Temperature and Humidity control

Now that first light had been and gone it was time to start looking at ways to make the observatory more functional so as to exploit its advantages to the full.

Three things demanded priority :  

  1.         Since astrophotography and Deep Sky Objects were my main targets, an f-reducer upgrade to the optical lineup and some sort of autoguiding was essential. A decent DSLR was necessary and it would have to be one with a proven track record in astrophotography and most of all affordable. Some optical filters e.g. a CLS for streetlight pollution and a Neutral density for lunar work might come useful. A low-light video camera for planetary and lunar work would also be desirable. Some complementing free or reasonably priced software would be required to allow control of the shooting from the PC corner.  
  2.         It became rapidly obvious that,  handle or no handle, for a 5ft 7in tall and mostly sedentary member of the species, dragging at the main shutter to open and close it was making unrealistic demands on my shoulder muscles and was possibly responsible for an extended course of physiotherapy.  The greatest difficulty was in pulling the shutter up from its fully open position and dragging it across the zenith and then slowing down its momentum as it fell to its final closed position. This was easy to do with the dome at ground level but lifting it 1.2m above ground it became a nightmare. The angle of attack was lost. A chair provided some help but at some risk. 
  3.          Thirdly and this in fact was probably the most urgent was provision of a constant and reliable control of the internal environment for periods when the observatory was to be on standby. This naturally would be a huge part of the time. Temperature and more especially Humidity regulation was always high on the agenda. 

The running expenses would prohibit installation of a full A/C so I had opted for the four extractors to provide a down draft and heat release from inside by convection.. and  the white paint on the outside to favor reflection. This is not entirely satisfactory of course and falls well short of good control but it does provide an acceptable range of temperatures the year round.

Humidity control was more crucial because in Malta this averages higher than optics feel happy with. The solution was a 230W small mobile dehumidifier. I routed its condensation water by means of a small plastic pipe that drained to the outside from beneath the back wall of the observatory and from there to the roof. This water would track down the roof drain and end up in my well to water my lemons… ahh the cycle of life ! 

Dehumidifier and IR heater

One problem with dehumidifiers that rely on condensation is however that when the Temperature falls much below 15°C their efficiency falls too and at lower temperatures they stop working altogether.

To avoid this I added a home-made infrared heater. This was  based on three 50W IR ceramic bulbs that I mounted on a short section of rectangular aluminum.  This I fixed vertically above a round wooden base and added four small castors.

I surrounded this with a cylindrical tube of 13 mm square “chicken wire” that was anchored by small brackets to the bottom and capped on top with more chicken wire. This shape was adopted to limit its overall footprint and allow me to get close to the dehumidifier.

I position these close to the central pier when the observatory is “off duty”. When actually inside and observing there is little point in leaving these on with the hatch open. So I override the control and turn them off. The dehumidifier then retires under the desk and the heater in the wedge between the desk and the wall leaving the arena clear.

The design allowed me to keep the centre of gravity of the IR unit low and although tall it is very stable and will not topple over easily. It tends to skate instead!


By using this approach and making my own heater I could experiment on how much heat was just sufficient and therefore optimise on my energy requirements. Infrared lamps are available of different wattages and one can mix and match.

In this way the total drain from the dehumidifier/heater was kept at around 400W and this only when they are actually ON because they are under control by relays operated from the sensors on the Control Panel described in a previous chapter.

Another useful fact is that the internal motorized webcam that I installed beside the Control centre can see the invisible infrared glow emanating from the bulbs and confirm that all three are OK.

Some IOT charts to indicate effect of environmental control


Recent chart showing triggering of Dehumidifier/Heater at  66%RH and approx 4% hysteresis
20170622_Temp1_Temp2_ comp_IOT
Comparative  display of diurnal fluctuations in temperature between the inside (Left) and the outside (Right) for one week in June 2017


Comparison  between a hot dry Summer day and a recent humid one

All charts are produced via data uploaded from the observatory to the IOT via appropriate microprocessor controlled homemade sensor units and software that I will hopefully find time to describe at a later stage.




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