$\lambda5007$ lines of the calibrated spectra are constant with a mean of $0.96\times10^{-13}{\rm~erg~s^{-1}~cm^{-2}}$ and a scatter within 2.5%. The slit was fixed at a projected width of $2.5^{\prime\prime}$ and the spectra were extracted using a uniform window of $8.5^{\prime\prime}$. [The raw spectroscopic data are reduced using standard IRAF version 2.16 routines before absolute flux calibration, which includes bias subtraction, flat-field correction, and wavelength calibration.]{} The details for the data reduction and analysis will be presented in a separate paper [that describes the reverberation mapping analysis (see also @Du2014)]{}. We obtained [100 spectroscopic observations in total]{}, with a typical exposure [time]{} of 30 minutes. [The 5100 [Å]{} continuum flux is measured in a band of 20 [Å]{} wide around 5100 [Å]{} in the rest frame. To be consistent with the other datasets, the H$\beta$ flux is measured as follows: we first subtract the continuum underneath the H$\beta$ line by interpolating between two continuum bands 4760-4790 [Å]{} and 5000-5020 [Å]{}. We then integrate the continuum-subtracted H$\beta$ flux between 4810 [Å]{} and 4910 [Å]{}.]{}

The [previous published]{} spectroscopic observations were only to 2005, so there is no temporal overlap with our observations. As a result, to align the fluxes with the historical data, 