PocketCloudWatcher test report

This month we bring a sort of personal weather guard: a small station that works along with a mobile application to inform us of local meteorological conditions in real time.

Lunático Astronomía markets a small meteorological station for the observatory and the astrophotographer. In a nutshell, it measures several environmental parameters and transmits the data to your phone via Bluetooth, so any device running the Pocket CW application can read it. There is also a version with a mini-USB output to monitor the readings on a PC. In short, a mini meteorological station with no wires and alerts on your phone: alerts that warn you if the sky clouds over, if there is an excess of ambient light, if there is a risk of condensation, if the battery is discharging, or if the temperature varies outside the margins you have established.

Before starting

At first sight you can see two sensors that measure IR and sky brightness; the other detectors are inside. Pocket CW measures ambient temperature, sky temperature (based on the infrared radiation it receives), ambient light in luxes, relative humidity, dew point, and battery charge level. It's a nice touch that it measures humidity and, together with the temperature, the dew point, the temperature at which humidity will begin to condense on the surfaces of the telescope. That's something we should always be aware of, since a fogged lens, Schmidt plate, meniscus, or mirror is a real burden.

Cloud detection is done by measuring the sky temperature with an IR sensor, and it's the only reading that requires some practice in order to determine the appropriate temperature range. Lunático advises setting the alarm about fifteen degrees above the reading when the sky is clear. That way, when the sky temperature rises fifteen degrees, it will be because of clouds, and the alarm will sound.

Pocket CW is powered by a 12 V DC supply through a standard 5.5 mm / 2.1 mm jack with positive in the center. This same input is used to charge the optional internal battery, which according to specifications lasts almost 48 hours of continuous operation. Lastly, for Bluetooth communication to work properly, it is recommended not to go farther than fifteen meters away.

Figure 1. The Pocket CW device is very small: it measures barely 70 × 70 × 27 mm and weighs 85 grams. There is also a version with a mini-USB output to connect it to a computer.
Figure 2. At the top of the Pocket CW there are two detectors: the small one is an IR detector that alerts us to the presence of clouds, and the large, transparent one is the luminosity detector, expressed in luxes.

Figure 3. The temperature and humidity sensors are located inside the Pocket CW; when it's turned on, a blinking red light lights up inside. It communicates via Bluetooth with our mobile phone; we only need to download the Pocket CW app.
Figure 4. The Pocket CW's range is about twenty meters without obstacles, and about three meters if you are in a car. It is important to place it at some height so that the coverage is correct: on the ground, it barely reaches three meters without obstacles.

Weather guard

Whenever I have done deep-sky astrophotography sessions, they have always been long. In fact, to obtain an adequate signal in each shot and enough shots to satisfactorily lower the noise level, sessions almost always run two to three hours of capture, or more.

And you are always a bit preoccupied when leaving the equipment working without your supervision: "Maybe the lens or the secondary mirror gets fogged, or maybe that thing that looks like a threatening band of clouds ruins my photos…"

There are environmental factors we cannot control if we are resting inside the vehicle or simply don't have access to the telescope. The ones I consider most important are cloudiness and dew point. Cloudiness is obvious: if it clouds over mid-session we go home with a bad taste in our mouths, since we won't have recorded the shots we wanted and will have wasted time on the mountain under the clouds. The Pocket CW has an IR sensor that detects the appearance of clouds and alerts us with an alarm on the mobile phone. The same happens with the humidity and temperature sensors that determine the dew point, also warning us with an alarm when the temperature gets dangerously close to that point.

The question of cloudiness requires some learning. Pocket CW measures the temperature of the sky, in height, with an IR sensor. We know that when the sky clouds over, the temperature increases, so Pocket CW detects cloudiness based on the sky's temperature. Suppose it measures −30 ºC: a cloudy sky is a warmer sky, so when it clouds over the temperature will rise. It is recommended to set the alarm 10 or 15 ºC higher than the temperature of a clear sky for the same location. That way, when Pocket CW measures a sky temperature of −20 ºC or −15 ºC, it will interpret that it has clouded over and the alarm will ring. These IR readings may vary from one observing site to another. By the way, high clouds do not emit heat, so Pocket CW does not detect them.

The dew point is presented in degrees of temperature. For example, if it reads 6 ºC, it means that that night, with the existing humidity level, condensation will occur if the temperature drops to 6 ºC. Astronomy lovers hate condensation.

We may not mind much if the tube or mount condenses moisture, to the point of having to dry them a little when packing up. But we are very bothered if moisture condenses on the lenses or mirrors of the optical tube, since that forces us to stop the observation or photographic session. A fogged lens kills the contrast, scatters light, and reduces luminosity to the point of having to cancel the observation. That's why a dew cap is so important to delay condensation a little, especially on closed tubes with large apertures. Refractors typically have a small aperture and a long dew shield, so they are slow to fog.

The secondary mirror of a Newtonian is more exposed than it seems, and in very humid environments it can fog more easily than expected. It is less common, but on very wet nights the primary mirror can fog too, worse still on a Truss tube. On nights with lower humidity, refractors are ideal, while Newtonians condense less than a Schmidt- or Maksutov-Cassegrain. A closed Cassegrain with an aperture of 200 mm or more will tend to fog very easily. In any case, all telescopes, sooner or later, are sensitive to condensation on their optical surfaces, so knowing the dew point value is essential if we want to anticipate it.

Coverage

I spent several nights with the Pocket CW. The first thing you notice is that you have to place it at a certain height off the ground so the signal has a decent range. On top of a table, the Bluetooth transmission reaches fifteen to twenty meters seamlessly, as long as there are no obstacles. Putting it on the ground is a bad idea: it barely reaches two or three meters. We usually don't sleep bivouacking in the open, but rather in a tent or a vehicle. I don't have a tent handy, but I do have my van: I go inside, close the doors and windows except for a small slit to breathe, and check the Pocket CW's coverage. It's enough, but be careful: three meters away from the van, the mobile still receives data from the Pocket CW, but move further and you may have problems. In any case, the Pocket CW doesn't need to be next to the telescope: it measures environmental factors, so putting it next to the tent or the car is more than enough.

Battery

It lasts a long time and charges wonderfully. All you have to do is connect the power jack of the battery that supplies the telescope to the Pocket CW, and that's it. It accepts standard power plugs, the same as those used by Sky-Watcher, Orion, many Dobsonian GoTo mounts, etc. I had about twenty hours of total use across roughly three nights of testing with the battery at 99%, and on the third day there was still some charge left. The duration of the Pocket CW's internal battery is a very positive point.

Figure 5. Pocket CW has an optional internal battery that can be charged with our 12V telescope battery. The plug is the standard power-supply type with positive in the center. Its durability is high.
Figure 6. The test unit's ON/OFF switch was reversed from the letters painted on the outside, an assembly mistake, I suspect. Nothing serious in any case, since if the internal red light is on, the Pocket CW is ON, and vice versa.

Alarms

One of the trial nights, an alarm went off. I checked, and it was the dew point alarm. Great! Very useful, since the temperature those late-February days was falling fast at sunset while the humidity rose rapidly. The Pocket CW alarm alerted me that I was five degrees from reaching the dew point. There are a number of different alarm sounds to choose from. You cannot leave the mobile in standby mode, because if the screen darkens the application stops receiving data, so keep the mobile charged or plugged in, with its screen brightness at a minimum.

Other notices

As mentioned, the Pocket CW also gives a reading of ambient temperature and luminosity in luxes. We don't usually back down because of the cold, so unless we want to know whether it's icing over, this won't be the most significant data. In summary, the Pocket CW is a great "toy" to warn you of environmental problems that may ruin an astrophotography or observing session.