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

Fliegel, K.

Koten, P.

Páta, P.

Vítek, S.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Acta Polytechnica Vol. 52 No. 2/2012Utilization of Image Intensiﬁers in AstronomyS. V´ıtek, K. Fliegel, P. Pa´ta, P. KotenAbstractIn this paper we present the properties of image intensiﬁers, used together with fast TV cameras for astronomicalpurposes within the MAIA project(Meteor Automatic Imager and Analyser, primarily focused on observing meteoricevents with high time resolution). The main objective of our paper is to evaluate the suitability of these devices forastronomical purposes in terms of noise, temporal and spectral analysis.Keywords: image intensiﬁer, astronomy, meteors.1 IntroductionAn interesting technique (that has became relativelyinexpensive in recent years) for increasing the timeresolution of any astronomical instrument is the useof a modern CCD camera with a fast framerate of 50or more frames per second. This type of system has asigniﬁcant deﬁciency in terms of reduced sensitivity;the solution may to use an image intensiﬁer. Thispaper describes our experience with a device of thiskind.The MAIA astronomical instrument [2] uses asecond generation Mullard XX1332 image intensiﬁer.The tube assembly of this 50/40mm inverter (typicalgain 30 000 to 60 000 lm/lm, resolution 30 lp/mm) isdesigned to be incorporated in night vision devices,in particular in tank driving periscopes. This leadsto some properties which signiﬁcantly deﬁne the lim-its and the possibilities of using a device of this kindin astronomy.2 ElectroopticalcharacteristicsThe instrument is equipped with an input aperturelens (Pentax SMC FA 1.4/50 mm) and an inner cam-era lens (Pentax H1214-M 1.4/12 mm) [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 spectrumof the light at the output of the image intensiﬁer, thespatial resolution of the input lens and the image in-tensiﬁer, and also the spatial resolution of the wholesystem are among the most important parameterstested and presented in this paper.10−4 10−3 10−2 10−1 10010−210−1100Normalized input power [−]Normalized gain [−]0 200 400 600 800 1000 120000.20.40.60.81Spatial frequency [LW/PH]MTF [−]Overall MTFInput lens andimage intensifier(a) (b)Fig. 1: The normalized gain of the system (measured at 650 nm) describes the automatic gain control as a nonlinearityin the image intensiﬁer (a), the overall MTF of the system, including the camera (solid line) and the partial MTF of theimage intensiﬁer with the lens (dashed line) (b)56Acta Polytechnica Vol. 52 No. 2/2012400 500 600 700 800 900 100000.20.40.60.81Wavelength [nm]Relative response, Spectral transmission,Relative intensity [−]                   Cameraw/o lensCamerawith lensRelativepowerLens400 500 600 700 800 90000.20.40.60.81Wavelength [nm]Relative sensitivity, Spectral transmission [−] LensB=0.11B=0.23B=0.36B=0.47B=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 theoutput screen of the image intensiﬁer (a), relative overall spectral sensitivity of the system for diﬀerent digital levels Bin the output image (B = 1 white, B = 0 black) (b)Fig. 3: Temporal changes in stellar object ﬂux2.1 Spectral responseThe spectral response was measured independentlyfor all parts of the system [3]. The experimental setupconsisted of the LOT-Oriel collimated halogen lightsource, the LOT-Oriel Omni 150 computer controlledmonochromator, 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 resolutionThe 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 aslanted edge (an approx. 5◦ slightly slanted blackbar used to measure the horizontal or vertical spa-tial frequency response), or a line square wave sweepwith the spatial frequency range 100–1000 LW/PH(line widths per picture height). In our case, slantededges were used to determine the spatial frequencyresponse — see Figure 1(b).3 Noise performanceThe noise analysis based on the acquisition of test-ing video sequences in various light conditions is de-scribed in [5]. We choose the Generalized LaplacianModel (GLM) for heavy-tailed noise.4 Temporal analysisFigure 3 shows the temporal changes in stellar objectﬂux over 100 frames of videosequence. The changesare mainly due to the automatic gain control. Thisfact puts greater demands on image calibration. Wehave proposed adaptive ﬂat-ﬁelding for any light con-ditions.5 ConslusionsThe biggest disadvantage of the image intensiﬁer de-scribed here is the built-in automatic gain control. Asis shown Figure 1(a), the gain decreases rapidly withincreasing input power, i.e. if any bright stellar ob-ject appears in the ﬁeld of view. However the image57Acta Polytechnica Vol. 52 No. 2/2012intensiﬁer is not a bottleneck of the MAIA device —all measured parameters of an image intensiﬁer arefar better than the parameters of the CCD cameraused for the project.AcknowledgementThis work has been supported by grant No.205/09/1302 Study of sporadic meteors and weakmeteor showers using automatic video intensiﬁercameras of the Grant Agency of the Czech Re-public. We would also like to acknowledge grantNo. 102/09/0997 from Grant Agency of the CzechRepublic.References[1] Koten, P.: Software for processing of me-teor video records. Proceedings of the Asteroids,Comets, Meteors 2002 conference, 197, 2002.[2] Vı´tek, S., Koten, P., Pa´ta, P., Fliegel, K.:Double-Station Automatic Video Observation ofthe Meteors. Advances in Astronomy, 2010 (Ar-ticle ID 943145), 4 pages (2010).[3] Fliegel, K., Havl´ın, J.: Imaging photometer witha non-professional digital camera. Proc. SPIE7 443, 74431Q, 2009.[4] Fliegel, K., Sˇvihl´ık, P., Pa´ta, P., Vı´tek, S.,Koten, P.: Meteor automatic imager and ana-lyzer: current status and preprocessing of imagedata. Applications of Digital Image ProcessingXXXIV, Proc. SPIE, 2011.[5] Sˇvihl´ık, P., Fliegel, K., Koten, P., Vı´tek, S.,Pa´ta, P.: Noise Analysis of MAIA Systemand Possible Noise Suppressio. Radioengineering,Vol. 20, pp. 110–117, 2011.Stanislav Vı´tekKarel FliegelPetr Pa´taFaculty of Electrical EngineeringCzech Technical UniversityPavel KotenAstronomical Institute of theAcademy of Sciences of the Czech Republic58

Utilization of Image Intensifiers in Astronomy

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