When OASIS is used in conjunction with GLAS (the laser guide star AO system), a special 'notch' filter is inserted into the light path. This filter selectively reflects light at ~5145Å (the wavelength of the laser) towards the wavefront sensor, while transmitting the remainder of the light towards the science arm.

The key characteristics of the notch filter are:

• The filter is centred at 5145 Å, where the transmission to the science arm is ~ 1 x 10^-5.
• At ±75 Å from this central wavelength, the transmission is ~80%. (This is the range illustrated in the following figure.)
  
• At wavelengths outside the notch (4000 Å - 2.5 μm), the transmission is ~92%.

The following figure illustrates the wavelength range (green shaded region) where the transmission of the filter is below ~80% . Four spectral configurations are affected (HR502, HR530, MR516 and LR610).