MUSKY: Multispectral UV Sky camera. Valentina Caricato, Andrea Egidi, Marco Pisani and Massimo Zucco, INRIM

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1 MUSKY: Multispectral UV Sky camera Valentina Caricato, Andrea Egidi, Marco Pisani and Massimo Zucco, INRIM

2 Outline Purpose of the instrument Required specs Hyperspectral or multispectral? Optical design Realization and tests

correct cosine error typical of commercial UV spectroradiomters The target could be

3 Purpose of the instrument Purpose of the device is to create a complete spectro-gonoiometric map of the irradiance of the sky in the UV The map will be used to correct cosine error typical of commercial UV spectroradiomters The target could be acheived with a scanning spectroradiometer, but an imaging technique is preferrable because is much faster

Specifications Goal Specs Field of view > ± 80 Spatial resolution 1 Spectral range 300-400 nm Spectral resolution 5 nm

4 Specifications Goal Specs Field of view > ± 80 Spatial resolution 1 Spectral range nm Spectral resolution 5 nm Good dynamic range > 10 4 The instrument must be compact and transportable to be easily calibrated and operated in the field

Hyperspectral or Multispectral? A hyperspectral imaging system (HI) is a combination of imaging device (a digital camera) and a spectrophotometer.

5 Hyperspectral or Multispectral? A hyperspectral imaging system (HI) is a combination of imaging device (a digital camera) and a spectrophotometer. The obtained data set, known as hyperspectral cube, is a 3D matrix formed by a 2D image combined with a third dimension that is the spectral composition of each pixel of the image.

The INRIM imaging spectrometer: a F-P resonator used as a two beams interferometer The intensity modulated light signal is captured in a video during the F-P cavity length scanning.

6 The INRIM imaging spectrometer: a F-P resonator used as a two beams interferometer The intensity modulated light signal is captured in a video during the F-P cavity length scanning. The cavity length starts fro zero i.e. mirrors in contact The spectral composition is calculated by means of a Fourier Transform based algorithm from the interferogram. M. Pisani and M. Zucco, Optics Express, 17 (10), pp , (2009)

7 Hyperspectral device main elements F-P interferometer Digital camera for video acquisition Photogrphic objective to focus the image on the camera sensor

8 Spectral analysis in the visible range

9 Spectroscopy in the IR region: Long distance atmospheric absorption Direct sunlight Clouds

10 Hyperspectral in the UV Difficulties encountered in finding the right substrates for the F-P mirrors and mostly the right coating After several attempts we proceeded with our original device made from metal coated glass UVNet Workshop, Davos 16 July 2014

11 Test with 5 UV LEDs

12 Hyperspectral analysis of LED target UVNet Workshop, Davos 16 July 2014

13 Test with solar spectrum UVNet Workshop, Davos 16 July 2014

14 Test with solar spectrum UVNet Workshop, Davos 16 July 2014

15 Hyperspectral in the UV Difficulties encountered in finding the right substrates for the F-P mirrors and mostly the right coating After several attempts we proceeded with our original device Preliminary results are good but limited to 320 nm because of the glass substrate UVNet Workshop, Davos 16 July 2014

16 Multispectral solution: safer option 11 band-pass filters covering the nm range UVNet Workshop, Davos 16 July 2014

$the UV is not commercially available must be designed from scratch A refractive design although possible would be extremely$

17 Design of the optical system A fisheye objective with good efficiency in the nm range is required A fisheye objective in the UV is not commercially available must be designed from scratch A refractive design although possible would be extremely complex

$A catadioptric system combines a traditional refractive system with a mirror Catadioptric scheme from an early 20 th$

18 Catadioptric solution A wide angle image can be easily obtained by looking «through» a convex mirror. A catadioptric system combines a traditional refractive system with a mirror Catadioptric scheme from an early 20 th century patent

with aluminium protected by a thin layer of SiO 2

19 Realization of the mirror The miror has been realized starting from a glass lens vacuum coated with aluminium protected by a thin layer of SiO 2 The reflectivity exceeds 80% in the range of interest

20 The objective is made by UKA optics from quartz lenses coated MgF 2. Is a 25 mm f= 2.8 lens with a transmittivity of 85% from 200 to 300 nm The objective

21 CCD/camera requiriments sufficient absolute quantum efficiency down to 300 nm; high frame rate in order to acquire a sufficient number of frames in a small time (in case of hyperspectral imaging). small transversal size, because we have to minimize the shadow projected by the camera itself on the spherical mirror in order to scan the maximum portion of the sky above the system;

Selected: Kodak KAI 4022 CCD Scientific CCD with sufficent responsivity in the UV (>5%

22 Selected: Kodak KAI 4022 CCD Scientific CCD with sufficent responsivity in the UV 300nm), good dynamic range (16 bit) and speed, excellent spatial resolution (4 Mpixel)

23 Ascent 4000 camera Kodak sensor (version without microlenses) is integrated in the Ascent 4000 camera with dual 16 bit ADCs. Compact and cooled

24 Building a compact filter wheel

25 Integration of the filter wheel Objective Step Motor Filter wheel with box UVNet Workshop, Davos 16 July 2014

26 Assembled system Aluminium frame Honeycomb plate UVNet Workshop, Davos 16 July 2014

27 Resolution and sensitivity test Camera set: 4x4 binning = 512x512 pixel image Objective aperture f=8

28 Resolution at zenith Angular resolution << 1 Angular sensitivity 4.4 pixel/deg 1

29 Resolution close to the horizon 1 1 Angular resolution < 1 Azimuth sensitivity 4.4 pixel/deg Zenith sensitivity 2.5 pixel/deg

30 Angle of view The angle of view has been evaluated theoretically and verified experimentally. Sky coverage exceeds ± 80 as required. Edge of the mirror 6.5 above the horizon

31 Resolution and mapping With 4x4 binning the resolution exceeds 2pix/deg (worst case), the image dimension is 512x512x16bit A complete mapping of the angular coordinate of each pixel must be obtained experimentally

32 Angle-Pixel calibration UVNet Workshop, Davos 16 July 2014

33 Method to calibrate Central Catadioptric Systems UVNet Workshop, Davos 16 July 2014

α( ) Calibration using a laser Distance r α 90 80 70 60 50 40 30 20 10 0 Light spot

34 α( ) Calibration using a laser Distance r α Light spot UVNet Workshop, Davos 16 July 2014 α vs r Image Centre r (pixel)

35 Zenith Angle-Pixel calibration UVNet Workshop, Davos 16 July 2014

36 Responsivity calibration at IMU MUSKY Mario Massimo IMU Reference Spectrogoniometer UVNet Workshop, Davos 16 July 2014

37 Calibration at IMU (with strong wind!) «active» sun shield UVNet Workshop, Davos 16 July 2014

38 Calibration Procedure W 90 We have compared the reference specta with the spectra from Musky image taken in the same coordinate at about the same time S N UVNet Workshop, Davos 16 July E

39 Calibration Procedure Filters transmission Raw reference Weighted reference Resampled reference MUSKY output UVNet Workshop, Davos 16 July 2014

40 Rm Calibration Procedure 9,0 Calibration Parameter Ascent elevation 60 8,0 7,0 6,0 5,0 4,0 3,0 2,0 1,0 0, l (nm) UVNet Workshop, Davos 16 July 2014

41 Sky measurements in Davos UVNet Workshop, Davos 16 July 2014

42 Sky measurements in Davos Increasing wavelength UVNet Workshop, Davos 16 July 2014

Conclusions 390 370 A Multispectral Fish-eye

Spectral resolution: 11 bands in the 300-400

Zenith angle An UV Hyperspectral device has

43 Conclusions A Multispectral Fish-eye camera in the UV has been built and tested Spectral resolution: 11 bands in the nm range Angular resolution < 1 up to 83 Zenith angle An UV Hyperspectral device has been realized obtaining good preliminary results

44 Thank you!

Chemistry 524--"Hour Exam"--Keiderling Mar. 19, pm SES

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