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.
Thanks for reading!