Two Hours Under the Stars: Capturing the Barred Spiral Galaxy NGC 7640

This past imaging season, I set my sights on NGC 7640, a rather faint but intriguing barred spiral galaxy nestled within the Andromeda constellation. While the plan was for a longer, more comprehensive capture, astronomical realities (and a well-timed tree) meant I ended up with a valuable ~2.5 hours of integration. Nevertheless, it was enough to reveal some of this galaxy's hidden beauty and test the limits of my equipment.

The Target: NGC 7640

NGC 7640 is characterized by its elongated shape and a prominent central bar. Its dust lanes are a fascinating feature, hinting at the dynamic processes occurring within this cosmic structure. There's also a subtle, irregular star-forming activity that adds to its complexity.

My Imaging Setup

For this session, I relied on a familiar setup:

• Camera: Sony a6400 (unmodified APS-C)
• Telescope: Celestron NexStar 8SE SCT with a Celestron f/6.3 reducer
• Mount: Celestron NexStar SE (wedge-mounted and guided)
• Guide Setup: ZWO ASI120MM Mini with a ZWO 30mm guide scope, using PHD2 for guiding.

The Imaging Process

Imaging Date: August 27, 2025 Integration: 115 × 75s subs (approximately 2 hours 24 minutes) Filter: Broadband (no specific filter used) Exposure Details: ISO 1600, 75s subs, with dithering applied every 4 frames to help reduce patterned noise.

Calibration: Standard calibration frames (Flats, Darks, Bias) were used to process the raw data.

Software Suite: My workflow involved NINA for capture, PHD2 for guiding, Siril for stacking and initial preprocessing, GraXpert for effective gradient removal, and finally, Photoshop for post-processing.

Challenges and Triumphs

This was one of my first longer broadband sessions specifically targeting NGC 7640. The Bortle 4 skies provided a decent canvas, but the faintness of the galaxy meant every photon counted. The guiding performance was crucial, and after a mid-session restart, the seeing conditions improved, which was reflected in reduced star trailing and a better FWHM (Full Width at Half Maximum) across the subs.

However, the universe doesn't always cooperate! About two hours into my planned six-hour capture, the target gracefully slid behind a rather inconveniently placed tree. This meant I had to cut the session short, leaving me with less data than initially hoped for.

Post-Processing and Results

After calibration and stacking, the raw data presented a faint rendition of NGC 7640. The subsequent steps of gradient removal and star color calibration were key to bringing out the galaxy's core details and its subtle halo.

The final result, despite the limited integration time, clearly shows the central bar structure, hints of faint spiral arms, and the surrounding star field. The color balance is relatively clean, giving a good representation of the target.

While more integration time (ideally 5-6 hours or more) would undoubtedly help deepen the faint outer structures and smooth out any remaining background noise, I'm quite pleased with the amount of detail that emerged from this ~2.5-hour session. It's a testament to careful planning, robust guiding, and effective processing techniques.

This experience reinforces that even shorter sessions can yield rewarding results with the right approach, and it certainly motivates me to revisit NGC 7640 when the opportunity arises for a more extended observation period!