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Question: How will microshutter arrays be used in the James Webb and future space telescopes? Are they acting as a sort of moving pinhole or slit, or is the pattern more complicated, like a coded aperture telescope? I can't imagine that would work though, as the application seems to include spectroscopy rather than just image reconstruction.


The NASA Goddard news item NASA to Demonstrate New Star-Watching Technology with Thousands of Tiny Shutters says:

The technology, called the Next-Generation Microshutter Array (NGMSA), will fly for the first time on the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy, or FORTIS, mission on October 27. The array includes 8,125 tiny shutters, each about the width of a human hair, that open and close as needed to focus on specific celestial objects.

and

The microshutter technology gives scientists the ability to produce multiple spectra at once. This capability improves productivity on both sounding rocket missions, which offer only six minutes of observing time, or large space-based observatories, which can take up to a week to observe faint, far-away objects and gather enough light to obtain good spectra. With observing time at a premium, the ability to gather light from multiple objects at once is paramount.

Webb, scheduled to launch in 2021, will carry NASA's first-generation microshutter technology — four 365-by-172 microshutter arrays that together total 250,000 shutters. They will allow Webb to obtain spectra of hundreds of objects simultaneously.

What distinguishes the next-generation array on FORTIS from the one flying on Webb is how the shutters are opened and closed. Webb’s arrays employ a large magnet that sweeps over the shutters to activate them. However, as with all mechanical parts, the magnet takes up space and adds weight. Furthermore, magnetically activated arrays can’t be easily scaled up in size. As a result, this older technology is at a disadvantage for supporting future space telescopes larger than Webb.

Magnet Eliminated

To accommodate future missions, Goddard’s microshutter-development team eliminated the magnet. The shutters in the pilot 128-by-64 array that will fly on FORTIS open and close through electrostatic interactions. By applying an alternating-current voltage to electrodes placed on the frontside of the microshutters, the shutters swing open. To latch the desired shutters, a direct current voltage is applied to electrodes on the backside.

Without a magnet, the next-generation array can be dramatically scaled up in size — and that’s precisely what the team is attempting to accomplish. Particularly, Greenhouse and Li are using advanced manufacturing techniques to create a much larger, 840-by-420 array equipped with 352,800 microshutters, dramatically increasing an instrument’s field of view.

“The array that is flying on FORTIS is a technology development prototype for the big one,” Greenhouse said.

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The role of the microshutters is NOT to act as pinholes or coded aperture and to produce a focused image, similar to a mirror or lens.
Its job is just to separate the the light from different sources. This is achieved simply by opening only the shutters corresponding to the sources of interest. See the following image:

enter image description here

This is important for spectroscopy as here the light is split in its wavelengths and dispersed in one direction by a grating or prism. That is why Webb's micro-shutter is only used for spectroscopy. In order to not have a bunch of overlapping spectra you need some shutters to block everything you are not interested in. The spread out image of multiple sources on the detector than looks like this:

enter image description here

In other words: As you can not detect a 3D image (2 spacial + 1 spectral) on a 2D detector, you have to make some sacrifice by blocking much of your field of view.

This sketch makes the process quite clear:

enter image description here

Further sources: Wikipedia Closed-source micro-shutter decription

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  • $\begingroup$ Thank you for your answer! To double check, does the shutter array takes the place of a traditional linear slit? I remember placing a slit of a high-dispersion spectrometer across Jupiter's equator in an undergraduate lab exercise (a zillion years ago) and measuring the Doppler-induced tilting of the resulting lines. In this case, do the shutters just form a non-linear slit? Moving in the direction perpendicular to the dispersion, only zero or one is opened at each "height"? $\endgroup$ – uhoh Oct 24 '19 at 23:45
  • $\begingroup$ One reason for writing my previous comment is that some vision-impaired people use text-to-speech conversion, so while the "non-linear slit" concept might be self-evident in some images, it doesn't hurt to state it explicitly in text as well, perhaps referring to the images for backup. $\endgroup$ – uhoh Oct 25 '19 at 3:36

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