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Exterminating Dew: Review of the Celestron Dew Control System

Celestron Dew Heater Ring connected to Smart Dew Heater Controller. Credit: Celestron

I have been testing the Celestron’s dew control system on my Celestron EdgeHD 9.25” for the last two months at my home in Ohio, where humid, dewy nights provide a good stress test for a dew control system. This system, designed to keep the corrector plate of Celestron SCTs from fogging up, consists of the Dew Heater Ring, the Aluminum Dew Shield, and the Smart DewHeater Controller (the 2x version in my case).

Overview

I was using a traditional dew strap running on 12V power from an analog dew controller box before trying out the new system. I kept the box turned up high all the time since conditions often change throughout the night. But this extra heat can cause heat currents inside of the telescope tube, which can reduce the clarity of your images. Even cranked up high, set up on the grass at the local dark sky site, the heat didn’t always make it to the center of the corrector plate, leaving a ring of dew around the secondary.

The Celestron dew control system brought a few major changes to the game. First, the dew heater ring is in direct contact with the glass of the corrector plate, applying heat directly instead of through the metal of the outside of the tube. This helps the heat conduct better through the entire corrector plate, keeping even the center free from dew. Second, the dew controller is set to apply just enough heat to keep the corrector plate a little above ambient, preventing the formation of dew without applying lots of heat that might cause heat currents. The temperature and humidity sensor are built-in, and a thermistor cable attaches to the dew ring to monitor the actual temperature of the glass, allowing precise automatic adjustments. Third, the sturdy aluminum dew shield also helps keep dew from forming, which means the dew controller can use less power (important when you are running on battery!). The dew shield is lined with velvet to prevent collected water from sliding down onto the telescope.

Dew Heater Ring Installation

The dew heater ring installation was relatively simple – removing the corrector plate retention ring (with the scope facing up so the corrector won’t move), slide on the dew heater ring, and rotate to a specific position where the cables will feed through a clip that holds them in place and feed out of the dew shield. There is a notch on the ring where you slide in the peg that holds the telescope’s cap in place; there is only one notch though, so the ring must be rotated to drop the other half of the ring past the other peg (see illustration below).

Top: Drop the dew heater ring through the peg at the notch; rotate around to the other peg; this will allow you to maneuver the ring down past both pegs. Bottom: There is no notch on the opposite side. Credit: AstronoMolly

With one of the sections of text aligned with the dovetail as described in the instructions, the cables were on the lower left side of the corrector, just in the right place to feed out through a raised gap in the dew shield. The dew heater ring packaging also includes a sheet of paper to place over the corrector plate to protect it from detritus and fingerprints, which is a nice touch. The power cable for the ring is included in the box; the cable for the thermistor comes with the dew controller and is quite long (it ran all the way down from the top of the tube to the ground through my cable bundle).

Celestron 9.25” Edge HD with the dew heater ring installed. Credit: AstronoMolly

Dew shield installation

The aluminum dew shield has two large clips that slide over the wide metal lip at the end of the OTA to latch it into place, and it has a wide notch cut into it for the dovetail (with extra room). There is a raised segment on the side where the dew ring cables are, allowing them to pass through; it can also be used to run camera cables in the Hyperstar/Fastar configuration.

The Celestron aluminum dew shield installed on my 9.25” Edge HD. Credit: AstronoMolly
Opening for the cables (the glued-on label came off in the heat). Credit: AstronoMolly

The dew shield comes with its own cap, which means you can leave the shield on and still cover the scope’s corrector plate. The shield is quite long, however, and I have to detach it and stow the cables every morning in order to cover the scope with my outdoor scope cover. The ring has slots to hold the cables in place when they’re not connected. Fortunately, it’s not too difficult to take on and off.

Dew Controller Installation

The dew controller can be attached to the mount by strapping it to a tripod leg or sliding it onto a dovetail (either on top, or any front-hanging portion of the bottom dovetail). It fits both Vixen and Losmandy-width dovetails.  

Dew controller attached onto a tripod leg (left) and on the dovetail plate (right). Credit: Left: AstronoMolly; Right: Celestron

The controller has a built-in fan for heat dissipation, and a “bridge” over the top where you can wrap up extra cable. The bridge also has dim red status lights, as well as the temperature/humidity sensor (to keep it away from the heat exhaust fan).

The 2X version of the controller I tested has several ports: two 12V RCA-style dew heater ports (one for the dew ring, one for any other 12V dew device such as a strap for a guidescope or eyepiece), two thermistor ports, two AUX ports (a cable is included to connect the controller to a Celestron mount), a USB type B port to connect to a computer, and a 12V DC port to supply power to any other device. Connecting to a computer is optional, but you can use Celestron’s CPWI software (available free on their website) to communicate with the dew controller to see power and environment graphs, change the power supplied to each dew device, and activate/deactivate the spare 12V port. To connect to CPWI, select “Mount USB” as the device type in CPWI, and it will detect the controller automatically.

I initially had some trouble getting a tight connection to the input 12V power port, even with a 2.1mm barrel. Celestron sells a 5A power supply with the same screw-on connector used on their mounts, though, and that gives a much more solid connection.

Performance

The complete system does the job of preventing dew very well. With the controller supplying just the amount of power the dew ring needs, I have yet to see any dew on the corrector plate in the morning, and I’ve noticed no heat currents inside the tube from arbitrarily high heat. I use the other dew device port to run the dew strap for a camera lens on a tripod beside my main scope. The dew controller’s connection to CPWI has been very stable but the controller will continue to operate even if I disconnect it from the computer.

Monitoring the dew controller using CPWI. Credit: AstronoMolly

Any one of these components – the dew shield, the dew heater ring, and the dew controller – can be used independently; the dew heater ring can be powered by a different device, or the dew controller can power any type of 12V dew device – but they do work nicely together, particularly with the thermistor information going from the dew heater ring to the controller.

Conclusion

The system has worked quite well, even on very damp nights. The dew controller is easy to operate using the free CPWI application; the dew heater ring was straightforward to install and does the job well; and the dew shield is quite nice, particularly with the built-in gap for either the dew ring cables or other cables from a Hyperstar/Fastar configuration.

The dew heater ring is available for several sizes of Celestron’s Schmidt-Cassegrains and RASAs (5”, 6”, 8”, 9.25”, 11”, and 14”), and the dew shield is available in all of those same sizes except for the 5”. The Smart DewHeater Controller 2X is also available in the 4X variant, which has three USB 3.2 ports, 4 dew heater ports (each with a thermistor port), three 12V output ports plus one variable DC output, and a third AUX port to serve as a control and power hub for all of your devices.

MSRP:
Celestron Dew Heater Ring, 5” to 14”: $39.95 to $159.95
Celestron Aluminum Dew Shield with Cover Cap, 6” to 14”: $109.95 to $299.95
Celestron Smart DewHeater Controller: $259.95 (2x); $439.95 (4x)

Website: www.celestron.com

 

About AstronoMolly

I got into astrophotography in July 2015 after receiving my first telescope as a gift. Much trial and error later, I now have three astrophotography rigs set up in my backyard just north of Berkeley, CA, in the San Francisco Bay area, including one dedicated to variable star and exoplanet transit observations. I love doing STEM and astronomy outreach, and I've accrued more than 150 hours of volunteer activities reaching over 20,000 people, both in-person and virtually. I am an AAVSO Ambassador (American Association of Variable Star Observers), an Explore Alliance Ambassador, and a panelist and broadcaster for The Astro Imaging Channel weekly YouTube show. I have a B.S. in Physics from Washington State University, and am currently pursuing my PhD in Physics at University of California, Berkeley, studying neutrinos with my two cats, Orion and Apollo.

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