Thursday 9 January 2014

Horsehead (Orion special - part III)

I was like 9 years old when the Hubble Space Telescope started to bring wonderful pictures of the deep sky. I used to have a look at my father's magazines and when I first saw them I was astonished with an image which looked like the head of horse and I thought to myself, it must be wonderful to take a picture of that thing (whatever or wherever it is). Well, a "couple" of years later and with less hair on my head, finally, I managed to take a shot of this wonder of the sky.


The Horsehead nebula is located next to the star Alnitak, the easternmost star in the belt of Orion. This object is also known as Barnard 33 in the emission nebula IC 434. It is a dark nebula located at approximately 1500 ly from earth. This darkness comes from the blocking of the light from the stars being formed behind it. The reddish are surrounding it is caused by hydrogen gas ionized from the close star Sigma Orionis.

This picture also shows the Flame Nebula (designated as NGC 2024) located about 900 - 1500 ly from earth. The bright of this nebula is caused by the star Alnitak shining ultraviolet light through it and the dark lanes are dust and dark gas located in front of the nebula causing this flame shape to appear.

The annotated image shows different nebulae associated to this cloud complex such as NGC 2023.

A closer look  in this 2x Drizzle processing:

Taking this picture with the ETX-70 is pretty tricky and to be honest I didn't believe that the result could be so wonderful. This telescope is a small 70mm with 350mm of focal length quite fast (f/5), far from apertures needed to picture this nebula but with the help of my new mount the HEQ-5, I managed to get enough exposure times to picture this image. It is composed of 24x240'' lights ISO 1600 (tracked, not guided), 10 darks and 40 bias with my unmodded DSLR Nikon d3100, all of them stacked with DSS and processed with StarTools and Photoshop.

As this image is the best I've taken so far I would also explain a little bit how these astrophotographs are taken and what means to stack and process them. With my short experience in astrophotography (less than 6 months now) I'm still developing a methodology for processing my pictures but I want to share with you my steps so those of you who are starting can see the benefits and those of you who, without any doubt, are better and more experienced than me can show me my errors.

Everything starts with the frames from the camera which are far away from the pictures shown above:

We can try to process this images directly, and without knowing any better, probably we can get a nice result (quick process with Lightroom):

Nice but again really far from the pictures shown above, then, what's the magic?

Astrophotography is not about creating a wonderful image from the camera, it is all about controlling the noise. If we keep stretching the histogram of above's picture maybe we can get some more colors and brightest nebula, but the noise will pop up and spoil the image. Stacking is the process that allows us to control that noise and also to increase the intensity of what we have pictured. By using a tool like DeepSky Stacker we can add all the images we took from the camera and also some special images (darks, bias and flats) that will subtract the noise generated by the interferences and heat in the sensor. This is the result of stacking explained above:

Well, nothing spectacular, but this is just the beginning. Now we have something to play with that is less noisy and with the information of all the lights summed up. We can now start processing with our favorite tool. In this case, because of the budget and because it's a great software for beginners and advanced users, I do that with StarTools.

First we stretch the histogram to see what we have:

Some dirt in the sensor (the black spot on the left) and some artifacts result of the stacking process (see the margins). We will crop the margins to isolate the nebula and also it will help to avoid this problems that would spoil the processing. Also we can see vignetting due to the quality of the ETX-70's lens. We can take care of this issues with the tool:

Ok, now way clearer, we can work with the image. After some deconvolutions, filterings and sharpenings we are closer to our objective:

But the colors are not correct. The image is too reddish. Time for balancing the colors:

A lot better. Almost there. Now with the help of the tool we can perform a noise reduction in the image.

By this time you can compare with only one frame processed and the result of stacking and processing. Of course there will be noise, we are pushing the images to the limit to get as much information as possible but we are several light years ahead of what we could achieve from the single shot. The final steps of every image depend on the author, his liking and, of course, the artistic point of view of each one.

Hope you liked the post if you managed to reach the end of it.

Clear skies!

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