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Archive for November, 2011

Following  older posts on astrophotography  where I have shown Lyra and Andromeda, here I present Canis Major, the constellation of the brightest star in the sky, Sirius. The M41 open cluster is seen clearly in the image. It has 90 seconds of total exposure time, a focal length of 30mm, and opening of f/4.0. I used Iris to calibrate and merge the raw images. The original  image has a resolution of about 8 times better.

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Python-histogram-histtype=’step’

Using histogram or better to say a probability density function (PDF) is a daily task for many people in science and engineering. Making a histogram is the same as a PDF: one just needs to add a keyword “normed=True”. Since making histogram is a standard example in Matplotlib, I do not repeat it here in detail. I rather discuss an annoying feature of it.  The problem is the following: Imagine you have two histograms on a plot and want to add a legend. If you select the step function for plotting the histogram and then add a legend, it creates a rectangle instead of a line in the legend, like below:

n, bins, patches = plt.hist(x, 50, normed=1, histtype=’step’, lw=2, color=’blue’, label=’plot a’)

n, bins, patches = plt.hist(y, 50, normed=1, histtype=’step’, lw=2, color=’red’, label=’plot b’)

Now if we use the legend command as usual:

plt.legend(loc=’upper left’)

we face the following which is very annoying for me:

As far as I know, there is no way to get rid of this rectangle. The only way I could figure out was to use a Line2D object to make fake legend like this:

legend([Line2D([0], [0], color=’b’,lw=2),Line2D([0], [0], color=’r’,lw=2)], [‘plot a’, ‘plot b’], ‘upper left’)

which leads to what I expected to eb a default behavior:

I hope in the next release of Matplotlib, they consider an improvement for this issue.

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In a seri of posts, I plan to show some of my simple astrophotos. Almost all of them were taken without any special equipment, e.g. telescope. I used only a camera, a tripod and a cable release. Using my  Canon EOS 400D camera, on Sep 2010, I have recorded  24 times 15 seconds exposure without tracking. Then, the raw images were merged in Deepskystacker. M31 and M33 are seen clearly at bottom and top, respectively, on the both sides of the Andromeda constellation. It simply shows that one does not need dedicated equipments for simple astrophotography.

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Green Flash

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot or rim is visible above the sun.  The reason is that for different part of the solar spectrum, i.e., different colors, there are different refraction index in the atmosphere. That means the earth atmosphere plays role of a prism. This phenomena is called the differential refraction.

The difference between the refraction index of green and red is very small. However, when the sun is very close to the horizon, the red and green beams are off by a few arc seconds. Certain atmospheric conditions can intensify this effect.

Green rim above the solar image seconds before the sunset.

In the above image, you see a green rim above the sun, seconds before the sunset.  It was captured with similar setup as I explained in a previous pots:  a Canon EOS 400D camera was attached to the Celestron Nexstar 4 inch Maksutov telescope in prime focus mode.

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