Introduction: TESS-W Night Sky Brightness Photometer

About: Physicist (Astrophysics) at UCM @unicomplutense Another amateur photographer. @stars4all_eu #LightPollution

TESS-W is a photometer designed to measure and continuous monitoring the night sky brightness for ligth pollution studies. It was created during the STARS4ALL H2020 European Project with an open design (hardware and software). The TESS-W photometer has been designed to send data via WIFI. The data is visualized in real time and shared (open data). Browse for more information.

This document contents some technical details of the TESS-W night sky brightness photometer and describes how to build it. It includes the electronic and optical schematics of the sensor and also the weather proof enclosure.

More information about the TESS photometer was presented in Zamorano et al. “STARS4ALL night sky brightness photometer“ at Artificial Light At Night Meeting (ALAN2016) Cluj, Napoca, Romania, september 2016.

TESS-W has developped by a team and the design is based in the work of Cristóbal García.

This is the first working version of the Instructables. Keep tuned.

Step 1: Description of TESS-W

The photometer is enclosed into a weather proof box that contents the custom made electronics and optical parts. TESS has a custom made Printed Circuit Board (PCB) with an ESP8266. The ESP8266 is a low-cost WIFI chip with full TCP/IP stack and microcontroller capability. The electronic is used to reads the frequency provided by the TSL237 light sensor (for night sky brightness data) and also the MLX90614ESF-BA Infrared thermometer module (for cloud cover information).

The sky brightness detector is a TSL237 photodiode that converts light to frequency. It is the same sensor used by the SQM photometers. However, the bandpass is more extended to the red range with the use a dichroic filter (labelled UVIR on the plots) with respect to the BG38 color filter of the SQM.

The light from the sky is collected with the optics that includes a dichroic filter to select the bandpass. The filter fully covers the collector (1). The sensor (not seen in this picture) is located on a printed circuit board along with the custom-made electronics (2). The WIFI module (3) with an antenna inside the box that extends the WIFI range. A near-infrared sensor (4) is used to measure sky temperature. Finally, the heater (5) is switched on when needed to get rid of condensation on the window or even to melt the ice or snow (6). The field of view (FoV) is FWHM=17 degrees.

The spectral response of the TESS-W is compared with the astronomical Johnson B, V and R photometric bands and with the spectra of a the light polluted sky of Madrid and the Calar Alto astronomical observatory dark sky.

Step 2: TESS-W Photometer Electronics

Electronic board

The main component of TESS is a custom made electronic board (PCB, printed circuit board).

The file needed for the PCB can be downloaded from

The PCB has been designed to fit inside the selected enclosure box (see later).

Main components

The PCBs electronic parts can be browsed on the accompanied the image and in the file provided.

Step 3: TESS-W Photometer Optics

Design and components

The light from the sky is collected with the optics that includes a dichroic filter to select the bandpass. The filter fully covers the collector. The photometer enclosure has a clear window that allow the sky light to enter into the photometer. The inside is protected with a glass clear window.

The optical design is depicted in the first figure. The light pass the clear filter window (1) and enter through a hole (3) of the enclosure cover (2). The clear window is glued to the enclosure cover. The dichroic filter (4) is located on top of the light collector (5). The detector (6) has been placed at the collector exit.

The clear window

The first component is a transparent window that allows the light pass to the rest of the components and seals the photometer. This is a window made in glass (BAK7) because it should resist the weather. The window has a thickness of 2 mm and a diameter of 50 mm. The transmission curve has been measured at the LICA-UCM optical workbench. It is almost constant ~90% in the wavelength range 350nm -1050nm, that means that the clear window does not introduce change in the colour of the light.

The dichroic filter

The dichroic filter is a rounded filter of 20 mm in diameter to cover completely the light collector. This assures that there is no unfiltered light reaching the detector. This is important since the TSL237 detector is sensible in the infrared (IR). The UVIR filter was designed to transmit from 400 to 750 nm, i.e. it cuts the ultraviolet response of the detector below 400 nm and the IR response over 750 nm. The transmission curve is similar to a combination of a long pass and a short pass filter with almost flat response reaching almost 100% as measured in LICA-UCM optical workbench (see the plots at the description)

The light collector

In order to gather the light from the sky TESS uses a light collector. This collector is very cheap because it is made in plastic using injection moulding. This lenses are used to beam the light in flashlights. The inside part is a transparent paraboloid reflector. The black holder prevents the stray light to reach the detector.

We are using black light collectors with nominal 60 degrees FoV. When used in TESS the FoV is reduced due to the position of the detector outside the collector. The final measured FoV (including the possible vignetting from the enclosure cover) has been measured in the optical workbench. The angular response is similar to a Gaussian function of 17 degrees full width at half maximum (FWHM).

The box

The electronics and optics of the TESS photometer are protected by a simple enclosure based on a commercial plastic box that is suited to be outdoor and to resist the weathering.

The box is small (outside: 58 x 83 x 34 mm; inside: 52 x 77 x 20 mm). The box has a screw cover to access the inside. The sealed construction provides enough level of protection against the entry of both water and dust. To prevent that the screws suffer from rusting, the original screws have been changed by stainless steel screws.


Step 4: TESS-W Enclosure

The box

The electronics and optics of the TESS photometer are protected by a simple enclosure based on a commercial plastic box that is suited to be outdoor and to resist the weathering.

The box is small (outside: 58 x 83 x 34 mm; inside: 52 x 77 x 20 mm). The box has a screw cover to access the inside. The sealed construction provides enough level of protection against the entry of both water and dust. To prevent that the screws suffer from rusting, the original screws have been changed by stainless steel screws.

Box machining

It is necessary to perform some simple machining on the box. The window that allow light to reach the light collector has a width of 20 mm in diameter. It is covered by a clear window that should be glued with weather resistant silicone. The small hole is the IR thermometer port and has 8.5 mm in diameter. On the other side of the box it is necessary a 12 mm hole for the cable gland. The two perforations of 2.5 mm are used to secure the heater to the box cover.

Step 5: Mounting the TESS-W Photometer

1. Preparation

1. Paint the box inside in black.

Box machining

2. Drilling:

● 1x 20 mm for the window.
● 1x 12 mm for the cable gland.
● 1x 8.5 mm for the thermopile.
● 2x 2.5 mm for the heater.
● 2x 1 mm on the side of the box.

3. Drill the aluminium diffuser plate (1 mm thickness) for the heater resistance,
4. Screw the resistance and plate to the cover.
5. Glue the 8mm spacers for the PCB.
6. Glue the clear window (the resistance heater should be screwed in place)


7. Remove the voltage regulator and connect both terminals by soldering a bridge.
8. Solder a single-head 4-pin wire to board connector of 60 mm length.
9. Glue the thermopile to the cover.


10. Drill a hole to secure the antenna to the box.
11. Trim the corners of the antenna.
12. Remove the ceramic antenna of the wifi module and also the antenna conector and the red LED.

2. Mounting

Please follow this ordered sequence:

1. Secure the antenna to the box using a screw.
2. Place the cable gland and the power cord.
3. Secure the collector (black cilinder) to the PCB (two screws).
4. Secure the PCB to the box (two screws).
5. Screw power cable to the green board connector. (Red wire to positive).
6. Solder antenna cable to wifi module.
7. Solder to resistance heater a single-head 2-pin wire to board connector cable of 55 mm.
8. Connect the thermopile and the resistance (be careful not to break the PCB).

The resistance acts as a heater and is connected to the cover with a aluminium plate. The pictures explain the next processes: The antenna should be screwed to the box, the regulator of the thermopile has been replaced by a bridge, and the two spacers (in black) for the PCB should be be glued to the box. The inside of the box is painted in black.

One of the figures shows the original WIFI module that has a ceramic antenna and a socket to connect an extra antenna (top). We use an antenna whose cable is soldered to the wifi module (bottom). Note that the ceramic antenna, the socket and red LED near the cable have been removed.

Step 6: TESS-W Photometric Calibration

The photometers should be calibrated to assure that the measurements from different devices are consistent. The TESS-W are cross calibrated relative to a master photometer at Laboratorio de Investigación Científica Avanzada (LICA) of the Universidad Complutense de Madrid.

The setup is an integrating sphere whose interior could be illuminated by a light source and with several optical ports to connect the photometers. The light source employed is a LED of 596 nm with 14 nm FWHM.

If you wish to calibrate your TESS-W photometer, you can contact LICA-UCM.

Step 7: TESS-W Software

WIFI module software

Communication and software

The complete system includes a sensor network and a software broker that mediates between information producers and consumers that is reserved to calibrated sensors. Once you have calibrated your photometer (see Step 6), STARS4ALL will provide you with the credentials to publish in the broker.

A sample consumer in Python to store data in an SQLite database has been developed. This consumer can be installed in one or many PCs or servers. The software main characteristics are listed below:

● Custom software for TESS developed in C .

● MQTT publisher software developed in the Arduino IDE and ESP8266 libraries.

● MQTT Broker either in an in-house deployment or an available third party (ie test

● MQTT subscriber software receiving data from publishers and storing it into a relational database (SQLite).

MQTT is an M2M / Internet of Things lightweight protocol suited for constrained devices that requires far less overhead than HTTP based communications.

Each sensor sends periodically measurements to a remote MQTT server through a local router. This server - named “broker” in the MQTT world - receives data from many sensors and redistribute to all subscribed parties, thus decoupling publishers from consumers. The remote server can be deployed either in-house in a central facility for the project. Alternatively, we can use available, free MQTT brokers such as

Any software client can subscribe to the broker and consume the information published by the TESS devices. An special MQTT client will be developed to collect all these data and store them in an SQLite database.

Device configuration

● Instrument configuration will be reduced to a minimum to help maintenance.

● Each device needs this configuration:

o WiFi SSID and password.

o Photometer calibration constant.

o MQTT Broker IP address and port.

o Instrument friendly name (unique per device)

o MQTT channel name (as described above)

WiFi configuration

When first connected to the power, TESS-W creates a WiFi access point. The user fills in the settings that includes the name (SSID) and password of the WiFi router, the zero point of the photometry and the Internet address and name of the broker repository. After a reset and switch off and switch on cycle, the TESS photometer begins to produce and to send data.

At first boot, TESS starts as an access point with the name TESSconfigAP. A mobile phone must connect to this access point.

● Browse with an Internet browser the following URL:

● Fill in the form with the parameters listed in 2.3

● Reboot the device, which will connect to the local router.

When the device loses the link with the WiFi router, reboot and configures itself again as an access point, which is convenient to change the configuration.


The TESS-W Firmware an documentation can be found at the github repository

For the ESP8266

For the microprocessor

Step 8: Final Remarks

The STARS4ALL Foundation is the continuation of the STARS4ALL project that is in charge of the operation of the TESS-W photometers network. This is a citizen science project which produce data of interest for light pollution studies.

Once your photometer is calibrated and configured will start to send the measurements to the STARS4ALL infrastructure. These measurements can be visualized from our platform ( Plus, all data generated in the network can be downloaded from our Zenodo community (