Phil Hart
03-06-2013, 10:20 PM
The commercial controllers for dew heaters have three issues as far as night sky photography is concerned:
Expensive: $100+ just for the dew heater controller. This might be ok when you need multiple outputs to protect a telescope and guidescope, and possibly finderscope/eyepiece/camera lens as well, but that's an expensive solution to keeping just one camera lens free of dew. These controllers also make sense when you need fine control over the amount of heat applied to avoid telescope tube heat currents and the thermal distortion of delicate telescope lenses and mirrors. However, with wide angle camera lenses these issues are not generally relevant.
Complicated: It's another black box and an extra length of cables and connectors to carry and hook up.
Require heavy batteries: These controllers are designed to run off lead-acid batteries providing greater than 12 volts (and are often powered by DC power supplies connected to mains power). All the newer controllers will cut-out if the voltage drops below 12 volts, to protect the lead-acid battery from being over-discharged. This is great for protecting your battery, but a lead-acid battery that can provide several amp-hours of heating in cold conditions and still stay above the cut-off voltage ends up being quite heavy and inconvenient to carry up a mountain for example.
Lithium-ion batteries offer a great lightweight alternative to lead-acid batteries, but because their supply voltage quickly drops below 12 volts, they cannot be used effectively with the existing commercial dew controllers. In fact with the high resistance of the small dew heaters for wide-angle lenses, they could be run directly from 12 volt supply without any controller, but there is no easy way to plug the standard RCA connector on a dew heater to a DC power supply (RCA was a dumb connector to choose as the standard for dew heaters).
The Solution
My solution is to simply hack the connectors and plug the dew heater straight into a lightweight 12v lithium-ion battery pack.
Rather than cut off the RCA end and stick on a 2.5mm DC connector, I've made up my own adapter cables so that I can still use the dew heaters with the controller when I'm using the scope and mains/big battery power etc (see attached pic). You can make this adapter cable very easily from components on ebay.
Power Use and Specifications
The Lithium-ion batteries run at around 11.5 volts (anything from 12.6 fully charged down to less than 10 at full discharge). The Dew-Not 2" heater has 38 ohms resistance. This draws current of 0.3A and power output of 3.4W which is plenty for just a lens. On paper you should get 22 hours run-time from the 6800mah battery but I would not assume anything like that. I do comfortably get 8 hours run-time out of them (perhaps less in extreme arctic cold conditions though). If they are flat, the batteries can take a *long* time to recharge so put them on to recharge early in the day if you want to reuse them again the next night.
The 2" Dew-Not heater (~20cm) plus elastic and velcro is long enough to go round all my lenses, although there can be a bit of a gap in the heater material with just the elastic reaching round the last bit of the lens, but in practice this is fine.
The 2" Kendrick heater (~15cm) is a bit shorter leaving a larger gap and in some cases even the velcro does not reach round to attach at all around very big lenses. But the higher resistance of 49 ohms draws less power (0.23A, 2.7W) meaning longer battery life. In many situations this may actually be preferable. The 3" Kendrick heater (~25cm) has a lower resistance of 29 ohms and probably draws more than ideal (0.4A, 4.5W) from the Lithium-ion battery.
The dew heater and battery can now all fit in one pocket - much easier to carry round and hook up!
I've copied this content from a post on my site (http://philhart.com/content/dew-heaters-night-sky-photography) for anyone who wants to do the same thing.
cheers
Phil
Expensive: $100+ just for the dew heater controller. This might be ok when you need multiple outputs to protect a telescope and guidescope, and possibly finderscope/eyepiece/camera lens as well, but that's an expensive solution to keeping just one camera lens free of dew. These controllers also make sense when you need fine control over the amount of heat applied to avoid telescope tube heat currents and the thermal distortion of delicate telescope lenses and mirrors. However, with wide angle camera lenses these issues are not generally relevant.
Complicated: It's another black box and an extra length of cables and connectors to carry and hook up.
Require heavy batteries: These controllers are designed to run off lead-acid batteries providing greater than 12 volts (and are often powered by DC power supplies connected to mains power). All the newer controllers will cut-out if the voltage drops below 12 volts, to protect the lead-acid battery from being over-discharged. This is great for protecting your battery, but a lead-acid battery that can provide several amp-hours of heating in cold conditions and still stay above the cut-off voltage ends up being quite heavy and inconvenient to carry up a mountain for example.
Lithium-ion batteries offer a great lightweight alternative to lead-acid batteries, but because their supply voltage quickly drops below 12 volts, they cannot be used effectively with the existing commercial dew controllers. In fact with the high resistance of the small dew heaters for wide-angle lenses, they could be run directly from 12 volt supply without any controller, but there is no easy way to plug the standard RCA connector on a dew heater to a DC power supply (RCA was a dumb connector to choose as the standard for dew heaters).
The Solution
My solution is to simply hack the connectors and plug the dew heater straight into a lightweight 12v lithium-ion battery pack.
Rather than cut off the RCA end and stick on a 2.5mm DC connector, I've made up my own adapter cables so that I can still use the dew heaters with the controller when I'm using the scope and mains/big battery power etc (see attached pic). You can make this adapter cable very easily from components on ebay.
Power Use and Specifications
The Lithium-ion batteries run at around 11.5 volts (anything from 12.6 fully charged down to less than 10 at full discharge). The Dew-Not 2" heater has 38 ohms resistance. This draws current of 0.3A and power output of 3.4W which is plenty for just a lens. On paper you should get 22 hours run-time from the 6800mah battery but I would not assume anything like that. I do comfortably get 8 hours run-time out of them (perhaps less in extreme arctic cold conditions though). If they are flat, the batteries can take a *long* time to recharge so put them on to recharge early in the day if you want to reuse them again the next night.
The 2" Dew-Not heater (~20cm) plus elastic and velcro is long enough to go round all my lenses, although there can be a bit of a gap in the heater material with just the elastic reaching round the last bit of the lens, but in practice this is fine.
The 2" Kendrick heater (~15cm) is a bit shorter leaving a larger gap and in some cases even the velcro does not reach round to attach at all around very big lenses. But the higher resistance of 49 ohms draws less power (0.23A, 2.7W) meaning longer battery life. In many situations this may actually be preferable. The 3" Kendrick heater (~25cm) has a lower resistance of 29 ohms and probably draws more than ideal (0.4A, 4.5W) from the Lithium-ion battery.
The dew heater and battery can now all fit in one pocket - much easier to carry round and hook up!
I've copied this content from a post on my site (http://philhart.com/content/dew-heaters-night-sky-photography) for anyone who wants to do the same thing.
cheers
Phil