We present the results of a programme of thermal-infrared imaging of nineteen z \sim 1 radio galaxies from the 3CR and 3CRR ( LRL ) samples .
We detect emission at L ^ { \prime } ( 3.8 \mu m ) from four objects ; in each case the emission is unresolved at 1-arcsec resolution .
Fifteen radio galaxies remain undetected to sensitive limits of L ^ { \prime } \approx 15.5 .
Using these data in tandem with archived HST data and near-infrared spectroscopy we show that three of the detected ‘ radio galaxies ’ ( 3C 22 , 3C 41 , and 3C 65 ) harbour quasars reddened by A _ { V } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 % pt \hbox { $ \mathchar 316 $ } } 5 .
Correcting for this reddening 3C 22 and 3C 41 are very similar to coeval 3C quasars , whilst 3C 65 seems unusually underluminous .
The fourth radio galaxy detection ( 3C 265 ) is a more highly obscured ( A _ { V } \sim 15 ) but otherwise typical quasar which previously has been evident only in scattered light .
We determine the fraction of dust-reddened quasars at z \sim 1 to be 28 ^ { +25 } _ { -13 } % at 90 % confidence .
On the assumption that the undetected radio galaxies harbour quasars similar to those in 3C 22 , 3C 41 and 3C 265 ( as seems reasonable given their similar narrow emission line luminosities ) we deduce extinctions of A _ { V } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 % pt \hbox { $ \mathchar 318 $ } } 15 towards their nuclei .
The contributions of reddened quasar nuclei to the total K -band light ranges from \sim 0 per cent for the non-detections , through \sim 10 per cent for 3C 265 to \sim 80 per cent for 3C 22 and 3C 41 .
Correcting for these effects does not remove the previously reported differences between the K magnitudes of 3C and 6C radio galaxies , so contamination by reddened quasar nuclei is not a serious problem for drawing cosmological conclusions from the K – z relation for radio galaxies .
We discuss these results in the context of the ‘ receding torus ’ model which predicts a small fraction of lightly-reddened quasars in samples of high radio luminosity sources .
We also examine the likely future importance of thermal-infrared imaging in the study of distant powerful radio sources .