Before reaching the detector, photons collected by OM are intercepted by one of the elements of the Filter Wheel. These are six wide-band filters, called (in increasing wavelength order) UVW2, UVM2, UVW1, U, B and V. Another filter, called White covers the full spectral band from UVW2 up to V (1800 Å to 6000 Å approximately). There is a Blocked (opaque) filter used in some engineering modes and also to protect the OM when there is a too bright object in the field of view. All these filters can be used in both observing modes, image and/or fast mode, with all the configurations described in section § 6.1.1.
The filter wheel has in addition two grisms, Grism1 or UV grism and Grism2 or Visible grism, which can be used to obtain low resolution spectra in the range 1800 to 3600 Å and 3000 to 6000 Å, respectively. Both grisms are used only in image mode. There is a user-defined default configuration intended for single object spectroscopy, in which a rectangular window is defined so that it contains the spectrum of the target sitting at the boresight. Alternatively, the grisms can be used in full frame low resolution to register the spectra of all objects in the field of view. Data obtained with the grisms are therefore image data with the same characteristics as defined above.
It should be noted that in early phases of the project some observations with the OM grisms were done using the image default configuration described above (§ 6.1.1). These few cases cannot be dealt with SAS. Even if grism data can be considered as normal image data, only the observations obtained as single object spectroscopy or in full frame as explained before, can be processed by SAS.