Capturing the Open Cluster IC 4665 Under Light Polluted Skies

Astrophotography often presents challenges, and perhaps one of the most significant is overcoming the pervasive issue of light pollution. Tonight, we're diving into a stunning capture of the open stellar cluster IC 4665, brought to us by an intrepid photographer who tackled the hurdles of Bortle 8/9 skies. This image is a testament to the dedication and skill involved in bringing the cosmos into view even from urban environments.

The Celestial Subject: IC 4665

IC 4665 is an open star cluster located in the constellation Ophiuchus. Open clusters are groups of stars that are loosely bound together by gravity, typically younger and less densely populated than globular clusters. They are often regions where stars are born, and IC 4665 showcases a beautiful collection of stars in various stages of development.

Tackling Light Pollution: Equipment and Setup

Achieving this level of detail under such challenging skies requires a well-thought-out setup. The photographer utilized:

• Telescope: A 130mm f5 telescope, providing a good balance of aperture and focal length for deep-sky objects.
• Camera: A modified Canon T2i, a popular DSLR camera often modified for astrophotography to enhance its sensitivity to astronomical wavelengths.
• Light Pollution Filter: A ZWO dual-band filter was employed. These filters are crucial for astrophotography in light-polluted areas, as they selectively allow the light emitted by specific nebulae (like H-alpha and OIII) to pass through while blocking much of the unwanted artificial light.
• Tracking: A Celestron logic drive engine was used to accurately track celestial objects, compensating for the Earth's rotation and allowing for longer exposures.

The Acquisition Process

To gather enough signal and overcome the noise inherent in light-polluted skies, a total of 69 "lights" (individual exposures) were taken, each with a 4-second exposure duration at ISO 800. This was supplemented by 12 "darks" – frames taken with the lens cap on, at the same exposure and ISO settings – which are essential for calibrating the image and removing sensor noise.

Processing the Data

The raw data, once acquired, requires careful processing to reveal the hidden beauty of the cluster. The workflow involved:

1. Stacking: The 69 light frames and 12 dark frames were stacked using Siril. Stacking multiple images helps to improve the signal-to-noise ratio, bringing out fainter details and reducing the impact of random noise.
2. Star Removal: StarNet was then used for star removal. This process isolates the deep-sky object from its surrounding stars, allowing for more targeted processing of the cluster itself. Often, star removal is followed by processing the stars separately and then recombining them.
3. Final Touches: The processed image was then brought into Adobe Lightroom for final adjustments, including color balance, contrast, and sharpening, to present the stunning final result.

A Triumph Over Light Pollution

This image of IC 4665 is a powerful reminder that even with the challenges of light pollution, compelling astrophotography is achievable with the right equipment, techniques, and a lot of patience. It inspires fellow enthusiasts to step out under the night sky, armed with their gear, and to discover the wonders that await, no matter where they are.

5 min read