code: import numpy as np
  from . import hst_observation, spectroscopy
  
  datasets = ['ld9m10ujq', 'ld9m10uyq']
  visit1 = hst_observation.Visit(datasets, 'cos', prefix='data/')
  
  line_list = spectroscopy.COSFUVLineList(wavelength_shift=.0,
                                          range_factor=1.0).lines
  
  tr = 'Si III'
  line = 0
  ref_wl = line_list[tr][line].central_wavelength
  
  shift = np.array([-23.623059845212502, -23.37932521889708])
  
  orbit_list = [visit1.orbit['ld9m10ujq'], visit1.orbit['ld9m10uyq']]
  test = hst_observation.CombinedSpectrum(
      orbit_list, ref_wl, 'cos', velocity_range=[-100, 100], doppler_corr=shift)
  
apis: numpy.array