| BZs >gnuplot.ps Acta Polytechnica Vol. 51 No. 2/2011 Improving the Photometry of the Pi of the Sky System A. F. Żarnecki, K. Ma�lek, M. Soko�lowski Abstract The “Pi of the Sky” robotic telescope was designed to monitor a significant fraction of the sky with good time resolution and range. The main goal of the “Pi of the Sky” detector is to look for short timescale optical transients arising from various astrophysical phenomena, mainly for the optical counterparts of Gamma Ray Bursts (GRB). The system design, the observation methodology and the algorithms that have been developed make this detector a sophisticated instrument for looking for novae and supernovae stars and for monitoring blasars and AGNs activity. The final detector will consist of two sets of 12 cameras, one camera covering a field of view of 20◦ ×20◦. For data takenwith the prototype detector at the Las Campanas Observatory, Chile, photometry uncertainty of 0.018–0.024 magnitudo for stars 7–10m was obtained. With a new calibration algorithm taking into account the spectral type of reference stars, the stability of the photometry algorithm can be significantly improved. Preliminary results from the BGInd variable are presented, showing that uncertainty of the order of 0.013 can be obtained. Keywords: Gamma Ray Burst (GRB), prompt optical emissions, optical flashes, novae stars, variable stars, robotic telescopes, photometry. 1 Introduction The “Pi of the Sky” experiment [1, 2] was designed for continuous observations of a large part of the sky, in the search for astrophysical phenomena vary- ing on scales from seconds to months, especially for prompt optical counterparts of Gamma Ray Bursts (GRBs). Other scientific goals include searching for novae and supernovae stars and monitoring blasars and AGNs activity. The large amount of data ob- tained in the project also enables the identification and cataloging of many different types of variable stars. The “Pi of the Sky” project involves scien- tists, engineers and students from leading Polish aca- demic and research units: The Andrzej So�ltan Insti- tute for Nuclear Studies, the Center for Theoretical Physics (Polish Academy of Science), the Institute of Experimental Physics (Faculty of Physics, Uni- versity of Warsaw), the Warsaw University of Tech- nology, the Space Research Center (Polish Academy of Sciences), the Faculty of Mathematics, Informat- ics and Mechanics (University of Warsaw), Cardinal Wyszynski University, the Pedagogical University of Cracow. 2 Detector The full “Pi of the Sky” system will consist of 2 sites separated by a distance of ∼ 100 km, which will al- low rejection by parallax of satellites and other near- Earth object. Each site will consist of 12 highly sus- tainable, custom-designed CCD survey cameras. The cameras will be placed on custom-designed paralac- tic mounts (4 cameras per mount) with high track- ing precision and two observation modes: “Deep”, with all cameras observing the same field (increas- ing measurement precision and/or time resolution) and “Wide”, when the cameras cover adjacent fields (maximizing FoV). Pairs of cameras will work in co- incidence and will observe the same field of view. The whole system will be capable of continuous observa- tion of about 1.5 steradians of the sky, which roughly corresponds to the field of view of the Swift BAT in- strument. The full system should be completed by the end of 2011. Fig. 1: The “Pi of the Sky” prototype detector located in the Las Campanas Observatory in Chile 112 Acta Polytechnica Vol. 51 No. 2/2011 Hardware and software solutions were tested with a prototype device installed in the Las Campanas Observatory in Chile in June 2004 and upgraded in 2006 (see Figure 1). It consists of two CCD cameras (2 000 × 2 000 pixels, 15 μm × 15 μm each) observ- ing the same field of view (20◦ × 20◦) with a time resolution of 10 seconds. Each camera is equipped with Canon lenses f = 85 mm, d = f /1.2, which en- ables them to observe objects to ∼ 11m (∼ 13m for 20 coadded frames). The prototype allows fully au- tonomous running including diagnostics and recovery from known problems. Human supervision is possible via Internet. 3 Data processing With each camera taking about 3 000 images per night, processing the large amount of data is a non- trivial task. The search for fast optical transients (e.g. GRB flashes) requires very fast data process- ing and identification of events in real-time. How- ever, nova star search and variable star analysis are based on precise photometry, which requires time- consuming detailed image analysis and data reduc- tion. To meet both requirements, two independent analysis paths were developed: the on-line part, which takes fast data scanning in real-time, and the off-line part, which performs a detailed data analysis. 3.1 On-line analysis On-line data analysis is based on dedicated fast al- gorithms optimized for transient search. In the full system, real-time frame by frame analysis will enable alerts to be distributed to the community for follow- up observations. After dark frame subtraction, an image is trans- formed by a special transformation called the Laplace filter. A new value for each pixel is calculated, taking into account the sum of the pixels around it and the sum of the pixels surrounding the central region. The idea of this transformation is to calculate the simple aperture brightness for each pixel (fast aperture pho- tometry algorithm). The resulting image, after the Laplace filter, is compared with the reference image stored in memory (based on a series of previous images). Any difference observed (above the estimated noise level) is consid- ered as a possible “candidate event”. All events are then processed through a set of selection algorithms to reject backgrounds such as background fluctua- tions, hot pixels, cosmic ray hits, or satellites. Coin- cidence between cameras is crucial for CCD related background and cosmic ray recognition. To allow for efficient background rejection, a multilevel selection system with pipeline data processing similar to trig- ger systems in particle physics experiments is used. 3.2 Off-line data reduction The aim of the off-line data analysis is to identify all objects in an image, and to add their measurements to the database. The reduction pipeline consists of three main stages: photometry, astrometry and cat- aloging. The data stored in the catalogue is then subjected to off-line analysis, which consists of sev- eral different algorithms. Algorithms optimized for off-line data reduction are applied to the sums of 20 subsequent frames, which is equivalent to an analysis of 200 seconds of exposure. After dark frame subtraction and flat correction, multiple aperture photometry is used, adopted from the ASAS [3] experiment. The pro- cedure prepares lists of stars with (x, y) coordinates on CCD and estimated magnitudes for each camera. The lists are then an input for the astrometry proce- dure. This is an iteration procedure where stars from the list are matched against reference stars from the catalog (the TYCHO catalog is currently used). Af- ter successful reference star matching, their measure- ments are used to calculate the photometry correc- tions (the final measurement is normalized to V mag- nitudes from the TYCHO catalog). Finally, all mea- surements are added to the PostgreSQL database. All data taken by the “Pi of the Sky” proto- type and stored in the project databases are pub- licly accessible. Two data sets are currently avail- able: the first database covers the period from July 2004 until June 2005, and contains about 790 mln measurements for about 4.5 mln objects, while the new one covers the period from May 2006 until April 2009, and includes about 2.16 billion measurements for about 16.7 mln objects. A dedicated web interface has been developed to facilitate public access [4]. 4 Photometry 4.1 Data quality cuts Off-line data reduction algorithms are designed for maximum efficiency. All collected data is stored in the data base. Additional cuts have to be applied in the analysis stage to select data with high measure- ment precision. It is necessary to remove measure- ments affected by detector imperfections (hot pix- els, measurement close to the CCD edge, background due to an opened shutter) or observation conditions (planet or planetoid passage, moon halo). Dedicated filters, taking into account all known effects, have been developed to remove bad object measurements (or whole images). The photometry accuracy obtained after applying standard set of cuts to remove bad quality data is il- lustrated in Figure 2. For stars from 7m to 10m, aver- age photometry uncertainty of about 0.018 − 0.024m has been obtained. 113 Acta Polytechnica Vol. 51 No. 2/2011 Fig. 2: Precision of star brightness measurements from standard photometry, for 200 s exposures (20 coadded frames) from the “Pi of the Sky” prototype in the Las Campanas Observatory in Chile 4.2 Spectral corrections Until 2009 the prototype detector installed in LCO was not equipped with any filter, except for an IR+UV cut filter1. This resulted in relatively wide spectral sensitivity of the “Pi of the Sky” detector, as shown in Figure 3. The average λ ≈ 585 nm is closest to the V filter, which we use as a reference in photometry corrections. When trying to improve the photometry precision for a BGInd variable star, we observed that the average magnitudo MP i of the reference stars, as measured by “Pi of the Sky”, is shifted systematically with respect to catalogue mag- nitudo V , depending on the star spectral type given by the difference of catalogue magnitudo B − V or J − K, see Figure 4. Fig. 3: Spectral sensitivity of the “Pi of the Sky” detec- tor, as resulting from the CCD sensitivity and IR+UV filter transmission, compared to the transmission curves of standard photometric filters B-V -0.5 0 0.5 1 1.5 2 2.5 - V P i M -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 Fig. 4: Average difference between the “Pi of the Sky” magnitudo MP i and the catalogue V magnitudo for the reference stars, as a function of the spectral type given by B − V The dependence of the average differences be- tween the measured and catalog magnitudo on the spectral type has been approximated by a linear func- tion. This corrects the measurement of each reference star so that, on average, the measured magnitudo Mcorr is the same as the catalogue V magnitudo, independently of spectral type. This so called “spec- tral correction” significantly reduces the systematic uncertainties in reference star magnitudo measure- ments. The distribution of the average magnitudo shift for the reference stars used in BGInd analysis, before and after spectral correction, is shown in Fi- gure 5. - VcorrM -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 5000 10000 15000 20000 25000 # stars corrected uncorrected Fig. 5: Distribution of the difference between the mea- sured reference starmagnitudo and the catalogue V mag- nitudo before (red) and after (blue) spectral correction Corrected reference star measurements are used to evaluate additional photometry correction for the studied object (the BGInd variable star is used as an 1Since summer 2009 one of the cameras has been equipped with a standard R filter 114 Acta Polytechnica Vol. 51 No. 2/2011 Phase 0 0.2 0.4 0.6 0.8 1 P i M -6.6 -6.5 -6.4 -6.3 Phase 0 0.2 0.4 0.6 0.8 1 c o rr M -6.6 -6.5 -6.4 -6.3 Fig. 6: Phased light-curveof thevariable starBGIndbefore (left) andafter (right) thenewcorrection proceduredescribed in this work example). To calculate the correction only reference stars with catalogue magnitudo 6 < V < 10, and with angular distance from the object smaller than 4 degrees are used. These cut values were found to result in the most precise and most stable photome- try corrections, resulting in the smallest uncertainty in the final BGInd brightness determination. Signif- icantly improved measurement precision is also ob- tained when the photometry correction is not calcu- lated as a simple average over all selected reference stars, but when a quadratic dependence of the cor- rection on the reference star position in the sky is fitted for each frame. The effect of the new photome- try correction procedure on the reconstructed BGInd light curve is shown in Figure 6. After applying the new corrections, the measurement quality improves significantly. Uncertainty of the order of 0.013m can be obtained. 5 Conclusions The “Pi of the Sky” prototype has been working since 2004, and has delivered a large amount of photo- metric data, which is publicly available [4]. With improved understanding of the detector and new fil- tering algorithms, the data quality and the stability of the photometry algorithm can be significantly im- proved. Work on the new photometry corrections is still ongoing, and further improvements are still pos- sible. Additional corrections can take into account the dependence of the magnitudo error of the star on its catalogue brightness, CCD pixel structure and pixel response non uniformity, as well as information on the correction quality from the fit. We hope to be able to obtain measurement precision of ∼ 0.01m for stars up to 10m (in optimal observation condi- tions). An independent study is also under way to prepare a photometry algorithm based on a detailed PSF (Point Spread Function) model. Acknowledgement We are very grateful to G. Pojmański for access to the ASAS dome and for sharing his experience with us. We would like to thank the staff of the Las Cam- panas Observatory for their help during the installa- tion and maintenance of our detector. This work was financed between 2005 and 2010 by the Polish Min- istry of Science and Higher Education, as a research project. References [1] Burd, A., et al.: Pi of the Sky-all-sky, real-time search for fast optical transients, New Astronomy, 10 (5), 2005, 409–416. [2] Malek, K., et al.: “Pi of the Sky” Detector, Ad- vances in Astronomy 2010, 2010, 194946. [3] Pojmanski, G.: The All Sky Automated Survey, Acta Astronomica, 47, 1997, 467. [4] “Pi of the Sky” measurement databases are avail- able at http://grb.fuw.edu.pl/pi/databases Aleksander Filip Żarnecki E-mail: filip.zarnecki@fuw.edu.pl Institute of Experimental Physics University of Warsaw ul. Hoza 69, 00-681 Warsaw, Poland Katarzyna Ma�lek Center for Theoretical Physics Warsaw, Poland Marcin Soko�lowski So�ltan Institute for Nuclear Studies Warsaw, Poland 115