# HPT: A High Spatial Resolution Multispectral Sensor for Microsatellite Remote Sensing
## 2 Sensor Characteristics
### High Precision Telescope (HPT)
Based on the results of thermal simulation analysis, both the tip of the telescope tube and the reverse side of the secondary mirror, which are exposed to the outside of the satellite, are covered by multilayer insulation to maintain the mirrors at room temperature passively. At room temperature, the Strehl ratio of the optical system measured with a laser interferometer is 0.92, which satisfies the criterion for a diffraction-limited system (>0.8). A monochromatic CCD image sensor module is used for imaging in the four spectral bands. Generally, CCD image sensors consume more power, but are more sensitive than complementary metal \begin{table}
\begin{tabular}{c c} \hline
**Size** & **380 mm \(\times\) 161 mm \(\times\) 124 mm** \\ \hline Weight & 3 kg \\ Focal length & 1000 mm \\ Aperture diameter & 100 mm \\ Ground sample distance & 4.6 m (at nadir from 628 km altitude) \\ Field of view & 0.28\({}^{\circ}\)\(\times\) 0.21\({}^{\circ}\) (3.1 \(\times\) 2.3 km at nadir from 628 km altitude) \\ \hline \multicolumn{2}{c}{Blue: 400–510 nm} \\ \multicolumn{2}{c}{Green: 520–600 nm} \\ \multicolumn{2}{c}{Red: 610–650 nm} \\ \multicolumn{2}{c}{NIR: 401 bands selectable at 1-nm intervals from 650 to 1050 nm} \\ \hline Image size & 659 \(\times\) 494 pixels \\ Data quantization & 10 bit \\ \hline \end{tabular}
\end{table} 
Table 1: Specifications of the HPT. oxide semiconductor (CMOS) image sensors. The most important characteristic of a CCD image sensor for satellite imaging is a global shutter that can capture the entire frame at the same instant, although many CMOS image sensors use a rolling shutter. The CCD module T065 manufactured by Watec Co., Ltd. (Tsuruoka, Japan) was developed originally for the optical mission payload instrumentation of the SPRITE-SAT. The unit cell size of this CCD module is 7.4 \(\upmu\)m\(\times\) 7.4 \(\upmu\)m, and the effective image size is 659 \(\times\) 494 pixels. The gain and exposure time of the CCD module are changeable from \(-\)6 to +36 dB and from 20 \(\upmu\)s to 34 s, respectively, enabling the HPT to be applied to a wide range of spectral radiances on the Earth's surface. An important function supported by this CCD module is the external trigger shutter that allows the synchronization of the three CCD modules for the blue, green, and red bands, in order to acquire a true color composite image. In the HPT on the RISING-2 microsatellite, LCTF technology was applied for the first time to a spaceborne multispectral sensor through collaboration with the Research Institute for Advanced Liquid Crystal Technology at the Aomori Support Center for Industrial Promotion in Japan. The LCTF is a type of optical band pass filter that uses liquid crystal for the birefringent plates of the Lyot filter, and the LCTF technology has been used for many applications, e.g., agriculture, healthcare, archaeology, and art (see review by [11] and references therein). The LCTF is composed of several stacked layers of liquid crystal sandwiched between crossed polarizers, and the transmission wavelength of the LCTF is controlled by square-wave voltages that are applied to each layer. On the RISING-2 microsatellite, the LCTF and the voltage-controlling circuit are separated into the HPT and the SHU, respectively. The LCTF developed for the HPT (Figure 4) has dimensions of 30.7 mm \(\times\) 30.3 mm \(\times\) 30.3 mm, and it weighs 80 g. The power consumption of the voltage-controlling circuit is 0.2 W. These features enhance the compatibility of the LCTF technology with nano/microsatellites. The LCTF is used for wavelength scanning in the NIR band, and the central wavelength of the NIR band is electrically selectable at 1-nm intervals from 650 nm to 1050 nm. In terms of the number of spectral bands, the HPT has 401 bands in the NIR. The peak transmittance and the full width at half maximum increase with the central wavelength from 7% and 12 nm (at 650 nm) to 29% and 30 nm (at 1050 nm), respectively.