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GLAS Notch Filter

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).

OASIS spectral configurations (with notch)
In general, it is possible to remove the notch filter from the light path if the blocked wavelengths are required for science observations. However applicants for OASIS should check the details of the specific call for observing time, as well as the OASIS web pages, for cases where this option is precluded. If the filter is removed, then wavefront sensing is only possible using a natural guide star rather than the GLAS laser guide star system.

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Contact:  (OASIS Instrument Specialist)
Last modified: 17 December 2010