SMALL-TELESCOPE SURVEYS Surveys with small optical telescopes Downloaded from https://academic.oup.com/astrogeo/article-abstract/60/6/6.14/5625003 by Institute of Child Health/University College London user on 04 March 2020 Tony Lynas-Gray reviews the odern astrophysics owes its exist- 1976) detectors, surveys were extended into history of surveys with small ence to small-telescope surveys the near-infrared. Moreover, images and Mdating back to the 18th century, spectra obtained using CCDs and digitized telescopes and explains why they for example Flamsteed (1725). In the 19th photographic plates are in a machine-read- are still vital to support the work century, Argelander’s survey based on able form and amenable to processing with of much larger telescopes. visual observations with small telescopes modern numerical methods. and meridian circles culminated in the 1859 In this article I summarize some of the publication of the Bonner Durchmusterung more important surveys – and results – (BD), a catalogue of northern hemisphere carried out so far with small telescopes stars brighter than ninth magnitude, with (having an aperture of no more than 2 m). accompanying charts (Batten 1991). The BD Modern networks of small tele scopes catalogue was later extended to the south- operated robotically are expected to ern hemisphere by the Córdoba Durch- continue survey work for the foreseeable musterung (CD; 1892 onwards) prepared future: small telescopes in general need to using Argelander’s method, and the Cape be retained for follow-up observations of Photographic Durchmusterung (CPD; 1885 brighter targets of interest discovered in onwards) based on photographic plates. continuing and future surveys. Surveys with small telescopes based on photographic plates have been carried Photographic surveys out for more than a century, starting with Glass photographic plates, invented in the the Carte du Ciel, which Bigourdan (1888) middle of the 19th century, proved to be a describes and for which Sementsov (2018) robust and durable medium with which provides further details in a contemporary to image the sky with telescopes of the context. More recently, with the advent time, which would generally be regarded of charge-coupled device (CCD; Sobieski as “small” by modern standards. Major 6.14 A&G • December 2019 • Vol. 60 • aandg.org SMALL-TELESCOPE SURVEYS Downloaded from https://academic.oup.com/astrogeo/article-abstract/60/6/6.14/5625003 by Institute of Child Health/University College London user on 04 March 2020 1 The Cerro Tololo Inter-American Observa- tory (CTIO) in Chile is one of three sites for the KMTNet survey telescopes. (Young-BeomJeon) observatories ran diverse research pro- galactic structure studies; it is now, being out with 1.2 m Schmidt telescopes at the grammes for which photographic plates superseded by the GAIA Catalogue (Gaia Mount Palomar Observatory (figure 2), the were used to image parts of the sky and Collaboration et al. 2016). European Southern Observatory and the record spectra of selected objects. Plate Three additional objective prism surveys Siding Spring Observatory have proved to archives became established and grew with small telescopes subsequently led to be research tools in their own right. Reid et throughout the 20th century; these in them- advances in astrophysics. The Hamburg al. (1991) describe the second survey carried selves defined surveys in the time domain. Quasar Survey (HQS; Hagen et al. 1995) out at Palomar, provide references and Digitization of photographic and its southern extension, compare their new survey with the first plates is ongoing, as Grind- “Small telescopes are the Hamburg/ESO (HE; Palomar survey. Briefly, the entire sky was lay et al. (2009) discuss, not needed for follow-up Wisotzki et al. 2000) Survey, surveyed with 6° × 6° fields to 20th magni- only helping to ensure their observations of aimed to provide a complete tude, making it much deeper than earlier preservation but also making brighter targets” survey of bright B < 16.5 high- surveys and an essential starting point for these older data available for redshift (2.2 ≤ z ≤ 3.2) quasars. extragalactic research. longer time-base studies Because bright quasars are comparatively A primary motivation for initiating The Henry Draper (HD) Catalogue was rare, the HQS and HE surveys identify photo graphic surveys at the end of the 19th based on photographically recorded objec- many other objects of interest, a couple of century was the prospect of comparing tive prism spectra (Moore 1918). A spectral which will be discussed below. Objective them with plates of the same field obtained temperature classification was therefore prism surveys were also used to search with the same instrument at a later date, included in the HD catalogue, to supple- for luminous stars in the Northern Milky thereby measuring proper motions for the ment right ascension and declination. As Way (Stephenson 1966) and in the Southern nearby less-luminous stars. The 13-inch such, the HD catalogue was an enormous Milky Way, as the Stephenson and San- photographic telescope, with which advance on the BD, CD and CPD catalogues duleak (1971; SS) catalogue; both surveys (Tombaugh 1946) discovered Pluto in 1930, which preceded it. For nearly a century, list objects of considerable interest, one of played a crucial role in proper-motion the HD catalogue has formed a starting which is also mentioned below. studies (Giclas 1980). Having a catalogue point for many (if not most) stellar and Photographic sky surveys carried of high-proper-motion objects, Luyten and A&G • December 2019 • Vol. 60 • aandg.org 6.15 SMALL-TELESCOPE SURVEYS others use colour estimates, photometry and spectroscopy to distinguish white dwarfs and M dwarfs as (for example) Oswalt et al. (1988) discuss in the context of common-proper-motion pairs. Green et al. (1986) describe another photographic survey made with a small Downloaded from https://academic.oup.com/astrogeo/article-abstract/60/6/6.14/5625003 by Institute of Child Health/University College London user on 04 March 2020 telescope, in this case the Palomar 18-inch (46 cm) Schmidt telescope; this came to be known as the Palomar–Green (PG) survey. The PG survey photographed a quarter of the celestial sphere using Johnson U and B filters which, after plate digitiza- tion and colour transformation onto the standard Johnson and Morgan (1953) system, allowed blue objects to be identi- fied for spectroscopic follow-up. Stobieet al. (1997) adopt a similar approach for their Edinburgh–Cape (EC) survey, except that the plates used had been obtained with the UK Schmidt telescope as part of the south- ern extension of the Palomar all-sky survey discussed above. Both the PG and EC surveys identify many objects of interest, a 2 Astronomers A G Wilson and Wilhelm Baade (right) inspect test plates for the first Palomar Sky Survey few of which are discussed below. (POSS I). Conducted between 1950 and 1957, this survey used the 1.2 m Oschin Schmidt Telescope at the The Kitt Peak Downes (KPD; Downes Palomar Observatory, California, USA. (RAS/SPL) 1986) survey provides another useful list of ultraviolet excess objects, mainly white stars, old white dwarfs or black holes 2019). Around 20 000 microlensing events dwarfs and hot subdwarfs, in the plane of (hereafter “massive compact halo objects” or have been found in the direction of the our galaxy. Johnson U and B photographic MACHOs) could not be directly detected if galactic centre, four of these being identi- plates were obtained with the 0.6 m Warner present in the halo, and could equally well fied as lensing black hole candidates. OGLE and Swasey Burrell Schmidt telescope provide otherwise unexplained dark matter. finds at least a few microlensing planets at the Kitt Peak National Observatory. Paczyn´ski (1986) proposes gravitational every year and continues to discover new Downes obtains follow-up spectra with the microlensing as a means of detecting variable stars of all types. The discovery of 1.5 m Mount Lemmon telescope, equipped MACHOs in the Milky Way halo. The idea blue large-amplitude pulsators BLAPs) by with the Robinson and Wampler (1972) was that a MACHO would in due course Pietrukowicz et al. (2017) may prove to be image scanner; to the best of my knowl- pass in front of a background one of the more important edge, this was one of the earliest attempts to star, temporarily amplify- “Monitoring the results to have come out directly record spectra in a digital form. ing its apparent bright- galactic bulge can be of the OGLE survey and is ness. Alcock et al. (1993a,b) done 24 hours a day, discussed further below. CCD surveys describe the MACHO weather permitting” A contemporary approach The replacement of photographic plates project, which was a survey is to establish a network of by the CCD as the detector of choice in looking for MACHOs in the direction identical small telescopes, distributed in astronomy was preceded by several other of the Large Magellanic Cloud using a longitude, that allows continuous monitor- detectors of interest. For example, Boksen- dedicated 1.27 m telescope at the Mount ing of a chosen field, weather permitting. berg (1972) describes the Image Photon Stromlo Observatory and what was, for One such example is the Korea Microlens- Counting System developed at University the time, a large-format CCD detector. ing Telescope Network (KMTNet; Kim et al. College London. Another development Seventeen MACHOs were found over a 2016). This consists of three 1.6 m telescopes was the RETICON (Jorden et al. 1982), pro- 5.7-year period, but the result is subject to at the South African Astronomical Observa- vided for spectroscopic observations with interpretation and comparison with simi- tory, the Siding Spring Observatory and the the 1.9 m telescope at the South African lar surveys (Calcino et al.