The near infrared presents several new and powerful advantages in the diagnostics of sunspot atmospheres: (1) increased magnetic sensitivity in Zeeman-split lines, (2) increased sensitivity of umbral brightness to temperature, and (3) reduced scattered light and seeing disturbances due to atmospheric turbulence. This has revealed a strong and consistent relationship between sunspot brightness and magnetic field strength.

We have made spatial/spectral observations of sunspots in the highly sensitive (g = 3) Fe I line at λ = 1.5649 μm to compare field strengths with continuum intensities. We find a characteristic but nonlinear relationship between magnetic field strength, B, and brightness temperature, Tb, in sunspots. In umbrae there is an approximately linear relation between B2 and Tb.

We report on new improved infrared (IR) imaging observations (1.6 μm) carried out with the National Solar Observatory's Vacuum Tower Telescope at Sacramento Peak, New Mexico (NSO/SP). Examples of high spatial resolution (up to 0.4″ images are shown, and results of comparisons between infrared and continuum at 526 nm and between infrared and the locations of enhanced magnetic field (“fluxtube regions” as defined by Mg I b1 line imaging) are given and discussed. Our results indicate that, close to disk center, magnetic elements have a positive contrast at the opacity minimum. This is contrary to the findings of several other authors (e.g., Worden 1975, Foukal et al. 1990). We emphasize the necessity of multi-color, high spatial and temporal resolution observations. The potential of present “almost-on-a-routine-basis” IR observations utilizing a fast video acquisition system developed at NSO/SP is pointed out.

The aim of the present communication is to draw attention to the value of simultaneous observations of sunspot umbrae and the quiet Sun in selected molecular lines. It is felt that such observations may lead to an array of sunspot models which account for sunspot sizes, magnetic field strengths, and the solar activity cycle.

I present a preliminary analysis of IR spectra of five K and M dwarfs and two RS CVn variables. Evidence for significant magnetic flux is found on several stars, a number of which are detected for the first time. Field strengths (B) on the RS CVn variables are lower than in the active dwarfs, consistent with the concept of pressure balance limiting B in stellar photospheres. I compare the results with previous measurements.

Some recent developments of atomic spectroscopy in the infrared spectral range are briefly summarized. Different topics of interest for astrophysics, such as term analysis, far infrared measurements, isotope shifts, hyperfine structure observations and transition probability determination, are briefly considered.

The ATMOS solar atlas covers the infrared spectrum of the Sun observed from space, over the frequency range from 650 to 4800 cm–1 at a resolution of 0.01 cm–1. The spectrum reveals a large number of molecular and atomic features that are not visible in spectra taken from the ground. The acquisition of the spectra, details of their presentation in the atlas, and some highlights of the solar features are described.

The Sun is the star we can observe with the highest spectral resolution and signal-to-noise. From studying the infrared spectrum we can learn about the Sun, about stars in general, and about atomic and molecular spectroscopy. We discuss the computer programs for spectrum synthesis, the infrared flux and central intensity atlases of the solar spectrum, and the atomic and molecular line data. Considerable work is still required to improve the observations and to improve the line data.

Line shifts and asymmetries of spectral lines have been found in the ATMOS spectra. These are due to the granulation at the solar surface. A two-component model for the solar photosphere is used to calculate theoretical profiles which are compared to the observations. The ATMOS lines provide additional constraints on models for the solar photosphere.

We briefly review the many indicators, atoms as well as molecules, of the photospheric abundances of C, N and O and present preliminary updated values of these abundances.

We present a detailed analysis of very high excitation lines (4f–5g) of Fe I which are present in the spectral region 2545–2585 cm–1 in high resolution spectra both in the laboratory and in the ATMOS solar spectra obtained from space. A value of the solar abundance of iron which agrees with the meteoritic value is derived.

The infrared solar spectrum is used to refine our knowledge of molecular constants of CH and CO and to test the accuracy of transition probabilities and dissociation energies of a few diatomic molecules.

We review the theoretical perspective for problems in adaptive optics, outline recent progress, and consider its application in the infrared. Techniques in adaptive optics are on the threshold of revolutionizing modern astronomy. These techniques are particularly applicable in the infrared, where refractive effects of turbulence are reduced, characteristic cell sizes are greater, and the isoplanatic patch diameter is increased. Adaptive techniques could be especially appropriate for modern large solar telescopes now under consideration that could operate in the infrared.

Optical interferometry has numerous applications in stellar and extragalactic astronomy. It also offers the potential for unique solar observations. This review describes current and planned activity in these areas and some possibilities for the future.

The Large Earth-based Solar Telescope (LEST) will be a powerful, next-generation telescope with unprecedented angular resolution, capable of highly accurate polarimetry of the Sun, covering the optical spectral range from about 300 nm into the near infrared to about 2.5 μm.

The telescope is a 2.4-m aperture, “polarization-free” concept based on a modified Gregorian optical system. A fast polarization modulator will be located close to the secondary focus of the system. An actively controlled NTT-type main mirror, a high precision pointing and tracking system, a helium-filled light path and a thin entrance window, together with an integrated adaptive optics system, will give the telescope near diffraction-limited performance in the visible. LEST will be sited on La Palma, in the Canary Islands, near the caldera rim on the Roque de los Muchachos Observatory, which often offers excellent seeing. A frequently occurring seeing parameter of ro = 15–20 cm in the visible will correspond to ro ≥ 1 m in the near IR.

The construction of LEST will begin in 1993, and the telescope is to be ready for “first light” in 1997. The telescope facility will accommodate a large number of focal plane instruments on a spacious instrument table. LEST will be made available for near-IR instrumentation from the start of its regular operation.

Having no window and a filled aperture, i.e., no occlusion by secondary optics, the all-reflective McMath telescope is a proven IR facility. Beyond about 2 μm, it is diffraction limited, however. Engineering studies show that the McMath building could accommodate an increase to a 4-m aperture with a 6-m alt-azimuth feed, permitting sub-arcsec resolution to 12 μm. The use of cooled, solid aluminum mirrors would eliminate “mirror seeing”, which plagues non-vacuum solar telescopes.

The anticipated requirements and operating conditions are considered for using a mid-infrared array camera in broad- and narrow-band solar imaging observations. The array camera system was designed for high-background 5–18 μm general astronomical imaging observations. The electronic and optical design of the camera, its photometric characteristics, examples of observational results, and the requirements for imaging in both high- and low-background solar applications are discussed.

Emission-line and K-coronal observations in the IR have the significant advantage of reduced sky brightness compared with the visible, while the effects of seeing are also reduced. Moreover, strong lines are available in the near-IR. Examples of the current capabilities of IR coronal observations using conventional Lyot coronagraphs are discussed briefly. Photometric measurements using the two IR lines of Fe XIII (10,747 Å and 10,798 Å), together with the Fe XIII 3,388 Å line, have provided a valuable electron-density diagnostic, but with low-angular-resolution. The 10,747 Å line has high intrinsic polarization. It has been used for extensive coronal magnetic field measurements, but only the direction of the field, and that with modest angular resolution, has been achieved due basically to flux limitations. Such studies suffer from the lack of high angular resolution and high photon flux. Moreover, the chromatic properties of a singlet objective lens preclude simultaneous observations at widely-differing wavelengths of the important inner coronal region. A coronagraph based on a mirror objective avoids such problems. Further, comparatively high-resolution and high-sensitivity arrays are now available with quantum efficiencies up to 90%. Reflecting coronagraphs with advanced arrays then provide the possibility of obtaining high-resolution images in the infrared to carry out a wide variety of studies crucial to many of the outstanding problems in coronal physics. A program for the development of reflecting coronagraphs is described briefly, with an emphasis on applications to IR coronal studies.