The PSPT image-acquisition system is constituted by a Xedar camera XS2048-A-12, which uses a CCD sensor Thomson TH7899M (2048×2048 pixel). This system is placed upon the focal plane of the telescope.
This camera enables the acquisition of images characterized by an excellent (0.1%) photometric accuracy thanks to the features of the individual pixels which make up the detector (collecting capacity = 200.000 e-/pixel, reading noise = 35e-, dynamic range of 12 bit in reading for individual exposures). The quality of photometric accuracy is also due to the high reading speed (4 Hz = 4 frame/s), allowed by the simultaneous use of 4 reading systems, each one employed for a 1024×1024 quadrant of the detector. In this way, we can quickly acquire images obtained by joining several exposure, thus increasing the signal-noise rates of the acquired data. In fact, if S/N indicates the ratio between the signal S and the noise N for an image obtained through a single exposure, the S/N relation for the image obtained, for instance, by the sum of 25 exposures is the following:
Granted that, during 25 exposures, the levels of both signal and noise remain roughly constant, then:
that is, the S/N relations is increased by a factor of 5.
Besides, the CCD has a coating which allows the increase of the quantum efficiency in the near UV.
In order to reduce the noise due to the dark current, esponentially dependent upon temperature, it is necessary to cool down the detector.
This cooling-down is made through a solid-state system inside the camera. By exploiting the Peltier effect, this system enables the sensor to work at a temperature of about -10°C. In order to cool down the gear, a water flow is activated by a pump. A vacuum is created inside the camera, in order to prevent steam condensing upon the CCD, which would affect the quality of acquired images.
The images acquired by the CCD detector are transmitted to the computer
used for data archiving through an optical-fibre cable.
The work done by the PSPT system can be divided into two main tasks: data acquisition and telescope control. These tasks are assagned to two different computers.
Data acquisition and archiving (including control of the CCD camera as well as acquisition parameters), is made by a Sun Ultra Sparc 1 workstation. Most of the CPU work, consisting in the integration of “on-the-fly” images, is done by a control programme of the camera, written in C language.
This programme runs as a background process, and is commanded both by the c-shell of the workstation and by an interface created in IDL language (through IDL “call-external” function).
The control of PSPT working parameters is made through commands of the IDL interface. In fact, this interface sets off the control programme of the camera (written in C), as well as of the telescope control programmes, and therefore it actually starts off all daily operations of the instrument.
The telescope is controlled by programmes written in C language and coordinated by a PC dedicated to hardware control. A series of programmes enables control over every element of hardware, inizialization of its configurations, enabling and disabling many safety checks upon
the function of mechanical components (i.e.: movements of mechanical trolleys, software operations, etc.).
The Sun Workstation and the PC communicate through a RS-232 line, the Sun assuming the role of “master” and the PC being the “slave”.