A Deep Dive into the Flying Bat and Squid Nebula – A Tale of Two Nights of Photon Count
Captured on 04. and 09. April 2025
In this project, I set out to capture one of the most captivating deep-sky pairings in the constellation Cepheus: the expansive H-alpha glow of Sh2-129, the Flying Bat Nebula, and the faint, ghostlike tendrils of the Squid Nebula (Ou4), a rare O-III structure hidden within. Spanning two nights of imaging and many hours of careful post-processing, this image is the result of combining RGB data for the Stars and narrowband data (O-III and H-alpha) to reveal both the subtle starfield and the vibrant interplay of hydrogen and oxygen emissions.
This post walks you through every step of the journey—from acquisition and calibration to advanced processing techniques like SpectroFlux calibration, masked hyperbolic stretching, and final composition in Photoshop. Whether you’re here for the technical details or just want to enjoy the final image, I hope this breakdown offers both inspiration and insight into the process of creating deep-sky astrophotography.
Object Details
Sh2-129, also known as the Flying Bat Nebula, is a large H-alpha emission nebula located in the constellation Cepheus. Embedded within it lies the faint and elusive Squid Nebula (Ou4), an oxygen-rich bipolar outflow of striking structure and color contrast.
Imaging Details
Two imaging sessions were conducted on the 4th and 9th of April 2025, each capturing broadband (RGB) and narrowband (H-alpha, O-III) data with the following parameters:
- Exposure per frame: 300 seconds
- Gain: 100
- Sensor temperature: -20°C (ASI 2600 MM Pro)
Frame Count per Filter:
Total Integration Time:
Processing Details
The overall processing approach for this image follows a structured and deliberate workflow: the broadband RGB star field is processed first, followed by the narrowband emission data from H-alpha and O-III. This separation is intentional and essential, as the techniques and goals for stars and nebulae differ fundamentally.
In deep-sky astrophotography, stars and nebulae require distinct processing paths. Stars benefit from calibration, color balance, and preservation of natural brilliance—whereas nebulae demand contrast enhancement, noise reduction, and dynamic stretching to reveal their faint structures. Working on these components separately allows for maximum control and precision.
In the following sections, I describe the complete processing path using PixInsight, from gradient removal to the final composition. Each processing step is listed in detail, accompanied by example screenshots that illustrate the progression and provide visual context for each major stage.
This methodical breakdown offers insight into how complex astrophotographic data is transformed into a harmonious image—balancing structure, depth, and color.
RGB-Stars
- Background extraction and gradient removal with AutoDBE (SetiAstro-script) for R, G and B-channel
- LinearFit using the G channel as reference (based on median & lowest MAD)
- RGB ChannelCombination
- BlurXTerminator
- NoiseXTerminator
- StarXTerminator
- HistogramTransformation
- SpectroFluxCalibration
H-alpha Data
- AutoDBE
- StarXTerminator
- BlurXTerminator
- NoiseXTerminator
- HT_Stretch_Unlinked2
O-III Data:
- AutoDBE (background extraction, gradient removal) a SetiAstro-script
- StarXTerminator
- BlurXTerminator
- NoiseXTerminator
- Star halo removal
- CloneStamp-Tool (in order to remove the dominant halo around the red supergiant star HD 202380)
- HistogramTransformation
- FAME Mask application for selective processing of the „squid“
- Generalized Hyperbolic Stretch
Combination of H-alpha + O-III data
The H-alpha and O-III narrowband data were combined using a classic HOO palette, in which the red channel is mapped to H-alpha and both the green and blue channels are assigned to O-III. This was achieved through an LRGB combination process in PixInsight, effectively translating the invisible wavelengths of ionized hydrogen and oxygen into a visually striking and scientifically meaningful color representation.
Bringing together the Nebula with the Starless Image
To blend the processed nebula (NEB) with the separately processed stars (STA), a custom PixelMath expression was used:
~( ~(NEB * 1.2 – median(NEB) * 0.0) * ~(1.2 * STA) )
At first glance, this formula may appear complex, but each part serves a specific purpose in creating a balanced and aesthetically pleasing composite image. Here’s a step-by-step breakdown:
- NEB * 1.2
This multiplies the nebula image by 1.2, slightly increasing its brightness and contrast to give the nebular structures more visual weight in the final composition. - – median(NEB) * 0.0
This part effectively has no impact on the result, since any value multiplied by 0 is zero. It’s just a placeholder/remnant of an earlier version of the formula, kept to maintain structure or for future tuning. I use this to substract background at other images. - ~(NEB * 1.2 – …)
The tilde~in PixInsight is the inversion operator. So this part inverts the brightened nebula image. Inverting allows for a blending approach that avoids simple linear addition, reducing the risk of overblown highlights or unnatural transitions. - 1.2 * STA
The star image is also slightly brightened by multiplying it with 1.2 to match the adjusted nebula brightness. - ~(1.2 * STA)
This inverts the star image as well, again preparing it for the blending process in an inverted space. - Multiplication of the two inverted images
The two inverted components—nebula and stars—are multiplied together. This step combines them in a way that maintains contrast and structure without blowing out details. - Final inversion: outermost
~(...)
The result of that multiplication is then inverted once more to bring the image back into normal tonal space. This „double inversion“ technique is a creative method often used in PixInsight to perform non-linear blending, ensuring both components contribute meaningfully to the final image while preserving dynamic range and detail.
In summary, this PixelMath expression blends the enhanced nebula and star images using a technique based on double inversion. It ensures a smooth and controlled combination where both structures retain their integrity, avoiding halos, overexposure, or unwanted blending artifacts. The result is a naturally balanced composite image that highlights both the delicate nebular filaments and the fine star field.
Revealing the hidden beauty of one of Cepheus’ most enigmatic regions
Final Touches in Adobe Photoshop
After all major processing steps were completed in PixInsight, the final image was exported to Adobe Photoshop for a series of subtle yet impactful refinements that help bring out the image’s full visual potential.
- Black Point Adjustment
The black point defines the darkest tones in the image. By carefully adjusting it, the background sky was gently deepened to a neutral black or dark gray—removing any residual haze or noise without clipping faint nebulosity. This enhances overall contrast and gives the nebula structures more presence against the background. - Hue and Saturation Balancing
With the black point set, the color balance was fine-tuned using the Hue/Saturation tool. This step allows for precise control over the visual appearance of each color channel, enhancing the contrast between H-alpha reds and O-III blues/teals. Care was taken to keep the colors vibrant but not oversaturated, maintaining a natural and harmonious look. - Camera Raw Filter
The Camera Raw filter was applied for selective global adjustments. Here, clarity and texture were subtly increased to enhance fine structures in the nebula. Highlights and shadows were balanced to retain delicate details in both bright and faint regions. This step also helps unify the image’s overall tonality, making the final result more coherent and aesthetically pleasing. - Annotation of the Final Version
To complete the project, the image was annotated with object names and key features, providing viewers with scientific context. This layer of information turns the final image into not just a piece of visual art, but also a useful astronomical reference.
These final steps in Photoshop elevate the image from a technically processed dataset to a polished and expressive representation of the deep sky—ready for both presentation and publication
This deep-sky image reveals both the glowing hydrogen structure of the Flying Bat and the delicate oxygen-rich bipolar outflow of the Squid Nebula. A showcase of contrast between subtle broadband starlight and striking narrowband structures—made possible by precise multi-session integration and detailed processing.
Comparison
The following comparison shows very impressively the effects of doubling the exposure time (on the left only one night and on the right the two nights)!
