Michael E. Dearborn

Optical testing of a membrane diffractive optic for space-based solar imaging

We describe imaging capabilities of a 0.2 m membrane diffractive primary (DOE) used as a key element in FalconSat-7, a space-based solar telescope. Its mission is to take an image of the Sun at the H-alpha wavelength (656nm) over a narrow bandwidth while in orbit. In this case the DOE is a photon sieve which consists of billions of tiny holes, with the focusing ability dependent on an underlying Fresnel zone geometry. Uniform radial expansion/contraction of the substrate due to temperature or relative humidity change will result in a shift in focal length without introducing errors in phase of the transmitted wavefront and without a decrease in efficiency. We will also show that while ideally the DOE surface should be held flat to within 5.25 microns, an opto-mechanical analysis showed that local deformations up to 32 microns are possible without significantly degrading the image quality.

FalconSAT-7: A Membrane Photon Sieve CubeSat Solar Telescope

We are currently constructing FalconSAT-7 for launch in mid-2014. The low-Earth, 3U CubeSat solar telescope incorporates a 0.2m deployable membrane photon sieve with over 2.5 billion holes. The aim of the experiment is to demonstrate diffraction limited imaging of a collapsible, diffractive primary over a narrow bandwidth. As well as being simpler to manufacture and deploy than curved, polished surfaces, the sheets do not have to be optically flat, greatly reducing many engineering issues. As such, the technology is particularly promising as a means to achieve extremely large optical primaries from compact, lightweight packages.

Optical Analysis of a Membrane Photon Sieve Space Telescope

This paper focuses on recent progress in designing FallconSAT-7, a 33U CubeSat solar telescope designed to image the Sun from low Earth orbit. The telescope system includes a deployable structure that supports a membrane photon sieve under tension as well as secondary optics. To satisfy mission requirements to demonstrate diffraction limited imaging capability of this collapsible, f/2 diffractive primary we have completed studying a number off effects on membrane material that can affect system imaging quality.

Analysis of stellar radiance contamination in observed satellite spectra

Reflectance spectra of Earth orbiting satellites can be readily observed with small diameter telescopes (D < 1 m) by utilizing a method known as slitless spectroscopy. Satellite spectra can be observed by simply placing a transmission grating within the collimated optical path of the telescope without the need to image through a slit. The simplicity of the slitless spectroscopy design makes it a promising alternative to spatially resolving satellites with larger and more expensive diameter telescopes for applications of space situational awareness. However, accurately observing satellite re ectance spectra without imaging through a slit requires a dark and homogeneous background. This requirement is frequently violated as background stars streak across the image due to the slewing motion of the telescope during satellite tracking. Rather than throwing out all images with noticeable stellar contamination, a principle component analysis of contaminated images from three geostationary satellite observations showed that it may still be possible to assess and identify satellite characteristics depending upon the amount of stellar contamination in the spectral region of interest. Additionally, a simple technique for automatic removal of contaminated frames is proposed based on an outlier analysis using Gaussian statistics and was found to successfully remove all signicantly contaminated frames.

Membrane photon sieve telescope

We have developed diffractive primaries in flat membranes for space-based imagery. They are an attractive approach in that they are much simple to fabricate, launch and deploy compared to conventional three-dimensional optical structures. In this talk we highlight the design of a photon sieve which consists of a large number of holes in an otherwise opaque substrate. We present both theoretical and experimental results from small-scale prototypes and key solutions to issues of limited bandwidth and efficiency that have been addressed. Our current efforts are being directed towards an on-orbit 0.2m solar observatory demonstration deployed from a 3U CubeSat bus.