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We present a general-purpose end-to-end image chain simulation (ICS) that enables to assess the image quality of a satellite imager for Earth observations. The image chain consists of four main components: radiometry, atmosphere, optics, and detector. In particular, ICS first computes the input radiance from the reflectance values of a high-resolution input, and then calculates the image radiance by using the optical transfer function (OTF) of the overall system. This OTF contains all the distortion effects due to atmosphere, optics, and detector. Finally, the signal on the detector, including the noise term, is computed and converted to digital counts. To evaluate the overall image quality, metrics such as peak signal-to-noise ratio and minimum resolvable contrast are used. To illustrate the utility and versatility of the developed ICS, several analyses are also performed that demonstrate the system performance of a generic satellite imagery. Our development provides a unified framework for the ICS developers of spaceborne Earth-observing systems. Such a complete end-to-end ICS is crucial for the effective development of an Earth observation satellite, especially in the design and test phases.

We present the optical transfer function (OTF) analysis of a spaceborne imaging sensor. This includes the effects of detector footprint, diffusion, smear, jitter, sampling, and time delay and integration. Each effect is analyzed under various design choices one of which corresponds to our designed complementary metal-oxide semiconductor imaging sensor. Effect of each subsystem is separately demonstrated using OTF values and corresponding degradation on a test image.

Collimator is an optical instrument used to evaluate performance of high precision instruments, especially space-born high resolution telescopes. Optical quality of the collimator telescope needs to be better than the instrument to be measured. This requirement leads collimator telescope to be a very precise instrument with high quality mirrors and a stable structure to keep it operational under specified conditions. In order to achieve precision requirements and to ensure repeatability of the mounts for polishing and metrology, opto-mechanical principles are applied to mirror mounts. Finite Element Method is utilized to simulate gravity effects, integration errors and temperature variations. Finite element analyses results of deformed optical surfaces are imported to optical domain by using Zernike polynomials to evaluate the design against specified WFE requirements. Both mirrors are aspheric and made from Zerodur for its stability and near zero CTE, M1 is further light-weighted. Optical quality measurements of the mirrors are achieved by using custom made CGHs on an interferometric test setup. Spider of the Cassegrain collimator telescope has a flexural adjustment mechanism driven by precise micrometers to overcome tilt errors originating from finite stiffness of the structure and integration errors. Collimator telescope is assembled and alignment methods are proposed.

We describe the design of a thermal refocusing method for spaceborne high-resolution imagers where Korsch optical design is usually implemented. The secondary mirror is made of aluminum, a high thermal expansion coefficient material, instead of conventional zero-expansion glass ceramics. In this way, the radius of the curvature can be controlled by means of temperature change of the mirror. Change in the radius of curvature also changes the effective focal length of the camera which is used for compensation of the defocus that occurred in space. We show that the 30 mu m despace of the secondary mirror in the optical system can be compensated by an similar to 10 degrees C temperature change of the mirror while the image quality is maintained. (C) 2016 Optical Society of America

Tubitak Uzay has started a spaceborne hyperspectral imager development project two years ago. As hyperspectral remote sensing is getting popular, Tubitak Uzay has R&D projects both on hardware development and software applications of hyperspectral imaging. Project is at the PDR level now. Many trade-off cases are evaluated during the system engineering and preliminary design phases. We are presenting a summary of trade-off cases and lessons learnt during those studies. Optical, mechanical and electronical designs are also shown at the end of paper.

We studied periodic aperture imaging with various subaperture configurations in terms of their point spread functions, optical transfer functions, and imaging quality. A circular-shaped unit cell provides the best performance compared to square- and cross-shaped unit cells when the total aperture size and collection area are kept the same. Moreover, decreasing the separation among the unit cells or the number of subapertures also improve the imaging performance. Results have potential applications in optical periodic aperture telescopes, diffraction masks, and coded aperture imaging. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Development of an imaging telescope having a large mirror generally requires a sound infrastructure including expensive devices such as collimators and interferometers. In this paper we investigate the development and characterization of a medium size telescope (140 mm aperture) using simple techniques. Alignment was done with a mechanical measurement device. Detector was placed using an image of a ruled outside building. Radiometric calibration was done with open-sky images. And finally MTF was measured using stellar images. We found that MTF of our telescope is greater than %15 all along the detector at 20 lp/mm, which is the Nyquist frequency of 25 μm pixel size. This shows that alignment and focus adjustments work well enough in the telescope. These simple techniques can be used for the development and characterization of a medium-size and medium-resolution imaging telescopes.

Hyperspectral imagers as remote sensing devices are getting more popular. Taking the image in different narrow spectral bands, instead of a single wide spectral band, gives much more insight information about the scene. But since the spectral band decreases considerably, signal reaching the sensor also decreases. This brings a trade-off between spatial resolution and spectral resolution. A detailed SNR (Signal to Noise ratio) analysis is commonly required to decide spatial and spectral resolutions and additionally to decide some optical parameters such as optical aperture and optical transmission. In this paper, SNR analysis of a spaceborne hyperspectral imager is realized through a preliminary optical design. We mainly focused on the effect of optical transmission (tau(optics)) to the SNR.

Main criterion for response time and cost of a remote sensing space mission is the size of the intended satellite and it is directly affected by the features of the payload. By an affective optical design and utilization of emerging detector technologies, such as time delay integration (TDI) sensors, higher resolution cameras can be designed with smaller sizes. In this paper, optical and electronic design of such an electro-optical system (4m resolution camera) is investigated. The simulated Modulation Transfer Function (MTF) and calculated Signal to Noise Ratio (SNR) values of the designed system are also provided.

MTF is the final and decisive parameter determining the image quality of optical imaging systems. To measure the MTF of a large-mirror telescope in a laboratory, an expensive collimator is necessary to make a collimated beam from a point or knife-edge source. Star-test is an old and traditional method giving some clues about wavefront error. In this study, we used the star-test to measure the MTF of an imaging telescope. We found the Line Spread Function (LSF) of a star on a line detector, and then derived the MTF using Fourier Transform of LSF. LSF is generated from the intensity distribution on the detector, by using different mathematical curve fitting methods. It is found that our telescope has an MTF greater than % 15 over the all fields at 20 lp/mm, which is the Nyquist frequency of 25 mu detector. We showed that a relatively large telescope (similar to 15 cm in diameter) can be built and tested without using expensive instruments such as collimator and interferometer. Also our telescope can be used with a 20 mu detector safely, considering % 10 MTF as the limiting case.

Radiometric calibration of an imaging telescope is usually made using a uniform illumination sphere in a laboratory. In this study, we used the open-sky images taken during bright day conditions to calibrate our telescope. We found a dark signal offset value and a linear response coefficient value for each pixel by using three different algorithms. Then we applied these coefficients to the taken images, and considerably lowered the image non-uniformity. Calibration can be repeated during the operation of telescope with an object that has better uniformity than open-sky. Also SNR (Signal to Noise Ratio) of each pixel was calculated from the open-sky images using the temporal mean and standard deviations. It is found that SNR is greater than 80 for all pixels even at low light levels.

Image blur caused by high speed motion is one of the main problems of aerial digital cameras. Removing image blur with special techniques is called Forward Motion Compensation (FMC) techniques. During the exposure time of the sensor, image of the same earth region moves more than many pixels on the sensor due to high speed, and causes image blur. Expensive TDI (time delay integration) sensors are now available and electronically compensate the motion within the sensor. In this article we propose mechanical compensation methods. During the exposure time, movement of the camera with proper angular speed will result in FMC. This can be achieved by pure older mechanical methods. Another method is to move the sensor linearly, proportional the speed of image walk. This can be provided by piezo-actuators. In this paper we present examples of these two mechanical FMC techniques. By these methods cheaper standard sensors can be used in digital aerial cameras.

Imaging from air has evolved from film cameras to digital cameras over the last years. Here we give the design steps of an aerial objective lens for a given CCD sensor. Matching to the pixel pitch of the sensor and providing desired field of view according to CCD size are traditional steps. Besides these, to minimize the environmental effects, design should be insensitive to the temperature and pressure as much as possible. Performance of a lens, designed at room temperature and pressure will be degraded if temperature and pressure changes. This is firstly caused by the change of relative refraction indices of materials, and secondly thermal expansion or contraction of both glass and housing materials. Here it is shown that according to the environmental working conditions, custom designs are necessary to provide images with sufficient resolution up in the air. These design principles are also valid for the fore optics of spectroscopic imaging tools. Since designs are insensitive to temperature and pressure, working environmental range is wide.