SAO's New Generation Multipurpose CCD Controller
In the 1990s considerable advances have been made in the world over technology of manufacturing scientific image detectors with high photoelectric characteristics, which called for the creation of new complex array controllers to operate such detectors.
The new generation controllers must ensure:
- flexible operation of different type detectors (CCDs, mosaic-type, IR-arrays and also technical CCDs for guiding and wave front sensing);
- very low readout noise;
- high accuracy and stability of conversion of optical radiation to digital image;
- fast readout of detectors.
Quite a few teams of researchers and engineers are ingaged in the development of controllers. The following controller designs have come to be well known in astronomy:
- ACE, FIERA, IRACE from European Southern Observatory,
- SDSU I and SDSU II from San Diego State University Astronomy Department CCD Lab,
- ARCON from Cerro Tololo Inter-American Observatory,
- Rutherford-SAAO CCD Controller from Rutherford Appleton Laboratory & South African Astronomical Observatory,
- CCD Controller of the National telescope Galileo" from Catania Observatory Laboratory for Detectors.
In 1995 the ADLab began realization of its own project on the development of a new controller intended to equip newly designed spectrographs and update the available instrumentation of the 6-m telescope.
The aim of the project is to develop a multipurpose controller which, when operated with a selected image detector, can provide:
- minimum readout noise;
- limiting precision of charge measurement needed for high accuracy photometry.
The explorative aspect of the project is directed towards the study of ways these characteristics can be attained and includes the following theoretical and experimental work:
- detailed study of random and systematic videosignal distortions in the process of charge readout (non-linearities and instabilities of transfer characteristics of up-to-date on-chip MOSFETs with high W/L, their noise spectral characteristics depending on the mode, and so on);
- examination of ways of construction of an optimum digital signal filter with the application of fast DSP to achieve higher characteristics than those with the use of the classical correlated double sampling (CDS) which is not an optimum filter;
- some other investigations.
The engineering aspect of the project includes:
- choice and substantiation of the architecture (processors, communications and topology) of the controller;
- development, SPICE modelling and design of modules;
- design of printed circuit boards;
- manufacturing and experimental testing of prototypes of the modules;
- manufacture of the test piece of the CCD system;
- manufacture of the systems for the 6-m telescope and to orders.
The controller is named DINACON (DSP-based Intelligent Array Controller).
Its intelligence is defined by the functions of adaptation to different on-chip MOSFETs and optimization of signal processing, including:
- measurement of transfer characteristics as dependent on the electric mode, selection of a regime with a maximum charge sensitivity, correction of non-linearity and instabilities in real time when reading out the signal;
- measurement of noise spectral characteristics, depending on the operation, selection of optimum conditions for MOSFET at different rates of charge readout;
- evaluation of the noise process parameters to optimize digital signal processing.
Status of the Project
The project is expected to be accomplished by 1998, the stages of its fulfilment are as follows:
Year Work 1995 Investigations on achieving a minimum readout noise and a high accuracy of signal charge-to-digital conversion. 1996 Investigations continued.
Development of architecture of the CCD controller, modules and software for Windows 95/NT.
Manufacture of prototypes of the modules and their experimental test.
Development of a new cryostat with a volume of 3,6 litres.
1997 Modules and controller software development continued.
Development of digital filtering algorithms.
Development of printed circuit boards of the modules.
Manufacture of cryostats.
1998 Manufacture of printed circuit boards.
Manufacture and test of one specimen of the CCD system with a CCD of 1024 x 1024 pixels.
Production of N specimens of the CCD system, using different type CCDs (the number depends on the amount of funds).
Development of IRAF-based version of the software.
Further development of the algorithms for increasing the controller intelligence.
The project is supported by the Russian Foundation for Basic Research (project N 95-02-04640) and the Federal Research-Engineering Programme Astronomy (topic N 220.127.116.11).