Small satellites (“SmallSats”) are a growing segment of the Earth imaging and remote sensing market. Designed to be relatively low cost and with performance tailored to specific end-use applications, they are driving changes in optical telescope assembly (OTA) requirements. OTAs implemented in silicon carbide (SiC) provide performance advantages for space applications but have been predominately limited to large programs. A new generation of lightweight and thermally-stable designs is becoming commercially available, expanding the application of SiC to small satellites. This paper reviews the cost and technical advantages of an OTA designed using SiC for small satellite platforms. Taking into account faceplate fabrication quilting and surface distortion after gravity release, an optimized open-back SiC design with a lightweighting of 70% for a 125-mm SmallSat-class primary mirror has an estimated mass area density of 2.8 kg/m2 and an aspect ratio of 40:1. In addition, the thermally-induced surface error of such optimized designs is estimated at λ/150 RMS per watt of absorbed power. Cost advantages of SiC include reductions in launch mass, thermal-management infrastructure, and manufacturing time based on allowable assembly tolerances.
Modern techniques in deterministic finishing employ devices, which provide geometrically well-defined removal functions for precision correction of fast aspheres. While stability of the removal function is essential, a commonly experienced consequence of such controlled removal is the creation of a residual trail, or signature of periodic surface “ripples” or textures that correlate to the characteristics of the removal function and tool path. The extent to which this signature exists in both amplitude and spatial frequency can have a profound impact on system imaging performance. Therefore, it is necessary to accurately characterize the spatial frequency content of surfaces and control its impact through proper specifications in order to guaranty image performance. Traditional specifications like Peak to Valley and RMS wavefront specifications cannot fully capture or predict image quality in fast aspheric optics unless perhaps they are specified over precise spatial scale lengths (or frequencies). In this paper we will explore a correlation of surface metrics and image performance using empirical data collected on a variety of fast aspheric mirrors produced by Aperture Optical Sciences Inc.
Recent experience with finishing off-axis parabolas and other conic surfaces is demonstrated by some examples that illustrate surface accuracy – not only in terms of traditional metrics, but also in terms of specified ranges of spatial frequency. Particular attention is given to the topic of interferometric metrology, and the extent to which we can effectively characterize mid-spatial frequency errors. The presence of mid-spatial errors can appear even more dominant in hard ceramics like SiC as compared with glass – reasons for this are suggested. This paper will discuss how controlled force grinding, robotic polishing, and surface smoothing can be employed to minimize and mitigate mid-spatial errors in fast silicon carbide aspheric mirrors. Recent experience and results are presented on two SiC mirrors finished by Aperture Optical Sciences Inc.
Students in photonics technology associate degree programs have two short years to prepare for employment as technicians. Recognizing that there is little in the traditional lecture/lab format of instruction that allows students to practice real-world project planning, time management and technical problem-solving skills, the authors have collaborated to provide students with authentic “real-world” industry problems in a final one or two semester capstone course. In this paper we present several student projects, describe barriers to successful project completion and strategies to improve outcomes.
Dual band infrared imagers require a similar set of filters as are needed by single band infrared imagers but with the
added requirement of high transmission in the mid and far infrared. The design of discrete layer filters with optimized
dual band transmission is investigated for three types of filters. These are a visible-infrared beamsplitter, a long
wavelength edge filter and a dual bandpass cold filter. These designs illustrate the role that harmonic reflection bands
can play in the design of dual band filters. The visible reflection beamsplitter design does not have harmonics in the
infrared but requires additional layers to reduce reflection at mid and long wavelengths. The long wavelength edge filter
requires suppression of the second and third harmonics while the sensor band pass cold filter can use harmonics to
advantage. Design techniques are discussed and the results of an initial set of fabrication runs are presented to assess the
sensitivity of example designs to manufacturing errors.
Sensor performance for dual band forward looking infrared (FLIR) imagers can be substantially improved by increased simultaneous throughput of both sensor bands in the optical systems. Currently available antireflection coatings (ARs) have optimized performance for either spectral band, but not both on the same optic. Where AR coatings cover the mid and long wave infrared (LWIR) bands, or the entire broad band spectrum from visible to LWIR, performance is not sufficient for future systems. A method of designing and fabricating high performance ARs has been developed. This paper presents a discussion of the trade-off of film thickness and complexity versus transmission performance. Fabrication results for high, medium and low index lens materials are also presented.
The various properties of polishing pitches have different advantages and are selected according to the type of work intended. It is important to check pitch properties before a pitch lap is poured; to ensure that the final polishing lap properties will be as desired. A simple penetrometer test is utilized as a quality control tool for measuring the hardness (viscosity) of various types of pitch as received from the manufacturer. Only a small sample, 20 grams, is needed for this test. Another simple method for determining pitch quality is the measurement of the softening point. A description of this method and typical results will be described. Lastly, the 'tackiness' of pitch and its importance will be discussed.