Beyond the Stars: Revealing the Double Cluster’s Faint Hα Glow

From a Shared View to a New Imaging Project

The story behind this image began with a shared view through a telescope at our club’s September Public Night. After horrible weather had plagued our public outreach programs, we finally had a clear night and a great turnout of visitors and member volunteers. A few clouds passed through near the scheduled end of the event. By then, most visitors had left, and members were observing or packing up. Outside the observatory, I found the unattended 20-inch Obsession and spent some time observing with it. But I wanted a wider-field view of M11, the Wild Duck Cluster, than that telescope could provide. I asked a member with a really nice 90 mm APO refractor whether he could show us M11. He had started packing up but agreed, and when the sky cleared again, we observed it with another member. Eventually, we turned to the Double Cluster. What a view! A switch to a wider-field eyepiece brought the X-Man asterism in Stock 2 into the same field. Absolutely gorgeous—and even better because I was sharing the experience with fellow members.

If you’re new to astrophotography, subexposures (or “subs”) are the individual exposures combined to make the final image. Broadband data provide the RGB—red, green, and blue—color image. Narrowband data isolate selected emission signals; here, the L-eXtreme filter captured Hα (hydrogen-alpha) and OIII (doubly ionized oxygen). For this image, I added the Hα signal to the RGB data, producing the RGBHα combination.

That eyepiece view set the goal for my next image: capture the Double Cluster and Stock 2 together. The member whose refractor we had used is also an astrophotographer and owns an Askar FRA400. When I bought my own FRA400, he strongly recommended its matching focal reducer. Back home after Public Night, I simulated my rig’s field of view around the Double Cluster. With the reducer, Stock 2 would fit. That sealed the deal—I ordered it. Once it arrived, I installed it and imaged the Double Cluster on the first clear night. During processing, I noticed very faint background nebulosity. I posted the image on our club’s forum and asked whether it was integrated flux nebula (IFN). The same member identified it as Hα emission. After looking at other images on repositories such as AstroBin, I decided to try bringing out more of that faint signal. A run of clear nights in October gave me the opportunity. I first collected about five hours of five-minute subexposures. After processing those data, I spent another night collecting about five more hours, this time with ten-minute subexposures. In the end, I had more than ten hours of narrowband data to add to the almost two and a half hours of broadband data from the original session. For the narrowband exposures, I used an Optolong L-eXtreme dual-narrowband filter, which collects Hα and OIII. In post-processing, I separated the Hα from the OIII and added the Hα to the broadband image. I hope you enjoy the result.

Image 1: The Double Cluster in RGBHα:

Image 1: The Double Cluster in RGBHα. Broadband RGB data capture the colorful star field; added Hα data bring out the faint red background nebulosity.

What is the Double Cluster?

Before getting into the equipment and processing details, it’s worth pausing on what makes The Double Cluster such a compelling subject. The Double Cluster is comprised of two open star clusters that are located very close to one another.

Object type: Open Star Clusters

The Double Cluster, also called Caldwell 14, consists of NGC 869 (h Persei) and NGC 884 (χ Persei). Open clusters contain anywhere from ten to a few thousand stars that formed from the same giant molecular cloud and have approximately the same age. These two clusters are young by astronomical standards—about 14 million years old. NGC 869 has a mass of about 4,700 times the Sun’s mass, while NGC 884 has a mass of about 3,700 times the Sun’s mass. Although their stars are young, the clusters have been known to humans since antiquity; they were first recorded by Hipparchus.

Location in the sky: Perseus, within the Perseus arm of the Milky Way Galaxy

The Double Cluster lies in the constellation Perseus, within the Perseus Arm of the Milky Way. To find it, look between Cassiopeia’s W asterism and Perseus’s Wishbone asterism. Here in Western New York, the clusters are circumpolar: their proximity to the North Celestial Pole means they never set. They are rewarding targets for late-summer and autumn observing or imaging, and appear highest in the sky during late fall and early winter.

Distance / size / scale

Both clusters lie approximately 7,500 light-years from Earth and are separated from one another by a few hundred light-years. In physical size, NGC 869 has a radius of about 35.5 light-years, while NGC 884 has a radius of approximately 50.8 light-years. Their apparent size on the sky is measured in arcminutes: each cluster spans about 18 arcminutes, and the pair together spans about 60 arcminutes. For comparison, the full Moon spans about 30 arcminutes.

Visual features

Unlike many of the objects that we capture with astrophotography, the Double Cluster is a superb visual target. Let’s look at how this target appears visually and in this image.

• Visually with an unaided eye: The Double Cluster appears as a fuzzy patch between the constellations of Cassiopeia and Perseus. As is almost always the case, dark skies and no Moon help.
• Visually with binoculars or a telescope: Binoculars make the Double Cluster easy to find and observe. In a high-quality refractor with a good eyepiece, the stars of both clusters look like diamonds on black velvet, with a couple of red stars adding the sparkle of rubies. A wider field reveals another part of the scene: a conspicuous line of stars leading from the Double Cluster toward the X-Man asterism in Stock 2.
• Astro Imaging: The camera reveals more stars than can be observed visually. As a result, Stock 2 doesn’t stand out to me in this image as it did through the eyepiece. The camera, with enough exposure time, can also detect the faint signal of Hα in the background. Adding narrowband data brought out more of this background as red nebulosity.

The stars’ brilliant pinpoints of color remain the heart of the image. To my eye, the added narrowband data provide a welcome complement: more color in the background and a better balance across the scene.

Why it matters

As the nights grow longer and cooler here in the Northern Hemisphere, it warms the soul to take out a telescope with a high-quality wide-field eyepiece and get lost in the Double Cluster. Whether I’m observing it visually or enjoying it through astrophotography, this object is always a pleasure.

Image 2: RGB versus RGBHα:

Move the slider to compare the broadband image with the version containing added Hα data. Look for the faint red background nebulosity and how it changes the balance of the scene around the stars.

Let me know what you think of this planetarium app in the comments!

Interactive Web Based Planetarium: Stellarium App

Use the live star chart below to locate the Double Cluster tonight. Type 'Double Cluster' into the chart's search bar to find its coordinates.

Tip: Click and drag to scroll the sky, scroll to zoom, or use the search bar to locate specific deep-sky targets.

Prominent Objects in the Field

Object 1: NGC 869 — Open Star Cluster — also known as h Persei, NGC 869 is an open star cluster that makes up the western part of the Double Cluster.

Object 2: NGC 884 — Open star cluster — Also known as χ Persei, NGC 884 forms the eastern part of the Double Cluster.

Object 3: Stock 2 — Open star cluster — Located 2° north of the Double Cluster, Stock 2 is an extended open cluster sometimes called the Muscleman or X-Man Cluster. It is one of 24 open clusters listed in Jürgen Stock’s catalog.

Image 3: Annotated image of the Double Cluster:

Image 3: Annotated view of the Double Cluster field, identifying NGC 869, NGC 884, and Stock 2.

Imaging Details

Capture dates:

• 10/5/2026
• 10/17/2026
• 10/20/2026

·       Location: Eden, NY

·       Target: The Double Cluster

·       Total integration: 12 hours and 54 minutes

·       Exposure breakdown:

• 10/5/2026: 288 subs at 30 seconds each
• 10/17/2026: 62 subs at 300 seconds each
• 10/20/2026: 32 subs at 600 seconds each
• All Exposures: Gain = 100, Offset = 50, and Sensor Temperature = -10 °C

·       Moon and sky conditions:

• 10/5/2026: Moon - 29% illuminated, waning crescent, transparency = good, and seeing = average.
• 10/17/2026: Moon - 43% waxing crescent, transparency = average, and seeing = below average.
• 10/20/2026: Moon — waxing gibbous; reference illumination 72.3% at 9:03 p.m. EDT in nearby Buffalo. Transparency = excellent, and seeing = above average.

Equipment Used

• Telescope: Askar FRA400 w/f/3.9 Reducer
• Camera: ZWO ASI2600MC Pro
• Mount: Sky-Watcher EQ6-R Pro

·       Filters: No Filter (10/5/2026) and Optolong L-eXtreme (10/17/2026 and 10/20/2026)

• Guiding: ZWO 30 mm Mini Guide Scope / ZWO ASI290MM
• Control and acquisition software: N.I.N.A & PHD2
• Other accessories: PrimaLuceLab Sesto Senso 3 & SvBony 241 Pro

Processing Details

The processing followed three paths: prepare the RGB image, extract a starless Hα image from the narrowband data, and process the RGB stars separately. I then added Hα to the starless RGB image and screened the processed stars back in. The steps below describe that workflow in detail.

Calibration frames: 50 Darks, 50 Flats, & 50 Dark Flats

·       Calibration and integration: All pre-processing was performed in PixInsight. I created three master light frames: one from the unfiltered data and two from the L-eXtreme data.

Star Alignment: All three master light frames were registered using the Star Alignment process.

·       Crop: I used Dynamic Crop to remove registration artifacts from the registered images. I then applied the exact same crop to the reference image so all images had matching crop boundaries.

Create Hα Image:

• The DBExtract script was used to separate the L-eXtreme data into separate grayscale narrowband images.

• Both resulting Hα images were averaged together with PixelMath to create the Hα image.

• Stars were removed (and discarded) with the StarXTerminator process.

RGB Image Processing:

• Background extraction: Before removing gradients, I plate-solved the image with the Image Solver script and performed Spectrophotometric Flux Calibration. With those prerequisites complete, I used the Multiscale Gradient (MSG) process to remove the gradients.

• Color Calibration: The Spectrophotometic Color Calibration process was used to color balance the image and neutralize the background.

• Deconvolution: Deconvolution and sharpening were applied with BlurXTerminator.

• Stretching: The image was made non-linear with the Multiscale Adaptive Stretch process.

• Star removal: I used StarXTerminator to separate the stretched RGB image into stars-only and starless images. These became the inputs for the separate processing steps below.

Starless Processing:

• Noise reduction: Noise was reduced/removed with the NoiseXTerminator process.

• Adjust Histogram: The histogram was optimized with the Histogram Transformation process.

• Add Hα to RGB:  Hα was added to the RGB image with the CombineHawithRGB script.

• Second Application of Noise Reduction: A second application of the NoiseXTerminator process was used to clean up remaining fine noise in the image.

• Contrast & Saturation: Contrast and saturation were enhanced with the Curves Transformation process.

• Sharpen: The BlurXTerminator process was used to sharpen the image (stellar sharpening and stellar halo reduction were set to zero and a manually entered PSF was used).

• Enhance Contrast: Contrast was further enhanced with the Curves Transformation process.

Star processing: 

• Increased star saturation with the Curves Transformation process.

Combine Starless and Star Images: The PixelMath process was used to screen the stars back into the starless image.

Conclusion

What began as a shared eyepiece view at Public Night became a project to reveal the faint Hα background around a familiar favorite. Capturing and processing that extra signal took time, but the result was worth the effort: the Double Cluster’s brilliant stars set against subtle red nebulosity. I still love this object as a visual target. Imaging it gave me another way to appreciate it—and another reason to spend a clear autumn night under the stars.