Automatic Plate Measuring Research Articles

Accurate photometry and variability of First Byurakan Survey blue stellar objects

A new method for the combined calculation of magnitudes based on accurate measurements of Palomar Observatory Sky Survey (POSS) epochs 1 and 2 (POSS 1 and POSS2) plates is given. The photometric accuracy of various surveys and catalogues has been estimated and established and statistical weights have been calculated for each one. To achieve the best possible magnitudes, weighted averaging of data from United States Naval Observatory (USNO-A2.0), Automatic Plate Measuring (APM), Minnesota Automated Plate Scanner (MAPS), USNO-B1.0 and Guide Star Catalog (GSC 2.3.2) catalogues has been used. The root-mean-square (rms) accuracy of magnitudes achieved for POSS1 is 0.184 mag for B and 0.173 mag for R, while that for POSS2 is 0.138 mag for B and 0.128 mag for R. We have derived the best POSS 1 and POSS2 magnitudes for First Byurakan Survey (FBS) blue stellar objects. We have refined the transformation formulae between POSS1 and POSS2 magnitudes and those for the Sloan Digital Sky Survey (SDSS) and a standard UBV system. Using these accurate magnitudes, we have estimated the variability of FBS blue stellar objects and revealed probable and possible variables. We have worked out methods with which to control and exclude the accidental errors that appear in any survey. We have compared and combined our results with those given in the Northern Sky Variability Survey (NSVS) data base and obtained better candidates for variability. Having excluded variables, we have combined POSS1 and POSS2 data for the remaining objects to achieve even better magnitudes and colours, the rms being smaller than 0.1 mag in both B and R and for B ― R colours. This approach has been applied to FBS blue stellar objects containing a significant number of white dwarfs (WDs) or cataclysmic variables (CVs), as well as extragalactic objects (quasars, Seyferts, BL Lac objects). Altogether 336 variable objects have been revealed, with POSS2―POSS1 ≥ 3σ of the errors. An electronic table of these objects is given. Candidate variables are divided into four classes: extreme, strong, probable and possible. For a more reliable sample of variable objects we excluded possible variables from the list and were left with 161 objects. By analysing the radio and X-ray properties of these objects we have revealed their nature and rediscovered or revealed candidate active galactic nuclei (AGNs), CVs, WDs and other objects.

Cross-correlations of X-ray and optically selected clusters with near-infrared and optical galaxies

We compute the real-space cluster-galaxy cross-correlation ξ cg (r) using the ROSAT-ESO Flux Limited X-ray (REFLEX) cluster survey, a group catalogue (2dFGGC) constructed from the final version of the Two-degree Field Galaxy Redshift Survey (2dFGRS), and galaxies extracted from the Two Micron All Sky Survey (2MASS) and Automated Plate Measurement (APM) surveys. This first detailed calculation of the cross-correlation for X-ray clusters and groups is consistent with previous works and shows that ξ eg (r) cannot be described by a single power law. We analyse the clustering dependence on the cluster X-ray luminosity L x and virial mass M vir thresholds as well as on the galaxy limiting magnitude. We also make a comparison of our results with those obtained for the halo-mass cross-correlation function in a ACDM N-body simulation to infer the scale dependence of galaxy bias around clusters. Our results indicate that the distribution of galaxies shows a significant anti-hias at highly non-linear small cluster-centric distances, b eg (r) ≃ 0.7, irrespective of the group/cluster virial mass or X-ray luminosity and galaxy characteristics. This shows that a generic process controls the efficiency of galaxy formation and evolution in high-density regions. On larger scales, b eg (r) rises to a nearly constant value of the order of unity, the transition occurring at approximately 2 h -1 Mpc for 2dF groups and 5 h -1 Mpc for REFLEX clusters.

Modelling the two-point correlation function of galaxy clusters in the Sloan Digital Sky Survey

We study the clustering properties of the recently compiled Sloan Digital Sky Survey (SDSS) cluster catalogue using the two-point correlation function in redshift space. We divide the total SDSS sample into two richness subsamples, roughly corresponding to Abell R≥ 0 and Automated Plate Measuring (APM) clusters, respectively. If the two-point correlations are modelled as a power law, ξ(r) = (r0/r)γ, then the best-fitting parameters for the two subsamples are r0= 20.7+4.0−3.8h−1 Mpc with γ= 1.6+0.4−0.4 and r0= 9.7+1.2−1.2 with γ= 2.0+0.7−0.5h−1 Mpc, respectively. Our results are consistent with the dependence of cluster richness on the cluster correlation length. Finally, comparing the SDSS cluster correlation function with predictions from three flat cosmological models (Ωm= 0.3) with dark energy (quintessence), we estimate the cluster redshift-space distortion parameter β≃Ω0.6m/b0 and the cluster bias at the present time. For the Λ cold dark matter case we find β= 0.2+0.029−0.016, which is in agreement with the results based on large-scale cluster motions.

Evolution of the galaxy luminosity function at z < 0.3

We measure the redshift-dependent luminosity function and the comoving radial density of galaxies in the Sloan Digital Sky Survey Data Release 1 (SDSS DR1). Both measurements indicate that the apparent number density of bright galaxies increases by a factor ≈ 3 as redshift increases from z= 0 to z= 0.3. This result is robust to the assumed cosmology, to the details of the K-correction and to direction on the sky. These observations are most naturally explained by significant evolution in the luminosity and/or number density of galaxies at redshifts z < 0.3. Such evolution is also consistent with the steep number-magnitude counts seen in the Automatic Plate Measuring (APM) Galaxy Survey, without the need to invoke a local underdensity in the galaxy distribution or magnitude scale errors.

Clustering of galaxy clusters in cold dark matter universes

We use very large cosmological N-body simulations to obtain accurate predictions for the two-point correlations and power spectra of mass-limited samples of galaxy clusters. We consider two currently popular cold dark matter (CDM) cosmogonies, a critical density model (τCDM) and a flat low density model with a cosmological constant (ΛCDM). Our simulations each use 109 particles to follow the mass distribution within cubes of side 2 h−1 Gpc (τCDM) and 3 h−1 Gpc (ΛCDM) with a force resolution better than 10−4 of the cube side. We investigate how the predicted cluster correlations increase for samples of increasing mass and decreasing abundance. Very similar behaviour is found in the two cases. The correlation length increases from for samples with mean separation to for samples with The lower value here corresponds to τCDM and the upper to ΛCDM. The power spectra of these cluster samples are accurately parallel to those of the mass over more than a decade in scale. Both correlation lengths and power spectrum biases can be predicted to better than 10 per cent using the simple model of Sheth, Mo & Tormen. This prediction requires only the linear mass power spectrum and has no adjustable parameters. We compare our predictions with published results for the automated plate measurement (APM) cluster sample. The observed variation of correlation length with richness agrees well with the models, particularly for ΛCDM. The observed power spectrum (for a cluster sample of mean separation ) lies significantly above the predictions of both models.

Optical identifications of 4000 radio sources at the 1mJy level

We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM (Automatic Plate Measuring) and FIRST (Faint Images of the Radio Sky at Twenty centimetres) surveys over the region 14813≲RA(2000)≲21813,−277≲Dec.≲225. Selecting radio sources down to 1 mJy, and adopting a uniform optical limit of bJ=21.5, we find 3176 have a counterpart in the APM catalogue, corresponding to 13 per cent of the radio sample. For bJ≤20.5 we can divide radio sources into resolved radio galaxies and stellar-like objects (principally quasi-stellar objects – QSO). We find the population of radio galaxies to be mainly composed of early-type galaxies with very red colours (bJ−R up to ∼4) and a radio-to-optical ratio 102≲r≲104. The contribution of starbursting objects is negligible. In general QSO show r≳104 and 0≲bJ−R≲1. On the basis of the R magnitudes, we estimate the sample of radio galaxies to be complete up to z≃0.3. We can therefore divide the whole sample of radio sources into a low-z and high-z population. The low-z one includes the objects identified as galaxies in the APM survey and the high-z one includes sources either identified as QSO or with no optical counterpart for bJ≤20.5. We find that radio galaxies are strongly clustered and are highly biased tracers of the underlying mass distribution. Models for the angular correlation function w(θ) show good agreement with the observations if we assume a bias factor b≃2 at z≃0.3.

New nearby stars among bright APM high proper motion stars

As part of a new southern sky survey for faint high proper motion stars based on Automatic Plate Measuring (APM) measurements of UK Schmidt Telescope plates, we have found a large number of previously unknown brighter objects. Spectroscopic follow-up observations with the European Southern Observatory New Technology Telescope of 15 of these new, relatively bright (12 0.45 arcsec yr - 1 ) show one-third of them to be nearby (d < 25 pc). Among the nearby stars is an M6 dwarf with strong emission lines at a spectroscopic distance of about 11 pc and an M4 dwarf at about 13 pc. Coupled with earlier South African Astronomical Observatory spectroscopic observations of three similar bright high proper motion stars, the success rate of finding nearby stars (d < 25 pc) is about 45 per cent. All newly discovered nearby stars have disc kinematics confirmed by radial velocity measurements from our spectra. In addition there are several high-velocity stars with halo kinematics in the sample, mainly subdwarfs, at about 60 to 110 pc distance. These high-velocity stars are interesting targets for further study of the Galactic escape velocity. One of the detected nearby high proper motion stars was formerly thought to be an M giant in the Small Magellanic Cloud. The spectrum of one M3 star shows a strong blue continuum, which is likely to signify the presence of a hot companion. Spectroscopic follow-up observations of high proper motion stars are shown to be an effective tool in the search for the missing stars in the Solar neighbourhood. Candidates for more extensive trigonometric parallax determination can be selected on the basis of the spectroscopic distance estimates.

Automated Selection of Metal‐poor Stars in the Galaxy—The HK‐II Survey

Automated Selection of Metal‐poor Stars in the Galaxy—The HK‐II Survey

Bumpy power spectra and T/T

With the recent publication of the measurements of the radiation angular power spectrum from the BOOMERanG-98 Antarctic flight, it has become apparent that the currently favoured spatially flat cold dark matter model (matter density parameter Ωm=0.3, flatness being restored by a cosmological constant ΩΛ=0.7, Hubble parameter h=0.65, baryon density parameter Ωbh2=0.02) no longer provides a good fit to the data. We describe a phenomenological approach to resurrecting this paradigm. We consider a primordial power spectrum which incorporates a bump, arbitrarily placed at kb and characterized by a Gaussian in log k of standard deviation σb and amplitude Ab, which is superimposed on to a scale-invariant power spectrum. We generate a range of theoretical models that include a bump at scales consistent with cosmic microwave background (CMB) and large-scale structure observations, and perform a simple χ2 test to compare our models with the COBE Differential Microwave Radiometer (DMR) data and the recently published BOOMERanG-98 and MAXIMA-1 data. Unlike models that include a high baryon content, our models predict a low third acoustic peak. We find that low l observations (20<l<200) are a critical discriminant of the bumps because the transfer function has a sharp cut-off on the high l side of the first acoustic peak. Current galaxy redshift survey data suggest that excess power is required at a scale around 100 Mpc, corresponding to kb∼0.05 h Mpc−1. For the concordance model, use of a bump-like feature to account for this excess is not consistent with the constraints imposed by recent CMB data. We note that models with an appropriately chosen break in the power spectrum provide an alternative model that can give distortions similar to those reported in the automated plate measurement (APM) survey as well as consistency with the CMB data. We prefer, however, to discount the APM data in favour of the less biased decorrelated linear power spectrum recently constructed from the Point Source Catalogue Redshift (PSCz) redshift survey. We show that the concordance cosmology can be resurrected using our phenomenological approach and that our best-fitting model is in agreement with the PSCz observations.

The Second APM UKST Colour Survey forz &gt; 4 quasars

We present the spectra, positions, and finding charts for 31 bright (R<19.3) colour-selected quasars covering the redshift range z = 3.85–4.78, with four having redshifts z>4.5. The majority are in the southern sky (δ<−25°). The quasar candidates were selected for their red (BJ−R≳2.5) colours from UK or POSSII Schmidt Plates scanned at the Automated Plate Measuring (APM) facility in Cambridge. Low-resolution (≳10 Å) spectra were obtained to identify the quasars, primarily at the Las Campanas Observatory. The highest redshift quasar in our survey is at z≈4.8(R = 18.7) and its spectrum shows a damped Lyα absorption system at z = 4.46. This is currently the highest redshift damped Lyα absorber detected. Five of these quasars exhibit intrinsic broad absorption line features. Combined with the previously published results from the first part of the APM United Kingdom Schmidt Telescope (UKST) survey we have now surveyed a total of ∼8000 deg2 of sky i.e. 40 per cent of the high galactic latitude (∼b∼>30°) sky, resulting in 59 optically selected quasars in the redshift range 3.85 to 4.78; 49 of which have z≥4.00.

Nonlinear Gravitational Growth of Large‐Scale Structures Inside and Outside Standard Cosmology

We reconsider the problem of gravitational structure formation inside and outside general relativity (GR), in both the weakly and strongly nonlinear regime. We show how these regimes can be explored observationally through clustering of high-order cumulants and through the epoch of formation, abundance, and clustering of collapse structures, using Press-Schechter formalism and its extensions. We address the question of how different these predictions are when using a nonstandard theory of gravity. We study examples of cosmologies that do not necessarily obey Einstein's field equations: scalar-tensor theories (STT), such as Brans-Dicke (BD), parametrized with ω, a nonstandard parameterization of the Hubble law, H2 = a-3(1+), or a nonstandard cosmic equation of state p = γρ, where γ can be chosen irrespective of the cosmological parameters (ΩM and ΩΛ). We present some preliminary bounds on γ, ω, and from observations of the skewness and kurtosis in the Automated Plate Measuring (APM) Galaxy Survey. This test is independent of the overall normalization of rms fluctuations. We also show how abundances and formation times change under these assumptions. Upcoming data on nonlinear growth will place strong constraints on such variations from the standard paradigm.

A deep radio survey in the ISO-ELAIS fields: optical identification with the APM catalogue/atl>

The optical identification programme is presented for a complete sample of 867 radio sources that were observed with the Very Large Array (VLA) within 4.22 deg2 to S 1.4 GHz ≥ 0.135 mJy (5 σ). The area covered by the VLA survey is part of the regions of the sky that have been surveyed in the mid and far Infrared over the wavelength range 6.7–175 μmwith ISO as part of the European Large Area ISO Survey (ELAIS). Optical identifications are made using the Automatic Plate-measuring Machine (APM) optical catalogue in E (∼ R) and O (∼ B) bands with approximate limiting magnitudes of ∼ 20.0 and 21.5 mag, respectively. For 220 out of the 867 radio sources, likely identifications are proposed with an a posteriori identification percentage of 25%. The sample of identification has a reliability of 96% and a completeness of 80%.

On the APM power spectrum and the CMB anisotropy: evidence for a phase transition during inflation?

Adams et al. have noted that according to our current understanding of the unification of fundamental interactions, there should have been phase transitions associated with spontaneous symmetry breaking during the inflationary era. This may have resulted in the breaking of scale-invariance of the primordial density perturbation for brief periods. A possible such feature was identified in the power spectrum of galaxy clustering in the automated plate measurement (APM) survey at the scale k ∼ 0.1 h Mpc − 1 and it was shown that the secondary acoustic peaks in the power spectrum of the cosmic microwave background (CMB) anisotropy should consequently be suppressed. We demonstrate that this prediction is confirmed by the recent Boomerang and Maxima observations, which favour a step-like spectral feature in the range k ∼ (0.06–0.6) h Mpc − 1, independently of the similar previous indication from the APM data. Such a spectral break enables an excellent fit to both APM and CMB data with a baryon density consistent with the big bang nucleosynthesis (BBN) value. It also allows the possibility of a matter-dominated universe with zero cosmological constant, which we show can now account for even the evolution of the abundance of rich clusters.

The European Large Area ISO Survey -- II. Mid-infrared extragalactic source counts

We present preliminary source counts at 6.7 and 15 μm from the preliminary analysis of the European Large Area ISO Survey, with limiting flux densities of ∼2 mJy at 15 μm and ∼1 mJy at 6.7 μm. We separate the stellar contribution from the extragalactic using identifications with automated plate measurement sources made with the likelihood ratio technique. We quantify the completeness and reliability of our source extraction using (a) repeated observations over small areas, (b) cross-identifications with stars of known spectral type, (c) detections of the point spread function wings around bright sources and (d) comparison with independent algorithms. Flux calibration at 15 μm was performed using stellar identifications; the calibration does not agree with the pre-flight estimates, probably due to effects of detector hysteresis and photometric aperture correction. The 6.7-μm extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. model underpredicts the observed source density by ∼0.5–1 dex, although the photometry at 6.7 μm is still preliminary. At 15 μm the extragalactic counts are in excellent agreement with the predictions of the Pearson & Rowan-Robinson, Franceschini et al., Guiderdoni et al. models and the evolving models of Xu et al., over seven orders of magnitude in 15-μm flux density. The counts agree with other estimates from the ISOCAM instrument at overlapping flux densities, provided a consistent flux calibration is used. Luminosity evolution at a rate of (1+z)3, incorporating mid-infrared spectral features, provides a better fit to the 15-μm differential counts than (1+z)4 density evolution. No-evolution models are excluded, implying that below around 10 mJy at 15 μm the source counts become dominated by an evolving cosmological population of dust-shrouded starbursts and/or active galaxies.

Inverting the angular correlation function

The two point angular correlation function is an excellent measure of structure in the Universe. To extract from it the three-dimensional power spectrum, one must invert Limber’s equation. Here we perform this inversion using a Bayesian prior constraining the smoothness of the power spectrum. Among other virtues, this technique allows for the possibility that the estimates of the angular correlation function are correlated from bin to bin. The outputs of this technique are estimators for the binned power spectrum and a full covariance matrix. Angular correlations mix small and large scales but after the inversion, small-scale data can be trivially eliminated, thereby allowing for realistic constraints on theories of large-scale structure. We analyse the automated plate measurement (APM) catalogue as an example, comparing our results with previous results. As a by-product of these tests, we find ‐ in rough agreement with previous work ‐ that APM places stringent constraints on cold dark matter inspired models, with the shape parameter constrained to be 0:25 ^ 0:04 (using data with wavenumber k # 0:1 h Mpc 21 ). This range of allowed values uses the full power spectrum covariance matrix, but assumes negligible covariance in the off-diagonal angular correlation error matrix, which is estimated with a large angular resolution of 0.58 (in the range 0.58 and 208).

Generalized cumulant correlators and hierarchical clustering

The cumulant correlators, Cpq, are statistical quantities that generalize the better-known Sp parameters; the former are obtained from the two-point probability distribution function of the density fluctuations while the latter describe only the one-point distribution. If galaxy clustering develops from Gaussian initial fluctuations and a small-angle approximation is adopted, standard perturbative methods suggest a particular hierarchical relationship of the Cpq for projected clustering data, such as that obtained from the Automatic Plate Measuring (APM) survey. We establish the usefulness of the two-point cumulants for describing hierarchical clustering by comparing such calculations against available measurements from projected catalogues, finding very good agreement. We extend the idea of cumulant correlators to multipoint generalized cumulant correlators (related to the higher-order correlation functions). We extend previous studies in the highly non-linear regime to express the generalized cumulant correlators in terms of the underlying ‘tree amplitudes’ of hierarchical scaling models. Such considerations lead to a technique for determining these hierarchical amplitudes, to arbitrary order, from galaxy catalogues and numerical simulations. Knowledge of these amplitudes yields important clues about the phenomenology of gravitational clustering. For instance, we show that a three-point cumulant correlator can be used to separate the tree amplitudes up to sixth order. We also combine the particular hierarchical Ansatz of Bernardeau & Schaeffer with extended and hyper-extended perturbation theory to infer values of the tree amplitudes in the highly non-linear regime.

Correlations in the Spatial Power Spectra Inferred from Angular Clustering: Methods and Application to the Automated Plate Measuring Survey

We reconsider the inference of spatial power spectra from angular clustering data and show how to include correlations in both the angular correlation function and the spatial power spectrum. Inclusion of the full covariance matrices loosens the constraints on large-scale structure inferred from the Automated Plate Measuring (APM) survey by over a factor of 2. We present a new inversion technique based on singular-value decomposition that allows one to propagate the covariance matrix on the angular correlation function through to that of the spatial power spectrum and to reconstruct smooth power spectra without underestimating the errors. Within a parameter space of the cold dark matter (CDM) shape Γ and the amplitude σ8, we find that the angular correlations in the APM survey constrain Γ to be 0.19-0.37 at 68% confidence when fitted to scales larger than k = 0.2 h Mpc-1. A downturn in power at k < 0.04 h Mpc-1 is significant at only 1 σ. These results are optimistic, since we include only Gaussian statistical errors and neglect any boundary effects.

The sedentary multifrequency survey - I. Statistical identification and cosmological properties of high-energy peaked BL Lacs

We have assembled a multifrequency data base by cross-correlating the NRAO VLA Sky Survey (NVSS) catalogue of radio sources with the RASSBSC list of soft X-ray sources, obtaining optical magnitude estimates from the Palomar and UK Schmidt surveys as provided by the Automated Plate Measurement (APM) and COSMOS on-line services. By exploiting the nearly unique broad-band properties of high-energy peaked BL Lacs (HBL), we have statistically identified a sample of 218 objects that is expected to include about 85 per cent of BL Lacs and that is therefore several times larger than all other published samples of HBLs. Using a subset (155 objects) that is radio-flux-limited and statistically well-defined, we have derived the V/Vm distribution and the log N—log S of extreme HBLs (fx/fr3×10−10 erg cm−2 s−1 Jy−1) down to 3.5 mJy. We find that the log N—log S flattens around 20 mJy and that 〈V/Vm〉=0.42±0.02. This extends to the radio band earlier results, based on much smaller X-ray-selected samples, about the anomalous cosmological observational properties of HBLs. A comparison with the expected radio log N—log S of all BL Lacs (based on a beaming model) shows that extreme HBLs make up roughly 2 per cent of the BL Lac population, independently of radio flux. This result, together with the flatness of the radio log N—log S at low fluxes, is in contrast with the predictions of a recent model that assumes an anticorrelation between peak frequency and bolometric luminosity. This scenario would in fact result in an increasing dominance of HBLs at lower radio fluxes; an effect that, if at all present, must start at fluxes fainter than our survey limit. The extreme fx/fr flux ratios and high X-ray fluxes of these BL Lacs makes them good candidate TeV sources; some of the brighter (and closer) ones are possibly detectable with the current generation of Cerenkov telescopes. Statistical identification of sources based on their location in multiparameter space, of the kind described here, will have to become commonplace with the advent of the many large, deep surveys at various frequencies currently scheduled or under construction.

An automated search for nearby low-surface-brightness galaxies -- I. The catalogue

We have developed an automatic technique to search for low-surface-brightness (LSB) galaxies in the local Universe (v ≤ 5000 km s−1) using the automated plate measuring (APM) scan data of UK Schmidt photographic plates. We optimized our technique and selection criteria by surveying the known LSB galaxies in the Fornax cluster. Plate-to-plate magnitude calibrations were carried out using independent CCD sequences. The galaxies we detected are brighter than 20 B mag, have scalesizes greater than 3 arcsec and a central surface brightness fainter than 22.5 mag arcsec−2. In total 2435 LSB galaxies were detected over a total area of 2187 deg2. The survey covers the Fornax cluster, NGC 1400, Sculptor and Dorado groups and the field between. We detect on average 32 LSB galaxies per 5.8 °× 5.8 ° field. We have estimated the background (v>5000 km s−1) contamination in three ways: by numerical modelling, using a limited redshift sample and comparing our Fornax data with those of Ferguson. The results indicate a contamination of about 19 galaxies per field.

Quasars from the 7C survey -- I. Sample selection and radio maps

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151 MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10 h 28 m, Dec +41 and covers 0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting `7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S151 > 100 mJy; POSS-I E-plate magnitude E approx R < 20; and POSS-I colour (O - E) < 1.8; the effective area of the survey drops significantly below S151 approx 200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al. (1998b), that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.