The applet now loading into this page is intended help you to plan your Michelle observations on UKIRT or Gemini by predicting the spatial and spectral coverage at the array for any of Michelle’s wide range of configurations. The sensitivity figures should be taken only as an optimistic guide, as the true performance won’t be known until February 2004. If you have any problems, the same information is available in the user guide here.
To use the applet, select the telescope and optical path (imaging or spectroscopy) you plan to use, and choose the observing wavelength using the scroll bar controls. The spectral resolving power and wavelength coverage for the observation can then be refined by selecting the appropriate diffraction grating and entrance slit. The diffraction order, grating blaze wavelength and the angle of incidence (in degrees) of light onto the grating are all reported in the box next to the grating selector.
For imaging, the slit and grating selection controls are replaced by a filter control, with the filters denoted by their centre wavelength and fractional bandwidth (expressed as a percentage of the centre wavelength). This control has now been updated to reflect the filters that are actually fitted in the instrument (see also the filter manifest in the filter wheel service manual). As more filters become available, they will be added to the planner.
The lower half of the panel shows the value of a selection of instrumental parameters as a function of position on the detector array,
Pixel Column – The data in this column refers to the left (short wavelength), centre or right hand column of the SBRC 320 column, 240 row detector array.
Wavelength[mm] – The detector’s wavelength coverage. You can also select Velocity [km/sec] as the displayed unit. Values outside the N and Q windows of good atmospheric transmission are now displayed in red text.
Spectral Resolving Power – The dimensionless ratio l/Dl. Spectral Bandwidth [mm] and Velocity Resolution [km/sec] units are also provided via the choice control.
Optical Efficiency [%] – This is Michelle’s approximate, estimated efficiency at transporting photons arriving at the top of the atmosphere from an astronomical target through to the detector. At certain wavelengths, especially when using the echelle, the wavelength dependent efficiency of the diffraction gratings will cause this figure to drop well below its maximum value of around 34% for the spectrometer. When the Fraction of PSF Sampled [%] item in this control is selected for the spectrometer, it calculates the fraction of the light (within the first dark diffraction ring) from a point source that is vignetted by the entrance slit.
FWHM in Spatial Direction [pixels] – This is the full width of the contour of half-maximum intensity for a point source, along a column for the spectrometer and across a diameter for the imager. The Area of Detector Sampled [pixels] is a figure that is used in calculating the instrument sensitivity, and refers to the unvignetted region used to calculate the ‘Fraction of PSF Sampled’ figure above.
Flux for S/N=1 in 1 minute [mJy] – This control displays an optimistic estimate of the predicted sensitivity for the planned observation in a variety of formats. The flux refers to the total flux from the object, incident at the top of the atmosphere. The Flux Density for S/N=1 in 1 minute [mJy/square arcsecond] refers to extended sources. NB: The sensitivities for the two broadband N and Q filters are very uncertain, and depending on testing at UKIRT in February 2004, could be twice as poor as given here.