Routine millimagnitude photometry may require a new approach to reduction of photometric errors. Such an approach is outlined in this paper; it stresses elimination of each error as close to its source as possible. The possibilities provided by modern technology are reviewed in this light. An engineering design group dedicated to photometry is a prerequisite and an on-site photometric technician may be necessary. In this concept, observers are mainly remote users of a database. Implied is the idea of accurate photometry necessarily developing into a single but multi-site astronomical facility (cf. VLBI) and the communal discipline that goes with it.

The technique of image sharpening which allows high resolution images to be produced from ground-based telescopes is applied to the problem of photometry in crowded field regions - such as close to the cores of globular clusters. The conditions for image sharpening are discussed and the technique is demonstrated using simple objects (close double stars). Preliminary results from image sharpening of M15 are presented.

Infrared arrays have been in use at a number of observatories for several years. They are more complicated in their construction than optical ones and more problems arise in obtaining good photometry from them. The types of arrays currently available are described together with the observational techniques and the problems encountered in obtaining accurate photometric results.

Astronomical photometry like other scientific methods is always forced to search for up-to-the-minute instruments and the most sophisticated devices. The photomultiplier tube (PMT) is the most widely used photon detector offering high sensitivity and gain with acceptable quantum efficiency and spectral range but it also has some disadvantages. During the past few decades all of these parameters have been improved and it seems likely that possibilities for their continued development are exhausted.

In the meantime, technical development — especially micro electronics and semiconductor technology — has produced a new detector, the avalanche photodiode (APD). The APD which is often referred to as a solid-state PMT is just at the start of its career and we may expect its rapid perfection. Although properties of APDs were first discussed almost twenty years ago their mass production did not start until 1990. Early silicon APDs had very limited dimensions compared to PMTs but unfortunately this also implied that their photosensitive area was less than 0.25 mm. One more year of development has been enough to increase the photon collecting surface by a factor of 1000 and APDs may soon become possible rivals of PMTs.

Following a brief description of the new device and discussion of its operation a comparison of photodetector performance will be given in order to draw attention to those parameters which are of great promise. Those sections of astronomical photometry which are ready to benefit from the suggested testing and use of this kind of photon detector will be highlighted.

A photon-counting detector with a 45 mm diameter active area has been developed using a microchannel plate (MCP) and the wedge and strip readout system. The detector uses a UV transparent glass input window proximity coupled to a stack of MCP. Quantum efficiency (QE) of the detector photo-cathode is ~ 20%. The resolution of ~ 60 μm FWHM at a gain of 5.107 has been achieved.

We present the design of a new 4 channel, fiberoptic spectrophotometer which allows to measure the object, two comparison stars and sky background simultaneously throughout the whole optical wavelength range with high efficiency. The overall spectral resolution of about 50 in combination with over-sampling allows to eliminate colour dependent extinction effects to a high degree and guarantees accurate transformations to other broad band photometric systems.

A new photometer is presently being developed at the O.C.A. Observatory. It consists of four arms and a CCD camera situated in the focal plane of the telescope. Each arm can move in both directions and support a diaphragm and a liquid optic guide that directs the light to a photomultiplier. The simultaneous acquisition of the four signals enables to obtain magnitude differences between the objects in real time. A typical use of this photometer is to observe at the same time one or two variables, comparison stars and the sky background.

We briefly describe the history of robotic observatories, give details on an example of the current state-of-the-art in robotic observatories, and suggest several key areas for future development.

We present various methods for determining night quality from observations obtained with totally unattended automatic photoelectric telescopes at the Fairborn Observatory site on Mt. Hopkins, AZ. Telescopes are the Phoenix 10-inch (P10) and the Four College 75-cm (CAPT). Filter systems used are Johnson UBV, Kron-Cousins RI and Strömgren uvby. As a preliminary data filter for the P10 “rent-a-star” data, all observations with standard errors of the mean (SEM) above 20 mmag are discarded; a summary of nightly SEM’s, number of aborts and total observing time is sent to each user. The CAPT data are evaluated by standard deviation of magnitudes or counts for photometric groups and for an entire night. The Geneva photometric statistics are computed for these latter data and have proved useful as a preliminary screen for bad data. Various criteria for good and bad groups and nights are discussed. Scientific results are presented for several magnetic CP stars that were observed using both telescopes. Differences in precision of the data and strategies for observing are discussed.

From Oct. 1983 through Dec. 1987 most of the observing capacity of the Boyd-Genet prototype 10-inch APT was devoted to a program of 92 variable stars the output of which was sent to us for analysis. From Nov. 1987 through the present the entire observing capacity of the VU-TSU 16-inch APT has been devoted to a program of 136 mostly magnetically active variables. We review the technical performance of both APTs in terms of malfunctions, down-time, resulting photometric accuracy, phase coverage, and scientific results to date.

Copenhagen University Observatory operates a 52cm Cassegrain telescope at ESO, La Silla (Chile) for photoelectric photometry in a completely automatic mode. Alternatively, the telescope may be operated, remotely, from Europe. We report on the operational experience from more than four years of successful use and on the decision by ESO to establish a similar system on their photometric telescope.

Observational evidence was found that even in good photometric sites the extinction coefficient can display large variations during a night. The authors propose a method to determine instantaneous values of the extinction coefficient: it is based on the knowledge of the extra-atmosphere instrumental magnitude m0 for a reference star. An analytical procedure is proposed to determine a reliable value for m0 and results are very satisfactory.

A retrospective investigation covering about twenty years of wide-band and intermediate-band photometry at Jena University observing station allows an analysis of variations of atmospheric extinction at different time scales. A comparison with an earlier investigation by Wempe (1947) permits to draw conclusions about the evolution of air pollution by aerosols over nearly half a century. Complementary weather observations obtained at the meteorological observing station of the Jena University allow to find correlations between the variation of extinction parameters and the changing weather situations. Parallel Mie calculations allow to interpret the variations of the measured extinction data in terms of size variations of water-rich aerosols.

The passbands of the Johnson JHKL broadband photometric system used at a number of major observatories have been compared to the atmospheric window transmissions calculated by MODTRAN, and a family of solar-composition model stellar fluxes from Kurucz (1991 private communication) have been used as input to model the atmospheric extinction under different water vapor content, altitude, and airmass conditions. A figure of merit related to the slope of the extinction curve at zero airmass describes the sensitivity of the response function to atmospheric extinction. We have compared passbands used at several observatories, and have designed an improved set of passbands.

All of us “have-nots” need more telescope time, for ourselves and for our students. There are also many programs where a global linkage is needed to accomplish the objectives. In addition, the world needs more and better science education; astronomy can be a leader if it has adequate facilities to do so. A global network of automatic telescopes can help supply these needs, which are global, spanning all countries. A new non-profit organization (GNAT, Inc.) has been formed to be the catalyst to develop and implement such a concept. We hope that many astronomers and organizations will become “members” of GNAT.

A summary of the results from seven global runs using the Whole Earth Telescope is presented, together with an evaluation of the scientific results obtained to date. Factors such as the distance of the target star from the equator, the nature and timescales of its intrinsic variability, etc. are shown to affect the value and quality of the results, as well as the traditional factors such as brightness and long-term coherent behaviour. Experience with the network shows that, taken as a whole, it enjoys far better weather than any one of its sites, and provides unprecedented ‘uncluttered’ resolution of time-series power spectra.

To promote fuller awareness we should aim to globalize observations. In a more immediate sense, this addresses the planetary distribution of observing facilities and their access. What components in such facilities occur in ‘global networks’? What factors optimize the growth and scientific viability of such a network? What kinds of progress can be expected?

Transformations are a central issue in making a global network do more than simple monitoring of low-amplitude variability. I explore the approach of observing at a much narrower instrumental bandwidth than is required for the scientific problem; such an approach would have the following advantages:

1. • transformations can be handled in standard fashion at the instrumental level; at the scientific level, they can be avoided entirely,

2. • users have almost complete freedom in specifying the shape of the scientific passbands, hence comparison of observational data with stellar atmosphere models can be maximally effective,

3. • standard star observations can be used repeatedly, for programmes running concurrently in different scientific photometric systems,

4. • an observer can use existing standard-star observations to create his own specially-tailored photometric system from scratch,

5. • all-sky homogeneity of the instrumental system can be tested against space photometry such as that provided by HIPPARCOS; this will benefit other scientific systems synthesized from the same instrumental system.

Key components in the hardware will be array detectors with low readout noise and a calibration lamp system designed specifically for this application. A data base provides the link between the observations (in the instrumental system) and the results (with scientific passbands defined by the end user).

Lausanne University’s Institute of Astronomy has been collecting stellar photometric data since 1970. Our collection contains data for some 170,000 stars in 78 photometric systems. This database has been used in various applications: comparison of systems, Hipparcos Input Catalogue, studies of star samples, etc. With CCD detectors we are facing a new challenge, since a great deal of information is obtained, mainly for star clusters. But unfortunately, a policy for archiving these data has not yet been established. This is an urgent problem to solve, all the more so as in many cases only a part of the data is published. A solution to this problem is discussed here.

Characteristics of telescopes, photometers, CCDs and data acquisition systems relevant to achieving high quality photometry are discussed.