Astronomical Twilight
Observations for Night Vision
should be taken when the sun contributes no light to the SQM's sky glow
reading. You have to wait until after
astronomical twilight for total natural darkness.
 |
For a custom calendar with (For the "Display Twilight
Times" option,
Linked website by Steve Edwards |
Do we really have to wait that late, especially in the summer?
Astronomer Ferdinando Patat of the European Southern Observatory writes about using SQMs for Night Vision:
"To be on the safe side, one should always wait for the end of astronomical twilight (sun elevation <=-18deg),
since in these conditions, at least at zenith, one is sure the sun is not contributing.
More practically, one can relax this condition to -15 degrees, as you can see from the plots in our recent
paper. On the other hand, the choice also depends on the light pollution at your sites. In heavily polluted sites,
the contribution by the sun might become negligible (with respect to the artificial contribution) much earlier.
... In principle, you could verify this with a simple test. At least on one good night (no clouds), you should prepare your instrument
at sunset and start measuring at zenith as soon as the instrumental dynamical range allows it, and record the brightness
values as the sun goes down, taking note of the time each measurement is done. Keep doing this
into the astronomical twilight. Then, you will be able to produce plots like ours and you will be in the
condition of judging when the sun contribution becomes negligible.
...In a pollution-free site, the difference between nautical (-12) and astronomical (-18) twilight in the V passband is more than
a magnitude, i.e. indeed significant.
... More details on night sky brightness and a general introduction to the field are at
http://www.eso.org/~fpatat/science/skybright/paperII.pdf."
Thank you, Nando.
See also http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0604/0604128.pdf
Another Way of Looking at Astronomical Twilight
| The table below from the U.S. Naval Observatory lists the non-dark hours, beginning in the morning at astronomical twilight and ending that night at astronomical twilight. Between these hours, the sun contributes light to the sky. Observations for the Night Vision program should not be taken between these hours. The times below are from a 24-hour clock (the first two digits are hours, the second two digits are minutes). During Daylight Saving Time (DST), you add one hour to the values given below. For example, if you were to observe on May 23, with DST in effect you should take a reading of the sky glow either after 2306 (11:06 p.m.) or before 0418 (4:18 a.m.). |
o , o , SOUTH BEND, INDIANA Astronomical Applications Dept.
Location: W086 15, N41 40 Astronomical Twilight for 2006 U. S. Naval Observatory
Washington, DC 20392-5420
Eastern Standard Time
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
Day Begin End Begin End Begin End Begin End Begin End Begin End Begin End Begin End Begin End Begin End Begin End Begin End
h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m h m
01 0632 1905 0622 1936 0548 2008 0453 2046 0354 2131 0308 2219 0305 2233 0346 2156 0432 2056 0509 1959 0543 1914 0613 1854
02 0632 1906 0621 1937 0546 2009 0451 2048 0353 2133 0307 2220 0305 2232 0347 2154 0434 2054 0510 1957 0544 1912 0614 1854
03 0632 1907 0620 1938 0545 2010 0449 2049 0351 2134 0306 2221 0306 2232 0349 2152 0435 2052 0511 1956 0545 1911 0615 1854
04 0633 1908 0619 1939 0543 2011 0447 2050 0349 2136 0305 2223 0307 2231 0350 2151 0436 2050 0513 1954 0546 1910 0616 1854
05 0633 1909 0618 1940 0541 2012 0445 2052 0347 2138 0304 2224 0308 2230 0352 2149 0438 2048 0514 1952 0547 1909 0617 1854
06 0633 1909 0617 1942 0540 2014 0443 2053 0345 2139 0303 2225 0309 2230 0354 2147 0439 2047 0515 1951 0548 1908 0618 1854
07 0633 1910 0616 1943 0538 2015 0441 2054 0344 2141 0303 2226 0310 2229 0355 2145 0440 2045 0516 1949 0549 1907 0619 1854
08 0633 1911 0615 1944 0536 2016 0439 2056 0342 2143 0302 2227 0311 2228 0357 2143 0442 2043 0517 1947 0550 1907 0619 1854
09 0633 1912 0614 1945 0535 2017 0437 2057 0340 2144 0302 2228 0312 2227 0358 2141 0443 2041 0518 1946 0551 1906 0620 1854
10 0632 1913 0613 1946 0533 2018 0435 2059 0338 2146 0301 2228 0314 2226 0400 2140 0444 2039 0519 1944 0553 1905 0621 1855
11 0632 1914 0612 1947 0531 2020 0433 2100 0337 2147 0301 2229 0315 2225 0401 2138 0446 2037 0520 1942 0554 1904 0622 1855
12 0632 1915 0611 1948 0530 2021 0431 2102 0335 2149 0300 2230 0316 2224 0403 2136 0447 2035 0521 1941 0555 1903 0623 1855
13 0632 1916 0610 1949 0528 2022 0429 2103 0333 2151 0300 2231 0317 2223 0405 2134 0448 2033 0522 1939 0556 1902 0623 1855
14 0632 1917 0608 1951 0526 2023 0427 2105 0332 2152 0300 2231 0319 2222 0406 2132 0449 2031 0524 1938 0557 1902 0624 1855
15 0631 1918 0607 1952 0524 2024 0425 2106 0330 2154 0259 2232 0320 2221 0408 2130 0450 2029 0525 1936 0558 1901 0625 1856
16 0631 1919 0606 1953 0523 2026 0423 2107 0328 2156 0259 2232 0321 2220 0409 2128 0452 2027 0526 1935 0559 1900 0625 1856
17 0631 1920 0605 1954 0521 2027 0421 2109 0327 2157 0259 2233 0323 2218 0411 2126 0453 2025 0527 1933 0600 1900 0626 1856
18 0630 1921 0603 1955 0519 2028 0419 2110 0325 2159 0259 2233 0324 2217 0412 2124 0454 2023 0528 1932 0601 1859 0626 1857
19 0630 1922 0602 1956 0517 2029 0417 2112 0324 2200 0259 2234 0326 2216 0414 2122 0455 2021 0529 1930 0602 1859 0627 1857
20 0630 1923 0601 1957 0515 2031 0415 2114 0322 2202 0259 2234 0327 2214 0415 2120 0457 2019 0530 1929 0603 1858 0628 1858
21 0629 1924 0559 1958 0513 2032 0413 2115 0321 2203 0300 2234 0329 2213 0417 2118 0458 2017 0531 1927 0604 1858 0628 1858
22 0629 1925 0558 2000 0512 2033 0411 2117 0319 2205 0300 2234 0330 2212 0418 2116 0459 2015 0532 1926 0605 1857 0629 1859
23 0628 1926 0557 2001 0510 2034 0410 2118 0318 2206 0300 2234 0332 2210 0420 2114 0500 2014 0533 1925 0606 1857 0629 1859
24 0628 1927 0555 2002 0508 2036 0408 2120 0317 2208 0300 2234 0333 2209 0421 2112 0501 2012 0534 1923 0607 1856 0630 1900
25 0627 1928 0554 2003 0506 2037 0406 2121 0315 2209 0301 2234 0335 2207 0423 2110 0502 2010 0535 1922 0608 1856 0630 1900
26 0626 1929 0552 2004 0504 2038 0404 2123 0314 2211 0301 2234 0336 2206 0424 2108 0504 2008 0537 1921 0609 1856 0630 1901
27 0626 1930 0551 2005 0502 2040 0402 2125 0313 2212 0302 2234 0338 2204 0425 2106 0505 2006 0538 1919 0610 1855 0631 1902
28 0625 1932 0549 2007 0500 2041 0400 2126 0312 2214 0302 2234 0339 2202 0427 2104 0506 2004 0539 1918 0611 1855 0631 1902
29 0624 1933 0458 2042 0358 2128 0311 2215 0303 2233 0341 2201 0428 2102 0507 2003 0540 1917 0612 1855 0631 1903
30 0623 1934 0456 2043 0356 2129 0310 2216 0304 2233 0343 2159 0430 2100 0508 2001 0541 1916 0613 1855 0632 1904
31 0623 1935 0455 2045 0309 2218 0344 2157 0431 2058 0542 1915 0632 1904
Add one hour for daylight time, if and when in use.
| Twilight: Before sunrise and again after sunset
there are intervals of time, twilight, during which there is natural light
provided by the upper atmosphere, which does receive direct sunlight and
reflects part of it toward the Earth's surface. Some outdoor activities
may be conducted without artificial illumination during these intervals,
and it is useful to have some means to set limits beyond which a certain
activity should be assisted by artificial lighting. The major determinants
of the amount of natural light during twilight are the state of the
atmosphere generally and local weather conditions in particular.
Atmospheric conditions are best determined at the actual time and place of
events. Nevertheless, it is possible to establish useful, though
necessarily approximate, limits applicable to large classes of activities
by considering only the position of the Sun below the local horizon.
Reasonable and convenient definitions have evolved.
Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities. Complete darkness, however, ends sometime prior to the beginning of morning civil twilight and begins sometime after the end of evening civil twilight. Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct. Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible. |