Reconstruction techniques for intrinsic quasar continua are crucial for the precision study of Lyman- \alpha ( Ly- \alpha ) and Lyman- \beta ( Ly- \beta ) transmission at z > 5.0 , where the \lambda < 1215 Å emission of quasars is nearly completely absorbed .
While the number and quality of spectroscopic observations has become theoretically sufficient to quantify Ly- \alpha transmission at 5.0 < z < 6.0 to better than 1 \% , the biases and uncertainties arising from predicting the unabsorbed continuum are not known to the same level .
In this paper , we systematically evaluate eight reconstruction techniques on a unified testing sample of 2.7 < z < 3.5 quasars drawn from eBOSS .
The methods include power-law extrapolation , stacking of neighbours , and six variants of Principal Component Analysis ( PCA ) using direct projection , fitting of components , or neural networks to perform weight mapping .
We find that power-law reconstructions and the PCA with fewest components and smallest training sample display the largest biases in the Ly- \alpha forest ( -9.58 \% / +8.22 \% respectively ) .
Power-law extrapolations have larger scatters than previously assumed of +13.1 \% / -13.2 \% over Ly- \alpha and +19.9 \% / -20.1 \% over Ly- \beta .
We present two new PCAs which achieve the best current accuracies of 9 \% for Ly- \alpha and 17 \% for Ly- \beta .
We apply the eight techniques after accounting for wavelength-dependent biases and scatter to a sample 19 quasars at z > 5.7 with IR X-Shooter spectroscopy , obtaining well-characterised measurements for the mean flux transmission at 4.7 < z < 6.3 .
Our results demonstrate the importance of testing and , when relevant , training , continuum reconstruction techniques in a systematic way .