The readout system for the ArTeMiS camera

摘要

During ArTeMiS observations at the APEX telescope (Chajnantor, Chile), 5760 bolometric pixels from 20 arrays at 300mK, corresponding to 3 submillimeter focal planes at 450μm, 350μm and 200μm, have to be read out simultaneously at 40Hz. The read out system, made of electronics and software, is the full chain from the cryostat to the telescope. The readout electronics consists of cryogenic buffers at 4K (NABU), based on CMOS technology, and of warm electronic acquisition systems called BOLERO. The bolometric signal given by each pixel has to be amplified, sampled, converted, time stamped and formatted in data packets by the BOLERO electronics. The time stamping is obtained by the decoding of an IRIG-B signal given by APEX and is key to ensure the synchronization of the data with the telescope. Specifically developed for ArTeMiS, BOLERO is an assembly of analogue and digital FPGA boards connected directly on the top of the cryostat. Two detectors arrays (18~*16 pixels), one NABU and one BOLERO interconnected by ribbon cables constitute the unit of the electronic architecture of ArTeMiS. In total, the 20 detectors for the tree focal planes are read by 10 BOLEROs. The software is working on a Linux operating system, it runs on 2 back-end computers (called BEAR) which are small and robust PCs with solid state disks. They gather the 10 BOLEROs data fluxes, and reconstruct the focal planes images. When the telescope scans the sky, the acquisitions are triggered thanks to a specific network protocol. This interface with APEX enables to synchronize the acquisition with the observations on sky: the time stamped data packets are sent during the scans to the APEX software that builds the observation FITS files. A graphical user interface enables the setting of the camera and the real time display of the focal plane images, which is essential in laboratory and commissioning phases. The software is a set of C++, Labview and Python, the qualities of which are respectively used for rapidity, powerful graphic interfacing and scripting. The commands to the camera can be sequenced in Python scripts. The paper describes the whole electronic and software readout chain designed to fulfill the specificities of ArTeMiS and its performances. The specific options used are explained, for example, the limited room in the Cassegrain cabin of APEX has led us to a quite compact design. This system was successfully used in summer 2013 for the commissioning and the first scientific observations with a preliminary set of 4 detectors at 350μm.