# Small Satellite Tools for High-Resolution Infrared Fire Monitoring
## 3 The FireBIRD Main Payload
### Bi-spectral Sensors of TET-1 and BIROS
#### 3.1.1 Design and Cooling of the Bi-Spectral Sensor Heads  
Figure 4 shows a semi-transparent perspective view of the three sensor heads of the BIROS main payload, i.e., the multi-spectral camera system.  
On the right side of Figure 4 is shown the MIR sensor head with its compact entrance lens and its detector cooler assembly, the so-called Integrated Detector Cooler Assembly (IDCA). On the left side of Figure 4 is shown the TIR sensor head with the baffle attached to its entrance lens and its IDCA. In this view, both IR sensor heads are missing the flaps for radiometric on-board calibration, which are shown in Figure 1.  
In the center of Figure 4 can be seen the VIS-NIR sensor head, which is described in the following subsection.  
Figure 5 shows a sectional view of the sensor heads of the BIROS main payload, providing a detailed insight into its opto-mechanical interior. Among other things, one can see:  
* on the right and left the yellow-colored so-called baffles, which protect the entrance optics of the MIR and TIR sensor head from stray light, and
* the gray-colored Dewar vessels with their brown and gray input windows, behind which the chips of the CMT line detectors, represented by two small circles, are \"flying\" in the focal plane of the input lenses.
* Comments on Figure 5:
* The use of the term \"flying\" is due to the fact that the cold fingers of the small refrigeration machines are not shown in this sectional view, but the CMT detector chips are mounted on the tips of the two cold fingers.  
Figure 4: Semi-transparent view of the three sensor heads of the BIROS main payload, comprising: the MIR sensor head with its compact entrance lens and its Integrated Detector Cooler Assembly (IDCA) on the right, the TIR sensor head with its entrance lens (plus baffle in its housing) and its IDCA on the left, and the VIS-NIR sensor head with its entrance lens (plus open baffle) and three spectral filters in front of the VIS-NIR focal plane, which is equipped with three CCD linear arrays, in the center.  
(2) Numerous details of the VIS-NIR camera--shown in the center of Figure 5--will be discussed further below.  
The MIR detector is cooled to 80.6 K (\(-192.4\)\({}^{\circ}\)C) by means of its small refrigeration machine, and the TIR detector is cooled to 71.2 K (\(-201.8\)\({}^{\circ}\)C) by its small refrigeration machine. Figure 6 shows the time diagram of the cooling processes of the BIROS MIR and TIR sensor heads.  
Figure 5: Sectional view of the sensor heads of the BIROS main payload, providing a detailed insight into its opto-mechanical interior with: the MIR sensor head with its entrance lens (plus baffle) and its IDCA on the right, the TIR sensor head with its entrance lens (plus baffle) and its IDCA on the left, and the VIS-NIR sensor head with its entrance lens (plus baffle) and three spectral filters in front of the VIS-NIR focal plane, which is equipped with three CCD linear arrays, in the center.  
Figure 6: Time diagrams of the cooling processes of the BIROS MIR/MWIR and TIR/LWIR sensor heads.  
To ensure stable in-flight working temperatures of the CMT detectors from the beginning of each data acquisition cycle, the small Stirling coolers are switched on about 20 min before the start of each Earth observation data registration.