Tuesday, August 12, 2008

M57 - The Great Ring Nebula


M57 is a probably one of the best know planetary nebulas in the night sky, and observing it with pretty much any telescope will tell you why. Whether it's just a tiny Polo in the sky in an 80mm refractor, or a birds nest of filaments weaved into a ring through an 18" Dobsonian, it never fails to inject intrigue into the observer as to what exactly happened to that ageing star. Did any exotic life forms had their existence extinguished in, what must have been, a catastrophic shedding of the stars outer atmosphere as it strived to find a new equilibrium after exhausting its main hydrogen reserves? Where there even any planets in the system to witness this metamorphosis of what was most likely a pretty regular star into one of natures finest and strangest objects, a white dwarf.

With dark enough skies, and a large enough telescope, you may even be able to glimpse this white dwarf at the centre of the ring, shining at a mere magnitude 15.7, but what a tiny object to be able to see at such a distance of 2300 light years!

We had the good fortune to be able to image this magnificent object using the Faulkes Telescope as part of ties between the Bristol Astronomical Society and the Faulkes team. This was the first "regular" object that we've imaged with the robotic telescope in Hawaii - normally we image more obscure groups of galaxies from the Hickson catalogue - and boy were we in for a treat!

We took ten 60 second exposures though red, green, and blue filters, and combining just the reds quickly showed we had caught more than we'd bargained for! By stretching the resulting image one could easily make out a very faint glow of nebulosity surrounding the main ring - material that was shed before the final stellar collapse and outpouring that we see the result of visually. In fact there is another, even fainter ring of material around this outer shell, but we were not able to record that with the exposure times we obtained. The challenge in processing this object was going to be to try to show this faint "nebulosity" alongside the traditional Ring.

Well, here is my result:


Now, I must have messed up at some point because the stars have all lost their differing hues - something to go back and fix at a later date - but I am very pleased at the result. Something interesting to note is that we managed to resolve the 3 stars just above the outer faint section directly above the ring - quite often these will blend together into one "bloated" star, so it goes to show the quality of the optics the Faulkes Telescope has.

Now on to the details of processing:

The initial processing was achieved using good old Deep Sky Stacker, giving 3 master RGB channels that I could tweak in PixInsightLE. The red was processed extensively to get a nice core ring image to use as a luminance channel - just curves and a little wavelet processing to bring out the knots in the gas sufficed. A second red image was then stretched to its limit to bring out the outer nebulosity, which needed a little noise reduction to smooth out the grainy nature of the tenuous gas. These 2 were blended together in Photoshop as a source luminance channel, then the "regular" RGB frames were added to produce the final image. Hot pixels, lines, and other artefacts where then removed with the spot healing tool before the final colour balance was achieved. I initially thought the ring seemed rather mute in colour, but I actually like it that way - more akin to what one would see through a telescope rather than in a glossy magazine.

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