a dew shield, the reduced heat loss will allow a lower setting of the controller without dew forming. This not only saves precious battery power, but also improves optical performance. Many dew shields also act as insulators and when fitted over the dew heater can improve heat transfer into the telescope and reduce power consumption.
Where is the best location for the heater strip on an SCT?
Around the aluminum tube just behind the corrector plate casting as shown in the photo below. This warms the tube keeping the air within from getting cold and robbing heat from the corrector plate. The corrector plate will then receive warmth from the air inside the telescope tube. See Telescope Cooldown for more information.
Why not place the heater around the corrector plate casting or inside the dew shield to warm the air where the dew forms?
The corrector plate does not make very good thermal contact with the casting because there are spacers between the metal and glass. Heat applied to the corrector plate casting usually warms the air inside the dew shield but this air becomes less dense when heated and thus rises up escaping into the atmosphere. It is much more effective to warm the telescope tube because the heat energy travels into the telescope warming the air within and transferring this heat to the corrector plate and offsetting heat loss via radiation cooling by the night sky. See Telescope Cooldown for more information.
If my scope has rails or bars that interfere with placing the heater strip just behind the Corrector Plate Casting then where should I place the heater strip?
In many cases the Corrector Plate Casting has a larger outside diameter than the telescope tube so there is about a 1/16" gap between the tube and the rail or bar as shown in the upper portion of the diagram below. Try sliding the thinner Velcro end through the gap first and then work the heater strip through by sliding back and forth along the rail. To avoid wear on the fabric, just leave the heater in place rather than removing it after each observing session. If however there is no gap or it is too small, then you may put washers or spacers between the rail and the casting as shown in the lower portion of the diagram below to create a gap. Adding spacers or washers will most likely require longer screws than those supplied with the rail (short screws will not fully engage the threaded holes and will strip out). In general the screws must be longer by the same amount as the spacer or washer thickness you added, but to be sure you should check the length by looking through the corrector plate while turning the focus knob and make sure the screws protrude into the scope a little but not so far that they hit the primary mirror as it moves back and forth. If the screws are too long you can either use a thicker spacer under the rail or cut the screw shorter (run a nut onto the screw before cutting and filing the burrs, then check the threads by unscrewing the nut).
What is the Temperature Sensor Cable for?
The Temperature Sensor Cable (see photo above) measures both the ambient air temperature and the telescope temperature. The DewBuster™ Controller uses this information to constantly adjust the power output to the Temperature Controlled Heater in order to maintain the telescope slightly warmer than the air temperature. The control knob sets the exact number of degrees Fahrenheit that the telescope will be warmed above the air temperature. For easy attachment, the Scope Sensor clips onto the heater strip which keeps it in place against the telescope tube. The Air Sensor is located on the end of a brown extension wire which keeps it suspended in air and away from the heater for an accurate air temperature reading. This is much better than having the air sensor inside the controller where it would be affected by heat from the electronic components. Since the two sensors are combined into one assembly with a single 6-foot cable, wiring clutter can be minimized by routing the sensor cable alongside the heater strip cable (you may tape them together if you wish). The cable length (may be shortened by the end user) or you may specify a shorter length when ordering.
How do I attach the temperature sensor to a heated dew shield?
As shown above, the brown wire of the sensor should be bent into a U shape so that it will protrude out the back of the dew shield when the sensor is clipped onto the back edge of the heated dew shield. Insure that the Scope Sensor (two little bumps under the black heat shrink band on the clip) are able to make contact with the telescope tube.
Can my controller be configured with more than two Temperature Sensor inputs?
No, but you rarely need more. Medium Power outputs are better suited for small heaters such as eyepiece, finderscope, Telrad, because they apply a little extra heat to prevent dew without temperature sensors. Temperature Control is only needed for items that affect optical performance such as the objective lens or corrector plate because they allow the heat to be reduced to the minimum that keeps it above the air temperature and prevents dew. These are also your largest heaters so reducing heat saves substantial energy making your battery last longer.
When would I need two Temperature Controlled heater outputs (white) for each Temperature Sensor Cable?
Most users will only need one Temperature Controlled Output (the default configuration) but the extra Temperature Controlled jack may be used in special situations such as when using two heater strips on the main scope (sometimes useful on C-14's) or for a large aperture guidescope. When ordering you may also select "Custom Configuration" and request 2 Temperature Controlled Outputs on one side and 1 Temperature Controlled Output on the other side.
Should I buy or build my heaters?
If you enjoy building projects then making your own heaters can be fun and if you don't like the results all you've wasted is your time. A few dollars worth of resistors will be enough to build heaters for every accessory you own. This not only saves you money, but once you see how easy it is to build heater strips you will probably build heaters for many things you would not have bought a heater for. One particular advantage of heaters built with the instruction on this web site is that they produce the same amount of heat for a given diameter as do commercial heaters. That means you can mix commercial and home made heaters on your telescope without worrying about one creating too much heat and the next being underpowered. It also means that you can build heaters for everything and if you like the commercial heaters better then your can replace them one or two at a time as your budget allows. One exception is the corrector plate heater, commercial heaters are usually thinner than home made heaters and thus easier to slip between the rail and telescope tube so many Schmidt-Cassegrain owners opt to buy the corrector plate heater and build the smaller heaters themselves.
Should I put a heater on my Star Diagonal?
If your telescope is normally used with a Star Diagonal then I recommend putting a heater strip around the diagonal as shown below and then you will not need an eyepiece heater because the diagonal will conduct heat to the eyepiece. Since the diagonal is bigger around than an eyepiece, you will need to use a larger heater. A 1.25" diagonal will probably need a 2" heater and a 2" diagonal will probably need a 3" heater. Another advantage of placing the heater on the diagonal is that the heater does not need to be removed when changing eyepieces. This not only makes it easier, but you won't get dewed up because you forgot to put the heater back on. If you are building your own heaters, take a look at my home-made diagonal heater instructions
How do I heat a Telrad since it has a plastic housing?
Heaters which fit the outside of the Telrad are not efficient or effective because the plastic sides of the Telrad do not conduct heat well. The best Telrad heaters apply heat directly to the glass like the Dew-Not Telrad heater. You may also wish to consider building your own Telrad heater.
Can I use an RCA "Y" splitter to plug 2 heaters into one RCA jack?
Most commercial RCA splitters and patch cords are "shielded" cables meaning they have very small wires designed to block electrical noise. Unfortunately the SMALL WIRES CAN'T HANDLE MUCH CURRENT AND WILL GET HOT AND POSSIBLY MELT if used with dew heaters. If you need a splitter or extension it is best to make your own using #24AWG (small heaters) to #18AWG (larger heaters) wire.
TELESCOPE PERFORMANCE & COOLDOWN
Is a dew shield important?
A dew heater is a supplement to a dew shield, not a replacement. A dew shield is an important part of any telescope, especially a Schmidt-Cassegrain. It not only reduces heat loss to the night sky, but also blocks stray light and improves image contrast. The dew shield should extend at least one lens diameter in front of the lens to be effective, but even a 1/2 diameter dew shield is better than none . Most refractors have built in dew shield but if it does not extend at least one lens diameter past the objective then an extension is recommended. A dew shield does not have to be expensive either, a home made Reflectix™ dew shield is easy to make yet very effective. Many dew shields have insulating qualities and when fitted over the dew heater can reduce heat loss so that more heat travels to the telescope where it is needed.
What are tube currents and telescope cool down?
Your telescope is at "thermal equilibrium" while stored inside your home, meaning all parts are at the same temperature. When taken outside the parts release stored heat to "cool down" to the outside air temperature. Metal parts conduct heat well and cool rapidly, however glass does not so the optics cool down slowly and the center of the glass will be warmer than the outside surfaces resulting in temporary distortions of the optical components and imperfect images. Compounding the problem, the escaping heat warms air within the telescope tube causing air movements (tube currents) as warm air rises and cold air sinks. Illustrated below is a Schmidt-Cassegrain telescope undergoing "cool down". Tube currents within the OTA are shown (red for warm and blue for cool air, the more arrows the greater volume of air movement). This drawing shows "worse case" conditions such as when the telescope is brought from inside a warm house out into the cold night air.
The metal telescope tube has a large surface area and conducts heat well so it rapidly cools down to the outside air temperature. Air inside the telescope tube is in contact with the inside surfaces of the tube so it cools off, becomes heavier, and sinks to the bottom of the tube (blue arrows). This air then absorbs heat from the primary mirror, warms up making it lighter, and rises to the top of the tube (red arrows). It then gives up heat to the corrector plate, becomes heavier and travels back down the tube and the cycle repeats. These "tube currents" are beneficial in that they assist in cooling the primary mirror and heating the corrector plate to prevent dew, but they also blur the highly magnified images in the telescope. It would seem that the primary mirror would eventually cool to the temperature of the air inside the telescope tube, but unfortunately radiation cooling by the night sky keeps robbing heat from the corrector plate and tube bringing its temperature down. Unless a heater is added, the telescope will become cooler than the outside air temperature and dew will form on the tube and corrector plate.
What is Radiation Cooling?
The laws of thermodynamics tell us that heat flows naturally from hot to cold objects. This means that the telescope (warm) gives up its heat to outer space (cold). A dew shield helps because it reduces the area of night sky that is robbing heat from the corrector plate. The telescope tube is also affected by radiation cooling because it has a large surface area exposed to the night sky and metal is very efficient at radiating heat (this is why metal objects dew up more quickly). While beneficial during telescope cooldown, it now works against us by cooling the telescope below the dew point and dew then forms.
Why do conventional dew heaters cause problems?
Conventional dew heater systems do not adjust themselves for constantly changing conditions, but instead apply "worst case" power levels which prevent dew later in the night but result in too much power early on. Although they may be adjustable, you must still guess what level and users tend to apply excessive power. The drawing below illustrates this situation.
As shown above, a heater strip and dew shield have been installed, both good for dew prevention. The problem is that too much heat is being applied which causes image degrading air currents in front of the corrector plate. The telescope will perform poorly and the owner will blame it on telescope cooldown or poor seeing conditions. Since the dew shield is open to the air, the excess heat escapes as rising warm air, causing air currents and degrading telescope performance. To correct this problem, the heat must be reduced so that the corrector plate is warmed only slightly above the air.
Another problem is that the heater strip was incorrectly placed around the corrector plate casting rather than the tube itself. While this would seem like a good location, most of the heat is wasted warming the air inside the dew shield which then escapes into the atmosphere (warm air rises) only to be replaced by an endless supply of cold air. Very little heat conducts to the corrector plate because it is insulated from the aluminum casting by shims. Also, since it is made of thin glass (an insulator) and has a large surface area, most of the heat never conducts to the center of the corrector plate but instead is drained away by the cold air inside of the tube and within the dew shield. Another problem with this heater location is that none of the heat warms the air inside the telescope tube to counteract radiation cooling, so the corrector plate has cold air on both sides of it and the center of corrector gets cold and dews up first. Unaware of this, the telescope owner generally cranks up the heat creating more air currents and destroying telescope performance.
The above situation can be improved by moving the dew heater back so that it is around the telescope tube, and reducing heat to the minimum needed to keep the corrector plate slightly warmer than the air.
How does the DewBuster™ Controller prevent tube currents and overheating?
In the drawing below, we have a telescope fitted with a properly positioned heater strip being thermostatically controlled by a DewBuster™ Controller. The DewBuster™ Controller is not heating up the telescope, it applies just enough heat energy to replace what is lost to the night sky by radiation cooling and keep the telescope at the air temperature.
The DewBuster™ Controller measures both telescope and air temperatures and applies only enough heat energy to maintain the telescope tube slightly warmer than the outside air temperature (user selectable 0 to 20 °FΔT). Thus it is only replacing the radiation cooling heat losses to the night sky, so the air within the telescope tube is only slightly warmer than the outside air temperature. This results in a blanket of slightly warm air inside the tube and the large surface area of the corrector plate allows it to absorb heat from the air inside the tube thus warming it evenly from center to edge and counteracting radiation cooling of the corrector plate by the night sky. Compared to previous drawings, there are fewer arrows because the smaller temperature differences result in less air movement. The warmest part of the telescope will be the telescope tube underneath the heater strip (red arrows) but the heater strip will not even feel warm because it is temperature controlled (you can only "feel" heat when an object is much warmer than the air temperature). The violet arrows are heat energy conducting down the telescope tube preventing radiation cooling from chilling the tube colder than the outside air. This helps stabilizes the air temperature within the tube so that the primary mirror can reach temperature equilibrium (violet arrow from primary indicates very little heat coming off primary).
The above photo shows the heater strip correctly placed around the telescope tube, not the corrector plate casting. The Telescope Temperature Sensor must be placed under the heater strip and in good contact with the tube in order to accurately measure the temperature. The Air Temperature Sensor may be routed into the dew shield or left outside, whichever is most convenient. Efficiency can also be improved by wrapping an insulating material around the outside of the heater strip to reduce heat loss and force more of the heat into the telescope. The dew shield is not shown in this photo, but it is very important to use a dew shield. A home made Reflectix™ dew shield is very easy to make but very effective.
BATTERIES AND POWER SUPPLIES
How many Amps does the DewBuster™ Controller use?
The controller is basically a switch, it applies voltage to the heaters but the heaters determine how many Amps of current will flow from your battery. With no heaters plugged in the controller will draw less than 0.1 Amps. To determine how many Amps your controller will draw with heaters plugged in, check the heater manufacturer's web site for the Amps used by each heater, then add them together. If you can not find this information, you can use a multimeter to measure the Ohms of each heater, then divide 12 Volts by the Ohms to get the Amps used by each heater.
What is the maximum current of the DewBuster™ Controller?
The main fuse (8 Amps on cigarette plug cords, 16 Amps on HD power cords, 10 Amps on Big Dob models with RCA power input) limits total current, but each of the DewBuster™ Controller's 3 output channels are limited to 6 Amps maximum by the short circuit protection circuitry (prevents damage if a heater shorts out). The white output jack(s) on left side are wired to one channel, the white output jack(s) on the right side are wired to another channel, and the black Medium Power output jacks on both sides are wired to a third channel. The red 12V Power output jacks are wired directly to the controller's main fuse. In addition, the Temperature Controlled and Medium Power outputs alternate pulsing, so a DewBuster™ Controller with HD Power Cord can deliver 6 Amps at 50% duty cycle to all 3 channels while never exceeding 12 Amps at any moment.
Do I need the Heavy Duty Power Cord?
Small refractors and the typical 8 inch SCT will be usually be fine with the standard power cord. Large Dobsonians do not usually have any large heaters so they will also work fine with a standard power cord or the Big Dob's RCA jack power input. However 10 inch and larger SCT's need a Heavy Duty Power Cord preferably with banana plug or ring terminals. If you select the HD Power Cord with Cigarette Plug the controller is internally fitted with the 16 Amp PTC fuse but the cigarette plug has an 8 Amp fuse in the tip. This allows you to later replace the cigarette plug with your own connector (this will NOT void your warranty) to utilize the full 16 Amp capability. If you prefer a plug and play approach, you can order the HD Power Cord with Powerpole® Connector and a Cigarette Plug Pigtail. Then if you switch to a different power source you can order a pigtail to fit it (pigtails alone are the same price as on the controller order form, to order a pigtail contact Me for a quote). Do not try to use a cigarette plug on 14" or larger SCT's, these big scopes really need the Heavy Duty Power Cord with Ring Terminals or Banana Plugs. If your power source only has a cigarette socket then it is probably not up to the task of powering a 14" SCT.
What's wrong with a cigarette plug?
A cigarette plug is best suited for about 3 Amps or less, any more and it starts getting warm because the contacts do not make sufficient metal to metal contact to handle the current. A good electrical connection has a quality known as "low-resistance" which keeps the connector from getting hot and delivers higher voltage at your heaters (important for large scopes). So if you find your cigarette plug gets hot after a while then the connection is unable to handle the amount of current flowing through it. In addition, cigarette plugs have standard fuses in their tip and since all standard fuses have resistance this creates a voltage loss. If you absolutely must use a cigarette plug with a large scope then I would suggest ordering the Heavy Duty Power Cord with Powerpole® Connector and a Cigarette Plug Pigtail. Then you can always order another pigtail later if you decide to switch to a better power source (to order pigtails alone contact Me for a quote).
What's different on the Heavy Duty Power Cord?
DewBuster™ Controllers equipped with Heavy Duty Power Cords use larger gauge #16 AWG wire and have a solid state 16 Amp "PTC fuse" within the DewBuster™ Controller. PTC fuses never need replacement, they self-reset a few minutes after the over-current condition is removed. Another advantage is that a PTC fuse has extremely low resistance so higher voltage reaches the heaters allowing them to produce more heat at full power. The Heavy Duty Power Cord also offers a variety of connectors as shown below.
What are Powerpole® connectors?
The Anderson Powerpole® is a premium connector widely used in Emergency Vehicles and Radio Controlled Aircraft where poor connections can be fatal. Powerpole® connectors are second to none when it comes to carrying large amounts of current with ultra-reliable low resistance connections. DewBuster™ Controllers use Powerpole® PP15 connectors which are compatible with PP30 and PP45 connectors. These connectors all use the same housings and all have 45 Amp contact surfaces for excellent connections, however the PP15 connector is de-rated to 15 Amps because it fits #16 wire which can not handle 45 Amps.
What is a Powerpole® pigtail?
DewBuster™ Controllers with Powerpole® Power Cords can be used with just about any 12V battery or 13.8VDC power supply via a Powerpole® "Pigtail". The pigtail's Powerpole® connector plugs into the Powerpole® connector on the power cord. The other end of the pigtail has a connector to fit your power source (stripped&tinned leads, ring terminals, banana plugs, RCA plug, cigarette plug, etc.). For example, you could fit your 13.8VDC Power Supply with a pigtail for home use, and then fit several 12V batteries with pigtails so that you can swap batteries in seconds. You could even fit your battery charger with a Powerpole® connector to make charging batteries easy. Another reason for ordering your DewBuster™ Controller with a Heavy Duty Powerpole® power cord is that you can connect to your present power source and later order another pigtail to fit a new power source. For independent reviews of Powerpole® connectors read the Cloudy Nights Powerpole® article and the VARA Powerpole® article.
The Heavy Duty Power Cord and Pigtails are available with the following connectors:
Anderson Powerpole® PP15 Connector - If you need to connect to several different power sources, or if you plan to switch to a different power source in the future, you may order your DewBuster™ Controller with Powerpole® connectors so that you can easily and quickly interchange between various connectors.
Cigarette Plug - As mentioned above, the Cigarette Plug is not the best choice, but if you need to connect to a cigarette socket then I do offer the Cigarette Plug on a HD Power Cord. DewBuster™ Controllers with Heavy Duty Power Cords have a self resetting 16 Amp PTC fuse inside, however the Cigarette Plug has an 8 Amp glass fuse in its tip because most cigarette sockets can not handle large amounts of current. If your cigarette socket is capable of handling more current, you may change to a larger size glass fuse, but it is your responsibility to insure that you do not exceed the current that your wiring and connections can handle.
Ring Terminal Pigtail - Fits a screw terminal making a solid connection that won't come lose by accident. Specify the size of the hole in the terminal. Standard sizes are 1/4", 5/16", and 3/8" but other sizes are available upon request. Many Marine Batteries use 3/8" Positive and 5/16" Negative terminals. Many power supplies accept 1/4" ring terminals but check that the red and black plastic nuts unscrew completely so that you can slip on the ring terminal (if the plastic nuts will not come off then use banana plugs instead). TIP: A thin coating of lithium grease on the battery terminal connections will prevent corrosion.
Banana Plug - a universal connector that will fit the Pyramid Power Supplies as well as most others. It is a good choice if you operate from a power supply that accepts banana plugs.
Stripped & Tinned Leads - If you prefer to connect to your own terminals you may order your DewBuster™ Controller without a connector. The end of the cord will have the wires stripped & tinned and marked for polarity.
RCA Plug - the same type plug used on heater strips. This plug can plug into the Power Distribution Panel on many Dob telescoopes.
Molex Connector - Although I have switched over to the superior Powerpole® Connector, Molex Connectors are still good connectors and if your DewBuster™ Controller has a Molex Connector you may still order Pigtails or an additional DewBuster™ Controller with a Molex connector to maintain compatibility with your previous DewBuster™ Controller and pigtails. I can also upgrade any DewBuster™ Controller and Molex pigtails to Powerpole® Connectors. Contact me if you need a quote.
Should I use a battery or a power supply?
Batteries must be replaced every few years so if reliable 120VAC power is available a good 13.8 VDC Power Supply is a better choice and can serve you for many years to come. If you do not have a reliable source of 120VAC power then a 12V battery can be used with a charger to maintain the battery at full charge while it is powering your DewBuster™ Controller. If 120 VAC is interrupted the battery will provide uninterrupted power.
Why a 13.8V Power Supply instead of a 12V?
A 12V battery will vary from 10.5 VDC when completely discharged to 13.8 VDC or more when installed in a vehicle and the engine is running. The DewBuster™ Controller will operate at 12 VDC as well as 13.8VDC, but your heater strips will produce more heat at 13.8 VDC. Also avoid "switching power supplies" because they are intended for constant current operation such as computers and have difficulty with the current surges caused by dew heaters switching on and off. For best results use a "linear regulated" type 13.8V power supply rated for at least twice as many amps as your heaters will draw at full power.
Do you offer a 5.5mm DC Plug for power input?
DC connectors are commonly used for telescope mounts, laptop computers, and other devices that use a few amps of current. However they are not suitable for high current which is why I do not recommend them for powering your DewBuster™ Controller. In many cases when someone is interested in powering their DewBuster™ Controller with a DC plug it is because they have found a "laptop type" power supply that is equipped with a DC plug. These are "switching" type power supplies and they should be avoided as they can not handle the current surges that a dew controller will create.
Can I connect the DewBuster™ Controller to my telescope's DC OUT connector?
NO! Many telescopes have a DC power output but it is only intended for low current accessories. These CAN NOT HANDLE THE HIGH CURRENTS THAT YOUR HEATERS will draw and may damage your telescope. The DewBuster™ Controller's power cord should always be connected as close to the battery as possible and preferably not share a cigarette plug extension cord with any other accessories.
How often should I recharge my battery?
Recharge your battery as soon as possible after each use because discharging dissolves the lead plates forming lead sulphate. If left discharged the lead sulphate settles to the bottom as sludge and can't plate back when you try to recharge the battery so the battery will not store as much energy. I recommend leaving a "smart charger" connected to the battery at all times so that it will charge the battery and then switch to "float" to maintain the battery at 13.6V to 13.8V so that it is ready for use when you need it. Conventional chargers should not be left connected all the time as they will overcharge and damage the battery.
What size battery should I get?Add together the amps for each of your heaters (see table below) and divide by two (heaters normally run at half power). Add the Amps used by your telescope or any other accessories. Multiply by the hours you wish to operate. This will give you the minimum battery size but it is best to select a battery at least twice this size because it is not good to run a battery down completely every time. Extremely cold weather will also reduce battery capacity so factor that in as well. For example, an 8" SCT (1.6A Corrector Plate + 0.2A Eyepiece + 0.2A Finderscope) has heaters that use 2 Amps at full power or 1 Amp at normal half power. It will also operate the telescope mount which will consume about 1 Amp so the total power used will be 2 Amps and to operate for 5 hours would take about 10 AH. Since it is best to only run the battery down about half way, a 19AH battery would be a good choice.
Heater Size Amps at Full Power
0.965" to 2" 0.2
3" to 4" or Hand Control 0.8
5" to 8" 1.6
0.965" to 2" 0.2
3" to 4" or Hand Control 0.8
5" to 8" 1.6
When battery shopping, what numbers are important?
Only buy a deep-cycle battery because they have thicker plates that can tolerate deep discharge and recharge. Standard car batteries are not suitable for "deep-cycle" use because they have thin plates that will deteriorate when deeply discharged. The only spec that we are interested in is the battery capacity, usually stated in Amp-Hours (AH). The AH can be divided by the number of Amps being used to find out how long it takes to completely discharge the battery. Some batteries will specify Reserve Capacity (RC) which is the number of minutes the battery can supply 25 Amps before it completely discharges. RC can be multiplied by 0.4 to calculate AH, but do not assume the battery is a deep-cycle unless it says so.
Can I use a "Jump-Start" portable power unit as my battery pack?
These work well if they are of sufficient capacity. As with regular deep-cycle batteries, for long life, avoid running them completely dead during each observing session and recharge between uses. Most will specify their battery's Amp-Hour rating somewhere in the specifications. Most contain a 17 to 21 AH battery which works well with small to medium sized telescopes, but read the side of the box to be sure of the battery size.
Do you recommend a Power Tank?
These contain the same AGM batteries as the Jump-Start units which can be found at department and automotive stores for 1/3 the price. So unless you need the extra features you are just paying extra for the same size battery. As with the Jump-Start units, read the battery specs and get a unit with at least a 17 AH battery.
Can I run the DewBuster™ Controller on 120 VAC power?
Yes, but you will need a 13.8 VDC Power Supply to convert 120VAC house current to lower voltage DC and you must power it from a GFI (Ground Fault Interrupter) to prevent electrical shock. Click here for more information about power supplies.
Can I use a battery and inverter to generate 120 VAC so I can plug everything in just like at home?
This is not recommended because it wastes 4 times as much battery energy. Each voltage conversion is only about 50% efficient. So going from 12V battery to 120 VAC and then back to 12 VDC will run the battery down at least 4 times faster. Put another way, you'll need 4 batteries to do the same job. So anything that is native 12 VDC should be powered directly off the battery and use the inverter only for things that are 120 VAC.
Can I power my telescope from one of the DewBuster™ Controller's 12V Power Output jacks?
Maybe. Non-computerized telescopes or computerized telescopes 6" or smaller will probably work. However on large computerized telescopes you may have problems because as the large heater turns on and off the voltage at the DewBuster™ Controller will dip and this may cause erratic behavior of the computer. The heavy duty power cord option helps reduce these voltage fluctuations, but very large telescopes may still experience problems. There is no harm in trying and some computers are more tolerant of voltage dips than others, so if the computer misbehaves then connect it as close to the battery or power supply as possible so that the power to the telescope will not have to travel through the same wires as the DewBuster™ Controller.
Does the DewBuster™ Controller really save battery power?
Yes, because it does not waste power during the early part of the night when the air temperature is dropping. During this period very little power is sent to the heaters conserving battery power and allowing the scope to cool down. As night sets in and the sky starts slowly sucking the heat out of your scope, the DewBuster™ Controller's temperature sensor detects this and starts gradually applying power but only what is needed to keep the telescope warmer than the air. Late at night when radiant cooling and dew are at their worst, the battery will have more energy left for the heaters and the DewBuster™ Controller will still be preventing dew while conserving power.
Can I still start my car when the low battery light comes on?
I do not suggest powering your dew controller or anything else from your car battery because car batteries are not designed for deep cycle use and will soon fail if repeatedly deep discharged. It is best to use a dedicated deep-cycle battery to power your telescope. The answer to the question however is that it is simply not possible for a low battery indication to give any meaningful information on whether the battery is able to start a small car engine much less a large one. The DewBuster™ Controller's low battery circuitry is intended to prevent battery damage, not to reserve any amount of energy.
How much time will my heaters operate after the Low Battery light comes on?
When the yellow "Low Battery" light illuminates, the DewBuster™ Controller is reducing heater power to prevent running the battery completely dead and damaging it. Depending on your battery size (AH) and the current drain of your heaters (Amps), you may get minutes or hours. When the "Low Battery" light begins to flicker you should consider disconnecting non-essential heaters. When the light remains on almost constantly you do not have much time left. The DewBuster™ Controller will not damage your battery even if it is left on for an extended period of time with the yellow light on, however other devices and anything plugged into the DewBuster™ Controller's 12V ouptputs should be disconnected or they may drain your battery completely and damage it.
Can the low battery circuit be disabled when using a dedicated battery?
No. When a 12-Volt battery is drained below 10-Volts it will be permanently damaged, so this feature is always active on the DewBuster™ Controller.
What method does the DewBuster™ Controller use to regulate power to the heaters?
The DewBuster™ Controller uses Pulse Width Modulation (PWM) meaning full battery voltage is cycled on and off several times per second. The ratio of on time to off time is what determines how much heat is generated. If it is on 40% of the time then this would be 40% power level which generates 40% of the heat that would be produced if it were on all the time.
Will the DewBuster™ Controller cause electrical noise that will interfere with my CCD imaging?
The DewBuster™ Controller is designed to minimizes EMF interference. I have had no reports of EMF problems and many users have reported that their DewBuster™ Controller eliminated any CCD interference that their previous controller was causing. I have seen a few cases of using inadequate power supplies and when the DewBuster™ Controller switches heaters on and off the voltage fluctuations created RF noise that affected the camera images. This is easy to pinpoint, simply turn the DewBuster off and if the problem disappears then try powering the camera from a separate power source than the DewBuster™ Controller. If you experience problems I will work with you to solve them. If you are not satisfied, you may return your DewBuster™ Controller within 30 days for a refund (shipping is not refundable).
Can I use a cigarette plug splitter?
It is best to avoid splitters in the power feed to the DewBuster™ Controller whenever possible. If you must use a splitter, get a heavy duty one rated for 10 amps or more as standard splitters are intended for low current applications like cell phone chargers. Before resorting to a splitter, check whether your power source has alternative connections. Many have terminals that allow connecting wires so you can cut the cigarette plug off of a heavy duty splitter and connect the wires directly to the terminals of the power supply. This eliminates having two cigarette connectors in series.
Can you build a DewBuster™ Controller that operates on 24VDC?
No because your heater strips would burn up. Applying 24V to a 12V heater creates 4 times the heat and the nichrome heating wire inside would glow red hot and melt the heater strip. So power your DewBuster™ Controller with a 12V battery or 13.8VDC Power Supply.
Can the DewBuster™ Controller handle a 14" SCT?
Yes, the DewBuster™ Controller works well on a 14 and eliminates the need for a second controller. Efficiency is very important so be sure to place the heater strip around the aluminum tube just behind the corrector plate casting and use a dew shield. The heavy duty power cord is strongly recommended to reduce voltage losses and allow the heater to produce as much heat as possible when needed.
Will the cigarette plug power cord work on a 14" SCT?
The big 14" heater demands quite a bit of electrical current which requires very low resistance in all wiring and connections. DewBuster™ Controllers equipped with heavy duty power cords have a solid state PTC fuse and heavier gauge wire for low resistance and efficient current flow. This delivers full voltage to the heater allowing it to produce maximum heat. For best results I recommend ordering your controller with the heavy duty power cord and banana plugs, stripped&tinned leads, ring terminals, or a Powerpole® connector. Note that while a Heavy Duty Power Cord with Cigarette Plug is available, the cigarette plug will be the bottleneck as it is not a low resistance connector. If your DewBuster™ Controller has a standard cigarette plug power cord it can be sent in for an upgrade to the Heavy Duty Power Cord (see DewBuster™ Controller order form for prices and return shipping, then contact me for instructions).
How can a DewBuster™ Controller keep my 14" SCT from dewing up if my old controller couldn't? Can it produce any more heat than other brands?
Small changes in battery voltage cause large changes in heat output (see chart below). Most controllers have cigarette plug power cords and standard glass fuses, both of which cause voltage losses when large amounts of current are flowing. This prevents your 14" heater from producing the maximum heat it is capable of. In addition, the resistance in the cigarette plug causes it to get very warm and this heat is wasted energy. A DewBuster™ Controller equipped with the heavy duty power cord has heavier gauge wiring, a solid state PTC fuse, and low resistance connections which will not get hot when large currents are flowing. This means full battery voltage reaches your heater producing maximum heat with very little wasted energy. The DewBuster™ Controller's also conserves battery power by using the temperature control to only apply heat when needed. This results in energy savings early in the night when the outside air is cooling down. The energy savings translate to higher battery voltage later in the night when you need it the most. For maxim performance, be sure to place the heater strip properly and use a dew shield. Also consider wrapping the heater with insulation to reduce heat loss.
How much does battery voltage affect heater wattage?
As shown in the graph below, a fully charged battery will produce almost twice as much heat as a discharged battery. Note that the heat generated depends on the actual voltage reaching the heater, not just the battery voltage. Because a cigarette plug power cord has resistance in both the glass fuse and the cigarette plug connections, the voltage at the heater will probably be about one volt less than battery voltage which translates into a 20% reduction in heat. A heavy duty power cord has much less resistance which means the heater voltage is much closer to the battery voltage and your heater will be able to produce more heat.
Can a second heater strip be added?
The 14" heaters are marginal in heating capacity, so if the DewBuster™ Controller's Temperature Controlled heater LED remains on constantly this tells you the heater strip is not producing enough heat to warm the telescope to the desired temperature setting. Before adding a second heater, try to reduce heat loss by wrapping your heater with insulating material to improve its efficiency. A DewBuster™ Controller with banana jacks or ring terminals can handle the extra current for a second 14" heater (no need for a second sensor, just plug heater into second RCA output just below first heater). Since the DewBuster™ Controller monitors temperature, once temperature is reached it will reduce power so two heaters will use no more power than a single heater would.
Why does my C-14 dew up even with a dew heater running at full power?
A 14" heater is barely able to keep up with the heat energy being lost to the night sky so insure your heater strip is placed in the most efficient position, around the aluminum tube just behind the corrector plate casting. It is also very important to use a dew shield as this will decrease the radiation cooling. You may also overlap the heater strip with insulation to help reduce heat escape so that most of the heat will enter your telescope where it is needed.
Also make sure you have a robust battery or power supply because voltage plays a huge factor and as shown in the graph above. A fully charged battery or 13.8V power supply will generate 30% more heat than a 12V power source. If the power source can't supply enough current the voltage will drop each time the big heater turns on and that means it is not producing full heat. The heavy duty power cord also helps deliver full battery voltage to your heater. In contrast the cigarette plug power cord will lose about a volt which translates into 20% less heat.
If after following the above recommendations you still experience dew, then check to see if the DewBuster™ Controller's LED remains on for the Temperature Controlled output driving the 14" heaters. If so, this means the heater is not warming the telescope up to the desired temperature. Try wrapping around the outside of the heater with insulating material to reduce heat loss. In some cases a second heater strip may be needed and this can be plugged into the second Temperature Controlled output (you do not need another temperature sensor since the first heater's sensor can control both). Together the two heaters will be able to warm the telescope up and once the desired temperature is reached the DewBuster™ Controller will automatically reduce power so that the pair of heaters uses no more power than a single heater would.
What is the best way to prevent dew on a Newtonian primary mirror?
It is OK to heat the Newtonian secondary mirror, but the primary is a big chunk of glass that was of equal temperature throughout when it was ground. So unless the entire mirror is at the same temperature it will not obtain its ideal shape and concentrate starlight into pinpoint star images. Applying heat in any fashion will result in smeared stars and blurry planets. The good news is that cooling fans blowing air against the back of the primary mirror will not only cool a warm mirror to the air temperature, but after it reaches the air temperature it will prevent it from cooling any further and keep the mirror at an even temperature throughout. If your telescope is not equipped with a cooling fan, a temporary solution is to position a floor fan behind it so that it blows air against the back of the primary mirror. A shroud is also a necessity and when combined with the fans should adequately prevent dew.
What if I don't need a temperature sensor?
The DewBuster™ Controller can be used without a Temperature Sensor in "manual mode". If no sensor is plugged into the sensor input jack, the Temperature Controlled outputs switch to Medium Power and operate at 40% duty cycle (40% heat) when the control knob is within the normal range (below 15). "Medium Power" provides enough heat to prevent dew in most cases. However extra power can be achieved by turning the control knob higher. As the control knob is increased above 15 the duty cycle of the Medium Power heaters gradually increases from 50% to 100% (full power). It will still utilize the Low Battery circuitry to prevent running your battery too low and damaging it.
Which type temperature sensor should I use on a Schmidt-Newtonian telescope?
Schmidt-Newtonians have a corrector plate and use an SCT type heater strip so the SCT/Refractor sensor is used. Follow the same instructions as Schmidt-Cassegrains.
Where is the best place to mount a DewBuster™ Controller on a Newtonian telescope?
The controller weighs about a pound, so on small Dobs if the controller is mounted near the eyepiece a counterweight may be required to maintain balance, however this may make the scope less balance-sensitive when changing eyepieces. On larger Dobs, mounting the controller on the upper cage reduces the need to run heater cables down the truss tubes and the optional RCA Power Jacks can power accessories such as the AstroSystems DewGuard. A Special "Big Dob" DewBuster™ Controller is available with options that many large Dob owners may find useful.
Why does my primary mirror stay dew free all night yet it dews up in the morning ?
Large Dobs may experience condensation when the morning sun rapidly warms the air and then moisture condenses onto the primary mirror which is still cold from the night before. The most efficient way to prevent morning condensation is by keeping the moist morning air from reaching the cold mirror until the mirror has warmed up to daytime temperatures. You may have success using the mirror cover over the front and a plastic bag over the back of the mirror box.
Can the DewBuster™ Controller be used on a Newtonian telescope?
Yes but you should avoid heating the primary mirror as explained above. The secondary mirror will often dew up because it is closer to the open end of the tube so the shroud is not an effective dew shield for it. The secondary mirror is also a much thinner glass and mounted in an aluminum housing which robs the mirror's heat and radiates it to the night sky. If the only component you need to heat is the secondary mirror then the AstroSystems Dew Guard my be the easiest solution. If you have a stalk mounted secondary mirror or if you need to heat other components, then the DewBuster™ Controller may be used with a Newtonian Temperature Sensor to control a heater attached to the back side of the secondary mirror or stalk. The Newtonian Temperature Sensor may be substituted for the standard sensor when ordering your DewBuster™ Controller or it may be purchased later using the Accessories Order Form.
What is the BDDB "Big Dob" DewBuster™ Controller?