It’s been about a year and a half now since the 16” scope up at Orion’s Belt saw first light. Now, finally, the very first imaging project with that telescope is completed! The “Bubble” nebula, or NGC 7635, it’s official designation in the “New General Catalog” of deep space objects, is a true bubble in space, about 7000 light years from us in the constellation Cassiopeia! Not science fiction! The bubble is created by “stellar wind” or gas ejected from the upper atmosphere of a very hot centrally located star in the nebula. Surrounding the bubble is a huge cloud of hydrogen gas. Since all of you who have been reading these posts are now experts in spectroscopy 😊 you know that hydrogen gas after it absorbs radiation can then emit that radiation back into space in the “hydrogen alpha” wavelength of light which is the visible red portion of the spectrum.
This data was a total bear to process and I almost bailed on the whole thing when I realized that the bubble was hidden in a dense star field! (see below). We forget how many stars are out there. 250 billion in our galaxy alone! It really seemed to me that they were all congregated right there in one frame. The processing challenge was to somehow remove the stars to bring out the nebulosity. Folks, if you are going to image a nebula in a dense star field my suggestion is to use narrow band filters all the way. I felt I could get the natural color of the nebula using just the hydrogen alpha filter, which is narrow band ( in other words just allows the light in the H alpha region to pass through), but only one channel, combined with the regular band width red-green-blue filters. Problem is the regular band width filters do not filter out the stars! The details of the processing are beyond the scope of this one post but I tried to show the essence of what was done in the images shown here. Star removal is still not a perfect “science”. I have yet to see one method that does a complete job with no artifacts, but the method I used seemed to work pretty well. The software Pixinsight has a number of processes that in combination can achieve the desired result. The “star mask process” can be applied repeatedly to “mask” different sized stars which can then be removed by applying the mask back to the image as a “defect map” (another process) to do the actual removal. Anyway it took weeks of trial and error to get it right enough where the residual artifacts were manageable!
If you click on the thumbnail under “My Astro images” on the right side of this blog (scroll down a bit) you will go to the full resolution version.
This is the Hydrogen alpha version. Notice it is black and white. You have to combine this with one or more other channels for color. The detail in the nebula is able to be detected with this filter since it is only letting the narrow 3nm width of light through in the H alpha region of the spectrum. Most of the star field is filtered out however to achieve the red color you have to combine this with another channel, typically red ,but in my case it was the regular band width red channel which is full of stars!
This is the standard “tricolor” RGB image! This is how we obtain color images in general with astroimaging, combining individual red, green and blue filtered images. Amazing to see how the nebula is really hidden in this image behind all of these stars! Also notice the slightly pink color of the nebula which is from the blue channel
Going through the star removal process for the RGB image above, then combining that with the Hydrogen alpha image (first one above) I was able to get to this workable solution which then had to be tweaked some to arrive at the final result many hours later. I did look at trying to sharpen things up a bit as it does get a little soft at full res, but that wasn’t possible due to the processing artifact in the background. Overall I think this was a decent salvage effort!
Thanks for reading!
The Trifid nebula, whose name means “divided in 3 lobes” is an amateur favorite. An island of HII region which is the red portion (HII regions are emission nebulae created when young, massive stars ionise nearby hydrogen gas clouds with high-energy UV radiation causing the gas to emit red light) , a blue “reflection “portion which occurs when light of the nearby massive blue stars reflects off of the dust, and a “dark nebula” portion which divides the nebula into its 3 lobes. There is also an open cluster of stars in the field. The Trifid is 5000 light years away toward the center of out galaxy, hence the very dense surrounding star fields and dust.
I completed my test of the Tak FS120 imaging capability and you can see the result below. I was pretty happy with it, especially considering the very modest set up I am using (see last entry) . For the full resolution image you can click on the thumbnail on the right side of the blog under “my astroimages”. I am looking forward to more projects with it!
Until next time. Thanks for reading!
M20, the Trifid. Single raw image 5 minutes.
This is the fully processed result! 33 x 5 minutes or close to 3 hours of data. I tried not to overprocess it but to preserve the natural nebula colors and bring out the background dust behind the dense star fields.
Orion the Hunter arrives heralding the start of Winter in the northern hemisphere. He brings with him a host of glorious star-forming regions of our galaxy. Finally I believe I can complete an imaging project on the popular Horsehead nebula which I have been trying for years now. Back on the east coast Orion is fairly low on the horizon and the Horsehead is at the lower end of Orion’s belt so not a particularly favorable target. Here in the Southwest it is better positioned. I have been able to finally test my 3nm HA (hydrogen alpha) filter! It’s a perfect target for that with the broad emission nebula behind the dust.
Constellation Orion which dominates the winter sky in the northern hemisphere. Arrow points to the approximate location of the Horsehead nebula
Single 15 min raw image of the Horsehead taken with SBIG6303E and Astrodon 3nm HA filter!