1. Oskar von der Lühe and the VTF Team
The Visible Tuneable Filtergraph at the Daniel K. Inouye Solar Telescope
Oskar von der Lühe and the VTF Team
J. Baumgartner, A. Bell, N. Bello Gonzales, M. Ellwart, A. Fischer, D. Gisler,
C. Halbgewachs, F. Heidecke, T. Kentischer, W. Schmidt, M. Schubert,
T. Scheiffelen, B. Schäfer, M. Sigwarth

2. Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop
Contents
Overview
Science Capabilities
Technical challenges
Making The Impossible Possible
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

3. Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop
VTF Summary
High-resolution imager (limited only by the telescope) with the diagnostic capabilities of a tunable spectro-polarimeter.
Spectral coverage 520 and 860 nm.
Within a field of view of 41,000 km x 41,000 km on the Sun, the VTF measures
• the magnetic field vector,
• the mass flow and
• the brightness
with 20 km resolution within ~10 seconds.
Overall dimensions are about 2.5m x 3.5m x 5.5m, with a weight of about 5 tons. Compares well with an elephant!
The instrument is developed and built at the Kiepenheuer-Institut in Freiburg as a first-light instrument for the Daniel K. Inouye Solar Telescope with contributions from NSO and MPS.
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

4. VTF as a DKIST Facility Instrument
VTF configuration at the DKIST Coude lab
VTF is the only instrument penetrating the floor into the mezzanine level.
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

5. Example of VTF-type data products - ImaX
Intensity
Velocity
VTF: 4 x better resolution
Circular
polarization
Linear
polarization
Image: Sunrise ImaX, Solanki et al., 2010

6. Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop
VTF Science Cases
Topic
What the VTF provides
disappearance of sunspots
Sufficiently large field of view
Magnetic field and mass flow
Fast measurement cadence (10 s)
Quiet sun magnetism
High magnetic sensitivity (20 G)
Heating of the chromosphere
Measures different layers of the atmosphere (photo- & chromosphere)
Fast cadence for 2 lines (20 s)
Magnetic flux tubes
High spatial resolution (20 km)
High magnetic sensitivity
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

7. VTF optical design FP1 FP2 Cam2 Cam1 FP2 FP1 L1 M5 M8 Cam1 Cam2
Field stop
Filter wheel
FP1
FP2
Cam2
Cam1 L1

8. VTF Science Capabilities
Description
Value
Comment
Wavelength coverage
520 – 870 nm (585 – 870 nm)
Spatial resolution
0.028” (0“014 / pixel)
diffraction limited
Spectral resolution
6 pm (3 pm sampling)
at 600 nm (Rλ = 105)
Spectral scan range
1 nm (0.15 – 0.4 nm)
at 600 nm
Cadence for full Stokes
12 … 15 s
SNR=650
Field of view
60” x 60 ”
4 k x 4k detector
Doppler sensitivity
85 m/s
Polarimetric sensitivity
3 x 10-3
SNR = 650
Red: single etalon mode
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

9. Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop
Cadences
VTF timing equation, single line
nk number of repetitions, determines SNR
nj polarization states (1, 2, 4)
nl number of wavelength samples
Time for prefilter change: 2 s
Examples
Intensity Imaging, burst of 50 images: s
Doppler maps, chromosphere: 51 ws, 1 rep s
photosphere, 12 ws, 4 rep s
Spectropolarimetry, 11 ws, 4 pol. states, 8 rep (->3 x 10-3 P/Icont) 12.1 s
(Single etalon, 11 ws, 4 pol. states, 10 rep (->3 x 10-3 P/Icont) s)
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

10. Instrument modes and parameter settings
Default (P-C1-M1-S8)
Supported modes
Expert modes
Instrument
Polarimetric imaging
Doppler mapping
Intensity imaging
Cameras
1x1 binning (C1)
2x2, 4x4
whatever the camera allows, e.g., subfields
Spectral sampling
Critical
non-equidistant patterns
equidistant undersampling
weird patterns
Scanning
Nested, starting from red continuum (M1)
sequential (red to blue or vice versa)
weird scan schemes
Repetitions
8 repetitions (S8)
1, 4, 12 repetitions
any reasonable number
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

11. VTF scan range
1 ET
2 ET

12. Simulated VTF observations
Blue: 3 Etalons, Red: 2 Etalons
1 Etalon, Blue: 0.3 nm Red: 0.2 nm prefilter
Comparison of synthetic Stokes V profiles with simulated VTF observations with 1, 2, and 3 Etalons; R = 100,000; ideal modulator; photon noise with SNR=670.

13. Performance from simulations
B field inversions for photosphere, chromosphere vertical and horizontal
Single and dual etalon
Line shifts caused by a large-scale air-gap variation of 5 nm RMS, expressed in Doppler velocity. The small-scale pattern seen in the figure is the (simulated) solar granulation pattern
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

14. VTF Selected Spectral Lines
Landé factor
Origin
Fe I 3
Photosphere
Fe I 543.4
Fe I 557.6
He I 587.5 ≠0
Prominences
Fe I 617.2
2.5
Fe I 630.3
H I 656.3
Chromosphere
Ni I 676.8
1.5
Ca II
1.1
The initial single etalon configuration will be provided with a limited selection of wavelengths for photospheric lines only
The VTF observes one line at a time. Up to 8 lines can be observed sequentially. All lines within VTF wavelength range can be observed. A narrowband (0.8 – 1.6 nm) filter is needed for each line.

15. VTF Instrument Performance Calculator
Tool for the observing scientist to plan an observation and to chose instrument parameters
Provides line scans, timing, SNR etc. for given instrument parameters
Supports up to ten prefilters
Will be available by end of 2017

16. Technical Challenges
The size of the etalons is determined by the étendue of the telescope
The 4m DKIST needs 250 mm free aperture etalons for the required FOV
No commercial vendor. We had to form an industrial consortium
We are building the World‘s largest etalons
The technology is the same as for LIGO
Commercial ET 50
350 mm plate diameter
100 mm plate thickness
0.55 mm air gap, mm steps
Plates made of highest-quality synthetic quartz (Heraeus Suprasil 312)
high-reflectivity coating on inner sides

17. Consortium to build the VTF Etalon
Etalon configuration
Etalon Development
Plate Figure & Polishing
Plate Coating
Etalon Assembly
Metrology & Spacing control
Etalon control
Etalon plate geometry
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

18. Challenge: Surface flatness of 3 nm (rms)
Full-size Etalon plate, 350 mm diameter, 250 mm high-reflectivity coating; done at LMA, Lyon
… on a area with a diameter of 250 mm and multi-layer coating.
Figure error 1.5 nm RMS (w/o power), measured at Zygo, Richmond

19. Challenge: Surface roughness: 0.1 nm
This is equivalent to Lake Michigan having waves smaller than 0.1 mm
PVD (Physical Vapor Deposition) produces very smooth surfaces, but also significant (compressive) stress.
CVD (Chemical Vapor Deposition) produces some micro-roughness, but no stress.
Before HR coating: nm RMS
After HR coating: nm RMS
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

20. Challenge: Tuning precision of 0.1 nm
Sensor: Heidenhain LIP382 optical sensor, >2 kHz measuring rate, 50 pm precision achieved in the lab setup (50 mm Etalon)

21. Etalon development status
All key technology problems are solved
Metrology system is procured
Final spacing system is ordered
Plates for first Etalon are polished and coated as specified
Final processing of lower plate to optimize the air gap matching is under way
Delivery of lower plate end of August 2017
Start of etalon assembly in October 2017 at KIS
Upper and lower Etalon plates after IBF coating with the high reflectivity broad band coating. Upper plate shows step cuts for the piezo actuators.
Images: LMA, Lyon
Visible Tunable Filtergraph for DKIST - 30Th Sac Peak Workshop

22. VTF Development Schedule…
2016
2017
2018
2019
2020
2021
2022
250 mm Etalon 1
Instrument frame
Optics
VTF setup at KIS
S/W & Control
Integration
1st Science
250 mm Etalon 2
DKIST Construction
Operation

23. Summary and Conclusions
VTF is a challenging instrument which stretches current technology
VTF is built by KIS with significant contributions from NSO and MPS
The end-to-end cost of the single etalon VTF is about € 12 M, the second etalon adds € 1M. This puts VTF in the class of VLT instruments
VTF provides more than 50% of the overall DKIST science data in certain configurations (1 hour/day only)
Studies of DKIST science cases indicate that VTF will be the most demanded instrument