Presentation Title

Characterization of Chrome-On-Glass Apodizer Test Sample

Faculty Mentor

Jacques-Robert Delorme, Dimitri Mawet

Start Date

18-11-2017 2:00 PM

End Date

18-11-2017 2:15 PM

Location

9-239

Session

Physical Sciences 4

Type of Presentation

Oral Talk

Subject Area

physical_mathematical_sciences

Abstract

With most of today’s space-based and adaptive optics-equipped ground based telescopes having obscured aperture systems with secondary mirrors held in place by support vanes that diffract starlight, well-designed custom apodized masks are particularly important in helping coronagraphs directly image exoplanets. A test microdot apodizer with 64 different patterns (step functions, gradients, and sinusoids) made of chrome-on-glass was characterized in optical transmission, infrared reflection, and with microscopy. Results in transmission showed that microdot density and transmission were mostly linear except with anomalously high transmittance around 50% density (where the pattern is periodic), which is potentially due to surface plasmon effects. Results from reflection potentially reveal anomalously high reflection at low dot density. Results from microscopy revealed the high craftsmanship of the microdots and thus ruled out poor fabrication as an explanation for the anomalies. Knowing these effects and understanding microdot apodization analytically will be helpful in optimizing future apodizer designs. High-dynamic-range imaging in reflection and further interferometer data will be taken to confirm previous results.

Summary of research results to be presented

In optical transmission, the step functions did not have steps of the same magnitude even though they were supposed to by design. The steps got significantly larger in the middle of the profile. Another unexpected result was that the linear functions had unexpected bumps in the middle of the profile. This is likely an unavoidable phenomenon when the dot density is at 50% and the microdots are laid down in a periodic pattern similar to a checkerboard. For future apodizer designs, other dot densities will need to be considered to achieve 50% transmission.

From initial infrared reflection imaging, the profiles reached lower bounds around 10% - 20% reflection. It also appeared as if the expected values below that value appeared to be reflected about a line parallel to the x-axis at the lower bound. This is particularly notable with the sinusoidal patterns with smaller dot sizes, but not observed or not very noticeable in the sinusoidal patterns with larger sizes. Initially we thought that meant that the squares with smaller dot sizes might’ve been underexposed. Some of the sinusoidal patterns with this effect were re-imaged at multiple exposure times (in a method known as high dynamic range) and final images were generated from the values in each individual image that were not overexposed or underexposed. Though the reflected behavior wasn’t seen, the HDR images showed that the reflectance could not go below a certain value. Such low reflectances are impossible due to limitations in the achromatic coating of the window.

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Nov 18th, 2:00 PM Nov 18th, 2:15 PM

Characterization of Chrome-On-Glass Apodizer Test Sample

9-239

With most of today’s space-based and adaptive optics-equipped ground based telescopes having obscured aperture systems with secondary mirrors held in place by support vanes that diffract starlight, well-designed custom apodized masks are particularly important in helping coronagraphs directly image exoplanets. A test microdot apodizer with 64 different patterns (step functions, gradients, and sinusoids) made of chrome-on-glass was characterized in optical transmission, infrared reflection, and with microscopy. Results in transmission showed that microdot density and transmission were mostly linear except with anomalously high transmittance around 50% density (where the pattern is periodic), which is potentially due to surface plasmon effects. Results from reflection potentially reveal anomalously high reflection at low dot density. Results from microscopy revealed the high craftsmanship of the microdots and thus ruled out poor fabrication as an explanation for the anomalies. Knowing these effects and understanding microdot apodization analytically will be helpful in optimizing future apodizer designs. High-dynamic-range imaging in reflection and further interferometer data will be taken to confirm previous results.