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Revista Mexicana de Astronomıa y Astrofısica, 46, 47–60 (2010)

A PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF

THE SITE AT TONANTZINTLA OBSERVATORY1

H. M. Hernandez-Toledo,2 L. A. Martınez-Vazquez,2 M. A. Moreno-Corral,3 and A. Pani-Cielo4

Received 2009 March 24; accepted 2009 October 15

RESUMEN

Se presenta una evaluacion sobre la calidad del cielo en el Observatorio As-tronomico Nacional de Tonantzintla mediante la calibracion fotometrica de unconjunto de estrellas estandar en M67 en las bandas B, V , R e I del sistemaJohnson-Cousins y la observacion del cielo a diferentes distancias cenitales con elespectrografo Boller & Chivens. El brillo del cielo se estimo (a) a partir de lasobservaciones CCD al cumulo M67 y (b) utilizando un metodo visual en direccionde estrellas de magnitud conocida. Se obtuvo un valor de 18.5± 0.6 mag arcsec−2.La curva de extincion atmosferica promedio presenta un comportamiento interme-dio entre la observada para el OAN-San Pedro Martir y la asociada a actividadvolcanica. Los espectros del cielo en el OAN-Tonantzintla permiten identificarlıneas asociadas a lamparas de HgI y NaI a baja y alta presion. Nuestros resultadospermiten justificar la actualizacion y compra de nuevo equipamiento para conver-tir al OAN-Tonantzintla en el Laboratorio para la Ensenanza de la AstronomıaObservacional.

ABSTRACT

Based on data obtained at the Observatorio Astronomico Nacional, Tonantz-intla, an evaluation of the quality of the local sky is presented. The evaluationis carried out through an absolute CCD calibration to a set of standard stars inthe field of M67 in the B, V , R and I Johnson-Cousins system and from Boller &Chivens long-slit spectroscopic observations of the local sky at various elevations.The sky brightness is estimated from our CCD frames yielding a mean value of18.5 ± 0.6 mag arcsec−2. The mean atmospheric extinction curve behaves midwaybetween the normal extinction and that related to volcanic outbursts. From thelong-slit spectra of the sky at OAN-Tonantzintla, HgI and NaI lines of high and lowpressure lamps are identified. These results allow us to justify a major upgradingof the observatory, to convert the OAN-Tonantzintla into the Laboratory for As-tronomy Education both for UNAM and for other educational institutions in thecenter-south part of the country.

Key Words: atmospheric effects — site testing — stars: fundamental parameters— stars: imaging — techniques: photometric — techniques: spectro-scopic

1Based on data obtained at the 1 m telescope of the Obser-vatorio Astronomico Nacional, Tonantzintla, Puebla, Mexico,operated by the Instituto de Astronomıa, Universidad Na-cional Autonoma de Mexico.

2Instituto de Astronomıa, Universidad NacionalAutonoma de Mexico, Mexico, D. F., Mexico.

3Instituto de Astronomıa, Universidad NacionalAutonoma de Mexico, Ensenada, B. C., Mexico.

4Instituto de Astronomıa, Universidad NacionalAutonoma de Mexico, Tonantzintla, Puebla, Mexico.

1. INTRODUCTION

The Observatorio Astronomico Nacional at To-nantzintla (OAN-Tonantzintla), under supervisionof the Instituto de Astronomıa at the UniversidadNacional Autonoma de Mexico (IA-UNAM), is lo-cated near the Cholula village in the state of Puebla,150 km away from the Instituto de Astronomıaheadquarters in Mexico City. Due to the increas-ing amount of light pollution from the growing

47

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48 HERNANDEZ-TOLEDO ET AL.

city of Puebla and nearby villages, the observatorybecame progressively inappropriate for deep astro-nomical observations. Observing priorities changedat Tonantzintla observatory and since the end ofthe 1970s the site has provided fundamental sup-port for (i) instrumental development and testing(c.f. Voitsekhovich et al. 2005), (ii) observing pro-grams of bright objects of various types (c.f. Pis-mis & Moreno 1984) and (iii) public outreach andphysics/astronomy university programs.

A close collaboration between Facultad de Cien-cias and the Instituto de Astronomıa at UNAMbrought the 1 m telescope of the OAN-Tonantzintlaas one of the first professional telescopes in Mexicoavailable at college level providing physics and as-tronomy students with real access to modern scien-tific instruments. The acquisition of a scientific CCDat the end of the 1980s opened new possibilities forboth photometric and spectroscopic observations atthe site. In this paper an evaluation of the localsky conditions by using the current photometric andspectroscopic instrumentation is presented and theresults are assessed in the context of promoting theOAN-Tonantzintla as a natural laboratory for astro-nomical and educational programs at UNAM.

The paper is organized as follows. In�

1.1 wepresent a brief synopsis of a historical investigationon the observatory’s beginnings (Moreno-Corral etal. 2009, in preparation) that allows us to bet-ter understand the role of the OAN-Tonantzintla asthe major astronomical facility in Mexico during the1960s. In

�2 we present the main characteristics

of the current photometric and spectroscopic instru-mentation available at the site. We also present theresults of our BVRI photometric observations of aset of standard stars from the M67 star cluster, ourestimate of the mean extinction coefficients and thesky surface brightness in the Johnson-Cousins bands.Additionally, the results of a campaign to estimatethe sky brightness in an area of ∼ 8 km2 around theOAN-Tonantzintla are presented. In

�3 the results

of the spectroscopic observations of the sky at var-ious elevations are given and compared against thespectra of other astronomical sites. The most promi-nent emission lines in the spectra of the sky thatcontribute to the light pollution are identified. Skyspectra acquired at the OAN-Tonantzintla at differ-ent epochs are also presented and compared. In

�4

a general discussion of the photometric and spectro-scopic results is presented, followed by some sugges-tions for more efficient observing runs at the site.Finally, based on an estimate about the actual ca-pabilities of the 1 m + current instrumentation, the

feasibility of in-situ/remote observing programs andtheir impact on the undergraduate/graduate physicsand astronomy programs at UNAM is commented.

1.1. Historical Overview

The important number of astronomical discover-ies carried out during the decade of 1947–1957 byGuillermo Haro and a group of collaborators usingthe 76-cm Schmidt camera of the Observatorio As-trofısico Nacional, subsidiary of the Secretarıa de Ed-ucacion Publica (Bok 1941; Mayall 1942), made himrealize the necessity of complementing and diversify-ing their observational projects. Therefore, in 1958,as a director of the Observatorio Astronomico Na-cional at the UNAM, Haro made an effort to pro-vide this institution with a modern reflector tele-scope. As the plans drawn by J. Brinkan show (Haro1958), it would be installed on the piece of land do-nated by the amateur astronomer Domingo Taboada(Moreno-Corral & Lopez-Molina 1992), contiguousto that occupied since 1942 by the ObservatorioAstrofısico Nacional in the town of Tonantzintla,Puebla.

In order to carry out this project, GuillermoHaro, in addition to the support provided by theUNAM authorities, sought external financing of sev-eral Mexican philanthropic foundations, and ob-tained from them, and principally from the MaryStreet Jenkins Foundation of the city of Puebla(Trueblood 1988), considerable resources which per-mitted him to commission the fabrication of amodern reflector telescope with fork mount andCassegrain focus. The office of the contractor en-gineer B.G. Hooghoudt of Leyden, Holland was des-ignated by Guillermo Haro to design the mount andthe rest of the mechanical parts of the new telescopewhich were then manufactured by the Holland firmRademakers Aandrijvingen of Rotterdam. The mir-rors for these instruments were polished in Califor-nia, USA, under the supervision of the prominentoptician Don O. Hendrix of the Hale Telescope Team(Osterbrock 2003). The diameter of the main mirrorwas 40 inches with a central aperture of 10 inches.

The members of the technical personnel of theObservatorio Astrofısico Nacional and the Obser-vatorio Astronomico Nacional installed, wired andtested the telescope which was ready for its first lightin 1961 (Poveda & Allen 1987). The first two in-struments of the telescope were a nebular spectro-graph and a photoelectric photometer. A few yearslater, the instrumental equipment of the telescopewas increased, including a Boller & Chivens spec-trograph built by Perkin-Elmer Corporation for the

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 49

Cassegrain focus, with the possibility of interchang-ing cameras and gratings. In addition, a photo-graphic Fabry-Perot interferometer with fixed plateetalons, and narrow band nebular filters was pro-vided. This instrument was essential for kinematicstudies of galactic nebular regions as discussed inPismis & Moreno (1984).

Two milestones were fundamental for the futureof the observatory after 1980: (1) the acquisition of afirst generation Thomson CCD detector in the mid-dle of the 1980s that, attached to the 1 m telescope,opened up new possibilities for observing, especiallyin photometric and spectroscopic modes, and (2) theopening of a graduate program in Astronomy & As-trophysics at UNAM at the end of the 1980s. The 1mtelescope and its instrumentation were made avail-able at college level providing fundamental supportfor physics and astronomy courses.

2. PHOTOMETRIC OBSERVATIONS OF THESKY AT THE OAN-TONANTZINTLA

The main telescope at the Tonantzintla obser-vatory is a 1 m f/15 Cassegrain with an equatorialmount yielding a plate scale of 13.53 arcsec/mm atthe focal plane. The range of movements is ±5.6 hin hour angle and from −60◦ to 80◦ in declinationwith a maximum pointing velocity of 1◦/sec and apointing error ≤ 1′ for zenithal distance z ≤ 60◦.The main optical detector is a thinned MetachromeII covered Thomson THX 31156 CCD of 1024×1024pixels, each of 19 µm in size. Among its character-istics are a deep well ∼ 175, 000 e−, a dark currentof ∼ 0.31 e−/hour/pixel at gain mode 4, a readoutnoise of ∼ 3.5 e− RMS, a linear response of 0.58%and a reading rate of 50KHz. The telescope and de-tector combination attain a maximum field of viewof 4.2′ × 4.2′. A set of broad-band B (λ 4300 A),V (λ 5400 A), R (λ 6400 A), I (λ 8900 A) filtersin the Johnson-Cousins systems is available in directimaging mode.

2.1. Broad-Band BVRI Photometry

BVRI images in the Johnson-Cousins systemwere obtained with the Thomson CCD detector in2× 2 bin mode attached to the 1 m telescope, cover-ing an area of 4.2′ × 4.2′, with a scale of 0.49′′/pixeland a typical seeing of 3′′. Good weather conditionsfor observing standard stars are difficult to find atthe OAN-Tonantzintla. For the sake of the presentreport we concentrate on the February 2005 observ-ing runs. Our monitoring program consisted of aCCD photometric follow-up of a set of 25 equatorialstandard stars from the “Dipper Asterism” M67 star

cluster (Chevalier & Ilovaisky 1991; Landolt 1992) atdifferent air masses in order to estimate local meanextinction coefficients, sky brightness and total ap-parent magnitudes of the stars. Due to the lack of afine guiding system, exposure times did not exceed5 minutes in the B band to avoid guiding problemsin longer exposure times.

Images were debiased, trimmed, and flat-fieldedusing standard IRAF5 procedures. First, the biaslevel of the CCD was subtracted from all exposures.A set of 10 bias images was obtained per night, andthese were combined into a single bias frame whichwas then applied to the object frames. The imageswere flat-fielded using twilight sky flats taken in eachfilter at the beginning and/or end of each night. Themost energetic cosmic-ray events were automaticallymasked using the COSMICRAYS routine. A finalstep in the basic reduction involved registration of allavailable frames in each filter to within ±0.1 pixel.This step was carried out by measuring centroidsfor stars on the images and then performing geo-metric transformations using GEOMAP and GEO-TRAN tasks in IRAF.

A total of 25 standard stars with a color range of−0.1 ≤ (B−V ) ≤ 1.4 and a similar range in (V − I)were observed at different air masses and were mea-sured with a 28 pixel diameter aperture to deter-mine, for a given frame, total apparent magnitudesin the B, V , R and I bands. Outside atmospherevalues for magnitudes were obtained after estimatingatmospheric extinction coefficients by means of lin-ear least-square fitting procedures. Linear transfor-mations to the Johnson-Cousins photometric systemwere also derived by least-square solutions. Figure 1shows a B band image of the selected standard starsin the field of the M67 star cluster. Number desig-nations were adopted following Chevalier & Ilovaisky(1991).

The reader should notice that the quantum effi-ciency of the Thomson CCD detector6 is significantlylower in the B than in the I band. However, a 5min integration time is enough to get a reasonablesignal-to-noise ratio in the B band for stars as faintas mB ∼ 15 mag within the M67 field.

Once the principal extinction coefficients in B,V , R and I were estimated, a transformation of theinstrumental magnitudes to a standard system was

5The IRAF package is written and supported by the IRAFprogramming group at the National Optical Astronomy Ob-servatories (NOAO) in Tucson, Arizona. NOAO is operatedby the Association of Universities for Research in Astronomy(AURA), Inc. under cooperative agreement with the NationalScience Foundation (NSF).

6See http://www.astrostnt.unam.mx.

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50 HERNANDEZ-TOLEDO ET AL.

4

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21 22 23

2425

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Fig. 1. B band image of stars in the field of the “Dip-per Asterism” M67 star cluster acquired with the 1 m+ Thomson CCD at the OAN-Tonantzintla covering anarea of 4.2′

× 4.2′, with a scale of 0.49′′/pixel. The num-bers correspond to the stars used in the current study.

calculated (Hiltner 1960; Massey & Davies 1992) ac-cording to the following equations:

B − b = αB + βB(b − v)0 ,

V − v = αV + βV (b − v)0 ,

R − r = αR + βR(v − r)0 ,

I − i = αI + βI(v − r)0 , (1)

where B, V , R and I are the standard magnitudes,b, v, r and i are the instrumental (and airmass-corrected) magnitudes. α and β are the transfor-mation coefficients for each filter. For more detailson our reduction procedures, see Hernandez-Toledo& Puerari (2001).

A comparison of our estimate of the apparentmagnitudes in the B and I bands against those re-ported in Chevalier & Ilovaisky (1991) for the 25stars in common is shown in Figure 2.

Figure 2 shows that non significant systematics(zero point and first-order color terms) are influenc-ing our magnitude estimates. The observed rangein apparent magnitudes goes from (9–15) mag andfrom (8–14) mag in the B and I bands respectively.Vertical error bars in each panel indicate the averageσ values per apparent magnitude bin. The B bandσ values per bin increase from (0.17–0.25) mag frombright to faint star magnitudes. On the other hand,the I band σ values per bin vary from (0.25–0.35)

Fig. 2. Comparison between our estimated B and I bandmagnitudes and those from Chevalier & Ilovaisky (1991)for 25 standard stars in common.

mag indicating the intrinsically noisy nature of thered filters as well as an increasing contribution of thesky background at redder bands. From the observedvariation of σ-values in Figure 2, typical values ∼ 0.2to 0.3 mag in the B and I bands are quoted as rep-resentative for the photometric calibration.

2.1.1. Errors

An estimation of the errors in our photometryinvolves (1) the procedures to obtain instrumentalmagnitudes and (2) the uncertainty when such in-strumental magnitudes are transformed to a stan-dard photometric system. For item (1), the extinc-tion corrections to the instrumental magnitudes andairmass estimates were considered. After a leastsquare fitting procedure, the associated errors to the

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 51

slope for each principal extinction coefficient are;δ(kB) ∼ 0.05, δ(kV ) ∼ 0.05, δ(kR) ∼ 0.06 andδ(kI) ∼ 0.06. An additional error δ(airmass) ∼ 0.005from the airmass routines in IRAF was also consid-ered. For item (2), the zero point and first order colorterms of the photometric transformation (equation1) are the most important. The associated errorsto the zero point and first order color terms were∼ 0.1 mag in the B, V , R and I bands. Total typ-ical uncertainties are ∼ 0.2 mag in B, V , R and Ibands, close to the typical values quoted from theσ variations. We notice that the observing roomhas various sources of light coming mainly from thecomputer screen and various electronic LEDs. Anappropriate shielding against that parasitic light atthe observing room is suggested. We also suggest toevaluate in further observing runs the contributionof scattered light along the optical path of the 1 mtelescope (c.f. Gutierrez et al. 1996). This contri-bution could affect the data calibration, particularlywhen a frame is acquired by exposing to a brightlight source (which was not the case in the presentpaper). Given the much fainter limiting magnitudesachievable with an autoguiding system, another rec-ommendation ad hoc to the actual major upgradingof the observatory is the implementation of such asystem.

2.2. Sky Surface Brightness

The surface brightness of the moonless night skyis a fundamental quantity of an observing site. Thesky brightness at the OAN-Tonantzintla has beendegraded with time, as the nearby population cen-ter of Puebla and its attendant light pollution hasbeen growing. Sky brightness was measured in ourCCD frames at different observing bands on moon-less conditions by excluding areas apparently freeof nebulosity, stars, cosmic-ray events or CCD de-fects. Since we have observations at different heightsabove the horizon (typically from 30◦ to 75◦), thesky brightness at those heights could be estimated aswell. Our results are presented in Table 1. Column(1) indicates the band of the observation, Column(2) is the airmass at the time of the observation,Column (3) the sky brightness values obtained atthe Observatorio Astronomico Nacional San PedroMartir (OAN-SPM) under dark sky conditions. Sim-ilarly, Columns (4) and (5) represent the airmass andthe corresponding sky brightness values obtained atOAN-SPM under bright (near full-moon) sky condi-tions. The airmasses at the time of our observationsat 30◦ and 75◦ above the horizon are presented inColumn (6). Finally, the corresponding sky bright-

ness estimates at the OAN-Tonantzintla in dark skyconditions are presented in Column (7).

The results in Table 1 indicate a sky sur-face brightness value in the V band of ∼ 18.5 ±

0.2 mag arcsec−2 in the line of sight towards the cityof Puebla and ∼ 19.0±0.2 mag arcsec−2 at the localzenith area. Compared to the representative V bandsky value for the OAN-SPM under moonless condi-tions, this difference of about 3 magnitudes is inter-preted as the amount of light pollution contributedto the site from both the local villages around To-nantzintla and from the city of Puebla. The effect isequivalent to losing the ability to distinguish by theunaided eye stars of magnitudes fainter than ∼ 4 ina clear dark night sky.

We warn the reader that although measuring thesky brightness via broad band photometry as shownhere represents a first step forward, it has to be takenwith caution since such measures encompass bothnatural air glow and artificial sources. Significantnight-to-night (and even hourly) variations of theintensity of OH emission and the OI λ 5577 auro-ral line are well documented and depend in part onsolar activity (see Pilachowski et al. 1989; Massey,Gronwall, & Pilachowski 1990).

To complement our sky brightness measures, inOctober 2008 we carried out a first campaign toestimate the sky surface brightness in an area of∼ 8 km2 around the observatory. With the aid of25 students from the Facultad de Ciencias and Insti-tuto de Astronomıa of the UNAM, simple, unaided-eye observations to look for the faintest stars to-ward the Orion constellation were carried out. Theexperiment was carried out on a moonless night,free of clouds, by matching the observed star con-figuration to one of six star maps of progressivelyfainter limiting magnitude that were prepared forthat purpose. The method uses two facts, namely(1) that a widely accepted value for sky brightnessat the zenith at a site completely free of man-madelight sources and near solar activity minimum isV ∼ 21.4 − 21.8 mag arcsec−2 and (2) that the un-aided eye detects stars of apparent magnitudes asfaint as 6 mag. Measures reading 6th magnitudestars would indicate a dark site with surface bright-ness near V ∼ 21.4 − 21.8 mag arcsec−2 and freeof light pollution, while a reading of 4th magnitudestars would indicate a site with a surface bright-ness worse than V ∼ 18 mag arcsec−2 and thusa more degraded, light polluted sky. Different po-sitions around the OAN-Tonantzintla were selectedfree of trees and tall buildings that could block thevisibility where students found latitude and longi-

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52 HERNANDEZ-TOLEDO ET AL.

TABLE 1

AIRMASSES, AND SKY BRIGHTNESS VALUES OBTAINED AT THE OBSERVATORIOASTRONOMICO NACIONAL SAN PEDRO MARTIR (OAN-SPM) UNDER DARK AND

NEAR FULL-MOON SKY CONDITIONS. SIMILAR QUANTITIES OBTAINED IN DARKSKY CONDITIONS AT THE OAN-TONANTZINTLA AT 75◦ ABOVE THE HORIZON AND

AT 30◦ TOWARDS THE CITY OF PUEBLA ARE ALSO PRESENTEDa

Sky Surface Brightness (mag arcsec−2)

OAN-SPM OAN-Tonantzintla

Band X(air mass) Sky X(air mass) Sky X(air mass) Sky

U 1.00 21.3 1.09 18.9 · · · · · ·

B 1.00 22.3 1.12 19.5 2.1/1.05 18.6/19.1

V 1.01 21.4 1.15 19.4 2.2/1.09 18.5/19.0

R 1.00 20.9 1.18 19.3 2.3/1.12 18.3/18.9

I 1.02 19.4 1.21 18.2 2.4/1.16 16.8/17.7

aOAN-SPM Sky Surface Brightness data from M. Richer http://www.astrossp.unam.mx/sitio/brillo_cielo.htm.

tude by means of a GPS. Once the students matchedtheir unaided-eye observations to one of our mag-nitude charts they moved to a new pre-selected lo-cation at least 1 km away from their original loca-tion. For a more detailed description on the ap-parent magnitude-surface brightness transformation,see Schaefer (1990).

Figure 3 (left panel) shows a 2.7 km × 2.7 kmarea around the observatory and the specific posi-tions (numbers) where the sky brightness was es-timated. The position of the observatory (OAN-T) is also indicated. The right panel of Figure 3shows a 2D representation of the data after fitting aspline surface function to the observed distributionof points in the 2.7 km × 2.7 km area. The final sur-face that passes through the original points is thenconverted into a color-scaled surface.

The data points were plotted over a map and atwo-dimensional function was fitted to the data us-ing a standard IDL routine. This routine uses thinplate splines to interpolate a set of values over a reg-ular two dimensional grid, from irregularly sampleddata values, in this case the measured visual mag-nitudes. The maximum at the surface (light yel-low color) corresponds to dark sky conditions whilethe orange tones represent brighter sky conditionsand its variations within the survey area7. Our re-sults indicate a mean sky surface brightness valueV ∼ 18.5 ± 0.6 mag arcsec−2 within the survey areaand V ∼ 19.1 ± 0.5 mag arcsec−2 only at the localzenith area of the observatory, consistent with the re-

7The figure can be viewed in color in the electronic versionof the paper.

Fig. 3. Left panel: 2.7 km × 2.7 km area around theobservatory and specific sites (numbers) where the skybrightness was estimated. The telescope site (OAN-T) isalso indicated. Right panel: A color-scaled 2D represen-tation of the data after fitting an spline surface functionto the observed distribution of points. Light yellow col-ors represent dark sky values while the various orangetones represent brighter sky zones and their variationsin the survey area. The darkest color corresponds to azone where there is no available data thus producing anartificial falling down that must not be interpreted as areal variation. The contours (without levels) are usedto identify maximum/minimum variations. x-y axes de-note Longitude and Latitude while Surface Brightness(mag arcsec−2) is indicated in the z-axis.

sults obtained from our CCD photometric estimates.This result confirms the feasibility of these visualprocedures and their potential to cover wider areasunder well-planned strategies and good weather con-ditions.

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 53

0 2000 4000 6000 8000

0

0.2

0.4

0.6

0.8

1

Fig. 4. The observed atmospheric extinction at the OAN-Tonantzintla (open circles) from our CCD frames in theJohnson-Cousins (BVRI) system. The mean extinctioncurve free of volcanic outbursts (empty triangles) andextreme extinction following the explosion of the volca-noes El Chichon and Pinatubo (empty squares and as-terisk symbols) at the OAN-SPM with the 13C and 4-color Stromgren photometric systems (Schuster & Parrao2001) are also presented. Filled circles denote the meanextinction (from 1998 to 2008 by Hernandez-Toledo) atthe OAN-SPM in the Johnson-Cousins (BVRI) system.

2.3. Mean Extinction Coefficients

Figure 4 shows the observed atmospheric extinc-tion at the OAN-Tonantzintla (open circles) from ourCCD frames in the Johnson-Cousins (BVRI) system.To compare our results, the mean extinction curvefree of volcanic outbursts (empty triangles) and ex-treme extinction following the explosion of the vol-canoes El Chichon in March 1982 and Pinatubo inJune 1991 (empty squares and asterisk symbols) arepresented. The OAN-SPM data were obtained overthe years 1973 through 1999 with the 13C and 4-color Stromgren photometric systems (Schuster &Parrao 2001). The extinction values are plotted ver-sus the equivalent wavelengths of the photometricband passes. For the 13C system these wavelengthshave been taken from Mitchell & Johnson (1970).We also present data obtained at OAN-SPM in thelast 10 years by Hernandez-Toledo from CCD BVRI

photometric data to validate the methodology usedat Tonantzintla.

The mean extinction BVRI curve and its varia-tions at the OAN-Tonantzintla at the time of our ob-

Aug Sep Oct Nov

Dec

Jan

Feb

2004 2005

Fig. 5. A histogram of the monthly number of exhala-tions of water vapor and gases from the Popocatepetlvolcano in the period august 2004-february 2005. Thedashed histogram indicates the frequency of “moderateintensity” ash exhalations during that period.

servations lies between the normal extinction (as ref-erenced by OAN-SPM data) and that related to vol-canic outbursts. Notice that our extinction measure-ments at the OAN-SPM confirm that the mean ex-tinction (free of volcanic outbursts) has not changedsignificantly in the last 10 years. While the mean ex-tinction curve at the OAN-SPM was modeled assum-ing that the extinction over 3200 A < λ < 6500 Acan be represented by three independent contribu-tions due to Rayleigh-Cabannes, aerosol scatteringsand ozone absorption, the extreme extinction wasinterpreted in terms of wavelength dependences forthe volcanic aerosols. The 13C extinction observa-tions from Schuster & Parrao (2001) showed clearevidence for an evolution and growth of the aerosolparticles from El Chichon. These results suggestthat the extinction at Tonantzintla could be relatedto the activity of the nearby Popocatepetl volcano.To investigate that possibility, we have used the on-line data from the government monitoring programof the Popocatepetl volcano, where daily/monthlyreports on the number of exhalations of water va-por, gases and ashes and an archive of daily imagescan be found8. Figure 5 shows a histogram of themonthly number of exhalations of water vapor and

8http://www.cenapred.mx.

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54 HERNANDEZ-TOLEDO ET AL.

Fig. 6. Archive image of the government monitoring pro-gram of the Popocatepetl volcano showing a moderateintensity ash exhalation in a date close to our observa-tions.

gases from the Popocatepetl volcano during a periodof six months previous to our observations at theOAN-Tonantzintla. The dashed histogram indicatesin addition, the frequency of “moderate intensity”ash exhalations reported during that period.

Although the number of monthly exhalations ofwater vapor and gases did not change significantlyfrom August to December 2004, a flag associatedto the intensity of exhalations of ashes turned onto “moderate” since the beginning of 2005 (dashedhistogram), reaching a maximum frequency at thetime of our observations in February 2005. Figure 6is an archive image from the government monitoringprogram showing an exhalation of the Popocatepetlvolcano at a time close to our observing dates.

Figure 7 shows a diagram of the site around thePopocatepetl volcano indicating the zone of proba-ble fall of ashes and sand as well as the dominantwind direction along the year. The Tonantzintla ob-servatory is located within a zone (mid gray) thatcould be affected by the fall of moderate amounts ofashes and sand.

A probable scenario explaining the observedanomalous extinction is that a “sea” of volcanicaerosols was coexisting in the local environment dur-ing our observations. With the aid of the govern-ment monitoring program, it is strongly suggestedto check the statistics and tendencies about the fallof ashes previous to any further observation at theOAN-Tonantzintla.

0 10 20

scale in km

Popocatépetl

N

Iztaccíhuatl

Cholula

México City

PueblaOAN-T

May - September October - April

Prevailing wind direction

Area with more chance of being hitby falling ash

Could be affected by the falling ofsignificant amounts of volcanic sandArea 1.

Could be affected by the falling ofmoderate amounts of volcanic sandArea 2.

ould be less affected by the fall ofmoderate amounts of volcanic sandArea 3. W

Fig. 7. Zones of probable fall of ashes and sand andthe dominant wind direction along the year around thePopocatepetl volcano.

3. SPECTROSCOPIC OBSERVATION OF THESKY AT THE OAN-TONANTZINTLA

A classic Cassegrain (Boller & Chivens) spec-trograph is also available at the OAN-Tonantzintla.Among its principal components are a set of diffrac-tion gratings of 150, 400, 600 and 830 groove/mm,and a He-Ar comparison lamp (for more details seehttp://www.astrostnt.unam.mx). Long slit spec-troscopy of the night sky at the OAN-Tonantzintlais also presented. The data were obtained with theBoller & Chivens spectrograph attached to the 1 mtelescope during various campaigns in 2005 and 2006.From these observations we present only those formoonless clear skies (October 2005). Sky spectraat different heights above the horizon were also ob-tained.

With the Thomson CCD installed and a 150groove/mm grating the useful wavelength coverageis from 3900 A to 6200 A with a dispersion reso-lution of 5.6 A/pixel. A slit width of 300 µm wasused to expose the sky. The measured resolution athalf maximum in some individual emission lines is∼ 50 A. Standard IRAF procedures were used follow-ing bias subtraction and wavelength calibration withHe-Ar arc lamps observed before and after each skytarget. Neither atmospheric extinction correction,flat fielding nor flux calibration were applied. Af-ter wavelength calibration, the identification of theemission lines was verified within the IRAF splot rou-tine. Exposure times were set to 1800s in all theacquired spectra.

The wavelength-calibrated night-sky spectrum atthe OAN-Tonantzintla is shown in Figure 8. No at-tempt has been made to correct it for the airmass.The upper panel shows two spectra: The one in redwas acquired from the east direction 30◦ above the

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 55

4000 5000 6000

Fig. 8. Upper panel: λ-calibrated spectrum acquired atan elevation of 30◦ above the horizon (in red) towardsthe city of Puebla and the spectrum at 70◦ above thehorizon (in black). Lower panel: Flux and λ-calibratedsky spectrum at the DDO observatory in Canada. Iden-tifying numbers below the emission lines are described inDe Robertis et al. (2002).

horizon towards the city of Puebla. The one in blackwas acquired at about the local zenithal sky area, 70◦

above the horizon. The x-axis shows wavelengths inAngstroms and the y-axis shows relative intensityunits per Angstrom. For a comparison, the lowerpanel shows a sky spectrum acquired at the DavidDunlap Observatory (DDO) by De Robertis, Finger-hut, & Blake (2002).

Notice a relative displacement of ∼ 3 pixels inthe sky spectra at the two extreme elevations (30◦

and 70◦). Possible causes could be movements of thelong slit on the focal plane or flexures at large zenithdistances. The line identification was made fromour λ-calibration and by combining the informationgiven in Massey & Foltz (2000), Osterbrock & Martel(1992), De Robertis et al. (2002) and Sheen & Byun(2004). The strongest lines in our night sky comefrom high pressure sodium (HPS), i.e. NaI emissions.Besides the very broad feature centered at 5893 A,strong lines are also seen at around 4980, 5685, and6157 A. Another kind of city lights, mercury-vapor

TABLE 2

LINE IDENTIFICATION OF THE SKYSPECTRA AT THE OAN-TONANTZINTLA.

THE WAVELENGTHS ARE IN ANGSTROMSa

Element

HgI 4358

NaI 4665,4669, NaI 4748,4752, NaI 4978,4983 (HPS)

NaI 5149,5153 (HPS)

HgI 5461

OI 5577 airglow (marginal)

NaI 5683,5688 (HPS,LPS)

NaI 5890,5896 (HPS), wings extend with self-absorption

NaI 6154,6161 (HPS,LPS)aCarried out from our λ-calibration.

lamps and their Hg emission lines are also presentin the OAN-Tonantzintla sky but appear to be notas strong as the HPs ones. We identified mainly HgImercury lamp lines and NaI sodium lines of high andlow pressure (HPS and LPS) lamps. Our identifica-tion of the emission lines in the sky spectrum at theOAN-Tonantzintla is presented in Table 2.

Among the strong lines of the natural sky emis-sion, we suspect a marginal detection of the [OI]5577 line. There is one emission line at around5080 A for which a solid identification could not bemade. The spectrum of Osterbrock & Martel (1992)does contain this line but without any identification.The DDO sky spectrum also shows this line clearly,but again without identification. An old study ofToronto sky by Lane & Garrison (1978) has a re-mark that multivapor lamps produce an emissionat 5073.08 A. Typical street-lamp spectra were pre-sented by Osterbrock, Walker, & Koski (1976).

The upper panel of Figure 9 shows the sky spec-tra at the OAN-Tonantzintla acquired from the eastdirection 30◦ above the horizon towards the nearbycity of Puebla, while the lower panel shows a plotof the sky spectrum at the OAN-Tonantzintla as re-ported by Torres-Peimbert & Robledo-Rella (1990).The 1990 spectrum was obtained with the 1.0 m tele-scope, the Thomson CCD + an intensified tube at-tached to the Boller & Chivens spectrograph using a600 grooves/mm with a blaze of 10◦. It is assumedhere that this spectrum was acquired towards thecity of Puebla.

Although there is no detailed information aboutthe 1990 spectroscopic observations, particularly theheight above the horizon and azimuthal direction, acomparison of these two spectra suggests that the ob-served relative intensity of the sodium and mercurylines and their contribution to the light-polluted lo-

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56 HERNANDEZ-TOLEDO ET AL.

4047 +

4078 H

g

4358 H

g

4979 +

4983 N

a

5461 H

g

5683 +

5688 N

a

5770 +

5791 H

g5890 +

5896 N

a

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1990

Fig. 9. Upper panel: The night-sky spectrum at theOAN-Tonantzintla. Lower panel: Night-sky spectrumat the OAN-Tonantzintla obtained in 1990 by Torres-Peimbert & Robledo-Rella.

cal sky at the OAN-Tonantzintla could have changedin the last 15 years. This is consistent with the factthat public lighting in the surroundings of the ob-servatory and in the nearby city of Puebla has beengradually changed from mercury to sodium lamps inthe last 20 years.

4. DISCUSSION AND CONCLUSIONS

Under reasonable weather conditions (cloudlessand moonless nights), an absolute photometric cal-ibration of standard stars of apparent magnitudesfrom (9–15) and (8–14) in the B and I bands, respec-tively, yielded no significant systematic shift of thezero points and color terms of the Johnson-Cousinsphotometric system. A total propagated error ∼ 0.2mag that slightly increases up to 0.3 mag at faintermagnitudes in the I band has been obtained. Theseerrors reflect the convolved behavior of various com-ponents of the photometric system, namely: the cur-rent transmittance of the filters, the status of theoptics of the telescope at the time of the observa-tions, the current efficiency of the CCD and also theamount of light pollution in the sky, among otherpossible factors. In spite of this, reasonable photo-metric work can still be done for bright stars. Wealso suggest to evaluate in further observing runsthe contribution of scattered light along the opti-cal path of the 1 m telescope (c.f. Gutierrez et al.1996). This contribution could affect the data cal-

ibration, particularly when a frame is acquired byexposing to a bright light source (which was not thecase in the present paper). Given the much fainterlimiting magnitudes achievable with an autoguidingsystem, we also recommend the implementation ofsuch a system as part of a major upgrading of theobservatory.

The sky brightness at the OAN-Tonantzintla wasestimated from two complementary methods: (1)from our CCD frames at the different observingbands and at different elevations and (2) from vi-sual estimates through a campaign covering an area∼ 8 km2 around the observatory. Both methods con-sistently yield a similar result of ∼ 18.5 mag arcsec−2

in the V band towards the city of Puebla and∼ 19.0 mag arcsec−2 at the more local zenithalarea of the observatory. Compared to the repre-sentative V band sky value for the OAN-SPM of∼ 21.4 mag arcsec−2 under moonless conditions, thedifference of about 3 magnitudes is interpreted asthe amount of the light pollution that is being con-tributed to the site from both the very local andnearby lighting of the villages around Tonantzintlaand from the city of Puebla. The effect is equivalentto loosing the ability to distinguish by eye stars ofmagnitudes ∼ 4 and fainter in a clear night sky. Theresults from method (2) above also confirm the feasi-bility of visual procedures to estimate the sky surfacebrightness and show their potential to cover widerareas under good weather conditions. Walker (1977,1991) and Garstang (1986, 1989) have estimated theincrease in brightness at zenith distance 45◦, in thedirection of a conurbation of population P at dis-tance D km to be ∼ PDα/C, where C is a coefficientthat does not depend on P and D but on factors suchas the light emission per head of the population andthe reflectivity of the ground. Garstang (1986, 1989)estimated the α index as the slope of the log (bright-ness) – log (D) relation for a value of the brightnessof 0.01 mag arcsec−2 times the natural background,reporting in addition, a linear interpolation of α andlog (C) as a function of log (P ) which allow us toestimate more appropriate values of α and log (C)for a given population P . Although Garstang’s re-sults are valid for intermediate/large distances, weuse them as a first approximation to estimate theexpected increase in brightness caused by the city ofPuebla as seen from the OAN-Tonantzintla.

A population census for the city of Puebla in 2005yielded a population P of ∼ 1,400,000 inhabitants.Given the extension of the city, a mean distance D of20 km to the observatory is considered as appropri-

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 57

ate. Since log (P) = 6.15, by using the interpolateddata in Garstang (1986, 1989) we estimate log (C)∼ 1.8 and α ∼ −3.6 yielding an increment in bright-ness ∆ mag ∼ 0.46. This variation is in agreementwith the observed variation of ∼ 0.5 mag arcsec−2 insky brightness from our measurements from the lo-cal zenith to the east direction 30◦ above the horizontowards the city of Puebla.

Being close to the cities of Puebla to the eastand Cholula to the north, both having potential forlarge growth, the OAN-Tonantzintla faces the dan-ger of increasing deterioration of its sky conditions.In order to maintain competitiveness for educationaland other scientific programs, it is important to pre-serve the sky brightness conditions through (1) ourawareness of the night sky characteristics in futuremonitoring campaigns (more measurements over thenext years will allow us to monitor changes) and (2)encouraging local authorities about the need to reg-ulate public lighting and at the same time, show-ing the benefits (economic impact) of such initiativeswhen well planned and correctly implemented.

The mean extinction coefficients at BVRI bandsand their variations are needed to correct observa-tions for the local effects of the atmospheric extinc-tion. It is found that the extinction curve at theOAN-Tonantzintla lies between the normal extinc-tion values and those related to volcanic outbursts,suggesting a possible connexion to the activity ofthe nearby Popocatepetl volcano. Compelling ev-idence in favor of this hypothesis came from dataon monthly exhalations of gas, ashes and sand avail-able from the government monitoring program of thePopocatepetl volcano. Since the OAN-Tonantzintlais located within a zone of probable fall of ashes andgiven the information on the dominant wind direc-tion along the year, a “sea” of volcanic aerosols wasprobably coexisting in the local environment duringour observations. It is therefore still necessary: (1)estimate far from outbursts extinction values and (2)study if aerosols other than volcanic ashes are con-tributing to the observed extinction, finding out ifthey show any seasonal pattern as it is the case ofspringtime winds stirring up dust to the local envi-ronment. It is known that the chemical compositionof the ashes can vary from volcano to volcano andeven from exhalation to exhalation. It would be alsointeresting to investigate the chemical compositionof the ashes at the Popocatepetl volcano in order toproperly model the optical properties of the corre-sponding aerosols and their contribution to the ex-tinction curve. This is the type of monitoring work

that could be done via our remote observing systemor in-situ with the new instrumentation that is beinginstalled at the observatory.

Light pollution at observatory sites (McNally1994; Holmes 1997) arises principally from tro-pospheric scattering of light emitted by sodium-and mercury-vapor and incandescent street lamps.The wavelength-calibrated night-sky spectrum at theOAN-Tonantzintla shows mainly HgI mercury lamplines and NaI sodium lines either of high and lowpressure (HPS and LPS) lamps. The relative contri-bution of the mercury and sodium lamps to the locallight pollution at the OAN-Tonantzintla might havechanged in the last 15 years as a comparison of thespectra acquired in 1990 with those obtained in thiswork suggests. This is consistent with the gradualchange from mercury to sodium lamps in the publiclighting in the last 20 years. The observed pattern ofsky lines has allowed us to detect a ∼ 3 pixel shift attwo extreme positions 30◦ and 70◦ above the horizonprobably caused by the instrument flexures when thetelescope is set at large zenithal distances.

The light pollution contribution to broad-bandoptical BVRI bands is affecting the range of wave-lengths between 4500 and 6000 A. The 4358 A Hgline can be a hazard for observers wanting to com-pare the intensities of the 4363 and 5007 A OIII lines(for measuring the temperature of astrophysical plas-mas). The 5460 A Hg line lies in the center of they band of the Stromgren ubvy system. The Na D5890/6 A line typically contaminates both the broadV and R bands.

Before discussing the feasibility of various astro-nomical projects, it is still necessary to mention aseries of practical problems, detected at the timeof our observations, that must be solved in orderto have more efficient local (and remote; see be-low) observing runs. For example, it is difficult tojudge how photometric the conditions at the OAN-Tonantzintla are, especially during moonless nights.It is important to have access (locally and remotely)to the meteorological data and to obtain informationon windspeed, temperature, humidity and pressure.Weather information complemented with a 1 µmfish-eye transparency camera can be very useful tomonitor cloud cover. Such cameras are experiencinglarge technical improvements and lowering in cost,and should be considered for acquisition within thecurrent upgrading program of the observatory. Twi-light flats are among the most demanding parts bothfor local and remote observations, because there islittle time available and the observer needs to know

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58 HERNANDEZ-TOLEDO ET AL.

0 1 2 3 4 521.5

21

20.5

20

19.5

19

18.5

18

17.5

Integration Time (hours)

U

B

V R

I

S/N > 10

seeing 3.0’’

pixel = 0.5’’

readout noise 7.5 e/pix

binning 2 x 2

Fig. 10. An estimate of the integration time required toobserve a point-like object of apparent magnitude 18.0mag in the UBV RI bands, given some CCD characteris-tics, with a S/N > 10 in a 0.84 m telescope and a typicalsky brightness of 18 mag arcsec−2 as observed in theOAN-Tonantzintla.

what the count level was in previous flats. The pos-sibility of including fast statistics estimators withinthe acquisition programs should be considered. Theacquisition of twilight flats in “service mode” bythe telescope operator at the OAN-Tonantzintla willbenefit from such improvements. Another problem-atic process is that of focusing. This process is car-ried out by using one or more exposures of prese-lected fields of bright stars (preferably close to thetarget). Since for this purpose it is not necessaryto read out the full CCD, the possibility of readingsmaller sections of the CCD when estimating focusshould also be considered and implemented. It isalso recommended that the image acquisition pro-grams let any basic information be included in theimage headers for further image reduction purposes.

The solution of these problems along with theimplementation of a fine guiding system for the 1m telescope will result in more efficient observationsand enable other observing possibilities at the OAN-Tonantzintla. Even more, our remote observing sys-tem will also benefit from those solutions since thejudging and transfer of images could be very quickand image quality could be analyzed either locallyor remotely as well. This opens new observing pos-sibilities for scientific and educational programs asdiscussed below.

Figure 10 shows an estimate of the integrationtime required to observe a point-like object of ap-parent magnitude 18.0 (typical in optical follow-upof gamma ray bursts) in the BVRI bands with a 0.84m telescope + CCD detector array of given charac-teristics. This approximation is intended to figureout the capabilities/limitations of the 1m telescope+ the current CCD at the OAN-Tonantzintla. Asignal-to-noise ratio greater than 10, a seeing of 3arcsec, a typical sky surface brightness value in therange of 18 mag arcsec−2 in the BVRI bands as ob-served in Tonantzintla, a CCD pixel size of ∼ 0.5arcsec and readout noise of ∼ 7.5 e/pix were con-sidered. The OAN photometric simulator9 has beenused for this purpose.

The above integration times were estimated byassuming CCD quantum efficiencies (QE) in theBVRI bands higher than it is actually the case forthe current CCD at the OAN-Tonantzintla. Thusthe curves in Figure 10 represent zero-order esti-mates to the real integration times required to reacha point-like object of a given magnitude at a givensignal-to-noise ratio. The feasibility of various in-situ and/or remote observing programs for objectsas faint as 20 mag is nevertheless apparent withoutrequiring prohibitive integration times to obtain wellsampled data over reasonably long time intervals.

4.1. OAN-Tonantzintla: a Laboratory for the

Teaching of Astronomy at College Level and

Graduate Programs at UNAM

In spite of the limitations imposed by light-pollution and given the current status of the instru-mentation at the OAN-Tonantzintla, the feasibilityof various educational and astronomical projects atcollege and graduate levels is nevertheless apparent.These possibilities could be increased if the currentphotometric and spectroscopic instrumentation (aswell as the new instrumentation that is being ac-quired for the actual major upgrading of the observa-tory) is combined with the implementation of a fineguiding system for the 1 m telescope. Other observ-ing possibilities at the OAN-Tonantzintla like the ac-tive Remote Observing System (hereafter ROS) forthe 1 m telescope and other observing strategies likeCCD differential photometry must also be consid-ered.

The ROS is operated in real time via Internet al-lowing us to control telescope pointing, guiding, fo-cusing and CCD image acquisition at the main and

9A. Watson (http://www.astrossp.unam.mx/indexspm.html).

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PHOTOMETRIC AND SPECTROSCOPIC EVALUATION OF THE SITE 59

auxiliary finder telescopes from the Instituto de As-tronomıa headquarters in Mexico City. The wholesystem was modeled within the Unified ModelingLanguage (UML) and the design has proved to beversatile enough for a variety of astronomical instru-ments and to offer a common framework for integrat-ing different operating system platforms (Linux, MSWindows, uIP). The remotized telescope is mainlydevoted to CCD photometry and spectroscopy withthe aim of optimizing the data-acquisition process,increasing the number of users that will have accessto the available instrumentation and giving supportto research and to our graduate astronomy program.

The ROS at the OAN-Tonantzintla can beused from simple observation up to integratingthe system into a carefully planned theme of aphysics/astronomy course. In fact, it opens up aresearch facility for our graduate program. Stu-dents could learn how to acquire and reduce datafrom upgraded instruments, thus providing excellentpreparation for graduate work. One specific projectthat could be immediately planned is the determi-nation of the optical atmospheric extinction coef-ficients through systematic remote observations inappropriate weather conditions. The access to thesefacilities provides a sense of participation and ac-complishment at a time when many students feeloverwhelmed by formal physics/astronomy courseswhere only textbooks and lectures are invoked.

The study of supernovae in-situ or via the ROScould yield important data to understand the deathof stars if high enough quality, frequently sampleddata are obtained over reasonably long time inter-vals. For example, the sharp rise in brightness andthe initially rapid color evolution allow the identifi-cation of the type of progenitor star involved. An-other important parameter is the estimate of thebolometric light curve, which in the case of the OAN-Tonantzintla would consist primarily of observationsof the optical continuum during one or two years.This is feasible since supernovae in the Virgo clusterhave an apparent visual magnitude m ∼ 11−13 magat maximum (depending on their type) and can inprinciple be monitored at Tonantzintla for at least ayear up to the limiting magnitude of the 1 m instru-mental set.

Optical multi-band observations of novae can alsobe used to built approximate bolometric light curves,as in the case of supernovae. For good results, theobservations must be done over a wide time scale.Well sampled bolometric and color curves can beobserved following them up to the late stages whichhave been less studied. The characterization of no-

vae in nearby galaxies is also possible, for example,bright novae in M31 can reach ∼ 15 visual magni-tude at maximum allowing us at OAN-Tonantzintlato follow their light curve up to the limiting magni-tude of the 1m telescope.

An important characteristic of quasi-stellar ob-jects (QSOs) and active galactic nuclei (AGNs) istheir rapid variability. Significant variations areobserved in such objects over time scales as shortas years, months or days. The utility of multi-wavelength continuum data will be enhanced whenanalyzed in conjunction with other ground-basedand space-based observations. One such recent ex-ample is the radio quasar 3C 454.3 (Villata et al.2006) that underwent an exceptional optical out-burst lasting more than 1 year. The maximumbrightness detected in the R band was R = 12.0,which represents the most luminous quasar statethus far observed. This type of multiwavelengthcampaigns are a clear example of a program wherethe 1 m telescope can be inserted. In the case of 3C454.3 continuous optical, near-IR and radio moni-toring was performed in several bands, followed bypointings by the Chandra and INTEGRAL satellitesproviding additional information at high energies.

The above are just a few examples of poten-tial astronomical applications where reasonably longtime intervals of observations are required. Consid-ering that the best window for observations in termsof weather at Tonantzintla includes the period ofOctober-April, and that some other smaller windowsare available along the year, decisions to observe ei-ther in-situ or remotely from Mexico City can beeasily assessed. A long-term program would producean extra benefit in the sense that we will be able tonotify other observers about the current status (c.f.brightness) of such objects.

The results presented here are relevant to provid-ing support for the actual major upgrading of the ob-servatory in terms of new photometric, spectroscopicand other instrumentation that is being acquired forscientific and educational purposes. The instrumen-tation will be capable of supporting the needs ofthe next generation of faculty and students of col-lege/graduate programs at the Universidad NacionalAutonoma de Mexico. This will convert the OAN-Tonantzintla to the natural Laboratory for Teach-ing of Astronomy at the college level and graduateprograms at the Universidad Nacional Autonoma deMexico.

Astronomers and Facultad de Ciencias staff andother science-related schools inside and outside theUniversidad Nacional Autonoma de Mexico are in-

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60 HERNANDEZ-TOLEDO ET AL.

vited to make use of these facilities for their observ-ing/eduacational programs. It is only on the basisof experience in using them and through their col-laboration that we will be offering an efficient andresponsive means of carrying out astronomical obser-vations for scientific/educative purposes. There arecertainly other important benefits to be gained fromthe proper use of these observing facilities. Publicoutreach goals through collaborative outreach effortswith the Universidad Nacional Autonoma de Mexicoand other public and private universities and schoolsare just another example. The OAN-Tonantzintlacould be eventually considered as a National Facilityfor Astronomy Education for the center-south partof the country.

H.M.H.T acknowledges support through grantsDGAPA PAPIME-PE103406 and Conacyt 42810/A-1. We have made use of NED and LEDA databasesthroughout. H.M.H.T thanks the night assis-tant A. Cielo for their help during all the ob-serving runs at the OAN-Tonantzintla 1 m tele-scope. H.M.H.T thanks Guadalupe Pardo fromCECADESU/SEMARNAT for discussions concern-ing the implementation of a first campaign to esti-mate the sky surface brightness and luminic contam-ination around Tonantzintla Observatory. H.M.H.Tthanks the municipal presidents of San Rafael Co-mac, Santa Marıa Tonantzintla and San FranciscoAcatepec for the support provided during this cam-paign. H.M.H.T thanks the students from the Facul-tad de Ciencias and the graduate astronomy programat IA-Universidad Nacional Autonoma de Mexicothat participated in this sky brightness campaign.H.M.H.T thanks the anonymous referee for his/hercareful reading and appropriate questioning thatgreatly improved this manuscript.

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