fahmi_dry_cyc_impr.eps Acta Polytechnica Vol. 52 No. 2/2012 Utilization of Image Intensifiers in Astronomy S. V́ıtek, K. Fliegel, P. Páta, P. Koten Abstract In this paper we present the properties of image intensifiers, used together with fast TV cameras for astronomical purposes within the MAIA project(Meteor Automatic Imager and Analyser, primarily focused on observing meteoric events with high time resolution). The main objective of our paper is to evaluate the suitability of these devices for astronomical purposes in terms of noise, temporal and spectral analysis. Keywords: image intensifier, astronomy, meteors. 1 Introduction An interesting technique (that has became relatively inexpensive in recent years) for increasing the time resolution of any astronomical instrument is the use of amodernCCD camerawith a fast framerate of 50 ormore frames per second. This type of systemhas a significant deficiency in terms of reduced sensitivity; the solution may to use an image intensifier. This paper describes our experience with a device of this kind. The MAIA astronomical instrument [2] uses a second generationMullardXX1332 image intensifier. The tube assembly of this 50/40mm inverter (typical gain 30000 to 60000 lm/lm, resolution 30 lp/mm) is designed to be incorporated in night vision devices, in particular in tank driving periscopes. This leads to some properties which significantly define the lim- its and the possibilities of using a device of this kind in astronomy. 2 Electrooptical characteristics The instrument is equipped with an input aperture lens (Pentax SMCFA1.4/50mm) and an inner cam- era lens (PentaxH1214-M1.4/12mm) [4]. The spec- tral transparency of the two lenses (input and cam- era), the spectral sensitivity of the image intensifier, the spectral sensitivity of the camera, the spectrum of the light at the output of the image intensifier, the spatial resolution of the input lens and the image in- tensifier, and also the spatial resolution of the whole system are among the most important parameters tested and presented in this paper. 10 −4 10 −3 10 −2 10 −1 10 010 −2 10 −1 10 0 Normalized input power [−] N o rm a liz e d g a in [ − ] 0 200 400 600 800 1000 1200 0 0.2 0.4 0.6 0.8 1 Spatial frequency [LW/PH] M T F [ − ] Overall MTF Input lens and image intensifier (a) (b) Fig. 1: The normalized gain of the system (measured at 650 nm) describes the automatic gain control as a nonlinearity in the image intensifier (a), the overall MTF of the system, including the camera (solid line) and the partial MTF of the image intensifier with the lens (dashed line) (b) 56 Acta Polytechnica Vol. 52 No. 2/2012 400 500 600 700 800 900 1000 0 0.2 0.4 0.6 0.8 1 Wavelength [nm] R e la tiv e r e sp o n se , S p e ct ra l t ra n sm is si o n , R e la tiv e in te n si ty [ − ] Camera w/o lens Camera with lens Relative power Lens 400 500 600 700 800 900 0 0.2 0.4 0.6 0.8 1 Wavelength [nm] R e la tiv e s e n si tiv ity , S p e ct ra l t ra n sm is si o n [ − ] Lens B=0.11 B=0.23 B=0.36 B=0.47 B=0.59 B=0.71 (a) (b) Fig. 2: Relative spectral response of the camera with (or without) a lens, and the relative power of the light at the output screen of the image intensifier (a), relative overall spectral sensitivity of the system for different digital levels B in the output image (B =1 white, B =0 black) (b) Fig. 3: Temporal changes in stellar object flux 2.1 Spectral response The spectral response was measured independently for allpartsof the system[3]. Theexperimental setup consisted of the LOT-Oriel collimated halogen light source, theLOT-OrielOmni 150 computer controlled monochromator, the expander to get even illumina- tion of the image sensor, and the Avantes AvaSpec- 3648 fiber optic spectrometer. The measurement re- sults are shown in Figure 2. 2.2 Spatial resolution The MTF was measured using a test chart accord- ing to ISO 12233. This chart can be used to eval- uate MTF with two different approaches, utilizing a slanted edge (an approx. 5◦ slightly slanted black bar used to measure the horizontal or vertical spa- tial frequency response), or a line square wave sweep with the spatial frequency range 100–1000 LW/PH (line widths per picture height). In our case, slanted edges were used to determine the spatial frequency response — see Figure 1(b). 3 Noise performance The noise analysis based on the acquisition of test- ing video sequences in various light conditions is de- scribed in [5]. We choose the Generalized Laplacian Model (GLM) for heavy-tailed noise. 4 Temporal analysis Figure 3 shows the temporal changes in stellar object flux over 100 frames of videosequence. The changes are mainly due to the automatic gain control. This fact puts greater demands on image calibration. We haveproposedadaptive flat-fielding for any light con- ditions. 5 Conslusions The biggest disadvantage of the image intensifier de- scribedhere is thebuilt-in automaticgaincontrol. As is shownFigure 1(a), the gain decreases rapidlywith increasing input power, i.e. if any bright stellar ob- ject appears in the field of view. However the image 57 Acta Polytechnica Vol. 52 No. 2/2012 intensifier is not a bottleneck of the MAIA device — all measured parameters of an image intensifier are far better than the parameters of the CCD camera used for the project. Acknowledgement This work has been supported by grant No. 205/09/1302 Study of sporadic meteors and weak meteor showers using automatic video intensifier cameras of the Grant Agency of the Czech Re- public. We would also like to acknowledge grant No. 102/09/0997 from Grant Agency of the Czech Republic. References [1] Koten, P.: Software for processing of me- teor video records. Proceedings of the Asteroids, Comets, Meteors 2002 conference, 197, 2002. [2] Vı́tek, S., Koten, P., Páta, P., Fliegel, K.: Double-Station Automatic Video Observation of the Meteors. Advances in Astronomy, 2010 (Ar- ticle ID 943145), 4 pages (2010). [3] Fliegel, K., Havĺın, J.: Imaging photometer with a non-professional digital camera. Proc. SPIE 7443, 74431Q, 2009. [4] Fliegel, K., Švihĺık, P., Páta, P., Vı́tek, S., Koten, P.: Meteor automatic imager and ana- lyzer: current status and preprocessing of image data. Applications of Digital Image Processing XXXIV, Proc. SPIE, 2011. [5] Švihĺık, P., Fliegel, K., Koten, P., Vı́tek, S., Páta, P.: Noise Analysis of MAIA System andPossible Noise Suppressio.Radioengineering, Vol. 20, pp. 110–117, 2011. Stanislav Vı́tek Karel Fliegel Petr Páta Faculty of Electrical Engineering Czech Technical University Pavel Koten Astronomical Institute of the Academy of Sciences of the Czech Republic 58