Channels
VTF has two optical channels through which the solar surface is simultaneously imaged: a spectrometer channel (narrow-band channel, NBC) and a continuum channel (broad-band channel, BBC).
The spectrometer channel supports the following measurements:
Polarimetric imaging (spectropolarimetry)
Line-of-sight fields (Stokes I and V)
Vector fields (Stokes I, Q, U, V)
Doppler imaging (spectroscopy)
Intensity imaging (monochromatic imaging at fixed wavelength and 6 pm bandwidth)
In the continuum channel, the same field of view is recorded strictly simultaneously with the same spatial resolution, exposure time and frame rate as in the spectrometer channel. The wavelength range in the continuum channel is selected by a pre-filter with a bandwidth of 1 - 2 nm near the spectrometer channel. If necessary, the luminous flux in the continuum channel is adapted to the dynamic range of the camera using crossed polarisers. The images of the continuum channel can be used to recognise dynamic changes in the observation conditions and to correct the spectrometer data accordingly.
Field of view and spatial resolution
The field of view of VTF is limited by a circle with a diameter of 60 arc seconds (43,000 km on the surface of the sun).
The spatial resolution is determined by the camera pixel size amounting to 0.014 arc seconds/pixel. This means that the diffraction limit of DKIST (0.028 arcsec) is critically resolved in the spectral range at 520 nm.
Spectral range and spectral resolution
The entire spectral range of the fully developed VTF is 520 - 870 nm. The free spectral range of the Etalon 1 + Etalon 2 combination is 0.5 nm (at 520 nm) or 1.0 nm (at 870 nm).
In the initial operation of VTF with an etalon, the spectral range is limited to 585 - 870 nm with the free spectral range coming to only 0.14 nm (at 520 nm) or 0.3 nm (at 870 nm).
Each wavelength interval within the entire spectral range can be selected using a pre-filter whose bandwidth must not exceed the free spectral range. Therefore, the usable scanning range is equivalent to the bandwidth of the pre-filter. The pre-filters have a diameter of 60 mm and are mounted near the first instrument focus.
The spectral resolution of VTF is 6 pm at 600 nm (R = 100 000).
The pre-filters for selected spectral lines are:
| Wavelength | Application | Landé factor | Comments |
| Fe I 525.02 nm | Photosphere | 3 | Not for 1 ET |
| Fe I 630.25 nm | Photosphere | 2.5 | |
| Ha 656.3 nm | Chromosphere | 0 | |
| Ca II 854.2 nm | Chromosphere | 1 |
Polarimetric performance and precision
VTF allows for polarimetric measurements of all four Stokes parameters to be performed in two channels. Orthogonal polarisation states are measured simultaneously in order to avoid crosstalk between Stokes parameters due to seeing. For spectral lines with large Zemman splitting (Landé factor ≥ 2.5), the instrument achieves an intrinsic precision of 3 x 10-3 P/Icont for each Stokes parameter U, Q, or V. This requires a signal-to-noise ratio of 650 for the photon noise. Based on the photon budget for DKIST and VTF, this requires an exposure time of 0.2 s or eight repetitions of 0.025 s elementary exposures.
The polarisation is modulated using a liquid crystal retarder and analysed with a polarising beam splitter.
Observation modes
The following parameters define the observation modes:
Spectrometer channel mode (see above)
Number of repetitions of exposures per polarisation state and spectral step
Spectral step size
Scan pattern
Binning of camera pixels
Target mode: Spectropolarimetry vector fields, 8 repetitions, dense spectral step size, interleaved scan pattern with equidistant steps, no binning.
The following modes are supported:
Number of repetitions: Any number. Large numbers reduce the cadence.
Spectral step size:
Critical: 3 pm equidistant
Not equidistant: Via user-defined wavelengths within the scanning range
Subcritical user-defined equidistant (for strong chromospheric lines)
Scan pattern:
Interleaved (Start at the red end, jump to the blue end, back and forth up to the centre of the scan area)
Monotone from red to blue
Camera pixel binning (identical for all cameras):
2x2
3x3
4x4
Number of spectral areas:
Up to nine pre-filters can be mounted into the filter wheel.
Temporal cadence
The time cadence (time interval between two consecutive scans) depends on the operating mode and the desired signal-to-noise ratio.
Cadence equations:
For single-line observations: T line = n λ ⋅ ( n k ⋅ (t e + t s) ⋅ n j + t w )
For observations with nf pre-filters: T tot = ∑ f =1n f nλ,f ⋅ (nk,f ⋅ ( te + ts) ⋅ nj + tw ) + (nf − 1) ⋅ tf
Specifications for spectropolarimetry:
nk = 8 (1 - 12, number of repetitions)
nj = 4 (fixed, polarisation states)
nl =12 (wavelengths per scan, 9 - 51, line-dependent)
nf ≥ 1 (number of pre-filters in the observation programme)
te = 25 ms (exposure time)
ts = 9,0 ms (modulator switching time)
tw = 34,0 ms (etalon response time, comes to 0 if nl =1, i.e. for intensity mapping)
tf = 2000 ms (pre-filter change time)
twait= 2000 ms - Tline (wait time between successive pre-filter changes if Tline < 2 s)
twait= 0 ms (wait time for Tline ≥ 2 s)
te+ts = tw = 34.0 ms, to stay in sync with camera readout te can be shortened, if needed, but cadence will not change. Frame cycle time = 34.0 m (frame rate = 29.41 Hz)
With the above values for ts, te, andtw, and for a single spectral line, the timing equation simplifies to
Tline = [nk x nj + 1] x nl
Examples
For a single spectral range:
Intensity Imaging, burst of 50 images (tw=0): T = 1.67 s
Doppler Imaging:
Chromosphere: 51 wavelength points, 1 exposure per wavelength: 3.4 s
Photosphere: 12 wavelength points, 4 exposures per wavelength: 2.0 s
Spectropolarimetric img: 11 wavelength points, 4 pol. states, 8 repetitions (->3 x 10-3 P/Icont) : 12.3 s
1 ET: 11 wavelength points, 4 pol. states, 10 repetitions (->3 x 10-3 P/Icont) : 15.3 s
For multiple spectral ranges:
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
Doppler imaging, Chromosphere, 51 wavelength points, 1 exposure per wavelength, 3 lines: [3,4 + 2,0 + 0] x 3 = 16,2 s
Spectropolarimetric img: 4 pol. states, 8 repetitions, Na I 589 (25 positions), Fe 630,25 (11 positions), Ca 854 (21 positions): 68,1 s
Operating examples
Scientific objective: Waves in the solar atmosphere; coupling between photosphere and chromosphere
Operating mode Polarimetric imaging in two lines, 630.25 nm (photosphere), 854.1 nm (chromosphere)
Operating parameters according to specifications (s. above)
Cadence 37.9 s
Total duration 60 minutes with 95 complete Stokes spectra
Data volume 95 * 11.8 GB = 3.4 TB
Options
2x2 binning for 854.1 for better SNR
Non-equidistant spectral scans for better coverage of the line wings
Scientific goal: Magneto-convection in the photosphere
Operating mode polarimetric imaging in one line, 630.25 nm (photosphere)
Operating parameters according to specifications (s. above)
Cadence 12.3 s
Total duration 60 min (295 2D Stokes spectra)
Data volume 295 data records * 11.8 GB = 10.44 TB
Options
Reduced number of wavelength steps for faster cadence or lower data volume
2x2 binning for better SNR and reduced date volume
Data structure and volume
Spectrometer channel:
Intensity mapping 12 exposures = 0.4 GB; minimum of 12 images. At least 40 images should be taken for adequate image reconstruction
Doppler imaging strong lines - 51 exposures = 1.6 GB, weak lines - 48 exposures = 1.5 GB
Polarimetric imaging 352 exposures per channel = 2 x 11.8 GB = 23.6 GB
Continuum channel:
As each exposure in the spectrometer channel is accompanied by a simultaneous exposure in the continuum channel, the data volume is 50% of that of the spectrometer channel.