Thursday, January 23, 2014

The Intelligent Telescope Dew Shield - iDewShield

Dew has always been a problem during telescope observation, especially so in the summer. The front lens of the telescope cools down rapidly due to emission cooling - the telescope points to the clear sky which readily absorbs any emitted heat by the front lens with nothing to reflect the IR radiation back - the deep-space literally sucks out the heat. As a result, the temperature of the lens drops past the dew point and then humidity in the air forms condensate on the glass.
Dew not only spoils the observation sessions but also dissolves contaminants in the air thus causing deterioration of the anti-reflective coatings. No matter how you slice it - it is a bad thing.
Dew Shields help marginally and they are big and clunky - this adds more weight on the mount too. Heater strips work but they need big marine batteries to haul around and you don't want to over-heat the lens assembly or SCT corrector plate/secondary mirror nor you want to be in the need to jump-start your car. After all, the telescope OTA should stay as close as possible to the ambient temperature to avoid internal air turbulence too.
So, here is the idea I have and it is what I have been working on lately - an Intelligent Dew Shield - iDewShield (again, the dreaded Apple Computers naming convention) based on the Arduino Controller.

The idea is rather simple but should be very effective. Relative Humidity and Ambient  Temperature sensors collect air data and the controller calculates the Dew Point. Then a few degrees of difference are added to the dew point temperature in order to makeup for acquisition inaccuracy and to introduce thermal "inertia". An angled Infrared Temperature sensor takes sample of the front lens (actually the back of the secondary mirror holder) temperature through a hole in the dew shield - no obstruction of the lens and safe way to sample the temperature from a distance. This data is fed to a comparator which controls a Pulse-Width Modulation circuit for the Heater Strip.
The main issue is that the anti-reflective coating and the metal cap in the middle, holding the secondary mirror will have too high reflectivity and low emissivity - this can be resolved either by calibration, factoring the reflective coefficient, or by covering the back of the metal cap of the  secondary mirror holder with black, non-reflective tape.
Knowing the exact Dew Point allows the system to heat just a few degrees above it to prevent condensation.
The main advantage of iDewShiled is that - there is no danger of excessively heating the lens and it will be a tremendous energy-saver when operated on battery. Because the lens temperature will be just what is needed to prevent dew from forming and not more, internal air movement due to convection will be minimized in the OTA. Finally, no modification to the telescope is needed - the iDewShield attached to the front with a Velcro strip just like a regular flexible Dew Shield. Adjusting the distance between the IR sensor opening and the front edge of the OTA allows for a precise "spot" measuring of the secondary mirror  temperature (which is mounted on the corrective lens) as the sensor has a tight directional pick-up pattern.

A red LCD display with variable brightness, will allow the user to monitor lens temperature, air temperature, air humidity and calculated dew point. The controller will also include low voltage alarm and a voltage shut-down threshold. Calibration constants for all 3 sensors can be entered using a simple 3-button interface.
I am also toying with the idea to include a "dumb" mode - the ambient temperature sensor will be removable from the main sensor unit and can be inserted under the Heating Strip or attached the lens retaining ring with tape/magnet. In the "dumb" mode the controller will operate just as a regular thermostat following a preset temperature.

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