Revisiting M17: The Impact of Dark Skies and Longer Exposures

Astrophotography is a journey of continuous learning and improvement, and sometimes revisiting a familiar target with new equipment or techniques can yield surprising results. Today, we're taking a look back at the stunning Omega Nebula (M17), showcasing the significant difference that moving to darker skies and utilizing longer guided exposures can make.

From Urban Glow to Rural Brilliance

My previous capture of M17 was done under the light pollution of a Bortle 9 zone, relying on relatively short 30-second sub-exposures. While it captured the brighter core of the nebula, much of the surrounding faint hydrogen gas and delicate filaments were lost to the skyglow.

Recently, I had the opportunity to revisit this celestial wonder from a Bortle 4 location, a significantly darker rural site. This move allowed for a fundamental change in my acquisition strategy: upgrading to 90-second guided exposures. The results were nothing short of remarkable.

What Changed? The Power of Integration Time and Dark Skies

The key differences in this revisit lie in two crucial factors:

1. Darker Skies (Bortle 4 vs. Bortle 9): Moving to a Bortle 4 location drastically reduced the background light pollution. This meant that the faint signal from the surrounding hydrogen cloud, which is much dimmer than the core, could be captured without being overwhelmed by ambient light. The signal-to-noise ratio significantly improved.
2. Longer Exposure Times (90s vs. 30s): By switching to 90-second guided exposures, I was able to gather significantly more light for each individual sub. This not only captures fainter details but also allows for more flexibility during processing. The longer exposures were made possible by accurate guiding, which keeps the telescope locked onto the target despite the Earth's rotation.

Capturing the Faint Details

The most striking improvement is the amount of surrounding hydrogen cloud detail that is now visible. Especially in the lower and outer filaments of the nebula, which were previously lost in the noise and light pollution of the Bortle 9 skies, these delicate structures are now beautifully rendered. The UHC filter also played a crucial role in isolating the hydrogen-alpha emission, further enhancing the contrast of these faint details.

Acquisition Details:

• Total Integration Time: 1 hour 55 minutes
• Camera: Sony a6400
• Filter: UHC
• Telescope: Celestron NexStar 8SE with f/6.3 reducer
• Mount: Alt-Az with EQ Wedge
• Guiding: ZWO ASI120MM Mini + PHD2
• Location: Bortle 4 (rural skies)
• Subs: 77 x 90s lights, 20 darks, 50 flats, 50 bias

Conclusion

This comparison serves as a powerful reminder that while equipment is important, the conditions under which you shoot, and the integration time you can achieve, are equally, if not more, critical in astrophotography. Investing in access to darker skies and mastering guiding techniques can unlock a whole new level of detail in your captures. The Omega Nebula certainly put on a spectacular show once given the chance to shine without interference!