A Photometricity and Extinction Monitor at the Apache Point Observatory The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Hogg, David W., Douglas P. Finkbeiner, David J. Schlegel, and James E. Gunn. 2001. “A Photometricity and Extinction Monitor at the Apache Point Observatory.” The Astronomical Journal 122 (4) (October): 2129–2138. doi:10.1086/323103. Published Version 10.1086/323103 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:33461897 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA THE ASTRONOMICAL JOURNAL, 122:2129È2138, 2001 October ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. A PHOTOMETRICITY AND EXTINCTION MONITOR AT THE APACHE POINT OBSERVATORY1 DAVID W. HOGG2 Department of Physics, New York University, 4 Washington Place, New York, NY 10003; david.hogg=nyu.edu AND DOUGLAS P. FINKBEINER,3,4 DAVID J. SCHLEGEL, AND JAMES E. GUNN Department of Astrophysics, Peyon Hall, Ivy Lane, Princeton University, Princeton, NJ 08544; dÐnk=astro.princeton.edu, schlegel=astro.princeton.edu, jeg=astro.princeton.edu Received 2001 May 23; accepted 2001 June 25 ABSTRACT An unsupervised software ““ robot ÏÏ that automatically and robustly reduces and analyzes CCD obser- vations of photometric standard stars is described. The robot measures extinction coefficients and other photometric parameters in real time and, more carefully, on the next day. It also reduces and analyzes data from an all-sky 10 km camera to detect clouds; photometric data taken during cloudy periods are automatically rejected. The robot reports its Ðndings to observers and data analysts via the World Wide Web. It can be used to assess photometricity and to build data on site conditions. The robotÏs automa- ted and uniform site monitoring represents a minimum standard for any observing site with queue scheduling, a public data archive, or likely participation in any future National Virtual Observatory. Key words: astrometry È methods: data analysis È methods: observational È surveys È techniques: image processing È techniques: photometric 1. INTRODUCTION signiÐcant cost that the observer does not have direct access More and more, astronomical research is being per- to observing conditions at the site. Most remote observa- formed remotely, in the sense that the observer, or perhaps tories and data archives keep logs written by telescope more properly ““ data analyst,ÏÏ is now often not present at operators, but these logs are notoriously nonuniform in the place or time at which observations are taken. The their attention to detail and use of terminology. All sites increase in remoteness has several causes. One is that for that plan to host remote observers or maintain public data many observatories, telecommunication is easier than archives must have repeatable, quantitative, astronomically travel, especially if telescope allocations are of short dura- relevant site monitoring. tion. Another is that several new telescopes are using or For these reasons, among others (involving photometric plan to use queue scheduling (e.g., contributions to calibration and direction of survey operations) the Sloan Boroson, Davies, & Robson 1996 and Boroson et al. 1998; Digital Sky Survey (SDSS; York et al. 2000), which is con- structing a public database of 104 deg2 of Ðve-bandpass Garzon & Rozas 1998; Tilanus 2000; Massey, Guerrieri, & 6 Joyce 2000), for which observer travel is essentially impossi- optical imaging and 10 optical spectra, employs several ble. Some new ground-based telescopes are partially or pieces of hardware for monitoring the Apache Point Obser- completely robotic (e.g., contributions to Filippenko 1992; vatory site, including a 10 km cloud-camera scanning the contributions to Adelman, Dukes, & Adelman 1992; also whole sky (Hull, Limmongkol, & Siegmund 1994), a single- Baruch & da Luz Vieira 1993; Akerlof et al. 1999; Castro- star atmospheric seeing monitor, a low-altitude dust parti- Tirado et al. 1999; Strassmeier et al. 2000; Querci & Querci cle counter, a basic weather station, and a 0.5 m telescope 2000). Possibly the most important reason for the increase making photometric measurements of a large set of stan- in remote observing is that many observatories, many large dard stars. This paper is about a fully automated software surveys, and some independent organizations are creating ““ robot ÏÏ that reduces the raw 0.5 m telescope data, locates huge public data archives that allow analyses by anyone at and measures standard stars, and determines atmospheric extinction in nearÈreal time, reporting its Ðndings to tele- any time (e.g., contributions to Mehringer, Plante, & 5 Roberts 1999 and Manset, Veillet, & Crabtree 2000). There scope and survey operators via the World Wide Web has been community discussion of a ““ National Virtual (WWW). This paper describes the software robot, rather Observatory,ÏÏ which might be a superset of these archives than the hardware, which will be the subject of a separate and surveys (e.g., contributions to Brunner, Djorgovski, & paper (Uomoto et al. 2001). Szalay 2001). Although the robot is somewhat specialized to work with Remote observing and archival data analysis bring huge the hardware and data available at the Apache Point economic and scientiÐc beneÐts to astronomy but with the Observatory, it could be generalized easily for di†erent hardware. We are presenting it here because it might serve ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ as a prototype for site monitors that ought to be part of any 1 Based on observations obtained at the Apache Point Observatory, functional remote observing site and of the National Virtual which is owned and operated by the Astrophysical Research Consortium. Observatory. 2 Also Institute for Advanced Study, Olden Lane, Princeton, NJ 08540- 0631. 3 Also Department of Astronomy, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720. ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 4 Hubble Fellow. 5 Information about the WWW is available at http://www.w3.org/. 2129 2130 HOGG ET AL. Vol. 122 TABLE 1 the interest of limiting stress on the real-time systems, the APPROXIMATE PT BANDPASS INFORMATION photometricity robot runs on a separate machine, obtaining its data by periodic executions of the UNIX RSYNC soft- Parameter u@ g@ r@ i@ z@ ware.6 The RSYNC software performs Ðle transfer across a Central wavelength (A )...... 3540 4770 6230 7620 9130 network, using a remote-update protocol (based on Ðle FWHM (A )................... 570 1390 1370 1530 950 check sums) to ensure that it transfers only updated or new Ðles, without duplicating e†ort. The output of RSYNC can TELESCOPE AND DETECTOR be set to include a list of the Ðles that were updated. This 2. output is passed (via scripts written in the UNIX shell lan- The primary telescope used in this study is the Photo- guage BASH7) to a set of software tools written in the data metric Telescope (PT) of the SDSS, located at the Apache analysis language IDL.8 Point Observatory (APO) in New Mexico, at latitude Virtually all the image processing, data analysis, Ðtting, 32¡[email protected], longitude 105¡[email protected] W, and elevation and feedback to the observer is executed with IDL pro- 2788 m. The PT has a 20 inch (0.5 m) primary mirror and is grams in BASH wrappers. This combination of software outÐtted with a 2048 ] 2048 pixel CCD with1A.16 pixels, has proven to be very stable and robust over many months making for a 40 ] 40 arcmin 2 Ðeld of view. The telescope of continuous operation. In addition, data reduction code and CCD will be described in more detail elsewhere written in IDL is easy to develop and read. Our only signiÐ- (Uomoto et al. 2001). cant reservation is that IDL is a commercial product, which The telescope takes images through Ðve bandpasses, u@, is not open source. We have responded to this lack of trans- g@, r@, i@, and z@, chosen to be close to those in the SDSS 2.5 m parency by writing as much as possible of the robotÏs func- imaging camera. Filter wavelengths are given in Table 1. tion in terms of very low level IDL primitives, which could The magnitude system here is based on an approximation be rewritten straightforwardly in any other data analysis to an AB system (Oke & Gunn 1983; Fukugita et al. 1996), language if we lost conÐdence in or lost access to IDL. We again because that was the choice for the SDSS imaging. have not found the lack of transparency to be limiting for The photometric system will be described in more detail any of the functionality described in this paper. elsewhere (Smith et al. 2001). Nothing about the function of the robot is tied to this photometric system; any system can 5. CORRECTING THE IMAGES be used provided that there is a well-calibrated network of Each raw image is bias-subtracted and Ñattened, using standard stars. biases and Ñat-Ðeld information archived from the most 3. OBSERVING STRATEGY recent photometric night. How the bias and Ñat images are computed from a nightÏs data is described in ° 11, along Site monitoring and measurement of atmospheric extinc- with the conditions on which a night is declared photo- tion is only one part of the function of the PT. The PT is metric. being simultaneously used to take data on a very large Because the CCD is thinned for sensitivity in the u number of secondary patches that will provide calibration bandpass, it exhibits interference fringing in the i@ and z@ information for the SDSS imaging. For this reason, the PT bandpasses. The fringing pattern is very stable during a spends only about one-third of its time monitoring stan- night and from night to night.