Archive for light pollution

Light Pollution

Posted in Astrophotography with tags , , , , , , , , , , on 2009/07/21 by computerphysicslab

Light pollution in suburban skies makes nearly impossible astrophotography. Nevertheless, there exists some computer techniques to make it possible. I have used a dark frame to catch the exact pattern of the light pollution in the photographed area. The way to accomplish this is not difficult. Shooting in continuous mode the DSLR camera facing the zenith in my home window, and forcing every frame to last 15 seconds I got a sequence of subframes to be processed afterward.

To get the dark frame I do some image arithmetic with Paint Shop Pro 9: I choose 3 or 4 distant subframes and compute them using “darkest” option. This way, stars become to fade until disappearing.

With Paint Shop Pro 9 and batch processor I apply a barrel lens distortion of 17 (empirical value to correct a 18mm focal lens like Canon’s) to every subframe and also to the dark frame.

Once got the dark frame DeepSkyStacker is needed to stack the single subframes and substract the dark frame. The sideral drift of the field is automatically compensated with the intelligent algorithm that DeepSkyStacker provides.

The resulting image is surprising taking into account this is an urban sky.

Light-pollution

From a light polluted city

Posted in Astrophotography with tags , , , , , , , , on 2009/07/19 by computerphysicslab

This image was taken in the center of a light polluted city with almost 1 million inhabitants. Using a dark frame of the polluted sky I could catch the Milky Way. Obviously it was impossible to observe it with naked eye.

The image is the result of adding 20 subframes of 15 seconds of exposure each one at ISO 1600. After applying a barrel distortion filter to compensate lens distortion the images become integrable. Using Nebulosity 2 software I could compensate the sideral motion of the sky.

The quality of the image is very low. The point here is not quality but the successful of avoiding light pollution in a widefield Milky Way shot. The area corresponds to the border of Lyra constellation towards Aquila constellation.

without-light-pollution

Background substraction

Posted in Astrophotography with tags , , , , , , , on 2006/03/25 by computerphysicslab

Astrophotos taken in light polluted skies use to show a noisy gray background that should be avoided mainly for aesthetic reasons, and also to gain contrast in deep-sky objects. Using some image software like Paintshop Pro or Photoshop it is simple to correct the image in 4 steps:

1.- Make a copy of the image.

2.- Level the copy from zero to the highest background noise value. This way bright stars will appear as dim as background.

3.- Gaussian Blur the copy.

4.- Substract the copy to the original picture. You may add a small offset to the substraction if the background removal is very hard.

Here is an example with Orion constellation:

Background Substraction

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