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NELIOTA (NEO Lunar Impacts and Optical TrAnsients)

NELIOTA Observations     References

NELIOTA is an activity launched by the European Space Agency (ESA) at the National Observatory of Athens (NOA) in February, 2015. It aims to determine the distribution and frequency of small NEOs (Near-Earth Objects) by monitoring lunar impact flashes. The NELIOTA project has established an operational system that started monitoring the Moon for faint NEO impacts in early 2017, using the 1.2 m Kryoneri telescope, located in the Northern Peloponnese, in Greece. The NELIOTA campaign will run until January 2021. 1)

The Kryoneri telescope was upgraded for the project in 2016. Specialized fast-frame cameras were installed and specialized software was developed to control the telescope and cameras, as well as process the resulting images to detect the impacts automatically. NELIOTA furthermore provides a web-based user interface, where the impact events are reported and made available to the scientific community and the general public.

The 1.2 m Kryoneri telescope is capable of detecting flashes much fainter than current, small-aperture, lunar monitoring telescopes. NELIOTA is therefore expected to characterize the frequency and distribution of NEOs weighing as little as a few grams.



The 1.2 m Kryoneri telescope at Kryoneri Observatory (established in 1972), located in the district of Corinth in the northern Peloponnese, was selected for the NELIOTA system. The telescope, in its original configuration, was a Cassegrain reflector with a primary parabolic mirror (1.2 m in diameter) and a secondary hyperboloid mirror (0.27 m in diameter) and a focal ratio f/13, manufactured and installed in 1975 by the British company Grubb Parsons Co., Newcastle. The mirrors were manufactured by Zerodur. 2)

In May 2016, the telescope was upgraded by DFM Engineering Inc. by the NELIOTA project. This upgrade included the replacement of some mechanical parts of the telescope, a new control system, dome automation and the installation of a Prime Focus Instrument (PFI) able to provide a large field-of-view as required for NELIOTA. The optics yield a focal ratio of f/2.8.

The PFI includes a dichroic beam splitter which directs the light into two paths and provides simultaneous imaging in two channels (R and I-bands). Two fast-frame sCMOS cameras are attached to the backend of PFI. The final field-of-view is 16.6 x 14.0 arcminutes (21.7 arcminutes in diagonal).


Figure 1: On June 27th, 2016, first light images of the Moon were obtained through the PFI with the cameras (image credit: NOA, ESA)

The design of the PFI can accommodate another option for imaging, hosting a "direct imaging" Apogee Aspen CCD where the light path is directed directly to the prime focus. With this option the field-of-view can be as large as 1 square degree (depending on the detector used). In this configuration, the field-of-view is 12.3 x 12.3 arcminutes (17.4 arcminutes in diagonal). The camera has a 1024 x 1024 format giving a pixel scale of 0.72 arcseconds/pixel.


Figure 2: Also on June 27th, 2016, first light images were obtained, imaging the globular cluster M13 (left), the galaxy M51 and its companion (middle) and Trifid Nebula (right), image credit: NOA, ESA


Figure 3: Photo of the Kryoneri telescope at Kryoneri Observatory (image credit: NOA)



NELIOTA uses a dichroic with two fast-frame sCMOS cameras (Andor Zyla 5.5; USB3.0) attached to the prime focus of the telescope to perform simultaneous imaging of the Moon in two channels (R and I-bands), at 30 frames per second. The cut-off of the dichroic is at 730 nm and the R and I-band filters (Astrodon 50-mm diameter, 3-mm thick) are in the Johnson-Cousins photometric system.

The cameras are used in 2 x 2 binning and have a pixel scale of 0.8 arcsec/pixel or 1.5 km (of the lunar surface) per pixel. The orientation of the cameras is such that East is up and North is to the left. Their characteristics are listed in the table below.

Sensor type

Front illuminated scientific CMOS



Gain settings

Low noise & high well capacity (16 bit)

Gain (e- per ADU)

0.5 (binning 1 x 1); 0.4 (binning 2 x 2)

Read noise (e- RMS)

2.8 (binning 1 x 1); 5.1 (binning 2 x 2)

Pixel readout rate (MHz)

2 x 280

Sensor size (mm)

16.6 x 14

Pixel size (µm)


Total pixels

2560 x 2160 (binning 1 x 1); 1280 x 1080 (binning 2 x 2)

Field of View (arcmin)

17.0 x 14.4 (R-band), 16.8 x 14.2 (I-band)

Pixel scale ("/pixel)

0.4 (binning 1 x 1); 0.8 (binning 2 x 2)

Table 1: Camera parameters


Figure 4: Camera mount (image credit, NOA, ESA)


Figure 5: The diagram for the linearity of the sCMOS cameras for the NELIOTA configuration (2 x 280 MHz, global shutter, low noise & high well capacity). The dark current is essentially zero (image credit: NOA, ESA)

The performance of the PFI (Prime Focus Instrument) optical system (dichroic, filters, cameras) is calculated taking into account the Quantum Efficiency (QE; blue solid line in Figure 5) of the cameras, the transmittance of the R and I filters (red and green solid lines, respectively) and the dichroic (black dashed and dot dashed lines). The dichroic is centered at 730 nm with a > 90% throughput. 3)


Figure 6: Photo of the Kryoneri Observatory, Greece (photo credit: Theofanis Matsopoulos) .


NELIOTA Observations

The Near-Earth object Lunar Impacts and Optical TrAnsients (NELIOTA) project looks for flashes where they are easiest to observe, on the dark side of the moon not illuminated by the sun.

NELIOTA is funded by ESA and operated by the National Observatory of Athens (NOA) at Kryoneri Observatory in Greece. It uses a 1.2 m telescope and a twin-camera system that splits the light of the lunar flash into two colors. This helps scientists to estimate another important feature of an impact, its temperature.

Since the project began, it has conducted a total of approximately 149 hours of lunar monitoring and detected 102 lunar flashes.

• April 14, 2020: Since March 2017, ESA's NELIOTA project has been regularly looking out for ‘lunar flashes' on the Moon, to help us better understand the threat posed by small asteroid impacts. The project detects the flash of light produced when an asteroid collides energetically with the lunar surface, and recently recorded its 100th impact. But this time, it was not the only one watching. 4)

Earth is constantly bombarded by natural space debris – fragments of comets and asteroids, also known as meteoroids. The majority burn up in our atmosphere, but some objects, particularly those larger than a few meters, are potentially dangerous and their number is not well known.

Smaller Earth impactors are too small to be detected directly with telescopes and too unpredictable to be captured reliably with ground-based ‘fireball' cameras. Instead, to get an idea of how common these objects are and their potential threat to Earth, we look to the Moon.

The Moon's atmosphere is negligible, with a total mass of less than 10 tons. As such, even tiny asteroids travelling at fast speeds leave an impact – as illustrated by its heavily cratered surface.

When meteoroids or small asteroids strike the lunar surface at high speed, they generate a flash of light that, if bright enough, is visible from Earth. Scientists can use the brightness of a flash to estimate the size and mass of the object that caused it, and improve our understanding of how often similar objects are colliding with Earth. Typically, asteroids weighing less than 100 g and measuring less than 5 cm create these observable lunar flashes. 5)


Figure 7: Locations of the 102 validated flashes observed by the NELIOTA project (in yellow) up to 27 March 2020. The flash in the red circle was also detected by the team of the Sharjah Academy for Astronomy, Space sciences & Technology, UAE. The lunar north pole is at the top (image credit: ESA / NELIOTA)


A second opinion

The NELIOTA project is not alone in its hunt for lunar flashes. Its 100th detection not only marked an impressive milestone for the project, but was also the first time that one of its detections was confirmed by another observatory.

Using a 35 cm telescope, a newly established team at the Sharjah Lunar Impact Observatory (SLIO) of the Sharjah Academy for Astronomy, Space Sciences & Technology, UAE (United Arab Emirates), detected a flash on 1 March 2020. It was later confirmed that this flash was from the same event as the 100th NELIOTA detection.


Figure 8: First joint-detection of lunar flash. Left: Lunar image showing the 100th flash (red arrow) detected by the NELIOTA project on 1 March 2020 at 16:54:24.09 UT. Right: Lunar image from the Sharjah Lunar Impact Observatory showing the same flash (red arrow). The numbered areas in both images indicate lunar features used for the comparison. The lunar north pole is on the right (image credit: ESA)

"Cross detections like this are very useful as they rule out the possibility of a slow, bright satellite being misidentified as an impact flash," says Detlef Koschny, co-manager of the Planetary Defence Office of ESA's Space Safety program. "While NELIOTA has other, less direct means of excluding such events, we're excited to have more eyes on the Moon, helping us to understand the rocky road our planet travels on".

Observing the same suspected lunar impact event from different locations is a very effective way to spot this type of false detection. Other lunar impact flash observers can cross-check their data with that of NELIOTA - all flashes detected by the system are posted on the NELIOTA website within 24 hours.

1) Alexios Liakos, Alceste Bonanos, Emmanouil Xilouris, Ioannis Bellas-Velidis, Panayotis Boumis, Vassilis Charmandaris, Anastasios Dapergolas, Anastasios Fytsilis, Athanassios Maroussis, Detlef Koschny, Richard Moissl, and Vicente Navarro, "NELIOTA Lunar Impact Flash Detection and Event Validation," Earth and Planetary Astrophysics, 2019, URL:


3) E. M. Xilouris, A. Z. Bonanos, I. Bellas-Velidis, P. Boumis, A. Dapergolas, A. Maroussis, A. Liakos, I. Alikakos, V. Charmandaris, G. Dimou, A. Fytsilis, M. Kelley, D. Koschny, V. Navarro, K. Tsiganis and K. Tsinganos, "NELIOTA: The wide-field, high-cadence, lunar monitoring system at the prime focus of the Kryoneri telescope," Astronomy & Astrophysics, Volume 619, id.A141, 14 pp., 16 November 2018,, URL:

4) "100th lunar asteroid collision confirmed by second telescope," ESA / Safety & Security, 14 April 2020, URL:

5) A. Liakos, A. Z. Bonanos, E. M. Xilouris, D. Koschny, I. Bellas-Velidis, P. Boumis, V. Charmandaris, A. Dapergolas, A. Fytsilis, A. Maroussis and R. Moiss, "NELIOTA: Methods, statistics, and results for meteoroids impacting the Moon," Astronomy & Astrophysics, Volume 633, January 2020, A112, Published online: 20 January 2020,, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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