Capturing the Prawn Nebula: A Low-Horizon Astrophotography Adventure
Astrophotography often presents unique challenges, and for this project, the target was the stunning Prawn Nebula, also known as IC 4628. This vibrant nebula, located in the tail of the constellation Scorpius, proved to be a particularly demanding subject due to its low elevation from my observing location.
The Challenge of Low Declination
From my vantage point, the Prawn Nebula only rises a mere 16 degrees above the horizon each night. This significantly limits the available imaging time, forcing a careful balance between acquiring sufficient data and working within the narrow window before the nebula dips below the horizon, obscured by surrounding houses. Despite the difficulties, the pursuit of this celestial gem was a rewarding experience.
Understanding the Colors: SHO Palette
This image was captured using the SHO (Sulfur, Hydrogen, Oxygen) narrowband filter set, which allows us to visualize the distinct emission lines of these elements. The warmer tones of red and orange predominantly represent the emissions from Hydrogen and Sulfur, while the striking blues highlight the Oxygen emissions. This false-color representation brings out the intricate details and structure of the nebula in a captivating way.
Discovering Galactic Tadpoles
Towards the bottom left of the image, you'll notice fascinating structures often referred to as 'galactic tadpoles.' These are likely stellar nurseries, the nascent stages of newborn stars, enshrouded within dense pockets of gas and dust. Their presence adds another layer of intrigue to the Prawn Nebula, offering a glimpse into the ongoing processes of star formation.
Equipment and Software
Achieving this image required a dedicated setup and meticulous processing:
Equipment:
- OTA: Stellarvue SV105T w/0.8x reducer (588mm fl at f/5.6)
- Mount: ZWO AM5N
- Imaging camera: ZWO ASI1600MM-Cool
- Guiding camera: ZWO ASI120MM-Mini OAG
- Autofocuser: ZWO EAF
Software:
- NINA
- PHD2
- PixInsight
Acquisition Details
- Location: Joshua, TX (Bortle 4)
- Dates: 7/16/25, 7/17/25, 7/18/25, 7/19/25, 7/20/25, 7/21/25, 7/28/25, 7/29/25
- Gain: 200
- Offset: 50
- Camera temp: -10°C
- Sii: 16x300"
- Ha: 180x300"
- Oiii: 18x300"
- Total integration time: 17hr 50min
- Calibration frames: 64 darks, 30 flats, 200 bias
Processing Workflow
Preprocessing:
- Batch PreProcessing script for calibration.
- StarAlignment, ImageIntegration, DynamicBackgroundExtraction, DynamicCrop.
Luminance Processing:
- Ha duplicated and processed as Luminance.
- BlurXTerminator, NoiseXTerminator, ArcsinhStretch (x2), HistogramTransformation.
- StarXTerminator used to create a starless version.
- Starless and original image combined using PixelMath for enhanced detail.
- LocalHistogramEqualization, MultiscaleLinearTransform.
Narrowband Masters (SHO) Preparation:
- BlurXTerminator, NoiseXTerminator, StarXTerminator, HistogramTransformation applied to each master.
SHO Combination and Tonemapping:
- ChannelCombination used to create the initial SHO tonemap (R: Sii, G: Ha, B: Oiii).
- CurvesTransformations with masks for color balancing.
- Invert > SCNR > Invert with mask to remove magenta.
- Further Curves and HistogramTransformations for color refinement.
Final Integration and Adjustments:
- LRGBCombination to integrate the tonemapped SHO data with the luminance.
- ColorSaturation for blues, CurvesTransformation.
- Invert > SCNR > Invert with mask to address remaining magenta tones.
- MorphologicalTransformation followed by UnsharpMask with a star mask to subtly reduce star sizes.
- Resampled to 75% for web display.
This meticulous process, from overcoming the low-horizon challenge to detailed color calibration, has resulted in a captivating view of the Prawn Nebula. You can explore more of my work on , , and .
