= Spectroscopic test of the Twin Quasar lens
The two resolved <quasar> images of 0957+561 provide an observational question: are they separate sources, or different light paths from one source? In May 1979, <spectrophotometry> used the McDonald Observatory 2.7 m <reflecting telescope>, the UVITS <spectrograph>, and an intensified dissector scanner at the f/9 focus of the <Cassegrain reflector>. A 600-groove-per-millimetre <diffraction grating> produced 4100-7100 <ångström> scans sampled every 2 <ångströms>, with about 7 <ångström> resolution over most of that range. Two <spectrograph slits>, about 52 arcseconds apart, recorded object and <sky background> simultaneously for subtraction. Interleaving observations of the two images reduced instrumental and atmospheric gain changes.<Spectrophotometry of the Double QSO, 0957 + 561 by Beverley J. Wills and D. Wills>[Section II, printed p. 2: instrument, grating, sampling, resolution, two apertures, sky subtraction and interleaving]{ref}
The readout was received light versus <wavelength> for each image. Closely matching <absorption line> <redshifts> supported a common source and absorber; differences in continua and line strengths motivated differential <interstellar extinction>. Spectral agreement supplied evidence for <gravitational lensing>, rather than proving the <point-mass gravitational lens> model. The authors discussed a physical double and discrepant optical and radio light ratios. Their note added in proof attributed the redder spectrum to foreground <galaxy> contamination and emphasized an extended <mass> distribution.<Spectrophotometry of the Double QSO, 0957 + 561 by Beverley J. Wills and D. Wills>[Abstract, printed p. 1; section III, pp. 4-5; section IV, pp. 6-7; note added in proof, p. 7, and Figure 3, p. 8: spectral agreement, model limitations and updated galaxy-contamination interpretation]{ref}
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