Grating line scan

In this example we consider a grating line scan of an unresolved line at 63 m. The continuum flux of the source is 50 Jy at this wavelength, the integrated line flux is W/m. The line should be observed with 4 times oversampling, and with 4 resolution elements on either side of the line. The requested S/N on the line is 10.

1. Calculate the required total S/N on line plus continuum (). For this it is important to realise that 50 Jy at 63 m corresponds to W/mm. The total S/N can then be calculated (see equation 6.19 in the LWS Observers Manual, page 28):

   

2. Calculate the S/N per ramp using equation 6

   

3. Calculate the number of required ramps using equation 8:

   

   The number of required ramps is thus 6.

4. Calculate the integration time per spectral point using equation 9. This gives in this case sec.
5. Calculate the number of points in the scan using equation 10. This gives .
6. Calculate the total integration time for the observation using equation 12: sec.
7. Finally estimate the total time for the observation, taking the overheads into account (including the 180 seconds slewing overhead), using equation 13 and section 6.2. The overheads for this observation are: 180 sec for the slew to the target, 18 sec for the instrument setup, 20 sec for the instrument switch off and 308 sec for the illuminator flashes. The total time for the AOT thus is: sec.