  BlackGEM (BG) is an array of telescopes, currently under development at the
Radboud University Nijmegen and at NOVA. It targets the detection of the
optical counterparts of gravitational waves. The first 3 BG telescopes are
planned to be installed in 2018 at the La Silla observatory. A single prototype
telescope, named MeerLICHT, will already be commissioned early 2017 in
Sutherland to provide an optical complement for the MeerKAT radio array. The BG
array consists of, initially, a set of 3 robotic 65-cm wide-field telescopes.
Each telescope is equipped with a single STA1600 CCD detector with 10.5k x
10.5k 9-mum pixels that covers a 2.7 square degrees field of view. The
cryostats for housing these detectors are developed and built at the KU Leuven
University. The operational model of BG requires long periods of reliable
ands-off operation. Therefore, we designed the cryostats for long vacuum hold
time and we make use of a closed-cycle cooling system, based on Polycold PCC
Joule-Thomson coolers. A single programmable logic controller (PLC) controls
the cryogenic systems of several BG telescopes simultaneously, resulting in a
highly reliable, cost-efficient and maintenance-friendly system. PLC-based
cryostat control offers some distinct advantages, especially for a robotic
facility. Apart of temperature monitoring and control, the PLC also monitors
the vacuum quality, the power supply and the status of the PCC coolers
(compressor power consumption and temperature, pressure in the gas lines,
etc.). Furthermore, it provides an alarming system and safe and reproducible
procedures for automatic cool down and warm up. The communication between PLC
and higher-level software takes place via the OPC-UA protocol, offering a
simple to implement, yet very powerful interface. Finally, a touch-panel
display on the PLC provides the operator with a user-friendly and robust
technical interface.
