The Andromeda galaxy is approximately 2.5 million light years from Earth and for the past 134 years, it has been the one of the most-observed galaxies by astronomers. This is why it's surprising that a recent oxygen nebula was uncovered by amateur astronomers Marcel Drechsler and Xavier Strottner – an arc almost as large as the galaxy itself, that hadn't been discovered until now.

To verify their claim, the duo enlisted French amateur astronomer Yann Sainty to help out. United States-based amateur astronomer Bray Falls was brought in to help verify as well. Together, Sainty and Falls put in roughly 110 hours, each, tirelessly taking images of the arc and stacking them to get a clearer picture of what they were observing.

It seems baffling that a new discovery of this magnitude took over a century. However in the video above, narrated by Falls, he explains why, regardless of all eyes on it over the decades – starting with Isaac Roberts in 1888 – it may have been overlooked until now. First and foremost, astronomers simply don't point their telescopes at the sky and hope for the best.

You can't point a telescope at the sky and hope for the best, you'll also need specialized filters. Image credit: screenshot from Bray Falls' YouTube video

As Falls explains, the night sky consists of different types of gases and dusts. Astrophotographers use narrowband filters to observe the light from specific gasses. The main three gas emissions photographers want to pay attention to are Hydrogen Alpha, Oxygen III and Sulphur II. When an external force or something pressures these gasses, they start to glow in certain colors.

The most prevalent gas in the universe is hydrogen. The other two, oxygen and sulphur, are extremely faint. Those that take the proper steps to observe the faint ones are more likely to come across new nebula. Notably, the Oxygen III filter used, which brings out a light blue color from the ionized oxygen, provided the hint that there was something new out there.

Not enough astrophotographers use an O-iii or Oxygen III filter, which is why there are still many nebulae to be discovered. Image credit: screenshot from Bray Falls' YouTube video

This blue tone is vulnerable to the effects of light pollution and it is faint. It takes a lot of time, dedication and patience to capture it properly. Falls notes that most astrophotographers don't collect data or images using this filter. If you do have the time, there are many undiscovered objects hidden.

The more exposure you have on an object in the sky, the clearer it will show up in your imagery. Falls and Sainty took tons of images at 5 to 10-minute exposure times and combined them in post-processing software to reduce the noise. Overall, the final image is the result of 110 hours worth of captures.

'When it comes to discovering new nebulae, you can't just image in the target filter and call it a day. There are some special treatments you're going to have to do to your image that not a lot of people know about,' Falls explains. One involves capturing blue images for continuum subtraction.

Falls helped verify this oxygen arc, almost as large as the Andromeda galaxy. Image credit: Bray Falls

Falls used his own Takahashi FSQ-106 telescope with a .73 reducer and a QHY600 CMOS camera. An F3.6 aperture and 400mm focal length gave him a (relatively) wide field-of-view (FOV) on the nebula at fast speeds. For those looking to get started, he recommends a wide FOV telescope, a lens with a focal length of 300–900mm, and a 3nm O-III filter. Finding an area with dark skies also helps.

What it comes down to is exposure time, along with the type of filter. Having that, along with patience, will yield the better results. I asked Bray what area of the sky should be focused on next and he emphasized it's not the sky, it's the filter.

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'A lot of astrophotographers use narrowband filters to shoot their images, and most prefer to focus on the Ha [Hydrogen Alpha] filter since it is bright and contains a lot of details. The O-iii filter which comes from oxygen gas is usually neglected, but as evidence by this discovery there are a lot of things to find with this filter. So more astrophotographers should be checking spots of the sky specifically with this kind of filter that not a lot of people use.'

A limited-edition print of the Andromeda with the oxygen arc can be purchased on Falls' site. Proceeds will support his future astrophotography endeavors.