Frequent Asked Questions

TESS-W photometer  set-up

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How to power the TESS-W photometer ?

The device should be connected to 5V power via an USB. The new version of TESS-W has a USB connector at the end of the cable.


You need an extra (not supplied) USB charger as those used to charge your smartphone. Do not use a cheap one, please refer to power consumption question.


Remember to place the charger in a covered outlet protected from the elements.
You can also power the TESS-W photometer using a smartphone external battery or the USB plug of your car.

Which is the power consumption of the TESS-W photometer ?

A DC power source of 5V is needed. The intensity of the current ranges from 50 to 100 mA (for different WiFi signal) when the ambient temperature is over 10ºC.


For lower temperatures, T < 10ºC, the resistance is activated to increase the internal temperature of the photometer in order to prevent fog in the window and also to melt frost.


In this situation the current ranges from 500 to 600mA. The power consumption varies from 0.25W and 3W depending on the external temperature

Can I use a TESS-W in a remote location with no WIFI nor electricity?

If you wish to measure sky brightness during one night while observing on the field, you can power the TESS-W with batteries (DC 5V) and record the data with the software for PC or (better option) with the smartphone app (download from the software page).


For  a permanent monitoring station you could use a simple solar power  kit to provide electricity.


The internet connection could be provided by a 3G router. A small travel router should be enough.

Which is the spectral response of TESS ?

The graph shows the spectral response of TESS compared with the Sky Quality Photometer (SQM) and the astronomical Johnson filters (B, V and R).


The spectra of some common lamps used in street illumination are also displayed.


TESS response has been extended to the red region to fully cover the High Pressure Sodium (HPS) spectrum.

What maintenance issues might my TESS-W get with time?

While the first TESS-W units (including the prototypes) have been resisting the weather for 6 years, some of them show defects after a while. The new version of TESS-W has a more reliable enclosure.

In the meantime, for those old photometers that are sending data at this moment we recommend the users a set of actions to mitigate the problems showed in the attached document.

How can I insert the TESS-W graph on my website?

Search your photometer at the Grafana TESS-W Dashboard page.


Select the panel you are interested to share, e.g. “Last 24 hours”. Click on the arrow on the top and select “share” on the drop down menu.


Copy the link or use the embed code to insert the iframe on your html website directly.

If the project stops at some point in the future, will I be able to continue using my TESS photometer?

Although we recommend to connect to our server so the data is uploaded to the network cloud rather than being stored locally this is not the only storage option for TESS-W photometers.

We serve data and graphs in real time so you don’t need to dedicate a computer for this task. We are open data so anyone can download all the files from Grafana, Google drive and/or Zenodo to use them.
However you can use your photometer locally and even decide not to share the data with the rest of the community. To do so, you will need to download tess.apk software and install it in a device (e.g. an smartphone) able to connect your TESS-W photometer via WiFi.

There are water droplets inside the glass window

These droplets are water condensation inside the photometer. There are small holes at the bottom that should be open.

TESS-W photometers have small holes at the bottom of the enclosure box. The holes (marked with blue ovals in the picture) assure the air connection between the inside and the outside to prevent condensation. Please be sure to leave space when mounting the photometer in the final location. When the holes are not open the enclosure of the photometer is airtight and condensation is possible.

Cloud cover estimate in grafana panels

TESS-W photometers have a near IR sensor to determine the ‘sky temperature’. The difference Tsky-Tambient is a proxy of the atmosphere water content. Grafana panels show an estimate of the cloud cover.

We calculate cloud cover as:
Cloud Cover [%] = 100 – 3* (“tamb” – “tsky”))
And graph as
– ‘clear’ (up to 40%),
– ‘partly cloudy’ (40-65%) and
– ‘overcast’ (greater than 65%)
The equation is approximate and depends on many atmospheric parameters.
An example for one night with clear skies at the beginning and clouds at the end is shown for grafana and plotting the temperature difference in the second graph.