SPIES - A Testbed for DL-NIRSP

The SpectroPolarimetric Imager for the Energetic Sun (SPIES) is an ever-evolving instrument that functions as a testbed for new concepts related to fiber-optic based imaging spectropolarimeters. SPIES, in its current state, is a scaled down version of DL-NIRSP. It operates with a single spectral channel using a BiFOIS integral field unit (IFU) that is 80% as large as the one planned for DL-NIRSP.

The DL-NIRSP design leverages a great deal of experience from SPIES, and the team continues to work with SPIES to develop optimal techniques of operation and data processing.

Observations from the Dunn Solar Telescope

The movie below illustrates the type of data one should expect from DL-NIRSP, albeit DL-NIRSP will have much higher spatial resolution than SPIES. This movie consists of SPIES data (without polarimetry) in the He I 1083 nm spectral window, acquired at the Dunn Solar Telescope with the BiFOIS-4K IFU (see below).

Each map results from the scanning of the solar image across the IFU spectrograph in a 5 x 5 pattern. Thus only 25 exposures were used to build one full spatial/spectral map of this sunspot. A comparable single-slit spectrograph would require 300 exposures. For this data, 250 spectral samples of a R~250000 spectra are recorded for every 0.3'' spatial pixel. Thus, a single map's data cube contains 300 x 320 spatial pixels and 250 spectral pixels. For these observations a full map is acquired every 30 seconds.

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The BiFOIS-4K is a fiber-optic IFU based on the Birefringent Fiber-Optic Image Slicer design. It contains 15,360 individual fiber cores and provides an imaging format of 60 x 64 spatial pixels for each placement of an image on its aperture.

Below are construction images of the BiFOIS-4K device. The entrance plane of the IFU consists of a closely packed bundle of 120 fiber ribbons shown magnified in the upper left image. Black regions in this image correspond to broken and/or damaged fibers. The unit reconfigures the entrance plane fibers into four long slits at the exit plane (lower right). The upper right and lower left images show the remapping of the fiber ribbons inside the IFU unit, which in these images has one of its side panels removed (side-view and an oblique-view, respectively). The distance between the entrance and exit planes is 14 cm.

SPIES He I 1083 nm Spectra

Below is a real exposure from SPIES using the BiFOIS-4K device in the He I 1083 nm spectral channel, which also contains the Si I 1082.7 nm spectral line as well as the 1083.2 nm telluric water line.

Just like for DL-NIRSP, encoded within this single frame is the radiative intensity of the spectra emerging from a two-dimensional spatial field. For the BiFOIS-4K device, the IFU reformats the 2D spatial field into 4 parallel slits, each of which in this case consists of 30 fiber ribbons. The ribbons are separated by 4 dark pixel rows. A Wollaston prism separates the two orthogonally polarized components of each slit, which results in 8 slit spectra in this image (instead of 4).

Dark areas between the spectra (especially near the bottom of the image) correspond to broken and/or damaged fibers. Still the yield of the fibers is great than 90 percent for this engineering device.