Several WWW sites offering topical databases of special types of radio source research will be mentioned briefly in this section.
A collection of well-chosen radio source catalogues has been used, together with optical sky surveys like the DSS, to develop a database of radio-optical information on Abell/ACO ([Abell et al. (1989)]) clusters of galaxies. It is managed by A. Gubanov at the St.-Petersburg State University in collaboration with the present author (Gubanov & Andernach 1997). At the URL www.astro.spbu.ru/CLUSTERS/, the user may interactively prepare schematic radio-optical overlays, charts from the FIRST, NVSS, APM or DSS survey data, or retrieve references for cluster data. Radio continuum spectra of cluster radio galaxies may be displayed and fitted with user-specified functions. Source luminosities may be derived assuming cluster membership.
5.2. VLBI and Astrometric Surveys
The VLBA Calibrator Survey (magnolia.nrao.edu/vlba_calib/) is an ongoing project to provide phase-reference calibrators for VLBA experiments. When completed it will contain astrometric (~ 1mas) positions and 2.7 and 8.4 GHz images of the ~ 3000 sources in the JVAS catalogue (Section 5.3).
The ``Pearson-Readhead'' (PR) and ``Caltech - JodrellBank'' (CJ)
imaging data base is a VLBI source archive at Caltech
(astro.caltech.edu/~tjp/cj/).
It offers images of over 300 VLBI sources at
>35° observed in
the PR
([Pearson & Readhead (1988)]),
CJ1 ([Xu et
al. (1995)]),
and CJ2 ([Henstock et al. (1995)])
surveys. Many of these sources are
excellent calibrators for the VLA and VLBA.
It has mostly 5GHz (6cm) and some 1.67GHz (18cm) VLBI images,
as well as 1.4 and 5-GHz VLA images of extragalactic sources.
Contour maps are publicly available as PostScript files.
A sample of 132 compact sources have been observed in ``snapshot'' mode with the VLBA at 15GHz (2cm; [Kellermann et al. (1998)]). At present it contains images at one epoch (www.cv.nrao.edu/2cmsurvey/ and www.mpifr-bonn.mpg.de/staff/azensus/2cmsurvey/), but it eventually will have multi-epoch sub-milliarcsecond data.
The ``Radio Reference Frame Image Database'' (RRFID) is maintained at the
U.S. Naval Observatory (USNO). Data obtained with the VLBA and the
``Global Geodetic Network'' at 2.3, 8.4, and 15GHz (13, 3.6 and 2cm)
are publicly available as over 1400 images for more than 400 sources,
at rorf.usno.navy.mil/rrfid.html.
In April 1998 first-epoch imaging of northern hemisphere radio
reference frame sources was completed. Images at both 2.3 and 8.5GHz
now exist for ~ 97% of the ``Radio Optical Reference Frame'' (RORF)
sources
([Johnston et al. (1995)])
north of = -20°,
which is ~ 90% of the ``International Celestial Reference Frame'' (ICRF)
sources in this region of sky.
A number of links are available from the RRFID page, in particular to the
RORF data base of sources
(rorf.usno.navy.mil/rorf.html).
The European VLBI Network (EVN; www.nfra.nl/jive/evn/) was formed in 1980 by the major European radio astronomy institutions, and is an array of radio telescopes spread over Europe, the Ukraine, and China. A catalogue of observations carried out so far can be retrieved from terra.bo.cnr.it/ira/dira/vlbinet.dat. The column explanation is available at www.ira.bo.cnr.it/dira/vlbinet.doc.
5.3. Gravitational Lens Surveys
About 2500 compact northern sources stronger than 200mJy at 5GHz have been mapped with the VLA at 8.4GHz in the ``Jodrell-Bank/VLA Astrometric Survey'' (JVAS; www.jb.man.ac.uk/~njj/glens/jvas.html). The goal was ([Patnaik et al. (1992)]) to find phase calibrator sources for the MERLIN interferometer (www.jb.man.ac.uk/merlin/) and gravitational lens candidates. If the redshift of both the parent object of the compact source and that of the intervening galaxy or cluster (causing the lensing effect) can be determined, and if in addition the compact source shows variability (not uncommon for such sources), the time delay between radio flares in the different images of the lensed object may be used to constrain the Hubble constant.
The ``Cosmic Lens All-Sky Survey'' (CLASS; astro.caltech.edu/~cdf/class.html), is a project to map more than 10,000 radio sources in order to create the largest and best studied statistical sample of radio-loud gravitationally lensed systems. Preliminary 8.4-GHz fluxes and positions are already available from www.jb.man.ac.uk/~njj/glens/class.html and www.jb.man.ac.uk/~ceres1/list_pub.html. The whole database will eventually be made public.
The ``CfA - Arizona Space Telescope Lens Survey'' (CASTLeS) provides information and data on gravitational lens systems at cfa.harvard.edu/castles/. It includes HST and radio images that can be downloaded via ftp. The service distinguishes between multiply imaged systems and binary quasars.
The ``VLBI Space Observatory Program'' (VSOP or HALCA;
www.vsop.isas.ac.jp/) has put an 8-m radio antenna
into a highly elliptical Earth orbit so as to extend terrestrial
interferometer baselines into space.
Several hundred sources in the VSOP Survey Program
(www.vsop.isas.ac.jp/obs/Survey.html) are listed,
together with their observational status,
at www.ras.ucalgary.ca/survey.html.
These were selected to have 5-GHz flux above 1Jy
and a radio spectrum flatter than S
~ 
-0.5.
Galactic masers are also being surveyed.
An image gallery, including the first images ever made in
Space-VLBI, may be viewed at
www.vsop.isas.ac.jp/general/Images.html.
Further images of EVN-HALCA observations are available
at www.nfra.nl/jive/evn/evn-vsop.html.
The same page will soon provide access to VLBA images of over 350
extragalactic sources observed with the VLBA at 5GHz prior to the VSOP
launch and some results of the pre-launch OH-maser survey.
5.4. Variable Sources and Monitoring Projects
Since 1997, about forty Galactic and extragalactic variable sources have been
monitored with the Green Bank Interferometer (GBI) at 2.25 and 8.3GHz
(HPBW 11" and 3", resp.), under NASA's OSIRIS project.
The instrument consists of three 26-m antennas, and the targets
are preferentially X-ray and
-ray active.
Radio light curves and tables of flux densities are provided at
www.gb.nrao.edu/GBT/GBT.html.
The ``University of Michigan Radio Astronomy Observatory'' (UMRAO) database (www.astro.lsa.umich.edu/obs/radiotel/umrao.html; [Hughes et al. (1992)]) contains the ongoing observations of the University of Michigan 26-m telescope at 5, 8.5 and 15GHz. A number of strong sources are frequently (weekly) monitored and some weaker sources a bit less often. Currently the database offers flux densities, and polarisation percentage and angle (if available), for over 900 sources. For some objects the on-line data go back to 1965.
An explanation of the types of solar bursts and a list of special events observed can be found at www.ips.gov.au/culgoora/spectro/. Daily flux measurements of the Sun at 2.8GHz (10.7cm) back to 1947 are offered at www.drao.nrc.ca/icarus/www/sol_home.shtml. The Ondrejov Solar Radio Astronomy Group (sunkl.asu.cas.cz/~radio) provides an archive of events detected with a 3.0GHz continuum receiver and two spectrographs covering the range 1.0-4.5GHz. The Metsahovi Radio Station in Finland offers solar radio data at kurp.hut.fi/sun, like e.g. radio images of the full Sun at 22, 37 and 87GHz, a catalogue of flares since 1989, or ``track plots'' (light curves) of active regions of the solar surface. For further data, get in contact with the staff at solar@hut.fi or Seppo.Urpo@hut.fi.
The ``National Geophysical Data Center'' (NGDC) collects solar radio data from several dozen stations over the world at www.ngdc.noaa.gov/stp/SOLAR/getdata.html. This ``Radio Solar Telescope Network'' (RSTN) of 55 stations has now collected 722 station-years worth of data. Information about solar bursts, the solar continuum flux, and spectra from RSTN may be retrieved and displayed graphically at the URL www.ngdc.noaa.gov:8080/production/html/RSTN/rstn_search_frames.html.
Since July 1992, the Nobeyama Radio Observatory (NRO) has been offering (at solar.nro.nao.ac.jp/) a daily 17GHz image of the Sun, taken with its Nobeyama Radio Heliograph. Also available are daily total flux measurements at five frequencies between 1 and 17GHz observed since May 1994, as well as some exciting images of solar radio flares.
Cracow Observatory offers daily measurements of solar radio emission at six decimetric frequencies (410-1450MHz) from July 1994 to the present (www.oa.uj.edu.pl/sol/index.html).
Measurements with the Tremsdorf radio telescope of the Astrophysics Institute Potsdam (AIP), Germany, based on four solar sweep spectrographs (40-800MHz) are available at aipsoe.aip.de/~det.
The Radio Astronomy Group (RAG) of the ETH Zürich offers the data from its various digital radio spectrometers and sweep spectrograph at www.astro.phys.ethz.ch/rag, and also hosts the homepage of the ``Community of European Solar Radio Astronomers'' (CESRA). The ``Joint Organization for Solar Observations'' (JOSO) offers a comprehensive list of links to solar telescopes and solar data centres at joso.oat.ts.astro.it.
A wide variety of solar data, including East-West scan images from the Algonquin Radio Observatory 32-element interferometer, and reports of ionospheric data, are provided by the University of Lethbridge, Alberta, Canada (holly.cc.uleth.ca/solar, or its ftp server at ftp://ftp.uleth.ca/pub/solar/).
Finally, there are many sites about solar-terrestrial processes and ``Space Weather Reports'', e.g. at www.sel.noaa.gov or www.ips.gov.au/. There are spacecraft solar radio data at lep694.gsfc.nasa.gov/waves/waves.html and www-istp.gsfc.nasa.gov/.