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 glass is insulated from the casting with cork spacers so heat will not transfer from the metal to the glass. It is much more effective to warm the telescope tube because the heat energy travels into the telescope warming the air within which then warms the corrector plate from the inside and offsets heat loss to the night sky. See Telescope Cooldown for more information.
My scope has rails that interfere with placing the heater strip just behind the Corrector Plate Casting, so where should I place the heater strip?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 this is not successful, then it may be possible to 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 (check the length by looking through the corrector plate while turning the focus knob and make sure the screws do not protrude into the scope so far that they hit the primary mirror as it moves back and forth).
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 holds it in place against the telescope tube. The Air Sensor is located on the end of the 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 heat from the electronic components would introduce errors. 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, and Telrad because they apply a little extra heat to prevent dew without needing a temperature sensor. Temperature Control is best used 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 use 2 Temperature Controlled heater outputs 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 binoculars (use the sensor on one objective and plug both objective heaters into that 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 my instructions for building heaters 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 you 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 (see Tips at bottom).
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 tiny 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 and the amount of energy (Amps) is depends on how many and what size heaters are plugged in. With no heaters plugged in the controller will draw less than 0.1 Amps. So check your heater manufacturer's web site for the Amps used by each of your heaters, then add them together to get the total Amps. If you can not find the specs on your heaters, you can use a multimeter to measure the Ohms (Ω) for each heater, then divide 12 Volts by the Ohms to get the Amps used by each heater.
How many Amps can the DewBuster™ Controller handle?Models with Heavy Duty Power Cord are limited to 14 Amps by the main fuse, Newtonian models with RCA jack power input have a 10 Amp main fuse, and Cigarette Plug models have an 8 Amp fuse in the tip of the plug. To prevent damage from shorted heaters, the short circuit protection limits each output channel (Left, Right, Medium Power) to 6 Amps each. Note that the Medium Power and two Temperature Controlled channels take turns pulsing their heaters, so under normal conditions only two of the three channels will be drawing current at any given instant and thus the average current is only 12 Amps.
What's wrong with a cigarette plug?A cigarette plug is best suited for only a few Amps, beyond that it starts getting warm because the plug can not 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 that means it can't handle the amount of current flowing through it. Another limitation of cigarette plugs is they use a conventional fuse which has resistance and this creates a voltage loss. DO NOT try to use a cigarette plug on 14" or larger SCT's because the voltage losses will result in low voltage at the big heater and it will not produce full power (if your power source only has a cigarette socket then it is probably not sufficient to power a 14" SCT anyway).
What is the Heavy Duty Power Cord?DewBuster™ Controllers with Heavy Duty Power Cords use large gauge #16 AWG wire and have an internal solid state 14 Amp "PTC" fuse. PTC fuses never need replacement, if tripped they will self-reset a few minutes after the over-current condition is remedied. Another advantage is that a PTC fuse has extremely low resistance so more voltage reaches your heaters producing more heat at full power. The Heavy Duty Power Cord is available with a variety of connectors.
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. The contacts are the same 30 Amp contacts but because the PP15 accepts smaller gauge wire it has been derated to 15 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. Pigtails can be ordered after the controller by using the Accessories Order Form. For independent reviews of Powerpole® connectors read the Cloudy Nights Powerpole® article and the VARA Powerpole® article.
Pigtails are available with the following connectors:
- Unassembled Powerpole® Connectors - You may order your DewBuster™ Controller with Powerpole® connectors so that you can fabricate your own power connections. One "Powerpole® Connector with 15A pins" consists of a red and black housing and 2 pins suitable for #16 wire.
- Cigarette Plug Pigtail - Cigarette Plugs are not "Heavy Duty" connectors but for small to medium sized scopes they will work fine. Because cigarette sockets can not handle large amounts of current, the Cigarette Plug has an 8 Amp glass fuse in its tip and thus it will blow before the 14 Amp PTC fuse inside the DewBuster™ Controller. If the fuse in the cigarette plug blows the LED on the cigarette plug will not illuminate.
- Ring Terminal Pigtail - Fits screw terminals and makes a solid connection that won't loosen by accident. Ring Terminal size is measured by the screw hole in the terminal and is available in 1/4", 5/16", and 3/8". Marine Batteries often use 3/8" Positive and 5/16" Negative terminals. PowerTank and many power supplies can accept 1/4" ring terminals if 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 I can "spade" the ring terminals so they can slip onto the post). TIP: A thin coating of lithium grease on battery terminal connections will prevent corrosion.
- Banana Plug Pigtail - a universal connector that will fit the Pyramid and most other power supplies. A good choice for low-resistance high-current connections on large telescopes. Banana Plugs are designed to plug into Banana Jacks having holes approximately 5/32 inch I.D. and a depth of at least 1/2 inch. Many power units have "Binding Terminals" which have a hole in the center that can accept Banana Plugs.
- Stripped & Tinned Pigtail - Has stripped & tinned wires for connecting to your wiring. Marked for polarity.
- RCA Plug Pigtail - the same type plug used on heater strips. This plug can plug into the Power Distribution Panel on many Dob telescopes.
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 deep cycle 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 measure 13.8 VDC when fully charged and your heater strips will produce more heat at 13.8 VDC. The most common power source is a Linear Regulated 13.8VDC power supply. Switching power supplies may also be used provided they can hande the current surges caused by dew heaters switching on and off. Regardless of type, get a power supply rated for at least twice the Amps your heaters 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 but they are not suitable for more than a few amps of current. However if you must connect your DewBuster™ Controller using a 5.5mm DC plug then contact me. You will also need to determine whether the center pin is 2.1mm or 2.5mm and verify whether the center pin is +.
Can I connect the DewBuster™ Controller to the DC OUT connector on my telescope mount? Many telescope mounts are equipped with a 2.1mm x 5mm DC power output connector. These connectors are for low current accessory items and trying to connect any dew controller to it may result in too many Amps of current flowing through the small wires inside your telescope mount. This may permanently damage your telescope and void your mount's warranty. It is best to power your DewBuster™ Controller as close to the battery as possible. If you must use a cigarette socket splitter then connect it as close to your power source as possible. Note that many big Dob telescopes have a power distribution "rail" with multiple RCA jacks intended for powering accessories. Since these outputs are usually capable of carrying at least 5 Amps, it is OK to power the DewBuster™ Controller from the power distribution rail as long as the manufacturer does not recommend against it.
How often should I recharge my battery?Recharge your battery as soon as possible after each use. This offers the best chance for all of the lead sulphate to replate back onto the battery's lead plates. 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 after it has recharged it will switch to "float" and 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? First determine your Total Amps by adding together the Amps for each of your heaters (usually listed on your heater manufacturer's web site, if not use the table below for typical Amps) and also add the Amps for any other items running off the battery (telescope mount, accessories). Then multiply Total Amps by the hours you wish to operate and this gives the minimum 12V battery size in Amp-Hours or AH. It is best to select a battery with at least double the Amp-Hours so you will not run the battery down completely every time. Extremely cold weather will also reduce battery capacity so factor that in as well. EXAMPLE: 8" SCT with 1.6A Corrector Plate heater and 0.2A Eyepiece heater will after 5 hours consume 9AH so choose a 17 to 21AH battery.
|Heater Size|||||Amps at Full Power|
|0.965" to 2"|||||0.2 A|
|3" to 4"|||||0.8 A|
|Hand Control|||||0.8 A|
|5" to 8"|||||1.6 A|
When battery shopping, what numbers are important? Buy only "deep-cycle" batteries because their thick lead plates can tolerate deep discharge without damage. Select a battery based on the amount of energy it can hold which is called Amp-Hours or AH. If a battery does not specify the AH then it is not a deep cycle but rather a "starting" battery. These have thin lead plates to produce plenty of Amps when starting a car but they can not tolerate deep discharge and will be damaged. To determine the AH needed, determine how many Amps will be used then multiply that by the number of hours you need to operate. For best results buy a battery with about twice the AH you need.
Can I use a "Jump-Start" portable power unit as my battery pack? These work well if they are of sufficient Amp-Hour (AH) capacity, but the AH rating is often difficult to find on the box, usually buried in the specifications. Most of the light weight units only have 7AH batteries which is quite small so look for a unit with 17 to 21 AH battery which is heavier but works well with small to medium sized telescopes.
Are the amazingly small Li-Ion "Jump-Start" units okay to use? The Li-Ion type Jump Start batteries are getting more popular and the prices are dropping so they are becoming a viable option for powering your telescope. Most of these units have "EC5" connectors for the 12V output so you'll need an EC5 cigarette lighter socket to power your DewBuster™ Controller. However the mAH ratings are deceiving because the internal batteries are not 12V. To calculate the true AH rating at 12V you will need to know the Watt-Hour or WH of the Jump-Start unit (if listed in milliWatt-Hour or mWH divide by 1000 to convert to WH). Then divide the WH by 12V to get the AH rating at 12V. For example, an 18,000mAH Li-Ion Jump-Start unit is 67WH. Dividing 67WH by 12V yields 5.6AH at 12V, so this unit will last slightly less than a conventional 12V 6AH battery . The 18,000mAH was for the internal 3.7V battery (67WH / 3.7V =18AH or 18,000mAH) but at 3X the voltage we get 1/3 the AH.
Do you recommend a PowerTank battery unit? These contain the exact same Sealed Lead-Acid Battery as a Jump-Start unit, so unless the extra bells and whistles are important to you, just get a Jump-Start unit from a tool or automotive parts store for 1/3 the price. As with Jump-Start units, avoid the 7AH models and get one with a 17AH or larger battery. You may connect to your PowerTank's rear binding posts using 1/4" Ring Terminals. I have had reports that the binding posts are too small to accept standard Banana Plugs which require 5/32 inch I.D. holes at least 1/2 inch deep.
Can I run the DewBuster™ Controller on 120 VAC power? You can use a 13.8 VDC Power Supply to run on 120VAC house current. For your safety, be sure to power it through a GFI (Ground Fault Interrupter) to prevent electrical shock. These are available at any home supply store. Click here for more information about power supplies.
Can I use an Inverter to generate 120 VAC from a battery so that I can plug everything into 120VAC just like I do at home?You could, but every voltage conversion is only about 50% efficient. So going from 12V battery to 120 VAC and then back to 12 VDC will drain the battery 4 times faster which means you'll need a battery 4 times larger to do the same job. So power your 12V devices directly from you battery and only use the Inverter for devices that require 120VAC.
Can I power my telescope from the DewBuster™ Controller's 12V Power Output jacks?It will probably work on small telescopes, however you may have problems on large computerized telescopes because the voltage at the DewBuster™ Controller will dip each time the large heater cycles on and off which may cause erratic behavior of the telescope computer. The heavy duty power cord option helps reduce these voltage dips, but very large telescopes may still experience problems. Some computers are more tolerant of voltage dips than others, so if the computer misbehaves then connect the telescope power as close to the battery or power supply as possible so that it will not have to travel through the same power cord wires as the DewBuster™ Controller.
Does the DewBuster™ Controller really save battery power? Yes by not wasting power. After sunset the scope will cool slower than the air and the DewBuster™ Controller's temperature sensor allows it to reduce power to the heater and save energy. Later when the night sky starts sucking the heat out of your scope, the temperature sensor allows it to gradually increase power as needed to keep the telescope from cooling below the air temperature. 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 only using the minimum power needed to prevent dew.
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 fail much sooner leaving you stranded. The answer to the question however is that the DewBuster™ Controller's low battery circuitry is intended to prevent battery damage, not to preserve any amount of energy for another purpose.
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 outputs 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 10.5 Volts it will be permanently damaged, so this feature is always active on the DewBuster™ Controller. Also, heater strips produce less heat at lower voltage, so I do not recommend running your DewBuster on a power source of less then 10.5 Volts.
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 interfere with my camera equipment? 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 interference that their previous controller was causing. However, I have seen a few cases of "switching" power supplies emitting interference due to the DewBuster™ Controller switching the heaters on and off which caused the switching power supply to change its frequency and create RF noise that affected the camera images. This is easy to verify by turning the the DewBuster™ Controller off and if the problem disappears then try powering the the DewBuster™ Controller from a battery or linear power supply. If you experience problems I will work with you to resolve them and if you are not satisfied, your DewBuster™ Controller has a 30 day return policy (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 requires very low resistance in all wiring and connections for it to produce full heat. 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 a Heavy Duty Power Cord with Banana Plugs, Stripped&Tinned, Ring Terminals, or Powerpole® connector. Note that when a Cigarette Plug pigtail is used on the Heavy Duty Powerpole Cord, the cigarette plug will be the bottleneck as it is not a low resistance connector. If your DewBuster™ Controller has a 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). Controllers with cigarette plug power cords will lose a great deal of energy in the cigarette plug (gets hot) and this is energy that is not being applied to the corrector plate heater. 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 also conserves battery power by using the temperature control to only apply heat when needed. This results in 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? The Big-Dob DewBuster™ Controller (BDDB) is a special version intended for large Dobs equipped with a wiring harness having a male RCA plug to mate with the BDDB's female RCA power input jack. Click here for more information.