Introduction to the concept: the CTIO Evryscope
Astronomy is moving into an era of extremely large time-domain surveys. Synoptic sky surveys like Large Synoptic Survey Telescope (LSST) generally cover very large sky areas to detect rare events. Since it is usually infeasible to cover thousands of square degrees with a single telescope, they repeatedly observe few-degree-wide fields, use large apertures to achieve deep imaging, and tile their observations across the sky over days. The resulting survey, such as the Palomar Transient Factory (Law et al. 2009)1 , Pan-STARRS (Kaiser et al. 2010)2 , SkyMapper (Keller et al. 2007)3 , CRTS (Djorgovski et al. 2011)4 , ATLAS (Tonry et al. 2011)5 , and many others, is necessarily optimized for events such as supernovae that occur on day-or-longer timescales. These surveys are not sensitive to the very diverse class of shorter-timescale objects, including transiting exoplanets, young stellar variability, eclipsing binaries, microlensing planet events, gamma ray bursts, young supernovae, and other exotic transients, which are currently only studied with individual telescopes continuously staring at relatively small fields of view.
Figure 1: Cutaway diagram of the CTIO Evryscope; 24 cameras are mounted into a 6ft-diameter hemisphere.
With these considerations in mind, UNC designed and developed the first Evryscope, now operational at CTIO. This Evryscope (Figure 1) uses a large array of small telescopes to cover the entire visible sky in each and every exposure, repeatedly imaging the sky and stacking images to achieve depth. This technique has been prohibitively expensive up to now because of the cost of the extremely large number of pixels required to cover the sky with reasonable sampling, and the consequent data-storage and analysis facility requirements. The rise of consumer digital imaging and low-cost hard disks offer solutions to both these problems. The Evryscope uses mass-produced compact CCD cameras, compact mass-produced camera lenses, and a novel camera mounting scheme to make a low-cost 691 megapixel robotic telescope that images 8,000 square degrees in each exposure – a 7cm telescope pointed at every part of the accessible visible sky simultaneously.
The Evryscope is designed to open a new parameter space for optical astronomy, trading instantaneous depth and sky sampling for continuous coverage of much larger sky areas. The Evryscope has 10% of the enormous planned LSST étendue, and the largest étendue of any current ground-based surveys (Figure 2). Also, the large number of pixels combined with an exceptional field of view makes The Evryscope unique among current surveys (Figure 3) and allows for surveys scales which were previously unreachable.
Figure 2: The Evryscope's étendue (collecting area * field of view) compared to other current ground-based sky surveys.
Figure 3: Evryscope's number of pixels vs. field of view compared to other current ground-based sky surveys.
1 Law NM, Kulkarni SR, Dekany RG, et al (2009) The Palomar Transient Factory: System Overview, Performance, and First Results. Publ Astron Soc Pacific 121:1395–1408. doi: 10.1086/648598
2 Kaiser N, Burgett W, Chambers K, et al (2010) The Pan-STARRS wide-field optical/NIR imaging survey. In: Stepp LM, Gilmozzi R, Hall HJ (eds) Ground-based and Airborne Telescopes III. Edited by Stepp. p 77330E–77330E–14
3 Keller SC, Schmidt BP, Bessell MS, et al (2007) The SkyMapper Telescope and The Southern Sky Survey. Publ Astron Soc Aust 24:1–12. doi: 10.1071/AS07001
4 Djorgovski SG, Drake AJ, Mahabal AA, et al (2011) The Catalina Real-Time Transient Survey (CRTS).
5Tonry J (2011) ATLAS: An Asteroid Warning System.