Luminous Landscape Forum

Site & Board Matters => About This Site => Topic started by: samirkharusi on June 21, 2006, 10:56:15 am

Title: 20min exposures and color of the night sky
Post by: samirkharusi on June 21, 2006, 10:56:15 am

I think it's interesting to note that Michael's 20min exposure of a Moon lit sky shows a blue sky. Because the DSLRs have accurate color balance and no reciprocity failure (film even has differential reciprocity failure in which different colors fail in different amounts) it's only quite recently that we have been able to state categorically what color is the night sky under no light pollution. And it ain't blue! Astronomical CCDs have always been difficult to calibrate accurately to human vision, especially since astro-important spectral lines (H-alpha and SII) demanded the use of filters that deliberately skewed response away from human photopic response. The holy grail of DSLRs has however been to replicate human photopic response as closely as possible (while still prettifying Caucasian and Japanese skin tones, slightly). The blue sky in Michael's pic comes from the Moon light. A really dark sky with no sunlight (be it reflected from the Moon or otherwise) and no artificial light anywhere is, surprisingly, toffee colored when you use Daylight White Balance. Have a look here for the transition from toffee to blue as dawn breaks: http://www.samirkharusi.net/network-veil.html (http://www.samirkharusi.net/network-veil.html)
I found it quite fascinating. It tends towards grey only if you use Auto White Balance.

Title: 20min exposures and color of the night sky
Post by: Andrew Teakle on June 21, 2006, 10:43:00 pm

Thanks Samir for the very interesting images. How did you get images of 5 min exposure times without the stars "streaking" (except the one with full exposure to mid-histogram)?

Thanks again,

Andrew

Title: 20min exposures and color of the night sky
Post by: Bill in WV on June 22, 2006, 10:53:48 am

Quote

Thanks Samir for the very interesting images. How did you get images of 5 min exposure times without the stars "streaking" (except the one with full exposure to mid-histogram)?

Thanks again,

Andrew
[a href=\"index.php?act=findpost&pid=68826\"][{POST_SNAPBACK}][/a]

I'll jump in here for just a second . . . his telescope has a clock drive which compensates for the rotation of the earth. That's how he gets pictures with no streaking.

Bill in WV

Title: 20min exposures and color of the night sky
Post by: samirkharusi on June 22, 2006, 10:57:47 am

Quote

How did you get images of 5 min exposure times without the stars "streaking" (except the one with full exposure to mid-histogram)?
Andrew
[a href=\"index.php?act=findpost&pid=68826\"][{POST_SNAPBACK}][/a]

I used an astro tracking mount, complete with an autoguider (a CCD camera that checks a star's position every couple of seconds and corrects the mount's tracking):
(http://www.pbase.com/samirkharusi/image/60883106/original.jpg)

Title: 20min exposures and color of the night sky
Post by: Bill in WV on June 22, 2006, 11:57:19 am

Quote

I used an astro tracking mount, complete with an autoguider (a CCD camera that checks a star's position every couple of seconds and corrects the mount's tracking):
(http://www.pbase.com/samirkharusi/image/60883106/original.jpg)
[{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=68873\")

THAT is quite a rig. I never got good enough at aligning my scope to enjoy much photography with it. Now, if I could just figure the best way to sell it, I could add some stuff for my 30D.
I did like your astrowork though. Twas quite nice.

Bill in WV
[a href=\"http://thenerdnook.com/astronomy2.htm]http://thenerdnook.com/astronomy2.htm[/url]
http://www.pbase.com/bill_in_wv (http://www.pbase.com/bill_in_wv)