How does IBIS work?

The following papers summarize the characterization and technical performance of IBIS:
IBIS Layout
The optical layout of IBIS, with the main instrumental components labeled. The filter wheel is located between the two interferometers. The primary optical path is indicated in red.

The main components of IBIS are two Fabry Perot interferometers, 50-mm in diameter, used in series in a collimated mount. The coating of the interferometers is optimized for the 550-860 nm spectral range, so that their combined transmission profiles account for a spectral resolution greater than 200,000 over the whole interval; this corresponds to a FWHM of the instrumental profile of 20-45 mÅ, depending on wavelength.
IBIS passband IBIS spectral res
Left: Transmission profile of the two interferometers: the combined profile is equivalent to the narrower one, with the free spectral range of  the broader one. Right: effective spectral resolution over the whole wavelength range (from Reardon et al. 2008).
The interferometers can be rapidly tuned, simultaneously, so to allow sequential scanning of the spectral lines of interest (see movie in IBIS Intro). The latter are selected via a set of narrowband interference filters inserted in a filter wheel positioned between the interferometers. The currently available filters are (wavelengths in nm):

Fe I 543.4 (g=0) He I D3 587.6 Na I D2 589.0 Na I D1 589.6 Fe I 617.3 Fe I 630.1 / 630.2
H I 656.3 (Hα) Ni I 676.8 Fe I 709.0 (g=0) Fe II 722.4 (g=0) K I 769.9  Ca II 854.2
(All prefilters are 0.3 nm FWHM, apart from the 0.5 nm FWHM filter for Ca II 854.2).

The collimated mount of IBIS allows the instrument to exploit the full resolution of the telescope while imaging a large field ov view (about 95" in diameter; this is the largest amongst similar instruments currently in operation). The pixel size is 0.098″ x 0.098″ and the diffraction limit at 600 nm is ∼0.2″. Further, the overall large transparency of IBIS allows for very short exposures, of the order of 25 ms, permitting the use of image reconstruction techniques such as speckle or blind deconvolution.The fast acquisition system consistently achieves 8 frames/sec (and up to 14 fps for image bursts at a fixed wavelength). Depending on the details of acquisition, a single spectral line can be sampled in matter of few seconds or less.

wl fe Na
Sample images acquired at different wavelengths over a quiet area. FOV is 80" diameter. Left: continuum near FeI 709.0 nm, showing granulation. Some smaller magnetic elements in the center of image can be infered from the distorted granulation. Middle: FeI 709.0 core, sampling the mid photosphere and hence showing reverse granulation (see e.g. Janssen & Cauzzi 2006) and bright magnetic elements. Right: core of NaI D1, with aureoles surrounding magnetic elements formed in high photosphere from 3D resonance scattering (Leenaarts et al. 2010)
Finally, IBIS can function in spectropolarimetric mode by modulating the incoming light with a pair of nematic liquid crystal variable retarders placed ahead of the instrument. The polarized light is further analyzed with a beam splitter positioned in front of the detector. Two orthogonal states of polarization are thus imaged onto the same chip, limiting the field of view to about 40" x 90". For each wavelength position six modulation states [I+S, I-S] are detected.

Comparison of polarimetric signal and structure over a network area, from IBIS (top panels) and the spectropolarimeter on the Hinode satellite (bottom panels). From Judge et al. 2010
Important notes:
  • A maximum of 6 filters can be used in any given observational sequence. This is limited by the slots available in the filter wheel. The 6 filters can be chosen freely among the ones listed in Table above. The wavelength sampling within each spectral line, on the other hand, is highly flexible, limited only by the incremental step size of the Fabry-Perots (4-6 mÅ).
  • When using IBIS in polarimetric mode, the final beamsplitter reduces the available field of view by half. Images in this mode are thus only 40" x 90"
  • The collimated mount of IBIS introduces a blueshift in the instrumental tuning across the field of view, wavelength dependent. It can reach up to 7 pm at the edge of the field for the bluemost lines, and 11 pm for the redmost ones. It is removed within the data reduction pipeline, so to produce fully monochromatic images.
  • The prefilters are narrow enough that true continuum is never accessible when sampling broad chromospheric lines such as Halpha or CaII 854.2 nm, though both lines are able to sample the photospheric structures in the wings of the lines.

Last Update: November 2015