An absolute calibration of DENIS (deep near infrared southern sky survey)

Astronomy and Astrophysics Supplement Series, Jul 2018

An absolute calibration of the DENIS photometric system is presented. It includes the determination of the overall transmission profiles in the 3 bands, namely i, J and Ks, combining contributions from atmosphere, telescope mirrors, instrument lenses and dichroics, filters, and detectors. From these normalized profiles, isophotal and effective wavelengths are computed, using the same synthetic Vega spectrum as that used to support the absolute calibration of many other ground-based and spaceborne photometric systems. Flux densities at zero magnitude are derived and integrated to give in-band fluxes, which are used to compute theoretical zero-points and compare them to observed ones, yielding estimates of the overall throughput of the whole system.

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An absolute calibration of DENIS (deep near infrared southern sky survey)

Astron. Astrophys. Suppl. Ser. An absolute calibration of DENIS (deep near infrared southern sky survey) P. Fouque 11 12 L. Chevallier 11 M. Cohen 9 E. Galliano 11 C. Loup 17 C. Alard 3 B. de Batz 16 E. Bertin 17 J. Borsenberger 17 M.R. Cioni 15 E. Copet 12 M. Dennefeld 17 S. Derriere 14 E. Deul 15 P.-A. Duc 19 D. Egret 14 N. Epchtein 18 T. Forveille 13 F. Garzon 4 H.J. Habing 15 J. Hron 7 S. Kimeswenger 8 F. Lacombe 12 T. Le Bertre 5 G.A. Mamon 6 17 A. Omont 17 G. Paturel 1 S. Pau 12 P. Persi 2 A.C. Robin 0 D. Rouan 12 M. Schultheis 17 G. Simon 3 D. Tiphene 12 I. Vauglin 1 S.J. Wagner 10 0 Observatoire de Besancon , BP. 1615, F-25010 Besancon Cedex , France 1 CRAL , Observatoire de Lyon, F-69561 Saint-Genis Laval Cedex , France 2 Istituto di Astro sica Spaziale, CNR , C.P. 67, I-00044 Frascati , Italy 3 DASGAL, Observatoire de Paris , 61 Av. de l'Observatoire, F-75014 Paris , France 4 Instituto de Astrof sica de Canarias, E-38200 La Laguna , Tenerife , Spain 5 DEMIRM, Observatoire de Paris , 61 Av. de l'Observatoire, F-75014 Paris , France 6 DAEC, Observatoire de Paris , 5 place J. Janssen, F-92195 Meudon Cedex , France 7 Institut fu ̈r Astronomie der Universit ̈at Wien , Tu ̈rkenschanzstrasse 17, A-1180 Wien , Austria 8 Institut fu ̈r Astronomie, Innsbruck University , A-6020 Innsbruck , Austria 9 Radio Astronomy Laboratory , 601 Campbell Hall , University of California , Berkeley, CA 94720 , U.S.A 10 Landessternwarte Heidelberg, K ̈onigstuhl , D-69117 Heidelberg , Germany 11 European Southern Observatory , Casilla 19001, Santiago 19 , Chile 12 DESPA, Observatoire de Paris , 5 place J. Janssen, F-92195 Meudon Cedex , France 13 Observatoire de Grenoble , 414 rue de la Piscine, Domaine Universitaire de Saint Martin d'Heres, F-38041 Grenoble , France 14 CDS , Observatoire Astronomique de Strasbourg, UMR 7550, 11 rue de l'Universite, F-67000 Strasbourg , France 15 Leiden Observatory, University of Leiden , P.O. Box 9513, 2300 RA Leiden , The Netherlands 16 DASGAL, Observatoire de Paris , 5 place J. Janssen, F-92195 Meudon Cedex , France 17 Institut d'Astrophysique de Paris , 98 bis Bd. Arago, F-75014 Paris , France 18 Observatoire de la C 19 CEA, DSM, DAPNIA, Centre d'Etudes de Saclay , F-91191 Gif-sur-Yvette Cedex , France 20 ote d'Azur, Departement Fresnel , F-06304 Nice Cedex , France An absolute calibration of the DENIS photometric system is presented. It includes the determination of the overall transmission pro les in the 3 bands, namely i, J and Ks, combining contributions from atmosphere, telescope mirrors, instrument lenses and dichroics, lters, and detectors. From these normalized pro les, isophotal and e ective wavelengths are computed, using the same synthetic Vega spectrum as that used to support the absolute calibration of many other ground-based and spaceborne photometric systems. Flux densities at zero magnitude are derived and integrated to give in-band fluxes, which are used to compute theoretical zero-points and compare them to observed ones, yielding estimates of the overall throughput of the whole system. surveys | instrumentation; miscellaneous - Send o print requests to: P. Fouque general | 1. Introduction The main goal of the DENIS survey (Deep Near-Infrared Southern Sky Survey, see Epchtein et al. (1994) for a complete introduction to DENIS) is to bridge the gap between the optical surveys on Schmidt plates and the far-infrared IRAS survey. Many aspects of astrophysics will bene t from such a survey, particularly studies of cool stars and heavily obscured regions. Each night, roughly 1 million stars are detected by at least one of the 3 cameras of the DENIS instrument. Photometric calibration is derived by observation of stan dard star elds. In order to compare our magnitude system to published ones, we need a precise de nition of our photometric bands and an absolute calibration of the DENIS photometric system. In Sect. 2, we describe the DENIS instrument and show the response curve for the complete system in the three bands. In Sect. 3, we estimate the conversion factors (ADUs to electrons) from the typical characteristics of DENIS images. Absolute calibration, based upon a synthetic Vega spectrum, is performed in Sect. 4, and observed and theoretical zero-points are compared. 2. Instrument characteristics The DENIS instrument has been described in detail by Copet et al. (1999) . A sketch of its main optical components is displayed in Fig. 1. It is located at the Cassegrain focus of the ESO 1 m telescope at La Silla Observatory (Chile). After reflection from the two telescope mirrors, the light beam goes through a eld lens at the telescope focus, covered by a protective blade, both of CaF2 and uncoated. Then a dichroic splits the i beam in reflection from the J /Ks beam in transmission. The i beam has two more reflections from coated mirrors before entering the objective (3 CaF2 and 2 silica coated lenses), then goes through the Gunn i lter, a shutter, the cryostat entrance window (BK7) and arrives at the Tektronix 1 K CCD detector, cooled to 180 K. The J /Ks beam is reflected o a microscanning mirror (uncoated Al), then J is reflected by a second dichroic and a coated mirror before entering the J objective (3 CaF2 and 2 fused silica coated lenses), then the cryostat entrance window (coated fused silica), the lter and nally the NICMOS-3 detector, both cooled to 80 K. The Ks beam is transmitted by the second dichroic, then reflects o two more coated mirrors, passes through the Ks objective (4 ZnS-Cleartran and 1 fused silica coated lenses), the cryostat entrance window (coated fused silica), the lter and the other NICMOS-3 detector. We have tried to obtain response curves for all these elements. When this was not possible, a reasonable estimate of the transmission was adopted. Filter response curves as provided by Barr Associates (U.S.A.) for J and Ks, at the nominal detector temperature (77 K), and by MTO-France for the Gunn i lter (at ambient temperature) are displayed in Fig. 2. The full system response curves (atmosphere, optics, lter and detector) are shown in Fig. 3. 3. Characteristics of DENIS images and conversion factors Band RON i J Ks e− The rst characteristics generally measured on astronom- Wavelength ( m) i J Ks ical images are the \sky emission" level (which includes l 0.802 1.248 2.152 telescope and instrument background in the Ks band), fad 0.795 1.235 2.160 and the noise on sky images and darks. e 0.788 1.221 2.144 Variations in sky level are observed in all three bands. iso 0.791 1.228 2.145 In i, they are related to the distance to, and phase of, the Moon. In J, they are due to variations in the hydroxyl raditcoalt'hseempaissssaiogne ionftednesnistiyty(OanHd−teMmepineerlatbuarnedps)e,rtcuornbnaetciotends TDaEbNleIS4.bFalnudxs densities of a zero magnitude star for the three through the upper atmosphere (Ramsay et al. 1992) . In Band iso F F Ks they come from the temperature variations. Also note m W/m2/ m Jy that, in crowded elds, the background value is set by i 0.791 1:20 10−8 2499 the confusion level (background of faint undistinguishable J 1.228 3:17 10−9 1595 stars). Ks 2.145 4:34 10−10 665 From these variations, the conversion factor between ADU (analog-to-digital units) and electrons can be deduced. Table 1 compares the result of these gain mea- 4. Absolute calibration surements with the expected values, calculated from the electronics characteristics of the chips, the preampli ers The next interesting characteristics to establish for absogains, and the analog-to-digital conversions. Table 2 gives lute photometric work is the flux of a zero magnitude star for each band the median value of the read-out noise in in the three DENIS bands. Many infrared systems and sevelectrons, the median and faintest values of the sky level, eral ways to calibrate them exist. We have decided to use in mag arcsec−2, and the median and minimal values of the calibration scheme described by Cohen et al. (1992) : the sky image noise in electrons, calculated from more they start from a model of Vega from Kurucz (1991) , takthan 2000 images taken during the last year of observa- ing into account its lower than solar metallicity, and nortions (April 1998 to April 1999), and adopting zero-points malize it to F5556 = 3:44 10−8 W m−2 m−1 from Hayes of 23.5, 21.3, and 19.2 in i, J, and Ks, respectively (see (1985). Additionally, we adopt V = 0.03 mag, V − I = 0, Sect. 4 and Table 5). The large values of sky and sky noise so I = 0.03 mag, but JHKLM = 0.00 mag for this star. in Ks come from the thermal background of the instru- For a more detailed discussion of Vega magnitudes and ment, which does not include cool stop optics. The \best" colours, see Bessell et al. (1998) . value of the sky level in J is suspect. We must now determine the isophotal wavelength of Note that some care must be taken in applying the con- each lter, taking into account the lter response curve, version factors and zero-points to the whole DENIS sur- the atmospheric transmission, the detector radiance revey: rst, J and Ks conversion factors seem to be slightly sponse and the Vega spectrum. Isophotal wavelengths are variable (11.4 to 15.3 in J, 51.6 to 57.0 in Ks), and second, preferred over e ective wavelengths, because the latter changes in the instrument have altered these values: the vary with input source spectrum much more than do pre-ampli er boards of J and Ks cameras were changed the former (see Golay 1974, for details and de nitions) . in June 1996, and a spare J camera has been in use from Results are given in Table 3. April 3, 1998 to May 9, 1999. Old conversion factors valid Using the isophotal wavelengths of our bands, and the until June 1996 were 12.47 in J and 39.3 in Ks (Chevallier Vega spectrum, we can compute the flux densities for a 1996) . Zero-point variations will be analyzed in a future zero magnitude star. Table 4 gives the results in wavepaper. length and frequency units. From these flux densities, we can estimate how many ADUs would be measured if the atmosphere, telescope and instrument totally transmitted the photons from this zero magnitude star. Comparing to the actually observed zeropoint will give the overall transmission of the system. We rst integrate the product of the Vega spectrum (shifted by 0.03 mag in i) by the transmission of the full system (fad), over the wavelength domain of our lters ( 0 and 1 correspond to the rst and last wavelengths where lter transmission reaches 0), to obtain the measured flux of a zero magnitude star: Ft = where A is the unobscured telescope collecting area (0.68 m2), t is the e ective integration time (8.998 s in i, 8.809 s in J and Ks), and G is the conversion factor. Table 5 gives the results. Zero-points are measured during calibration nights, where only photometric standards are observed, and routinely during survey nights, to follow possible instrumental variations and make a rough estimate of the extinction coe cients. They are measured from aperture magnitudes inside a 7 arcsec diameter circle around the standard star. To make a valid comparison with the theoretical zero-points, a rst correction is necessary to include flux falling outside this aperture. This has been estimated to amount to 0.1 mag in all three bands from a comparison of observations through a 15 arcsec diameter aperture. A linear t of the observed magnitudes vs. airmass (assuming that Bouguer's 1729, law is valid) gives the extinction coe cient as the slope and the zero-point as the (1) (2) intercept. However, it is well known that extrapolation to zero airmass leads to a systematic error in the nearinfrared (the Forbes 1842, e ect) , which has been quantied for the J and K bands by Manduca & Bell (1979) . For the La Silla typical water vapour contents (1 to 10 mm of precipitable water), the error is about 0.10 mag in J and 0.02 mag in K, and should be similar in Ks. Therefore, we also add this J correction to the observed zero-point, while we neglect the K correction, given the uncertainty in measured zero-points. The corrected zero-points (for in nite aperture and non-linear variation with airmass) are given in Table 5, and the derived overall transmissions follow. To interpret the measured overall transmission, we have tried to estimate the contribution of each component of the system, namely atmosphere, aluminium reflections (telescope mirrors, and microscanning mirror for J and Ks), thin blade protecting the eld lens, eld lens itself, dichroics, coated mirrors, objectives, cryostat entrance windows, lters, and detector quantum e ciency (converted to radiance response). For details about each value, see Galliano (1999) . Table 5 gives all these estimates, and their nal product. The agreement with the measured overall transmission is satisfying, and shows a good performance of the instrument, with overall throughput of 20 to 30% in all three bands. Acknowledgements. Thanks to Maria Eugenia Gomez for nding the original reference of Bouguer's law. The DENIS project is supported by the SCIENCE and the Human Capital and Mobility plans of the European Commission under grants CT920791 and CT940627, by the French Institut National des Sciences de l'Univers, the Ministere de l'Education Nationale and the Centre National de la Recherche Scienti que, in Germany by the State of Baden-Wu¨rtemberg, in Spain by the DGICYT, in Italy by the Consiglio Nazionale delle Ricerche, by the Austrian Fonds zur Fo¨rderung der wissenschaftlichen Forschung und P. Fouque et al.: DENIS calibration 317 Bundesministerium fu¨r Wissenschaft und Forschung, in Brazil by the Fundation for the development of Scienti c Research of the State of Sa~o Paulo (FAPESP), and by the Hungarian OTKA grants F-4239 and F-013990, and the ESO C & EE grant A-04-046. Bessell M.S. , Castelli F. , Plez B. , 1998 , A &A 333 , 231 (and erratum in A&A 337 , 321 ) Bouguer P. , 1729 , Essai d'optique sur la gradation de la lumiere, C. Jombert, Paris, reprinted in: Les ma^tres de la pensee scienti que, Paris, 1921 Chevallier L. , 1996 , Performances et qualite d'images du survey DENIS, DEA (master's) thesis . University of Strasbourg Cohen M. , Walker R.G. , Barlow M.J. , Deacon J.R. , 1992 , AJ 104 , 1650 Copet E. , Epchtein N. , Rouan D. , et al., 1999 , A& AS (in preparation) Epchtein N. , de Batz B. , Copet E. , et al., 1994 , in: Proceedings of a Conference held at Les Houches: \ Science with astronomical near-infrared sky surveys" , Epchtein N. , Omont A. , Burton W.B. , Persi P . (eds.), \ A Deep NearInfrared Survey of the Southern Sky" , september 1993 , p. 3 , Les Houches , France, reprinted from Astrophysics and Space Science 217 , 3 Forbes J.D. , 1842 , Phil. Trans. 132 , 225 Galliano E. , 1999 , Une calibration absolue du systeme photometrique du survey DENIS, DEA (master's) thesis . University of Grenoble Golay M. , 1974 , in: Introduction to astronomical photometry. Reidel , Dordrecht, p. 39 Hayes D.S. , 1985 , in: Calibration of Fundamental Stellar Quantities, Proc. IAU Symposium No. 111 , Hayes D.S. , Pasinetti L.E. & Davis Philip A.G. (eds.). Reidel, Dordrecht, p. 225 Kurucz R.L. , 1991 , quoted in Cohen et al. ( 1992 ) (as private communication) Manduca A. , Bell R.A. , 1979 , PASP 91 , 848 Ramsay S.K. , Mountain C.M. , Geballe T.R. , 1992 , MNRAS 259 , 751


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P. Fouqué, L. Chevallier, M. Cohen, E. Galliano, C. Loup, C. Alard, B. de Batz, E. Bertin, J. Borsenberger, M. R. Cioni, E. Copet, M. Dennefeld, S. Derriere, E. Deul, P.-A. Duc, D. Egret, N. Epchtein, T. Forveille, F. Garzón, H. J. Habing, J. Hron, S. Kimeswenger, F. Lacombe, T. Le Bertre, G. A. Mamon, A. Omont, G. Paturel, S. Pau, P. Persi, A. C. Robin, D. Rouan, M. Schultheis, G. Simon, D. Tiphène, I. Vauglin, S. J. Wagner. An absolute calibration of DENIS (deep near infrared southern sky survey), Astronomy and Astrophysics Supplement Series, 313-317, DOI: 10.1051/aas:2000123