  Attosecond photoelectron spectroscopy is often performed with interferometric
experimental setups that require outstanding stability. We demonstrate and
characterize in detail an actively stabilized, versatile, high spectral
resolution attosecond beamline. The active-stabilization system can remain
ultra-stable for several hours with an RMS stability of 13 as and a total
pump-probe delay scanning range of \sim 400 fs. A tunable femtosecond laser
source to drive high-order harmonic generation allows for precisely addressing
atomic and molecular resonances. Furthermore, the interferometer includes a
spectral shaper in 4f-geometry in the probe arm as well as a tunable bandpass
filter in the pump arm, which offer additional high flexibility in terms of
tunability as well as narrowband or polychromatic probe pulses. We show that
spectral phase measurements of photoelectron wavepackets with the rainbow
RABBIT technique (reconstruction of attosecond beating by two photon
transitions) with narrowband probe pulses can significantly improve the
photoelectron energy resolution. In this setup, the temporal-spectral
resolution of photoelectron spectroscopy can reach a new level of accuracy and
precision.
