Today searches for young stars are pursued throughout the entire electromagnetic spectrum as a result of the accelerating development of advanced technologies specifically for focal-plane instrumentation. Instrumental development is an essential part of stellar research, and astronomy in general always motivates the creation of new technologies. The goal is to gain increased sensitivity, increased spatial and spectral resolution, and increased wavelength coverage. Of course, one would like to achieve all this throughout most essential parts of the electromagnetic spectrum. For a short review readers are directed to J. Kastner's review on imaging science in astronomy . In starformation research, observing efforts today engulf almost the entire spectral bandwidth from Radio to 7-radiation. In essence, it took the whole second half of the 20th century to conquer the electromagnetic spectrum technologically in its entire bandwidth for star-formation research. The following offers a very limited review of the use of instrumentation over this time period, and for various wavelength bands without providing technical specifications, which are outside the focus of this book. More detailed explanations and a description of instrumental acronyms in the following sections and chapters can be found in Appendix F.
Observations of young stars are predominately performed in medium to short-wavelength ranges:
thermal continua, vibrational lines molecular/atomic lines molecular/atomic lines atomic lines, continua inner shell atomic lines
There are strong signatures at longer wavelengths as well but they are normally not understood in the context of the protostar itself. Dust envelopes around protostars are only visible in the far-IR and the sub-mm. Long wavelengths exclusively identify very young protostars in the context of thermal emission from dust and molecules and some non-thermal emission in the radio band . Ranges are:
Radio : 50 m < A < 5 mm non-thermal continua rotational lines mm : 5 mm < A < 1 mm rotational lines, dust
Sub-mm : 1 mm < A < 0.35 mm rotational lines, dust Far-IR : 350 | m < A < 20 | m rotational lines Mid-IR : 20 |m < A < 3.5 |m vibrational lines
Optical telescopes can be used from the near-UV to the mm-bandpass. Observing capability is dominated by the transmission properties of the atmosphere and thus only a few windows outside the optical band are really visible from the ground, with some becoming visible at higher altitudes (see Fig. 2.8). At even higher altitudes IR and sub-mm waves become accessible, though mirror surfaces are increasingly sensitive to daytime to nighttime temperature changes and require specific accommodations. Thermal background noise irradiated by instrument components is reduced detector cooling. Throughout the IR band, variable thermal emission from the Earth's atmosphere is a problem growing with increasing wavelengths. Modern facilities thus use choppers or alternate beams to subtract atmospheric radiation and filter out the difference signal for further processing.
The 1940s and 1950s observed primarily older regions of stellar formation such as open clusters, OB, and T Tauri associations (see Sect. 2.2.5). The first systematic studies of young stars and star-forming regions were performed with optical telescopes. Though most of these studies occurred long after the 1950s when more advanced photomultipliers became available, hypersensitive photographic plates were used even into the early 1990s, which due to immense advances in photographic techniques had not much in common with the original plates . To further enhance sensitivity the plates are sometimes submitted to a heating process (baking). The dynamical range of photographic plates is limited and so is their efficiency in comparison with photoelectronic devices. Thus some surveys used photographic plates specifically to cover wide fields, some used photoelectric detectors. Both methods produced uncertainties s ranging between 0.005 and 0.015 mag. Today large area charge-coupled devices (CCDs) with higher linear dynamic ranges and efficiencies have replaced most photographic plates. In order to obtain spectral information filter combinations  are applied. For higher resolution objective prism plates, objective grating spectrographs and slit spectrographs have been used [372, 376, 439, 2, 382, 493, 551]. Objective prisms had already been used early in the century  and were specifically useful for scanning extended stellar fields. Today grating spectrographs are used with edged gratings for Cassegrain spectrographs and Coude spectrographs that allow spectral resolutions of up to 100,000. Similar results can be achieved with lithographic reflection gratings in Echelle spectrographs [314, 858].
Many near-IR observations have been performed throughout the 1970s, notably [574, 821, 175, 733], which together with optical observations provided a vital database for further studies of young stars. For wavelengths < 3.5 |m nitrogen cooled InSb-photodiodes are commonly used. Wavelengths < 1.2 |m can be observed with optical photomultipliers, though nitrogen cooled photo cathodes are needed.
Research in the 1980s also systematically began to survey star-forming regions in the mid-IR up to the mm-band as more advanced electronics became available. Specifically CO surveys provided a direct probe of molecular clouds and collapsing cloud cores (see Chaps. 3 and 4 for more details). For observations in the sub-mm and mm band nitrogen cooling becomes insufficient and superfluid helium cooling needs to be in place instead, forcing focal plane temperatures to below 2 K. S. Beckwith and collaborators, for example, used a He-cooled bolometer to measure 1.3 mm continuum emission with the IRAM 30 m telescope at an altitude of around 2,900 m on Pico Valeta in Spain . The IRAM and VLA telescopes were used throughout the early 1990s to map molecular clouds, foremost the p Oph A cloud which hosts very recently formed stars with CO outflows . Other surveys and observations of recently formed stars also involve dust emission from Herbig Ae/Be stars , objects in the p Oph cloud , H2O emission in circum-
stellar envelopes of protostars , and from various collapsing cores with envelope masses < 5 Mq , to name but a few out of hundreds of these observations performed to date.
Today many measurements are performed with SCUBA on the JCMT, which is a state-of-the-art facility located in Hawaii and which came into service in 1997. Another even more recent sub-mm facility is the 10 m HHT on Mt. Graham in the USA. Major recent, currently active, and future observing facilities like CSO, BIMA, and OVRO with short characterizations are listed in Appendix F.
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