Kendrick Newtonian Heaters
We have primary mirror heaters for 8" to 18" mirrors. They are very effective in the prevention of dew on Newtonian telescope optics, particularly for those who have Dobsonian telescopes with open truss tube designs.
These are installed behind the mirror in between the mirror and the mirror cell and are held in place by putting a thin skim of silicone caulk (easily found at any hardware store) onto the heater and allowing the heater to dry to the back of the mirror. We do not recommend using heaters that would wrap around the outside of the optic as the thickness of Newtonian primary mirrors make this method ineffective due to the mirror mass. It is much better and much more effective to heat a primary mirror from behind.
The heaters require the use of a Dew Controller. Any Kendrick Controller model can be used for low-wattage heaters and for higher wattage, all but our Micro Controller (the small, light-duty model) are the perfect match. These heaters can be used with a temperature sensor on some models of our Elliptical and Primary mirror heaters, when used withour our DigiFire-12 Controller (#2001-12), and soon to be released Premier Controller (summer 2016).
It is recommended that the Primary Mirror heaters be used at a low controller setting (ie: 15% to 20% in order to prevent optical distortion. The high wattage is built into the heaters to help clear dew or frost that is already on the optic. Once cleared reset your controller to a lower setting.
Our Standard Elliptical heaters fit behind the secondary mirror and work best in Newtonian telescopes that have the removable type of secondary mirror housing.
The Split heater is also meant to be attached to the back of a secondary mirror but is to be used with larger secondary mirrors that are attached to mirror stalks. As the heater is flexible, it can easily be placed around the mirror stalk and then the heater is glued to the back of the mirror.
The Wrap-around heaters can be used with secondary mirrors that are attached to stalks* or can be wrapped around the outside of a mirror holder**.
*Some stalks connect to the secondary mirror using double sided foam tape. This tape will insulate the stalk from the mirror, making it difficult for the heater to do its' job.
**Using a Wrap-arpound heater on a typical secondary mirror holder will increase the size of the secondary mirror obstruction but it will be very effective. If your secondary mirror is larger than 1.83" in width, we recommend using the Standard Elliptical heaters.
Which one fits my scope?You will need to study how your secondary mirror assembly is constructed and take some measurements. As much as we would like to have one of every telescope in our shop, well, we can't do that. Manufactureres also don't share CAD drawings or even dimensions. Here are some basic guidelines to steer you in the right direction. NOTE: Heaters CANNOT be trimmed to fit (it will sever the heating element)!
Guide to choosing the right secondard heater We make several styles of secondary mirror heaters. Ideally, a heater that is able to make direct contact with the back-side of the mirror works best with lower power consumption:
More information on these various heaters can be found below.
These heaters are very effective at preventing and/or removing dew and frost from your secondary mirror. Using your Kendrick controller, you may turn the power up high to quickly clear the optics of moisture if they are dewed or frosted over or you may use a very low power setting to keep the optic barely above air temperature and prevent the formation of dew or frost.
These heaters work best in Newtonian telescopes that have the removable type of secondary mirror housing pictured here. They can also be used with secondary mirrors that are attached to stalks. To use them in this situation, the mirror will need to be removed from the stalk, the heater glued to the back of the mirror and the mirror (with the heater) glued back onto the stalk. If you do this, be careful that the mirror is firmly attached to the stalk again as you will not want it falling off onto your primary mirror!
Our Split heaters are designed to be used with larger secondary mirrors that are attached to mirror stalks (as opposed to being inside a secondary mirror holder). As the heaters are flexible, they can easily be placed around a mirror stalk. The heater is glued (glue not supplied) to the back of the mirror.
These heaters have plenty of power to clear a fogged over secondary when set to high power. Under normal operation, they should be run at 30% to 50% controller setting, depending on conditions.
To be effective to its full potential, it will need to be covered with foam or some kind of insulating material so that the heat is directed into the glass and not the atmosphere. A self adhesive, neoprene insulator is provided with the 2029-M, 2029-XM, 2029-L and 2019-XL heaters.
Specifications (note: All electrical specs stated at 100% controller setting):
The wattage fairly high for a heater of this type but this is necessary if you ever need to clear your secondary of dew that has already formed. With your Kendrick controller you can reduce the ceuurent draw of these heaters to very little. Our recommended controller setting for these heaters is 10% to 20%. Your observing environment will determine what setting you must use.
The Dew-Guard Temperature Sensing heaters (DG1, DG2 and DG3) have been out of production since 2012. Not to worry, there are still many options for keeping your seconday mirror dew & frost free!