A.

Overall Configruration

The overall configuration of the camera is present below.

B.

Optical system Design and Verifcation

The optical system of PMS camera has a long focal length, small big F number and wide spectral. Table I gives the specifications of the optical system. It should get high image quality at the 77lp/mm nyquist frequency. Also the method of splitting the light into multibands and the register of multibands should be considered. Optical manufacturing and alignment are two of the most important restriction factors. All these factors add the difficulties to design the optical system. Balance all the factors and choose the catadioptric Schmidt optical system. The optical elements include: swing reflective mirror, window, 45 ° reflective mirror, spherical reflective mirror, lens and prism. There are two imaging channels, one for panchromatic band and the other for multibands. The prism is used to split the light into three multibands. Fig. 2 gives the drawing of the optical system. Fig. 3 gives the prism sketch. Fig. 4 gives the design result of the optical system. Fig. 5 gives picture of the telescope tested in the lab.

Figure 2.

Optical system of PMS camera

Figure 3.

Light splitted in the Prism

Figure 4.

Desgin Results of MTF of Four bands

TABLE I.

OPTICAL CHARACTERISTICS

From the MTF curve above, the design MTF is almost coincide with the diffractive limit.

The Optical System Performance was estimated by using a ZYGO GPI/XP interferometer that works at the 632.8nm wavelength. The measurement of the optical Performance was performed only in this wavelength and it was used as a preliminary analysis that was carried out during the alignment phase.

Fig. 5 gives the test setup of the optical MTF.

Figure 5.

The Setup of the Optical MTF Test

From the Fig. 6, RMS of B1 band is 0.049 λ. The optical MTF is 0.47 at 77 lp/mm. The requirement is 0.45. The other three bands also reach the requirements.

Figure 6.

The Optical MTF Test Interface of B1 band

A high quality optical system is designed, aligned and tested. The test results reach the requirements.

C.

Design of the Opti-mechanical Body

From the design result of the optical system, the body is long and thin. It is difficult to draw the sketch. In order to have the sight ability of ±32°, the swing mirror needs to swing ± 16 ° off the track. The swing mirror is a flat and reflective mirror and it only provides the rotary direction. So the position accuracy is not very high as other optical elements. From the above analysis, we can get that the swing mirror and lens are separated mounted in the satellite. There is no fix connection between the two parts. Fig.7 gives the two separate assemblies. Fig.8 gives the whole opti-mechanical body.

Figure 7.

The Two Seperation Assemblies

Figure 8.

The Opti-Mechanical Body

The main function of the swing mirror is to swing ± 16 °. The main function of lens is to collect the light that reflected by the swing mirror and transmit the light into electrical signal. The separation mount also saves about 60kg mass. The mass of opti-mechanical Body is 218kg.

D.

Design and Verification of Thermal Control

The thermal control of the camera consists of passive thermal control and active thermal control. Passive thermal control is composed multi-layer heat insulation materials, thermal control coat and reasonable heat emission plane. Active thermal control means the heating loops on the steer mirror, lens and focal plane. The function of active thermal control is to ensure the suitable work environment temperature of the optical mechanism main body with the thermal controller.

The requirements of the camera are above:

In order to get a better image, there is a refocus mechanism with the prism, CCD and driving circuit boards. The prism and CCD will move some distance relative to the main body. But in order to keep the temperature of the CCD and prism, the heat generated by CCD should be dissipated. Conventional ways to dissipate the heat such as heat pipe or copper can not be used because they are fixed. Conductive rope is manufactured to solve this problem. The conductive is made of copper and has a high conductive efficiency. It is also like a rope and has freedom to move.

Fig.9 gives the scheme of the dissipation chain of the CCD. Table II gives the results tested in the lab.

Figure 9.

The dissipation chain of the CCD

Table II.

TEST RESULTS OF THERMAL CONTROL

A.

System MTF Test

The MTF is of high importance on the evaluation of the spatial frequencies transferred from object to the image of an imaging optical system.

The MTF is equivalent to a combination of the MTF of optical system (MTF_lens), CCD (MTF_CCD), amplification and signal processing electronic (MTF_electronics)

The system MTF test is a very important phase. The system MTF was obtained by using an on-axis collimator (EFL =7000 mm) and a square target. This target is positioned in the focal plane of the collimator and then is moved in steps of 500μm, which allows for determining the CTF of the target image around a specific pixel in discrete steps of 4.2 μm. MTF is calculated by the equation 2.

Table III gives the test results of globe MTF of PMS. Use equation 1 to get the prediction value.

Table III.

OPTICAL CHARACTERISTICS

B.

Calibration Test

Calibration test is to verify the radiometric quality of the camera. We can get the absolute calibration coefficients, relative calibration coefficients, linearity and S/N of the camera by the calibration test. A high steady and precision integral sphere is used to conduct the test. Fig.10 gives the test setup.

Figure 10.

Integral Sphere Test

Table IV.

ABSOLUTE CALIBRATION COEFFICIENT

Fig.11 gives the response curve of B2. Table IV gives the test result of absolute calibration coefficient at default gain.

Figure 11.

Response curve of B2 band at different radiance