Focal reducers provide efficient solutions to the drawbacks introduced by the large focal length of direct foci on large modern telescopes. They house a variety of spectroscopic devices, and lead to various families of spectroimagers. Rational filterings, BPM irmltisimultaneous bandwidths, extreme spectral resolution of the field scanner interferometers (Tully’s first design, Taurus, Cigale) and their image reconstruction, as well as the possibility to obtain the spectra of all image elements of the field owing to the new (Tiger and Silfid) integral field spectrographs sometimes combined with Fabry Perot Interferometers (Pytheas) are now within reach. In classical spectrography the focal reducer leads to the optimized size Baranne’s white pupil spectrograph.

The dimensional curse of having to pack 3D spectrophotometry data on solely 2D detectors is briefly discussed, and the main astronomical domains covered by 3D spectroscopy introduced. Finally, the scientific case for high spatial resolution 3D techniques coupled to Adaptive Optics capabilities is presented, with special emphasis on the observation of the nuclei of galaxies.

Focal reducers are nowadays widely used on large telescopes as multi-mode instruments. Some general aspects of the design of focal reducers are discussed. FORS, the focal reducer/low-dispersion spectrograph for the ESO VLT is presented as an example for such a multi-mode instrument on a large telescope.

The University Observatories of München and Göttingen and the Landessternwarte Heidelberg are building in cooperation with ESO two almost identical FOcal Reducer /low-dispersion Spectrographs (FORS) for the ESO Very Large Telescopes. FORS allows low-dispersion multiobject spectroscopy (19 slits) and longslit spectroscopy in the wavelength range of 330 to 1100 nm. A set of standard grisms with reciprocal dispersions of 45 ...230 Å/mm working in the first order are foreseen. With a slitwidth of 1 arcsec the resulting spectral resolutions range from 180 to 1800.

For further FORS details see Appenzeller and Rupprecht (1992) and Seifert et al. (1994).

The standard grisms are located in a grism wheel in the parallel beam between the collimator and the camera. Seven of eight positions are available for grisms. The free diameter of the grisms is 135 mm to cover the whole field of view of FORS. To avoid reflection ghosts the entrance surfaces are all tilted by .

Various optical solutions are presented for the Beijing Faint Object Spectrograph and Camera of Beijing Astrophysical Observatory 2.16 m telescope. They are based either on dioptric or catoptric designs including an adaptation of the existing Cassegrain spectrograph.

Planetary Nebulae are proving to be powerful tertiary distance indicators as well as sensitive probes of a galaxy’s velocity field and mass distribution. We present a new technique for determining the positions, luminosities and radial velocities of the PNe in an external galaxy which may enable a one-step measurement of all these parameters.

The present review consists of two parts. The first is mostly of historical interest: the largely-forgotten Lippmann color-photography technique is recalled. The second part is wholly devoted to a critical comparative study of the recently- developed techniques.

In this review the development of the Scanning Fabry-Perot for use in Astronomy, from its invention to the present day is described. The limitation of angle scanning and pressure scanning led to the development of piezo scanned etalons incorporating capacitance micrometers to form a servo-controlled system.

The performance of a new liquid Nitrogen cooled Fabry-Perot etalon for imaging at 3-5µm is described. Capacitance sensors monitor the etalon mirror spacing and parallelism, and error signals produced as a consequence of changes in these parameters are used in a feedback loop with piezoelectric actuators for active cavity control. These new cryogenic etalons are designed to be compatible with the Queensgate Instruments Ltd CS100/ET servo-stabilized Fabry-Perot system.

The cryogenic etalon has a clear aperture of 50mm and a nominal mirror spacing of between 5 and 60µm. It is coated for the 3 - 5µm spectral region, although coatings are also available for the 2 - 2.5µm and 8 - 13µm regions. Under servo-control at operating temperature the etalon has a response time of 30 msec and a minimum cavity tuning range of ±3µm about the nominal cavity length, corresponding to approximately 3 orders of interference at the midrange wavelength of 4µm.

During the last years, considerable improvements occurred in the field of multidielectric spectral filters. Nowadays we can conceive and realize practically any filtering system meeting extremely precise requirements necessary for optical instruments and particularly for astronomy. We will give here a short overview of the multiple factors leading to these improvements.

At optical and infrared wavelengths, imaging Fabry-Perot devices are used in three different ways: (i) to obtain a single spectrum of a diffuse source which fills a large fraction of the aperture, (ii) to obtain a monochromatic image within a field defined by the Jacquinot spot, and (iii) to obtain a spectrum at each pixel position over a wide field by tuning the etalon. We review the theoretical basis for Fabry-Perot photometry and summarize the calibration procedures for the different applications. We discuss methods for recognizing and dealing with artefacts (scattered light, atmospheric effects, etc.) which can seriously comprise the photometric integrity of the data if left untreated.

Several suggestions are made concerning Fabry-Perot data acquisition and reduction, in order to improve the quality of imaging Spectroscopy.

The Goddard Fabry-Perot Imager (GFPI) is an optical scanning interferometer and CCD imaging system developed at the Goddard Space Flight Center’s Laboratory for Astronomy and Solar Physics for use at various astronomical observatories. Besides being transportable, its most notable characteristic is the relatively low spectral resolution (3 – 30Å bandpass). The instrument may be used in scanning mode for objects with a large range of velocities but is most often employed to match the observed wavelength of a Doppler shifted emission feature for objects with any velocity.

We describe recent advances in the development of a scanning Fabry-Perot interferometer applied to the study of the kinematics of the interstellar medium at the Observatorio Astronómico Nacional at San Pedro Mártir, B.C. México.

Our interests in the general field of astrophysics lie principally in two areas: 1) the study of extremely faint emission lines from the interstellar medium, galactic halo, and intergalactic clouds, and 2) the study of neutral and ionized components of the outer atmospheres of solar system objects, including the earth. These studies require instruments of the highest possible area-solid angle product, but typically do not require extremely high spatial resolution. This paper highlights our past work in these areas, and discusses new instrumental approaches we are developing.

Imaging Fabry-Perot interferometers have a particularly useful role to play in studies of the extended regions of anomalous emission in active galaxies. It becomes possible to study the velocity and excitation structure of warm gas with full coverage of large fields. Observations and modeling of several good examples of transient activity will be described as a way of demonstrating the power of an operational facility.

The Hawaii Imaging Fabry-Perot Interferometer (HIFI) was used to produce a large data cube of the edge-on SBc galaxy NGC 3079 covering Hα + [NII] λλ6548, 6583. The complete two-dimensional coverage of the Fabry-Perot data allowed us to derive the general flow pattern of the nuclear gas that constitutes the energetic superbubble.