Knowing the thermodynamic phase of a cloud–whether it is composed of spherical water droplets or polyhedral ice crystals–is critical in remote sensing applications and in climate studies. We recently showed that we can determine cloud phase with visible-wavelength sky polarimetry, and in this presentation we extend that method to shortwave infrared wavelength bands near 1.6 microns. We describe the instrument, a passive, three-channel polarimeter with spectral bands at 1550 nm, 1640 nm, and 1700 nm with approximate width of 40 nm and how we are using it in experiments to discriminate between liquid-water and ice clouds. This portable polarimeter measures scattered sunlight using polarizers orientated at 0° , 45‡ , and 90° with respect to the solar vertical scattering plane. It has a 4.9° field-of-view and a motorized, computer-controlled pan-and-tilt mount that controls the positioning of the polarimeter so that it can measure any point in the sky.
For space optical systems that image extended scenes such as earth-viewing systems, modulation transfer function (MTF) test data is directly applicable to system optical resolution. For many missions, it is the most direct metric for establishing the best focus of the instrument. Additionally, MTF test products can be combined to predict overall imaging performance. For fixed focus instruments, finding the best focus during ground testing is critical to achieving good imaging performance. The ground testing should account for the full-imaging system, operational parameters, and operational environment. Testing the full-imaging system removes uncertainty caused by breaking configurations and the combination of multiple subassembly test results. For earth viewing, the imaging system needs to be tested at infinite conjugate. Operational environment test conditions should include temperature and vacuum. Optical MTF testing in the presence of operational vibration and gravity release is less straightforward and may not be possible on the ground. Gravity effects are mitigated by testing in multiple orientations. Many space telescope systems are designed and built to have optimum performance in a gravity-free environment. These systems can have imaging performance that is dominated by aberration including astigmatism. This paper discusses how the slanted edge MTF test is applied to determine the best focus of a space optical telescope in ground testing accounting for gravity sag effects. Actual optical system test results and conclusions are presented.
Getting students interested in science, specifically in optics and photonics, is a worthwhile challenge. We developed and implemented an outreach campaign that sought to engage high school students in the science of polarized light. We traveled to Montana high schools and presented on the physics of light, the ways that it becomes polarized, how polarization is useful, and how to take pictures with linear polarizers to see polarization. Students took pictures that showed polarization in either a natural setting or a contrived scene. We visited 13 high schools, and presented live to approximately 450 students.
As microbolometer focal plane array formats are steadily decreasing, new challenges arise in correcting for thermal drift in the calibration coefficients. As the thermal mass of the cameras decrease the focal plane becomes more sensitive to external thermal inputs. This paper shows results from a temperature compensation algorithm for characterizing and radiometrically calibrating a FLIR Lepton camera.