NGC 7635 – The Bubble Nebula
The Bubble Nebula was an astrophotography project I will not soon forget. In the fall of 2016, I spent almost 2 months straight capturing data on NGC 7635 from my backyard. The final image below utilizes narrowband Ha to bring out the faint details of the nebula.
The true color (broadband) images were merged with narrowband H-Alpha to create a composite photo. To learn how to create a composite photo like this using a DSLR, have a look at my HaRGB photoshop tutorial.
NGC 7635 – The Bubble Nebula
The Bubble Nebula is an H II region emission nebula in the constellation Cassiopeia. It lies very close to a star cluster known as Messier 52, as seen in the image above. Through a large telescope under dark skies, the Bubble Nebula appears as an extremely faint shell around the central star. I certainly can not visually observe NGC 7635 from the backyard!
The photo above is a wide-field view of the area, as seen through my Explore Scientific ED102 telescope. At this magnification, the star cluster M52 is also visible at the lower left.
The photo above is a composite image of the Bubble Nebula photographed in both RGB and Ha. The RGB version of the image simply means it was photographed in regular full color with a DSLR Camera. The (Hydrogen Alpha) was captured using a clip-in DSLR Ha filter in the camera. By combining the RGB and H-Alpha images together, more detail can be presented.
The filter I use is the Astronomik 12nm Ha Filter for Canon EOS cameras. It clips into my Canon T3i/600D camera easily with no extra adapters needed.
This filter blocks out almost all sources of light other than a narrow wavelength of Hydrogen Alpha. Because of this, I am able to capture images during the full moon, and through heavy light pollution.
Although the CMOS sensor in the camera captures color images, the ha filter renders a grayscale image to use as both a synthetic red channel and/or a luminance layer.
The Bubble Nebula in Ha (H-Alpha)
A camera with a mono sensor can harness the full power of narrowband astrophotography. However, the CMOS sensor found in your DSLR camera can also be used effectively. Learn more about capturing narrowband images with a color camera.
The photo above shows just how much the final image was cropped, and the size of this target at a focal length of 913mm. The size of the camera sensor changes the FOV by 1.6X, and the focal reducer brings it back by 0.8X.
Astrophotography from the city
This object is quite difficult to capture from a light polluted backyard! The Bubble nebula is extremely faint and requires many hours of exposures to pull forward. Even with the additional Ha data, the HaRGB version was a little disappointing to me after so many hours of work.
Adding additional images captured using an OIII filter with my DSLR should help reveal the full structure of NGC 7635.
RGB Photo Details
Total Exposure: 7 hours, 42 minutes (150 frames)
Photographed on Sept 15, Oct 3, Oct 7, Oct 8, 2016
Ha Photo Details
Total Exposure: 2 hours, 8 minutes (32 frames)
Photographed on Oct 31, 2016
I learned a lot about capturing Ha Data and combining it with RGB through this project. The toughest part of the process is finding the right balance between the 2 versions. Too much Ha – and you lose the natural color of the nebula. Too much RGB – and you are leaving out significant detail in the object.
One trick I found to be useful was to use the star colors from the true-color image as a final overlay. Meaning, I placed an RGB layer on top of my HaRGB composite using the “lighten” blending mode in Photoshop. This corrected many of the miscolored stars produced when merging the Ha data.
Astrophotography Tutorial: Processing H-Alpha with a DSLR
If you are interested in the techniques used to process the Ha portion of the image above, watch the following tutorial video: