n 
SS 
SS 
zht 
he 
IR 
Integrating Beam Collimator 
Sphere Splitter (VIS) 
Detector Detector 
Prisms (VIS) (SWIR) & Cooling 
  
Pointable Entrance Ground 
Mirror Imager 
Collimator Prisms Aspheric 
(SWIR) Mirrors 
  
approx. 60 am 
Figure 2. Schematic APEX Layout 
Finally detector lenses are selected to provide spatially and 
spectrally resolved images on the matrix detectors. The VIS 
detector is tilted by 30° to correct for image height 
differences at the individual wavelengths. The IR detector 
lens consists of three off-axis aspherical mirrors. The use of 
reflective elements simplifies the color correction, supports 
the compensation for image bending and improves the total 
throughput for better signal to noise ratios. 
2.2 Detectors and Front End Electronics 
In the APEX instrument two detectors are needed to cover 
the specified spectral range: 
* A VIS detector, sensitive in the spectral range 450 - 950 
nm (Si CCD) 
* An IR detector, sensitive in the spectral range 900 - 
2500 nm. (Choice of HgCdTe or InSb detectors) 
The detectors for the APEX instrument have to be two 
dimensional array detectors. To meet the specification on 
resolution, in the spectral and spatial direction the detector 
arrays must at least have 1000 pixels in spatial, and 200 in 
spectral direction. 
The FEE for the visual CCD detector will be realized by 
using photocapacitors and CCD readout structures. The 
analogue output signal of the CCD is sampled and 
converted to digital data by an ADC. 
The HgCdTe detector will be read out by a CMOS circuit. 
The array consists of a number of addressable pixels using 
MOS transistors as switches, controlled by an x and y 
multiplexer. 
The detectors will be glued in their holders. Alignment 
(translation) will be done making use of shims. To reduce 
dark current noise, the IR detector needs to be cooled to a 
temperature of -80? C. The cooling is achieved by means of 
a cooling engine. Care has to be taken that no vibrations 
caused by the cooling engine disturb the image quality of 
the system. 
23 Electronics Unit 
The electronics unit consists of the video electronics unit, 
the framegrabber unit, the data storage unit and the user 
interface unit which incorporates four basic operating 
modes to control the APEX instrument. 
* Mission preparation — before a data acquisition mission, 
all parameters and sequences are determined and stored 
in the system 
* Calibration mode - the calibration sequence is initiated 
and the calibration data are stored 
* Acquisition mode - the instrument stores the data 
coming from the detectors 
e Stand-by mode - all the values stored or transferred 
into the instrument stay active but no acquisition is 
done 
The proposed video chain of the systems consists. of an 
analogue ASIC that grants for the necessary stability. The 
next device is an internal offset loop correcting the offset 
recorded from some darkened border pixels of the 
detectors. Finally the ADC converter will take care of the 
signal conversion to be fed into the framegrabber. The 
framegrabber itself is capable of transferring 200 lines of 
1000 12 bit pixels each 40 ms. This throughput corresponds 
to a data-rate of 60 Mbit/sec. If the 300 channel option is 
used, the data-rate will increase to 90 Mbit/sec. 
In the mission preparation phase, the operator of the 
instrument must select 200 channels that will be recorded 
out of 400 lines accumulated by the detectors. The channel 
selection is stored in a PROM and multiple PROM's can be 
selected within one data acquisition flight. An alternative 
concept foresees the readout of 300 predefined channels, 
making the selection procedure obsolete. The calibration 
mode will be used in the laboratory where the operator has 
the choice of addressing basically all moving parts and 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 
33