Monday, April 20, 2015

The non-redundant masking testbed

Amid the coronagraphic, extreme starlight suppression experiments lies a simple setup with one mirror covered by a funny-looking mask. This is a non-redundant mask, or NRM.
This is the 7-hole mask design that will fly on the NIRISS (Near IR Imager and Slitless Spectrograph) on the James Webb Space Telescope. We have a scaled down version for our experiments in the Makidon Laboratory at STScI.

This mask blocks out about 90% of light that passes through. Why would we want to throw away light? We are using the pupil as an interferometer, allowing the light to interfere between every pair of holes. Each pair of holes forms a unique vector that contributes its own fringe pattern in the image, like the classic double slit experiment, which depends on the spacing and direction of the holes.
A few example "baseline" vectors are drawn onto the mask.
The NRM can help us measure signal from the sky like resolving close binaries, and looking at hot, young planets a few to a dozen Earth-to-Sun distances from their star. While coronagraphs can measure more extreme contrasts, NRM can can peer in closer to the star, behind the coronagraphic spot. The point source image formed with the NRM (Point Spread Function or PSF) is sensitive to deviations from a perfect point source, errors in the wavefront.

A short movie shows how successive holes builds up the fringe pattern we measure.

Building the pupil by A. Greenbaum
Building the PSF by A. Greenbaum

Our testbed is currently set up for wavefront sensing experiments that could be applied to future space telescopes. Because the NRM is so sensitive to how flat the wavefront is, it makes an excellent tool for diagnosing wavefront errors. Right now we are working on matching our theoretical framework with data from the testbed to see how helpful NRM will for measuring the wavefront on future space telescopes.

After theoretical work and simulations, it's nice so see that it all works in the real world too!
NRM testbed data from Summer 2014 taken by Noah Gamper

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