Index of /~jhollan/dirs/light/luminance/tempel

[ICO]NameLast modifiedSizeDescription
[PARENTDIR]Parent Directory  -  
[ ]tempel.zip2005-07-08 17:58 21Mall directory zipped
[IMG]04.jpg2005-06-28 23:13 1.1Mcamera-stored jpeg
[IMG]07.jpg2005-07-03 22:40 1.1Mcamera-stored jpeg
[IMG]11.jpg2005-07-03 22:43 1.1Mcamera-stored jpeg
[IMG]04.png2005-07-07 19:25 133Kcolour coded luminan.
[IMG]07.png2005-07-07 19:25 133Kcolour coded luminan.
[IMG]11.png2005-07-07 19:26 133Kcolour coded luminan.
[ ]04.eps2005-07-07 19:25 38Kgrid in PostScript
[ ]04_128.eps2005-07-07 19:29 3.2Kgrid in PostScript
[ ]07.eps2005-07-07 19:25 38Kgrid in PostScript
[ ]07_128.eps2005-07-07 19:28 3.2Kgrid in PostScript
[ ]11.eps2005-07-07 19:26 38Kgrid in PostScript
[ ]11_128.eps2005-07-07 19:29 3.2Kgrid in PostScript
[IMG]04d.jpg2005-07-07 19:51 67Kimage - darkframe
[IMG]07d.jpg2005-07-07 19:51 67Kimage - darkframe
[IMG]11d.jpg2005-07-07 19:51 70Kimage - darkframe
[TXT]04.txt2005-07-07 19:09 361 jhead (exif) info
[TXT]07.txt2005-07-07 19:08 361 jhead (exif) info
[TXT]11.txt2005-07-07 19:09 361 jhead (exif) info
[TXT]overview_r2l.txt2005-07-07 20:48 243 jhead (exif) info
[IMG]04g.png2005-07-07 19:26 4.6Mluminance grid
[IMG]07g.png2005-07-07 19:25 3.8Mluminance grid
[IMG]11g.png2005-07-07 19:26 4.8Mluminance grid
[IMG]04g128.jpg2005-07-07 19:29 1.2Mluminance grid, large
[IMG]07g128.jpg2005-07-07 19:28 1.4Mluminance grid, large
[IMG]11g128.jpg2005-07-07 19:29 1.2Mluminance grid, large
[TXT]info.txt2005-07-03 22:44 4.1Koriginal camera info
[TXT]info_d.txt2005-07-07 20:48 3.9Kresults overview
[ ]04.td2005-07-07 19:25 286Ktabelled photometry
[ ]04_128.td2005-07-07 19:29 14Ktabelled photometry
[ ]07.td2005-07-07 19:25 241Ktabelled photometry
[ ]07_128.td2005-07-07 19:28 14Ktabelled photometry
[ ]11.td2005-07-07 19:26 239Ktabelled photometry
[ ]11_128.td2005-07-07 19:29 14Ktabelled photometry

Sky over Brno Observatory, June 28 and July 3 and Comet Tempel 1

The air in the first night was very transparent, this leading to the minimum luminance near zenith about 2.1 mcd/m2 (2.1 mnt, i.e. two millinits) and just about three millinits at the position of Tempel 1 comet (just over Spica, the star which is faintly visible left from Jupiter). Let's remember that these two millinits mean a sky which is ``just'' eight times brighter than the natural one...

The second night was an average one. Minimum luminance near zenith was 3.2 mnt, and close to Tempel 1 it was six millinits. This is way too much.

No wonder the comet was invisible. Having about 10 mag and some 2' diameter, its luminance should be about one millinit. Within the millinit realm, human eye can not perceive a small spot which is just by 1/6 brighter than the surroundings. Quite probably, it would be still invisible at ``3mnt sky'' (being just 1/3 brighter). It could become to be visible just in those sites, where the surrounding sky has no more than one millinit (the comet having then twice the luminance of the sky). Of course, in our country the comet is so low, that it is at least one magnitude fainter due to extinction, so the prospect to see it is even worse.

For a 3mnt sky the comet sized two angular minutes should have nine magnitudes (including extinction) to become visible. On a 6mnt sky, 8 mag would be needed. However, after Deep Impact it did not brighten at all that much (it turned just several decimagnitudes less faint).

Similar computations can be done with my programme lum, as for 8 mag: 

lum m2 8 

The equivalent of Luminance expressed by a star of faintness of
 8.00 mag  defocused to a circle of angular diameter of   2.0 minutes
       means a Luminance of some
                                   0.00603 cd/m^2
The programme is accessible online as well, as lum.php within http://amper.ped.muni.cz/jenik/astro.

In fact, the inner part of the comet could be still seen under three-millinit sky with a 50cm telescope, assuming that the inner part has a much higher luminance. If a circle of 0.3' diameter would have 12 mag, its luminance would be 6 mnt allowing it to be perhaps noticed when magnified 100× (being then half a degree large). Still, it would be no easy task, as the luminance changes continuously from the comet centre to the ``normal'' sky.

Illumination of the horizontal surface is another result obtained from the images. It was 9 mlx (almost one centilux) on June 28, and to 17 mlx on June 3.

For any interpretation of the luminances, the extinction is to be determined. This is possible ``manually'' from the star images, but should be preferably done by some future software instead.

Any brighter segment due to astronomical twilight over northern horizon was completely invisible. Image times are UTC + 1 h 02 min, differing just 4 min from the local time, so the sun from Brno was at -17 degrees by the June image and at -16 degrees by the July ones.