The wavelength switching observing mode has been re-designed, this option is made available only in mapping mode. Based on the latest ground calibration campaigns we expect strong signal transients in wavelength switching mode. This is due to i) strong gradients in the spectrometer relative reponse function over the scale of the switching amplitude, and ii) the spectral line itself could generate transients when switching on/off the profile. This instrumental effects could be eliminated on a satisfactory level if we measure the response of the system on an emission-free reference position using exactly the same technique as on-source. Data obtained on off-position is subtracted from on-target frames. This technique could be further refined during AOT validation in the PV Phase, especially for bright lines.

- Implication for user: In the last AOR update campaign (Phase2) HSC asked proposers to enter a 2x2 raster map in order to mimic the current scheme for an atomic observation (2x1 raster + 2 off-positions). Since Phase 2 the grating switching cycle has been updated as well, the atomic block durations are evolved as

Taking the example of the shortest observation, this 4 position raster had the duration of 724 sec (252s on-source, 163s calibration, 292s instrument overhead, 180s observatory overhead) in HSpot v3.4:

In the new implementation the same AOR has a duration of 702 sec:

Now, in Phase 3, the 2x2 raster can be reduced to 2x1 because the real off-position is implemented. This has the following implications on timing:

Please note, in the auto-updated AORs the reference position is set to its defaults: zero degrees Ra/Dec offsets. A proper off-position has to be selected for each wavelength switching AORs by the observer. The slewing time to the off-position will increase the observation overhead, in the above example if the emission-free reference field would be 30 arcminutes away the total duration of the AOR would increase to 620 seconds. Concerning raster positions, for point-source observations a 2x1 raster can be specified with step size of few arcseconds (tipically 4-9"). In such a case the same sky position is seen twice and the corresponding rms sensitivity increases by sqrt(2).