From 5 to 9 October 2020, around 1,000 people participated in the Dark and Quiet Skies for Science and Society Workshop, which resulted in this report and recommendations for local governments to preserve dark and calm skies. The report addresses three classes of interference: Artificial Light At Night (ALAN), radio wavelength emission and Low Earth Orbit (LEO) satellite trails. Incorporating suggestions from participants in the workshop, the report has been prepared by more than 85 experts organised across five Working Groups. The Working Groups focused on, respectively: Dark Sky Oases, Optical Astronomy, Bioenvironment, Satellite Constellations and Radio Astronomy. Each Working Group has made recommendations for steps that can be taken to mitigate the particular effects of the human activities they focused on. Since many of these activities also have important benefits, such as increased safety thanks to urban illumination and network connectivity provided by satellites, care has been taken to recommend actions that reduce their negative impacts without compromising their effectiveness for their original purposes. This is essential for the quick development and implementation of actions to preserve dark and quiet skies, for both scientific exploration and cultural heritage.

Control quality of seeing measurements provided by the DIMMA at the ORM Antonia M. Varela, Casiana Muñoz-Tuñón & Julio Castro-Almazán Feb-2013- Technical report ------------------------------------- An automatic DIMM is installed near the GTC site. Data have been validated and the results presented in this document. Real time values are accessible at http://www.iac.es/site-testing/DIMMA_ORM. The comparison baseline spans from April to October 2012. Mean and median values in this period are 0.95” and 0.86” respectively.

INTRUSIONES DE POLVO SOBRE EL ORM Sky Quality Team Instituto de Astrofísica de Canarias Marzo 2010

The report concludes that the Canary Islands observatories (Roque de los Muchachos and Teide; ORM and OT) show an extremely clean air and pristine skies, only partially affected by some dust loaded African air mass intrusions in summer time (July‐September). Long term simulations (1958‐2006) of TSP (Total Suspended Particles) for OT and ORM show that no positive trend has been detected since 1958, in good agreement with the in‐situ observations. No positive trend in the inter‐annual variation of dust‐loaded air masses over the OT and ORM scientific facilities. Model simulations show a lower incidence of African air masses over ORM compared to OT, as it was expected by the geographical location of both observatories.

Concerning the astroclimatological comparison of the Paranal Observatory and El Roque De Los Muchachos Observatory (Temperature trends) J.A. Castro-Almazán, C. Muñoz-Tuñón and J.J. Fuensalida Instituto de Astrofísica de Canarias 15 Oct 2009 In response to the paper “The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos”. MNRAS, 399-2, p.783-793. 10/2009 (Lombardi, G.; Zitelli, V.; Ortolani, S.)

ANALYSIS OF SEEING MEASUREMENTS FROM THE LA PALMA RoboDIMM1* May 2009 Antonia M. Varela(1), Casiana Muñoz-Tuñón(1), Jesús J. Fuensalida(1) & Jean Vernin(2) (1) Instituto de Astrofísica de Canarias (2) Departement of Astrophysics; Nice University