Why the band area is the number, not the aperture
A strap heats a ring of tube. The power it delivers is the area of that ring multiplied by whatever heat flux the strap is built to produce, and the area is simply the circumference times the width: pi times the diameter, times the strap width. A 235 mm tube with a 50 mm strap wraps 36,900 square millimetres, which is 57 square inches.
This is why a wide strap on a small tube can want more power than a narrow strap on a big one, and why quoting heater wattage without the strap dimensions tells you very little. It is also why the flux figure has to come from the strap maker or from your own measurement — it depends on how the element is wound and how much of the strap is element at all, which varies between products and is not something this page can assert.
Duty cycle is where the whole range lives
Full output is a number the strap can produce. It is almost never the number it produces. A controller senses temperature or humidity, or simply gets set by hand, and it runs the strap at some fraction of full. On a dry breezy night that fraction can be very low. On a still, damp night under a clear sky it can sit near the top for hours.
| Conditions | What drives the controller |
|---|---|
| Breeze, dry air, wide dew point margin | Little or no heating needed — air movement keeps the glass near ambient |
| Still air, clear sky, wide margin | Radiative cooling still drops the glass below ambient, so some heating even in dry air |
| Still air, clear sky, narrow margin | The hard case — sustained high duty for hours |
| Cloud moving in | Cloud reduces the radiative loss, so demand usually falls as the sky closes |
The consequence for battery planning is that the average matters and the peak does not, right up until the night when the peak is the average. Size the pack for the wet night if you do not want to pack up early, and use the comparison field on the calculator to see what that costs.
Radiative cooling is why the glass goes below air temperature
An exposed piece of glass under a clear sky radiates heat upward and gets very little back, because the effective temperature of clear sky is far below the air temperature. The glass therefore settles below ambient, and if it settles below the dew point, water condenses on it. That is the whole mechanism, and it explains two things people find odd: that dew forms on nights when the humidity does not look extreme, and that a dew shield helps enormously despite doing nothing thermally active.
The shield works by blocking the corrector view of the cold sky, replacing part of it with a tube at roughly air temperature. Less radiative loss means the glass sits closer to ambient, which means less heating needed to hold it above the dew point. On a modest setup a long shield can do most of the job on its own and reduce the heater to an occasional top-up, which is the cheapest watt-hour you will ever save.
Heat costs seeing, so less is better
Warm air rising off a heated corrector goes straight up the light path and stirs the image. Run the heater harder than needed and you trade dew for a soft, boiling view, which is a bad trade when the dew was never going to arrive. The right target is the smallest setting that keeps the glass a degree or two above the dew point, not a comfortable margin.
The practical way to hit that is to measure rather than guess. A thermometer and a hygrometer give the dew point through the dew point calculator, and a cheap probe taped to the tube gives the glass temperature. Watching the gap between the two tells you what the controller should be doing far better than watching the percentage readout, which is only telling you what it is doing.
Working out the pack
Average watts times hours gives watt-hours. Divide by the bus voltage for amp-hours. Then divide by the fraction of the pack you are prepared to use, because nobody runs a battery flat and different chemistries tolerate different depths. That last figure is a decision plus a specification, and the specification belongs to whoever made the cells.
Two things then reduce what you actually get. Cold takes a bite out of delivered capacity, and the size of that bite depends on the chemistry. Discharge rate takes another, more on some chemistries than others. Neither is captured by a nameplate figure and neither is a constant this page could sensibly assert, which is why the battery runtime calculator exists as a separate page with those inputs. Plan the night with a margin, and treat the heater as the load that grows when conditions get worse rather than the one that stays put.
Questions people ask
How many watts does a dew heater strap need?
Work the band area first: pi times the tube diameter times the strap width, converted to square inches. Then multiply by the heat flux the strap produces, which is a figure from the strap maker or from your own measurement rather than a universal constant. A 235 mm tube with a 50 mm strap is about 57 square inches, so at 0.4 watts per square inch it is around 23 watts at full output — and it will spend most of the night well below full.
Why does dew form when the humidity is not especially high?
Because the glass gets colder than the air. Under a clear sky an exposed optic radiates heat upward and receives very little back, so it settles below ambient temperature. If it settles below the dew point, water condenses. This is why still clear nights are the worst and why a breeze or cloud cover reduces the problem, and it is the reason a dew shield helps so much despite being a passive tube — it blocks the view of the cold sky that the glass is losing heat to.
Does running the heater harder hurt the image?
Yes, and it is easy to overdo. Warm air rising off the corrector sits in the light path and makes the image boil, which costs exactly the fine detail you were trying to record. The useful target is the smallest setting that keeps the glass a little above the dew point, not the setting that keeps it comfortably warm. Measuring the dew point and the glass temperature and watching the gap is a much better guide than the percentage on the controller.
How big a battery do I need for a night with heaters running?
Average heater watts times session hours, plus everything else drawing from the same pack, gives watt-hours. Divide by the bus voltage for amp-hours, then divide again by the share of the pack you are willing to use. Size it on a bad night rather than an average one, because the heater is the load that grows when conditions turn. Note that a nameplate capacity is quoted warm and at a gentle discharge rate, and a cold night in the field gives you less than that.
Is a dew shield a substitute for a heater?
Often a large part of one, and it costs nothing to run. The shield blocks the optic view of the cold sky, which is where the heat is going, so the glass sits closer to air temperature and needs less help to stay above the dew point. On a mild night a long shield can do the whole job. On a still night with the air close to saturation it delays the problem rather than removing it, and that is where the heater earns its watt-hours.