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WAITING FOR THE
SHADOW
SOLAR AND LUNAR ECLIPSE OBSERVING |
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Precise extinction corrections are not easy. Extinction is calculated by estimating the number of air masses relative to the air mass you look through when looking straight up. variations due to air temperature, excessive pollutants, particulates or water vapour in the air. This seems to make the problem very complicated because the extinction varies greatly from place to place. The table is an average table that provides an estimate but does not have any way to take exact account of all the variable.
Normal TSE photography procedure would include starting your exposure at about 1/4000 f5.6 ISO 100 to capture Chromosphere then expose every shutter speed down to 4s or 8s to capture the outer corona. If you have a lot of extinction as we did in Central Australia in 2002 (elevation <2 degrees), you can estimate the extinction and start your sequence at a slower shutter speed but still leave a big safety margin. That way, you don't need to waste your time with very fast exposures that will only result in blank frames.
Here is how :
You can find a reprint of the ICQ 1992 paper on correcting for extinction here :
http://www.cfa.harvard.edu/icq/ICQExtinct.html
Read the extinction from the table (in magnitudes). Work out the factor. For convenience, I have converted Daniel Green's extinction table to both factor and aperture stops and tabulated relative to elevation from the horizon rather than zenith angle.
FACTOR = 2.512k
k is the extinction expressed in stellar magnitudes.
The FACTOR can be applied directly to the shutter speed. Multiply the factor by the shutter speed.
If you want to calculate the number of f-stops, f-stops are the square root of the FACTOR.
Go to Fred Espenak's exposure
table
http://eclipse.gsfc.nasa.gov/image/SEexpo.gif
For your equipment ISO
and f-stop, read the shutter speed you will need for the brightest
phenomena you want to photograph. For totality, this will be
Baily Beads or Chromoshpere. Multiply the Factor by the shutter
speed. Start your exposure sequence 3 shutter speeds faster than
this (safety factor) to allow for errors in the extinction estimates.
Example
ISO 100 f=5.6 ELEVATION 2 degrees Observer altitude 500m
Extinction (from ICQ table) 4.59 magnitudes
Chromosphere Exposure (From ESPENAK) = 1/4000
Corrected exposure
1/4000 x 2.512^4.59 = 1/58s (effectively 1/60 sec)
1/60s ÷ 8 = 1/500s (Safety Factor)
Start your exposure sequence at 1/500s and bracket down to the longest exposure you can manage. For low elevation eclipses, the recorded corona is less than higher elevations because the corona is lost in the higher sky fog or brightness found near the horizon. There is no point trying to do 1 minute exposures. Your camera meter will tell you what the sky brightness is. You can use this as a guide to the maximum exposure time.
At the 2002 TSE, the clear desert skies meant that I over estimated the extinction. I didn't have the "safety factor" and so I over exposed the inner corona. If you are observing from a very polluted site, you might elect to ignore the factor.
By way of comparison, when the Sun was setting just after the eclipse at our site, it was still quite bright & yellow to the eye not a dark red blob that was comfortable to look at.
Extinction tables
I've taken the table published in the paper of Green, Daniel W.E., International Comet Quarterly v 14 pp 55-59. In the first table I've reproduced it more or less as per the original article. I've converted zenith angle to elevation. When I've photographed low elevation eclipses, I find it more intuitive to think in terms of elevation rather than zenith angle. In the subsequent tables, I've converted the expression of extinction from magnitudes to the more photographically useful tables of multiplication factor and photographic stops.
To apply the multiplication factor to convert shutter speeds : -
NEW SHUTTER SPEED = OLD SHUTTER SPEED x FACTOR
eg at 2 degrees elevation the factor is 68.3
At ISO100 f5.6 the exposure for Baily's beads are 1/8000s, Chromosphere 1/4000 sec and Prominences(1/2000s)
1/8000s x 68.3 = 1/117s round to 1/125s
1/4000s x 68.3 = 1/58s round to 1/60s
1/2000s x 68.3 = 1/29s round to 1/30s
To allow for errors in the extinction estimates begin your bracketing sequence 2 stops faster than indicated at 1/500s (Baily's Beads) or 1/250s (Chromosphere) or 1/125 (Prominences) and bracket at all shutter speeds slower than indicated up to your maximum exposure.
To apply the photographic stop table to aperture value (Av)
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Example
Applying photographic stops to shutter speeds (Tv)
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Number of stops |
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Shutter speeds |
1/8000 | 1/6000 | 1/400 | 1/3000 | 1/2000 | 1/1500 | 1/1000 | 1/750 | 1/500 | 1/350 | 1/250 | 1/180 | 1/125 | 1/90 | 1/60 | 1/45 |
1/30 |
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Extinction in Magnitudes
From Green, Daniel W.E., International Comet Quarterly v 14 pp 55-59. The left hand column in the original article was expressed as zenith angle. I've converted it to elevation
Height of observer above sea level
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| 0 m | 500m | 1000m | 2000m | 3000m | |
| 89 | 0.28 | 0.24 | 0.21 | 0.16 | 0.13 |
| 80 | 0.29 | 0.24 | 0.21 | 0.16 | 0.13 |
| 70 | 0.3 | 0.25 | 0.22 | 0.17 | 0.14 |
| 60 | 0.32 | 0.28 | 0.24 | 0.19 | 0.15 |
| 50 | 0.37 | 0.31 | 0.27 | 0.21 | 0.17 |
| 45 | 0.4 | 0.34 | 0.29 | 0.23 | 0.19 |
| 40 | 0.44 | 0.37 | 0.32 | 0.25 | 0.21 |
| 35 | 0.49 | 0.42 | 0.36 | 0.28 | 0.23 |
| 30 | 0.56 | 0.48 | 0.41 | 0.32 | 0.26 |
| 28 | 0.6 | 0.51 | 0.44 | 0.34 | 0.28 |
| 26 | 0.64 | 0.54 | 0.47 | 0.37 | 0.3 |
| 24 | 0.69 | 0.59 | 0.51 | 0.39 | 0.32 |
| 22 | 0.75 | 0.64 | 0.55 | 0.43 | 0.35 |
| 20 | 0.82 | 0.7 | 0.6 | 0.47 | 0.39 |
| 19 | 0.86 | 0.73 | 0.63 | 0.49 | 0.4 |
| 18 | 0.91 | 0.77 | 0.66 | 0.52 | 0.43 |
| 17 | 0.96 | 0.81 | 0.7 | 0.55 | 0.45 |
| 16 | 1.02 | 0.86 | 0.74 | 0.58 | 0.48 |
| 15 | 1.08 | 0.92 | 0.79 | 0.62 | 0.51 |
| 14 | 1.15 | 0.98 | 0.84 | 0.66 | 0.54 |
| 13 | 1.24 | 1.05 | 0.91 | 0.71 | 0.58 |
| 12 | 1.34 | 1.13 | 0.98 | 0.76 | 0.63 |
| 11 | 1.45 | 1.23 | 1.06 | 0.83 | 0.68 |
| 10 | 1.59 | 1.34 | 1.16 | 0.91 | 0.74 |
| 9 | 1.75 | 1.48 | 1.28 | 1 | 0.82 |
| 8 | 1.94 | 1.65 | 1.42 | 1.11 | 0.91 |
| 7 | 2.19 | 1.86 | 1.6 | 1.25 | 1.03 |
| 6 | 2.5 | 2.12 | 1.83 | 1.43 | 1.17 |
| 5 | 2.91 | 2.46 | 2.13 | 1.66 | 1.36 |
| 4 | 3.45 | 2.93 | 2.53 | 1.97 | 1.62 |
| 3 | 4.23 | 3.59 | 3.1 | 2.42 | 1.99 |
| 2 | 5.41 | 4.59 | 3.96 | 3.09 | 2.54 |
| 1 | 7.38 | 6.26 | 5.4 | 4.22 | 3.46 |
| 0 | 11.24 | 9.53 | 8.23 | 6.42 | 5.28 |
Extinction as a multiplication factor
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| ALTITUDE OF OBJECT | 0 m | 500m | 1000m | 2000m | 3000m |
| 89 | 1.2 | 1.2 | 1.2 | 1.1 | 1.1 |
| 80 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 |
| 70 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 |
| 60 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 |
| 50 | 1.4 | 1.3 | 1.2 | 1.2 | 1.1 |
| 45 | 1.4 | 1.3 | 1.3 | 1.2 | 1.1 |
| 40 | 1.4 | 1.4 | 1.3 | 1.2 | 1.2 |
| 35 | 1.5 | 1.4 | 1.3 | 1.2 | 1.2 |
| 30 | 1.6 | 1.5 | 1.4 | 1.3 | 1.2 |
| 28 | 1.7 | 1.5 | 1.4 | 1.3 | 1.2 |
| 26 | 1.8 | 1.6 | 1.5 | 1.4 | 1.3 |
| 24 | 1.8 | 1.7 | 1.5 | 1.4 | 1.3 |
| 22 | 1.9 | 1.8 | 1.6 | 1.4 | 1.3 |
| 20 | 2.1 | 1.9 | 1.7 | 1.5 | 1.4 |
| 19 | 2.2 | 1.9 | 1.7 | 1.5 | 1.4 |
| 18 | 2.3 | 2 | 1.8 | 1.6 | 1.4 |
| 17 | 2.4 | 2.1 | 1.9 | 1.6 | 1.5 |
| 16 | 2.5 | 2.2 | 1.9 | 1.7 | 1.5 |
| 15 | 2.7 | 2.3 | 2 | 1.7 | 1.5 |
| 14 | 2.8 | 2.4 | 2.1 | 1.8 | 1.6 |
| 13 | 3.1 | 2.6 | 2.3 | 1.9 | 1.7 |
| 12 | 3.4 | 2.8 | 2.4 | 2 | 1.7 |
| 11 | 3.7 | 3.1 | 2.6 | 2.1 | 1.8 |
| 10 | 4.3 | 3.4 | 2.9 | 2.3 | 1.9 |
| 9 | 5 | 3.9 | 3.2 | 2.5 | 2.1 |
| 8 | 5.9 | 4.5 | 3.6 | 2.7 | 2.3 |
| 7 | 7.5 | 5.5 | 4.3 | 3.1 | 2.5 |
| 6 | 9.9 | 7 | 5.3 | 3.7 | 2.9 |
| 5 | 14.5 | 9.6 | 7.1 | 4.6 | 3.4 |
| 4 | 23.9 | 14.8 | 10.2 | 6.1 | 4.4 |
| 3 | 49 | 27.2 | 17.3 | 9.2 | 6.2 |
| 2 | 145.2 | 68.3 | 38.2 | 17.1 | 10.3 |
| 1 | 890.4 | 317.6 | 143.9 | 48.5 | 24.1 |
| 0 | 31069.3 | 6440 | 1946.7 | 368 | 128.9 |
Extinction in Photographic stops
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| ALTITUDE OF OBJECT | 0 m | 500m | 1000m | 2000m | 3000m |
| 89 | 1.1 | 1.1 | 1.1 | 1 | 1 |
| 80 | 1.1 | 1.1 | 1.1 | 1 | 1 |
| 70 | 1.1 | 1.1 | 1.1 | 1 | 1 |
| 60 | 1.1 | 1.1 | 1.1 | 1 | 1 |
| 50 | 1.1 | 1.1 | 1.1 | 1.1 | 1 |
| 45 | 1.2 | 1.1 | 1.1 | 1.1 | 1 |
| 40 | 1.2 | 1.1 | 1.1 | 1.1 | 1.1 |
| 35 | 1.2 | 1.2 | 1.1 | 1.1 | 1.1 |
| 30 | 1.2 | 1.2 | 1.2 | 1.1 | 1.1 |
| 28 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 |
| 26 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 |
| 24 | 1.3 | 1.3 | 1.2 | 1.1 | 1.1 |
| 22 | 1.4 | 1.3 | 1.2 | 1.2 | 1.1 |
| 20 | 1.4 | 1.3 | 1.3 | 1.2 | 1.1 |
| 19 | 1.4 | 1.3 | 1.3 | 1.2 | 1.2 |
| 18 | 1.5 | 1.4 | 1.3 | 1.2 | 1.2 |
| 17 | 1.5 | 1.4 | 1.3 | 1.2 | 1.2 |
| 16 | 1.5 | 1.4 | 1.4 | 1.3 | 1.2 |
| 15 | 1.6 | 1.5 | 1.4 | 1.3 | 1.2 |
| 14 | 1.6 | 1.5 | 1.4 | 1.3 | 1.2 |
| 13 | 1.7 | 1.6 | 1.5 | 1.3 | 1.3 |
| 12 | 1.8 | 1.6 | 1.5 | 1.4 | 1.3 |
| 11 | 1.9 | 1.7 | 1.6 | 1.4 | 1.3 |
| 10 | 2 | 1.8 | 1.7 | 1.5 | 1.4 |
| 9 | 2.2 | 1.9 | 1.8 | 1.5 | 1.4 |
| 8 | 2.4 | 2.1 | 1.9 | 1.6 | 1.5 |
| 7 | 2.7 | 2.3 | 2 | 1.7 | 1.6 |
| 6 | 3.1 | 2.6 | 2.3 | 1.9 | 1.7 |
| 5 | 3.8 | 3.1 | 2.6 | 2.1 | 1.8 |
| 4 | 4.8 | 3.8 | 3.2 | 2.4 | 2.1 |
| 3 | 7 | 5.2 | 4.1 | 3 | 2.4 |
| 2 | 12 | 8.2 | 6.1 | 4.1 | 3.2 |
| 1 | 29.8 | 17.8 | 11.9 | 6.9 | 4.9 |
| 0 | 176.2 | 80.2 | 44.1 | 19.1 | 11.3 |
Resources
Eclipse Photography
Fred Espenak's guide to eclipse photography
http://eclipse.gsfc.nasa.gov/SEhelp/eclipsePhoto.html
Bill Kramer's comprehensive guide on all aspects of eclipse photography
http://www.eclipse-chasers.com/Photo.html
Bill Kramer's eclipse photography exposure calculator includes a function
for estimating and correcting for atmospheric extinction
http://www.eclipse-chasers.com/tsePhotoExposure.html
Atmospheric Extinction References
http://www.asterism.org/tutorials/tut28-1.htm
http://www.cfa.harvard.edu/icq/ICQExtinct.html
http://ganymede.nmsu.edu/holtz/a535/ay535notes/node6.html
http://en.wikipedia.org/wiki/Extinction_(astronomy)
http://mintaka.sdsu.edu/GF/explain/extinction/extintro.html
http://www.starlink.rl.ac.uk/star/docs/sc6.htx/node15.html
http://www.skyandtelescope.com/howto/visualobserving/19712459.html
