Instrument description

Fig. 1: VTF optomechanical layout. Light path through is shown in yellow, starting from the main imaging lens; mechanical support structure is depicted in red; other optomechanical components are drawn in different colors. The locations of the large Etalons are highlighted, as well as those of the focal-plane detectors (2 for the dual-beam spectro-polarimeter, 1 for the broad-band channel).
Fig. 2: same as Fig. 1, but seen from the other side.
Fig. 3: Full-size Etalon prototype plate, diameter 350 mm, high-reflectivity coating applied; coated diameter is 275 mm.
Fig. 4: Upper plate of first Etalon, superpolished. The three cut-out segments will accommodate the piezo actuators needed to tune the distance between the two glass plates of the Etalon.
Fig. 5: Upper plate of first Etalon with high-reflectivity coating in the central 280 mm. The plate has a diameter of 350 mm and a thickness of 100 mm.

 

 

 

Mission

The central mission of the Visible Tunable Filter, VTF, is to spectrally isolate narrow-band images of the Sun at the highest possible spatial and temporal resolution from the DKIST telescope. The instrument should allow observations that allow rapid imaging spectrometry, Stokes imaging polarimetry, and accurate surface photometry. From the 2D spectral data, Doppler velocity maps, transverse flows, and maps of the magnetic field components will be derived to track evolutionary changes of solar activity on spatial scales between 20 km and 40000 km.

 

 At first light of DKIST, the VTF will be available in a configuration with one hi-res Etalon, while the final instrument will have two matched Etalons to maximize the free spectral range. The parameters for the 1-Etalon configuration (1 ET) are given in parentheses and in italics.

 

 

Etalon development

The VTF is based on a pair of large-format etalons with a clear aperture of 250 mm each. The development of these Etalons, by far the largest ever built world-wide, was a significant effort. In spring 2017, the first pair of etalon plates was successfully coated with unprecedented values for micro-roughness (0.3 nm RMS), and figure error (3 nm RMS) at LMA in Lyon. Super polishing and metrology is done at Zygo-Ametek in Richmond, California. The optical design of the instrument enables diffraction-lim­ited spectroscopic imaging with a bandwidth of 6 pm (at 600 nm). Tuning of the air gap be­tween the Etalon plates is performed with piezo actuators (built by Physik Instrumente, Karlsruhe) oper­ated in closed loop, using optical sensors to measure the distance between the plates. These optical sensors, procured from Heidenhain GmbH, were qualified for a precision of 0.1 nm by KIS. The integration of all components to a tunable Etalon, as well as the verification of the performance, will be carried out by the VTF team of KIS.

 

Instrument Modes Available:

Narrow-band channel (NBC):

·       Polarimetric Imaging (spectro-polarimetry)

·       Doppler Imaging (spectroscopy)

·       Intensity imaging (monochromatic imaging, i.e. fixed wavelength, 6 pm bandwidth

Broad-band channel (BBC):

In all modes, simultaneous continuum images are taken, using a 1-2 nm prefilter with a wavelength close to that of the NBC. The BBC images have the same field of view, spatial sampling, exposure time, and cadence as the NBC. Differences in light level are compensated by an internal "dimmer". BBC data are needed for Speckle reconstruction of the NBC data, and are available for scientific analysis.

 

Default mode: Spectro-polarimetric imaging, full Stokes, 8 repetitions, critical spectral sampling, nested scan with equidistant wavelength steps, no binning of camera pixels

Supported modes:

Camera pixel binning (identical for all cameras):

  • 2x2
  • 3x3
  • 4x4

Spectral sampling:

  • non-equi-distant sampling (user-defined wavelength points within available scan range)
  • undersampling (for strong chromospheric lines)

Wavelength scanning:

  • nested scan (start at red end, jump to blue end, continue toward line center)
  • red to blue

Multi-line observations

  • Up to nine pre-filters can be mounted in filter whelle and used in multi-line instrument programs

Spatial field of view and resolution

Optical:  60 arcsec x 60 arcsec (42,000 km x 42,000 km on the Sun)

Fixed spatial sampling:  0.014 arcsec/pixel; critically samples diffraction limited resolution of 0.028 arcsec @ 520 nm.  Longer wavelengths are oversampled.

 

Spectral range and resolution

Range:  520 – 870 nm  (1 ET: 585-870 nm)

Each selected spectral band requires its own narrow-band interference filter ("prefilter"); the width depends on wavelength (0.4 nm at 520 nm, 1,0 nm at 850 nm). The usable scan range corresponds roughly to the filter width, limited by the photon flux in the wings of the filter. Prefilters are placed near the entrance focal plane of the instrument, clear aperture of the filters is 60 mm.

 

Pre-filters for selected spectral lines:  Fe I 525.02 nm (photosphere, Landé factor 3, not for 1-ET configuration), Fe I 630.25 nm (photosphere, Landé factor 2.5), Ha 656.3 nm (chromosphere, Landé factor ¹ 0), Ca II 854.2 nm (chromosphere, Landé factor 1.1)

 

Spectral bands have free spectral ranges between 0.5 nm (520 nm) and 1.0 nm (854nm)

(1-ET configuration: between 0.14 nm (585 nm) and 0.3 nm (854 nm)

Resolution: 6 pm (3 pm sampling) at 600 nm (R = 100000)

 

Temporal cadence

Temporal cadence is coupled to the signal-to-noise ratio and depends on instrument mode:

 

Basic timing equation:

For single-line obs: Tline = [n_k x (te + ts) x nj + tw] x nl 

For multi-line obs: Ttot = {[n_k x (te + ts) x nj + tw] x nl + tf + twait } x nf

 

Default values for spectropolarimetric imaging:

te+ts = tw = 33.4 ms, to stay in sync with camera readout te can be shortened, if needed, but cadence will not change. Frame cycle time = 33.4 ms

 

With the above values for ts,  te, and tw, and for a single spectral line, the timing equation simplifies to

                                          Tline = [nk x nj + 1] x nl

Results

a) for a single line (examples):

 

1. Intensity Imaging, burst of 50 images (tw=0): T = 1.67 s  

2. Doppler Imaging:              a) Chromosphere: 51 wavelength points, 1 exposure per wavelength: 3.4 s

                                                 b) Photosphere:    12 wavelength points, 4 exposures per wavelength: 2.0 s

3. Spectropolarimetric img: 11 wavelength points, 4 pol. states, 8 repetitions (->3 x 10-3 P/Icont) : 12.1 s

                                          1 ET: 11 wavelength points, 4 pol. states, 10 repetitions (->3 x 10-3 P/Icont) :   14.7 s

 

b) for multi-line observations (examples):

1. Intensity Imaging, burst of 50 images (tw=0), 3 different pre-filters: [1.67  + 2.0  + (2.0-1.67)] x 3 = 12.0 s

2. Doppler imaging, Chromosphere, 51 wavelength points, 1 exposure per wavelength: [3.4 + 2.0 + 0] x 3 = 16.2 s

3. Spectropolarimetric img: 11 wavelength points, 4 pol. states, 8 repetitions: [12.1 + 2.0 + 0] x 3 = 42.3 s

 

 

Polarimetric capabilites and accuracy

  The VTF provides full Stokes vector polarimetry with a dual beam polarimeter to minimize seeing crosstalk. Using a spectral line with large Zeeman splitting (Landé factor ≥ 2.5), the instrument achieves 3 x 10-3 P/Icont  polarimetric signal. This requires a signal-to-noise ratio of ≥ 650. Based on the DKIST+VRF photon flux budget, 8 repeated exposures with 0.025 s exposure time each are required.

 

Polarization modulation: The VTF uses ferroelectric liquid crystal retarders together with a polarizing beam splitter.

 

Data structure & volume

Narrowband channel (NBC):

Intensity imaging: 12 images = 0.4 GB;   12 images or more. For post-facto reconstruction, single image will not have full spatial resolution, speckle needs at least 40 images

Doppler imaging: Strong line: 51 images = 1.6 GB; weak line: 48 images: 1.5 GB

Spectropolarimetric imaging:  352 images per beam (two orthogonal polarization st.); total:  2 x 11.26 = 22.52GB

 

Broadband channel (BBC): Takes the same number of images, in sync with the NBC, i.e. data volume is 50% of NBC.

 

Example modes of operation

(A) Purpose: Waves in the solar atmosphere, coupling between photo- and chromosphere.

Instrument Mode: Spectro-polarimetric imaging in two lines, 630.25 nm (photosphere), 854.1 nm (chromosphere).

Parameter settings: default, i.e., P-C1-M1-S8 for 630.25 and for 854.1

Cadence: 30.5 s, according to VTF timing equation

Duration: 60 min (115 2D Stokes spectra)

Data volume: 115 data sets * 11.26 GB/cam = 3.88 TB

Options:

(i) use 2x2 binning for 854.1 nm, to increase SNR

(ii) use non-equidistant line scanning or undersampling to get more line wing information.

 

(B) Purpose: Magneto-convection in the photosphere

Instrument Mode: Spectro-polarimetric imaging in one line, 630.25 nm (photosphere)

Parameter settings: default, i.e., P-C1-M1-S8 for 630.25

Cadence:12.1 s, according to VTF timing equation (see Sect. "Temporal Cadence")

Duration: 60 min (265 2D Stokes spectra)

Data volume: 265 data sets * 11.26 GB/cam = 9.96 TB

Option: use fewer wavelength points, either to decrease data volume or to speed up cadence. Use 2x2 pixel binning to increase SNR and decrease data volume.