21 March 2017
Jakson Hole - Gros Ventre Road - Wyoming - USA
Local circumstances
Location : North +43°35'48.7" , West 110° 41' 54.9".
Duration of totality : 2mn 20s
C1 : 18h16mn52.6s UT
C2 : 19h35mn00.3s UT , Alt = 50°
Max : 19h36mn10.2 sUT,
C3 : 19h37mn20.2s UT, Alt = 51°
Instrumentation
Telescope :
Skywatcher Maksutov 127 mm F/12 - Measured Scale : 0.761 arcec/pixel.
The optics were measured to lamda/8 P-V in red light : HASO-test.
The optical tube was insolated to reduce focus shift with temperature drop..
Focus accuracy is estimated to 0.1 mm (ie defocus aberration = lambda/6 P-V in red light).
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Seeing conditions :
Average seeing during the toatlity. No cloud, blue sky with some diffusion due to the presence of aerosols (dust ?).
Camera :
Basler 1920-155 with IMX 174 sensor - Field of view : 26.2 x 16.4 arcmin - Passive cooling with heat sink.
Aquisitions in 12 bits, with gain value set to minimum in order to maximise dynamic range and S/N max. Resulting dynamic range = 12.2 bits and S/N max = 7.5 bits.
Gamma value was set to 0.5 => it was a mistake. It results in a loss of grey levels differentiation in the high lights (isophotes in the inner corona).
Indicative IMX174 quantum efficiciency curve. It looks a bit optimistic ... Source : Point Grey : http://www.ptgrey.com/support/downloads/10414?
Filter :
Astrodon Sloan r' filter generation 1 (yellow color). Square transmission from 555 nm to 696 nm :
Telescope mount : Unitron equatorial mount with model 128 electric drive roughly aligned to the celestial pole with compass and inclinometer.
Processing :
- Correction of Fixed Pattern Noise and vigneting (and speckes of dust) by flat image.
- No offset correction.
- Accurate X /Y registration of video frames with Autostakkert 3 software directly on the solar corona : between 3 to 8 registration points. AP size = 400 pixels. All frames of each video files are stacked to maximize S/N (ie.no frame selection).
- Correction of radial gradient : Image = Image / Image convoluted by Gaussian filter (sigma 30 pixels).
- Visualisation : unsharp mask (sigma = 2.5 pixels, k = 25).
North limb
30 s acquisition centered on 17:35:22s UT, 77 fps, 2331 frames acquired, stacked frames = 2331 frames x 10 ms
"unsharp mask" version
Click on image for full-resolution image
North-West limb
30 s acquisition centered on 17h 37mn 02s UT, 53 fps, 1580 frames acquired, stack frames = 1580 frames x 19 ms
Magnitude 7.8 star visible
"unsharp mask" version
Click on image for full-resolution image
East limb
30 s acquisition centered on 17h36mn22s UT, 53 fps, 2000 frames acquired, stack frames = 1580 frames x 19 ms, moving thin clouds
Magnitude 10.2 star faintly visible on upper left corner
"unsharp mask" version
Click on image for full-resolution image