A New Approach to Dew Control
As will be well known to any astronomer using a refractor or a catadioptric telescope (that is any telescope with an optical component at the front of the optical tube), as the telescope cools down dew tends to form on that front component. This happens because the telescope cools by radiation to the sky more rapidly than the air does. If the temperature of the front component falls below the dew point of the air, then dew forms on it. There are conventionally two ways of combating this problem:
- A dew shield. This is a cylinder of plastic or metal fixed to the front of the telescope and sticking out in front of it. The idea is that this traps a layer of air in front of the telescope. What happens is that the small amount of water in that trapped layer deposits on a surface but is not replaced readily. Good examples have absorbent material on the inside and the water tends to form on it preferentially to the front lens. In addition the shield reduces the area of sky to which the front of the telescope is exposed. This reduces its radiative loss to the sky and so reduces the drop in temperature that occurs. This in turn reduces the tendency for the water vapour to condense.
- A heater. An electrical heater is wrapped around the front of the telescope and warms the front enough to prevent the formation of dew. It only needs to keep the temperature at or fractionally warmer than the air.
It seems to me that there are disadvantages to both these approaches.
It seemed to me there had to be another way.
- I have a commercial dew shield that fits my LX200 which is 30 centimetres (12 inches) long. Whilst this works, it does not work well and in an hour or so dew still forms on the front plate. I extended it to more like 60 centimetres which worked a lot better but, if there was any wind, it would catch the long dew shield and rock the telescope.
- I have not tried a heater, but in principle it seems to me not to be a good idea to heat the front of the telescope. More importantly, it consumes quite a lot of power, which is not too bad if mains power is available, but is quite a drain on a battery. Also the controllers that are generally needed are quite expensive.
My idea was to use a short dew shield and to blow a slow stream of dry air into it. Hopefully this would reduce the amount of water vapour in the air in front of the corrector plate to the point where it would not condense out. In addition, the flow of air at ambient temperature may reduce the extent to which the corrector plate cools below ambient.
- The desiccant
There are a number of desiccants that could be used and there would be an advantage, in my arrangement, if it was a liquid one. However I do not know of a liquid desiccant that would be both safe and convenient to use. To me the obvious desiccant was silica gel. This is a crystalline material which is inert and completely safe (it is a sort of hydrated sand), and it can be regenerated in an oven at 110°C. It can also be bought self-indicating—that is it changes colour when it needs regenerating. There are a number of different indicators, but in my view the best is one that is blue when dry and goes pink when the silica gel is about half saturated. This contains 0.5% Cobalt Chloride and Health & Safety consider Cobalt hazardous, but it is trapped within the silica-gel granules and I have been handling it on and off for the last 50 years and, as far as I can tell, I have suffered no ill effects. I use it without concern and it can be purchased without restrictions (in the UK anyway). I do have an alternative using an organic indicator which goes from yellow to green but this is unstable and only lasts a few regenerations. There is also one containing an iron salt which is stable, but I have no experience of that. The blue-pink variety turns fully pink at a relative humidity of 40%, which I believe may not be adequate, but if the top of the column is still blue, the relative humidity is below 20%, which I believe will always be adequate.
- Passing air through the desiccant
I needed a container that had an entry and exit port for the flow of air. My first idea was an in-line petrol filter from a car. The one I bought proved to be much too small, but then I woke up to the fact that I trained as a chemist, and chemists have gas-washing bottles. These consist of a tall glass jar fitted with a stopper that carries two glass tubes, one of which (the left-hand one in my picture) extends down to near the bottom of the jar. They are designed to pass a gas through a wash liquid. I found one on line in the UK for £16.67 (including VAT). Because it is tall and narrow and made of glass, I felt that a retort stand to hold it would be a good idea too (£17.84 from the same supplier). This assembly is shown in the picture. The glass joint needs to be greased which needs to be wiped off before the silica gel is poured out or small particles stick to it.
- Delivering air to the telescope
I use a pump designed to deliver an air stream to an aquarium. I started with a very small one I happened to have which delivered less than a litre per minute. I concluded that this was not adequate but I was using a different form of silica gel at the time and I'm not convinced that it had not become exhausted, so the jury is still out on that one. I bought another pump which delivers 3.5 litres per minute and that is certainly adequate. It has two outlets, so would be easy to use at half speed by only connecting one of them.
Suitable tubing is available from any shop selling aquaria and fits the pump, but is too small for the gas jar. Fortunately the external diameters of the tubing and the gas jar are about the same and I found some tubing with the corresponding internal diameter and I could use it as shown in the picture. This has the advantage that the connections are easily disconnected when the jar needs refilling.
- The Dew Shield
From the same place I got the pump I was able to buy some T-connectors. They came in packs of five. I needed one at the pump to join its two nozzles into one tube, and another at the dew shield to connect the delivery tube to a loop of tubing encircling the dew shield. This left three, and I used them to deliver air to the inside of the dew shield. I made three small holes in the shield (it is plastic) spaced approximately at 120° around the cylinder, and pushed the vertical part of the T-connector through it and used its other two legs to continue the loop.
I have run the system for three hours with an ambient relative humidity of 80 to 90% and got no dew whatever on the corrector plate. The rest of the telescope was wet. The effect on the silica gel can be seen in the first picture which was taken after that session. The layer at the bottom is pink but further up it is still blue. This shows that the relative humidity of the air was less then 20% by the time it exited from the top of the column. I don't have a suitable sensor to measure the actual humidity, but, at 20% relative humidity, condensation should not occur until the face plate reaches almost 20° below the temperature of the air.
Advantages and Disadvantages.
- It works with a shorter dew shield than without it. I've not experimented with shorter ones than the one I have.
- It is easily adapted to other piggybacked systems. For example I have adapted it to a lens hood for my Canon telephoto lens by simply drilling a hole near the base, disconnecting the delivery tube from the dew shield and pushing it into the hole in the lens hood. This keeps that lens free of dew too.
- It uses less power at the telescope than a heater. Overall it may use more because of the power needed to regenerate the silica gel.
- The pump is a little noisy (for the dead of night) and mine runs off the mains and requires alternating current. Most aquarium pumps do, but battery-driven ones are available.
- The gas jar is rather difficult to assemble when filled with silica gel. It is quite difficult to work the glass tube down into the crystals. I find that if I don't fill the jar more than ¾ full and tilt it sideways and gently shake it, I can work the tube down without too much difficulty. A container where the long tube remained in place when the top was opened would be more convenient. One could probably be made out of a jar and rubber bungs.
Suppliers in the UK (as of 18th March 2012).
- Silica Gel. GeeJay Chemicals Ltd, Sandy, Bedfordshire. http://www.geejaychemicals.co.uk/. They sell silica gel in bulk or in sachets in various configurations. Originally I bought the sachets to keep my spare parts dry in their cases, but the sachet material has not proved long-lasting. I also originally bought the organic version and when I remarked on its instability the company gave me a few blue sachets foc. I have found this company to be extremely helpful and they have answered all my questions. The sachets contained the gel in ~1 mm beads, but eventually I bought bulk 3-6 mm granules which were cheaper.
- Gas Jar and Retort Stand. Rapid http://www.rapidonline.com/Education/Science/Laboratory-Equipment (N.B. This does not work as a link, so go to http://www.rapidonline.com initially, then use the left-hand pane to navigate to Education and Science, then page down to Laboratory Equipment.) I also obtained from here a small plastic funnel with a wide stem to aid with pouring silica-gel crystals into the jar.
- Air Pump and Tubing. These are readily available from any aquarium supplier. I bought a Luby LB-7000 through Amazon UK but they do not list it any more. I have seen it listed as a Pisces pump at http://www.aquatix-2u.co.uk/acatalog/pisces_aquarium_air_pump.html for £7.99. Battery-driven ones are
available (e.g from Amazon). The T-pieces came from Angels Aquatics and Pet Supplies via Amazon. I obtained the tubing from a garden centre.
- Dew Shield. I've had this for years and don't remember where it came from but it is a standard flexible dew shield for the LX200.