2.1.

APIS Optical System

The optical system was a refractive objective working in the visible spectral band (450 to 650 nm). It was designed using the optical design software CODE V from Synopsis^® Inc. The objective consisted of a triplet with a conic surface and two meniscus lenses close to the focal plane (see Fig. 4). This configuration reduced residual aberrations in the image, such as astigmatism, spherical, and field curvature.

Fig. 4

APIS camera optical layout.

The system was designed with an external aperture stop and three different glasses from the Schott catalog: NFK51, NSF57, and NLASF41.

Figure 4 shows the optical layout which includes the protective window of the selected detector for APIS camera (the IBIS5-B-1300¹³ CMOS image sensor).

The optical system with a 2.5 $f$-number (20-mm focal length and 8-mm pupil diameter) was optimized, using the modulation transfer function (MTF) as merit function, for an asymmetric field of view (FOV) of $12.16\mathrm{deg}\times 9.25\mathrm{deg}$ and a pixel size of $6.7\mu \mathrm{m}$. This FOV was equivalent to a rectangular Earth swath between $128\mathrm{km}\times 97\mathrm{km}$ and $170\mathrm{km}\times 129\mathrm{km}$. It also provided a resolution between 201 and 268 m, depending on the orbit height.

Some additional studies, such as thermal behavior, tolerances, and sensitivity analysis, signal-to-noise ratio (SNR) and ghost images, were performed during the critical design phase because of their implications in the mechanical design and manufacturing phases. The main conclusions obtained from these studies are detailed from Secs. 2.1.1 to 2.1.4.

2.1.4.

Ghost images

This study was performed with the Advanced Systems Analysis Program (ASAP) from Breault Research Organization Inc. The objective of the study was to analyze the influence in the APIS point spread function (PSF) produced by retroreflections on the optical surfaces. The study consisted of finding the most relevant energetic paths and analyzing the relation between the energy collected by the ghost images and the PSF of the system.

The main contributions to the generation of ghost images were related to the interaction of the CMOS reflections and the flat surfaces (filter and detector window), and retroreflections of the meniscus lenses. The study verified that using high antireflection coatings in the optical components ($T>99\%$) resulted in a flux ratio of the ghost images/signal of ${10}^{-3}$.

Figure 5 shows the global PSF flux in the focal plane (with the ghost process included) for a 100-W energy incident. Figure 6 shows the corresponding flux of the ghost image process separately.

Fig. 5

Global PSF including the ghost image process (0.657046 W total flux).

Fig. 6

Ghost image process (0.000476 W total flux).

Therefore, the influence of the ghost images was negligible. Ghost images were on the same order as the inherent noise of the system.

2.2.

APIS Electronic Design

The selected detector for the APIS camera was the IBIS5-B-1300 model from Cypress Inc. (currently ON Semiconductor^®) in a monochrome version. The IBIS5-B-1300 is a CMOS image sensor that integrates several functionalities. It performs as an analog-image acquisition system, a digitizer, and a digital signal processing system. The main key performance parameters of the sensor can be seen in Ref. 13.

The APIS camera was optimized for a focal plane size of $4.3\mathrm{mm}\times 3.2\mathrm{mm}$ corresponding to a region of interest (ROI) of $640\times 480\text{pixels}$ in order to boost the readout speed. The sensor worked in snapshot shutter mode.

The electronic design of APIS was divided in two boards as shown in Fig. 7: the APIS focal plane board, which includes the CMOS image sensor and a temperature sensor, and the APIS main board with the rest of the electronic devices.

Fig. 7

APIS electronics diagram.

The APIS focal plane board was connected to the APIS main board by a flexible cable with 10 data lines, 14 control lines, and 2 power lines.

2.3.

APIS Optomechanical Design

The APIS optomechanical design had five important design challenges. (1) Geometrical: the APIS camera had to be installed inside a cubic prism volume of $65\mathrm{mm}\times 52\mathrm{mm}\times 36.4\mathrm{mm}$. (2) Weigh: the APIS camera had to weigh less than 150 g. (3) Tolerances: critical position tolerances of the optical elements had to be maintained. (4) Thermal: the mechanical design had to absorb the optical distance changes induced by the thermal expansion (passive athermalization system). (5) Stray-light protection: the APIS camera had to avoid the stray-light radiation by means of baffles at the entrance of the instrument and in the detector area.

2.3.1.

Mechanical subassemblies

The most important subassemblies of the APIS optomechanical design can be identified in Fig. 8 by different colors. The platform subassembly is formed by the base part (dark red part) that allows the system to be attached to the lateral wall of OPTOS CubeSat, the sliding part (green part), and two linear sliding elements to join them. Also shown is the optical subassembly, which includes the five mounted lenses, the tube to hold them (dark blue part) and the support for all of it (yellow part) with the venting slots. Next is the focal plane subassembly, which is formed by the electronic card with the detector itself. This subassembly provides the system with the ability to tilt the focal plane array with respect to orthogonal axes by means of a gimbaled platform. Finally, the baffle subassembly formed by the baffle itself (including the aperture stop) and the bandpass/neutral density filter that was glued on it.

Fig. 8

APIS optomechanical design: (a) CAD view and (b) exploded view.

2.3.6.

Stray-light analysis

The APIS camera included a real aperture stop. This stop guaranteed a good performance of APIS with respect to the stray-light rejection factor because it limited the light collected by the CMOS. However, a stray-light analysis of the optomechanical design had to be performed to check the amount of unwanted scattered light reaching the CMOS area.

The simulation was performed with ASAP considering a 1-W emitting Lambertian source covering the whole entrance pupil of the system. The ghost images and the preliminary stray-light behavior of APIS were studied considering the most relevant coatings and paintings of both optical and mechanical parts. Three kinds of coatings with different scattering properties were taken into account in the simulation. The Chemglaze 306 coating was applied to all internal parts of each optomechanical support and to those external parts that would be exposed directly to the optical beam, such as the baffles. The mechanical interface and the rest of the mechanical components were coated with black diffuse, which shows a much higher scattering level for different angles of incidence. The third coating for polished reflective surfaces was applied on some mechanical parts, such as adjustment mechanisms, screws, and the external surfaces of the mechanical structure.

The merit function used to analyze in detail the behavior of the stray-light in the APIS camera was the point source transmittance (PST) curve. The PST associated to the stray-light process was calculated as the ratio of the flux obtained on the focal plane for an incident on-axis collimated beam to the flux collected on the focal plane for an incident collimated beam outside the FOV of the instrument, i.e., from 7 deg to 50 deg (angles outside the maximum semi-FOV of APIS camera).

In the simulation, we separated the flux produced by the beams inside the FOV of the instrument, which produce the image in the CMOS, with respect to the flux produced by the beams scattered in any mechanical surface, which are related to the stray-light process. The results showed that the relative flux received in the image plane due to the scattering in the mechanical surfaces was $\le {10}^{-8}$ times the energy corresponding to the normalized flux directly collected in the CMOS area (inside the FOV). This means that the baffling proposed in the optomechanical design effectively attenuates the unwanted scattered radiation. Figure 9 shows the rejection factor [log (PST)] as a function of the incident angle (only angles out of the FOV are represented).

Fig. 9

Logarithm of the irradiance in the focal plane due to the stray-light process.

3.1.

Gamma Radiation Test on APIS Optical BB Models

The APIS camera included optical glass elements that could be affected by the ionizing radiation. The behavior in radiation environment was already studied for the optical glasses NFK51¹⁵ and NSF57¹⁶ within the LINES research activity for assessing and using COTS glasses in space environment. The behavior for NLASF41 glass was unknown. Therefore, gamma radiation test of this glass and the whole lens set were performed in order to correlate the transmittance loss observed during the mission lifetime.

To estimate the expected transmittance losses during the mission life, three different optical BB models (BB1, BB2, and BB3) were exposed to 5, 10, and 15 krad doses, respectively (corresponding to 1, 2, and 3 mission years). They were made up of optical parallel-plane plates of 8 mm diameter and thickness that closely simulate the elements used in the APIS camera optical design. The doses were calculated according to the mission orbital parameters and the standard models AP8/AE8¹⁷ for the Van Allen belts. The ESP/PSYCHIC^18–20 model for the probability of a solar particle event occurrence was also used. As a conservative approach, AP8/AE8 models were used considering a worst-case scenario of maximum solar activity.

The BB models were exposed to gamma radiation in the ${\mathrm{Co}}^{60}$ pool at the NAYADE facility²¹ belonging to the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT). The irradiation conditions were 15.7°C temperature, 4.86 krad (Si)/h dose rate, and 9.5% of maximum dose nonuniformity.

The NLASF41 glass showed higher transmission losses than NFK51 and NSF57 glass. The absorption induced by gamma radiation on NLASF41 glass followed the same behavior already observed on NFK51 and NSF57 glass. That means the transmission loss increases as a function of radiation dose following an exponential curve.

Transmission measurements versus wavelength of the BB models were carried out before and just after each irradiation test with a Perkin-Elmer Lambda 850 spectrophotometer ($\pm 0.08\%$ accuracy).

Table 1 shows the average transmittance data of the three optical BB models before irradiation and the transmittance data difference of each optical BB model before and after irradiation for several wavelengths into the work range.

Table 1

Transmittance loss data of the optical BB models.

Therefore, optical transmittance losses (on average) between 7% and 20% (corresponding to one and two mission years) would be enough to observe a performance degradation in the APIS image data and therefore to achieve the scientific goal of the instrument.

3.2.

Gamma Radiation Tests on APIS BB Focal Plane Boards

Two different BB focal plane boards (M01 and M02) were also exposed to gamma radiation in the ${\mathrm{Co}}^{60}$ NAYADE facility. The irradiation process was performed in six accumulated dose steps: 1, 2.5, 5, 10, 15, and 21 krad, respectively. The irradiation conditions were the same as the ones mentioned in Sec. 3.1.

Electrical characterizations were carried out with the BB focal plane boards before irradiation and after each irradiation step using the IBIS5 evaluation system provided by Cypress Inc. (see Ref. 13). A dark current analysis was performed in order to see the degradation of the active pixels by effect of the radiation. Degradation pixels were considered to have a charge level in dark current $>1.91\%$ of the full well charge ($\mathrm{62,500}{\mathrm{e}}^{-}$).

The measurements showed an increase on pixel degradation from 10 krad of accumulated dose on both specimens. Figure 10(a) shows the abrupt increment of the pixel degradation by effect of the gamma radiation from 10 krad, and Fig. 10(b) shows the zone between 0 and 5 krad in more detail.

Fig. 10

(a) Pixels with charges in dark current versus gamma radiation dose and (b) detail of the zone from 0 to 5 krad.

In addition, an image quality analysis was carried out in order to evaluate the contrast in the images obtained by the CMOS sensor with respect to the gamma irradiation dose. The setup was formed by an LED ($\lambda =565\mathrm{nm}$), a target (Foucault Test), an optical system, and the CMOS image sensor. Figure 11(a) shows the Foucault test image obtained before the CMOS gamma irradiation, and Fig. 11(b) the Foucault test image after 21 krad of accumulated gamma dose.

Fig. 11

(a) Foucault test image at 0 krad gamma dose and (b) Foucault test image at 21 krad gamma dose.

The procedure was to calculate the maximum and minimum intensity of the peak patterns obtained for the image central row. The Michelson contrast (visibility) was only estimated using the four central peaks. Table 2 shows the average contrast of the two samples as a function of the gamma radiation dose.

Table 2

Image contrast versus gamma radiation dose.

The obtained results guaranteed that changes in the IBIS5-B-1300 sensor could be considered negligible with doses of up to 10 krad of gamma radiation.

3.3.

Proton Radiation Tests on APIS BB Focal Plane Boards

Another two BB focal plane boards (M03 and M04) were exposed to proton radiation in order to study the effects of the nonionizing radiation damage combined with ionizing effects. These irradiation tests were carried out at the high-energy proton beam line of radiation effects facility, at Jyväskylä University (Finland).²² In this facility, protons in the energy range from 10 to 50 MeV can be delivered with a homogeneous beam size of $10\times 10{\mathrm{cm}}^2$. The irradiation process was performed at the maximum available energy of 50 MeV and in six accumulated fluence steps up to a total proton fluence of $1.9\times {10}^{11}{\mathrm{cm}}^{-2}$ and an equivalent total ionizing dose of $\sim 29.3\mathrm{krad}$ (see Table 3).

Table 3

50 MeV proton fluence data.

The results showed a similar behavior of the sensor as the one obtained with the gamma irradiation. From the $9.5\times {10}^{10}$ proton fluence step (14.6-krad ionizing equivalent dose), a strong increase of pixels degradation in dark current was observed. Moreover, 2 months after the last irradiation step further increase in pixel degradation was observed. This fact indicated an activation in the time of the samples irradiated with high-energy protons. Table 4 shows the number of pixels with charges in dark current detected in the M04 sample after the irradiation. (The M03 sample data are omitted because of a problem during the irradiation).

Table 4

Number of pixel with charges in dark current versus proton fluence for M04 sample.

On the other hand, the image contrast at the maximum proton fluence ($1.9\times {10}^{11}$) only showed slight variations as can be seen in Table 5 and Fig. 12.

Table 5

Image contrast versus proton fluence.

Fig. 12

(a) Foucault test image before proton irradiation and (b) Foucault test image at 1.90 ${10}^{11}$ proton fluence.

Therefore, the degradation in the sensor for proton fluences $<{10}^{11}$ (${\mathrm{p}}^{+}/{\mathrm{cm}}^2$) or $<15\mathrm{krad}$ ionizing equivalent dose can be considered negligible.

4.1.

Optical Characterization

The alignment and optical characterization of APIS optical units (without focal plane) were performed with the help of a visible MTF bench from Image Science Ltd. The evaluation of the optical performance was carried out through the measurement of the MTF at the Nyquist frequency (74.6 line pairs/mm) on and off axis. For this AIV phase step, the theoretical MTF had been calculated considering the contribution of three partial MTFs (diffraction, aberration, and alignment/manufacturing tolerances). This theoretical MTF value at the Nyquist frequency was 0.76. A thorough alignment of the lenses for each unit achieved experimental MTF values of $0.76\pm 0.02$.

The focal of the three optical units was also calculated from geometrical relationships between the positions of two pinhole images acquired in paraxial approximation. The focal values obtained were within an interval of $20\mathrm{mm}\pm 1\%$.

4.2.

CMOS Sensor Characterization

The three sensors were characterized in order to set the best configuration parameters for each one. Of particular importance were two parameters: DAC_FINE is used to tune the difference between odd and even columns and DAC_RAW is used to add a general offset (both even and odd columns) to the fixed pattern noise corrected pixel value. These parameters were independently adjusted considering the sensors’ behavior in both dark current and flat field simultaneously.

Some other parameters were selected during the characterization as the best default configuration for the sensors. For example, a synchronous (snapshot) shutter to have the light integration on all pixels in parallel; a slow frame/line calibration mode in order to cancel the thermal-KTC noise (reset noise) during the calibration of the output amplifier; a unity gain mode and the internal clock granularities to control the column/pixel readout; and the synchronous shutter sequencer.

4.3.

Electro-Optical Characterization

The alignment of the APIS focal plane was performed using a 300-mm focal length focusing autocollimator from Trioptics GmbH with $\pm 25\text{-}\mathrm{mm}$ focusing range. We established the equivalent relationships between a through-focus, performed around the autocollimator focus, and the corresponding APIS camera focus. To do that, a specific distance between instruments was set.

The theoretical MTF at this step of the AIV phase had been calculated adding the MTFs associated with the detector (footprint, spectral effects, array flatness, and tilt). This theoretical MTF value at Nyquist frequency was 0.46.

The alignment procedure of the APIS focal plane consisted in acquiring a number of images of the autocollimator tilted-cross hair. For each image, the cross hair was placed at different positions around the autocollimator focus. Then, an analysis software, developed in MATLAB, processed these images. The analysis was based on the sloping slit method,^23,24 which allowed finding the best focus position for each APIS camera unit. For each image, the line spread function (${\mathrm{LSF}}_{\text{Data}}$) of each cross hair arm was obtained and fitted to a Gauss function (${\mathrm{LSF}}_{\text{Gauss}}$). Figure 13(a) shows the cross hair image of the APIS camera with its labeled arms and Fig. 13(b) shows an example of ${\mathrm{LSF}}_{\text{Data}}$ and ${\mathrm{LSF}}_{\text{Gauss}}$.

Fig. 13

(a) Image of the tilted-cross hair and (b) example of ${\mathrm{LSF}}_{\text{Data}}$ and ${\mathrm{LSF}}_{\text{Gauss}}$.

Then, the full width at half maximum (FWHM) for each ${\mathrm{LSF}}_{\text{Gauss}}$ was calculated and these values were fitted to a parabolic function with respect to the autocollimator position [see Fig. 14(a) as an example]. In addition, MTF curves were calculated as the module of the LSF Fourier transform ($|f\{\mathrm{LSF}\}|$) for each image: $\mathrm{MTF}1=|f\{{\mathrm{LSF}}_{\text{Data}}\}|$ and $\mathrm{MTF}2=|f\{{\mathrm{LSF}}_{\text{Gauss}}\}|$. Finally, the MTF values at the Nyquist frequency were fitted to a parabolic curve and plotted as a function of the autocollimator position [see Fig. 14(b) as an example].

Fig. 14

(a) Example of the LSF FWHM fitting and (b) example of the MTF at Nyquist frequency fitting.

Therefore, the autocollimator position with the minimum FWHM value and the maximum MTF value showed the best focus position of the APIS camera (regarding the equivalent relationships between through-focus).

The alignment procedure of the focal plane for each APIS camera unit achieved MTF values $\ge 0.40$. This experimental electro-optical MTF value at room conditions agreed with a theoretical final MTF in orbit of 0.17 (added MTFs such as atmospheric, jitter, and operational perturbations).

4.4.

Vibration Test

The object of this test was to validate the APIS electro-optical design and demonstrate the availability of the specimen to withstand the stress, accelerations, and accumulated fatigue damage resulting from the specific vibration environment. For this purpose, one of the APIS camera units was assembled on the OPTOS structural thermal model (STM) that was subjected to its own vibration test. OPTOS STM was vibrated into the Poly-Picosatellite Orbital Deployer (P-POD).

Figure 15(a) shows the OPTOS STM model with APIS camera unit assembled on it and Fig. 15(b) shows the OPTOS STM model (into the P-POD) on top of the vibration platform.

Fig. 15

(a) APIS unit assembled on the OPTOS STM and (b) OPTOS STM ready on the vibration platform.

The test was carried out in the Mechanical and Environment Test laboratory at INTA. The following sequence for each axis was applied: low-level sine vibration, high-level sine vibration, low-level sine vibration, high-level random vibration, and low-level sine vibration.

An electro-optical characterization was carried out before and after the test. In addition, dark current and flat field analysis for different integration times were performed before and after the test. The results showed that the vibration environment test did not modify the optoelectronic performance of the APIS payload.

4.5.

Thermal-Vacuum Test

The object of this test was to check the vacuum induced focus shift, the APIS thermal-vacuum behavior, and the passive athermalization system. The test was carried out in the thermal vacuum chamber (TVC) at LINES ISO-7 cleanroom at INTA. This TVC has optical windows that allowed optical checks during the thermal-vacuum test.

The performed thermal profile, with a warming ramp of 0.2°C/min at ${10}^{-6}\mathrm{mbar}$ vacuum level, is shown in Fig. 16. The temperatures for image acquisition are marked in the figure with black points. The validation of the passive athermalization system was intended to be performed into the $-20°\mathrm{C}$ to $+20°\mathrm{C}$ temperature range.

Fig. 16

TVC performed thermal profile.

The APIS camera unit selected for this test was placed inside the TVC, and the focusing autocollimator was outside the TVC and facing the APIS camera through the optical window. Previously, the APIS camera focus at room conditions was shifted $-44\mu \mathrm{m}$. This value included the theoretical vacuum focus shift ($-26.5\mu \mathrm{m}$) and the thermal focus shift due to changing the temperature from 20°C to 0°C ($-17.5\mu \mathrm{m}$) according to CODE V.

An electro-optical characterization was carried out at the black points indicated in Fig. 16 following the procedure/algorithm presented in Sec. 4.3. The equivalent relationships between autocollimator and APIS through-focus were such that 1 mm displacement from the focus of the autocollimator was equivalent to $4.5\mu \mathrm{m}$ displacement in APIS focal plane. Therefore, the APIS camera focus depth ($\pm 7\mu \mathrm{m}$) was equivalent to $\pm 1.6\mathrm{mm}$ in the autocollimator.

The analysis of the results gave an experimental vacuum focus shift of $-22.1\mu \mathrm{m}$, which is according to the theoretical value obtained in Sec. 2.1.1. In addition, the passive athermalization system kept the APIS camera focus inside the focus depth for the complete temperature range. Experimental results can be seen in Table 6.

Table 6

Thermal-vacuum test experimental results.

After the AIV campaign, the APIS camera flight model was assembled in OPTOS CubeSat with the vacuum focus shift applied.