The Rice Hat Nebula, cataloged as Sh2-224, is a very faint supernova remnant (SNR) located in the constellation Auriga. Its delicate, filamentary structure is the result of a massive stellar explosion that occurred thousands of years ago, scattering ionized gas and dust across interstellar space. Unlike the more well-known supernova remnants such as the Veil Nebula, Sh2-224 exhibits an irregular shape with intricate shock fronts, making it a visually intriguing target for astrophotographers and researchers alike.
This elusive nebula is particularly interesting due to its interaction with the surrounding interstellar medium, forming distinct arc-like structures that hint at the dynamics of the explosion and the density variations in the surrounding space. Capturing Sh2-224 requires long exposure times and the use of narrowband filters, as its faint emission is primarily dominated by ionized hydrogen (Hα) and oxygen (O-III).
Visually the SNR is nearly impossible to see – even under Bortle 1 or Bortle 2 skies. Photographing the Nebula is both a technical challenge and a rewarding experience. This post explores the imaging process, data acquisition, and post-processing techniques used to reveal the details of this SNR under Bortle 4 skies.
Imaging Notes
Over the course of three nights, I captured data for the Rice Hat Nebula (Sh2-224) using narrowband filters. I collected a total of 17 hours and 40 minutes of exposure time (8 hours in H-alpha and 9 hours and 40 minutes in O-III). For this project, I used a Nexus 0.75x reducer with my f/4 Newtonian telescope, effectively reducing the focal ratio to f/3. This allowed for increased light-gathering efficiency and a wider field of view. The atmospheric seeing conditions varied between 2.4 and 2.8 arc seconds.
Processing without Star-Removal in HOO
The initial processing was done using the HOO palette without removing the stars. As a result, the star colors are not accurately represented in the final image. The focus was primarily on enhancing the nebular structures and emphasizing the contrast between the hydrogen-alpha and oxygen-III emissions.
96 x 300″ H-alpha und 116 x 300″ O-III (-20° Sensortemp. of the ZWO ASI 2600 MM Pro @ Gain 100)
Processing with Star Removal in HOO – Mode 3
A truly remarkable repository by SetiAstro provides a method for extracting accurate star colors from narrowband data, eliminating the need for additional RGB filter exposures to capture natural star hues. This technique, known as Foraxx for NB-stars, saves therefore R,G,B capturing time in order to focus on the narrowband details!
To achieve this, I first separated the nebula and stars using StarXTerminator (RC Astro). Once isolated, the stars were processed using the Foraxx method to restore their natural colors (see https://www.youtube.com/watch?v=UpG6bj68VPw). The nebula data itself was integrated using PixelMath, combining the H-alpha and O-III data in the usual manner.
One of the biggest challenges with extremely faint nebulae like Sh2-224 is merging the processed data without losing fine details. This requires careful adjustments and significant amount of trial and error…. Using background subtraction (median subtraction) helps move in the right direction, but achieving a seamless integration still demands practice.
In the end, the more meticulous workflow—where nebula and stars are processed separately—yields richer structural detail in the final image. However, from a purely aesthetic standpoint, the preference for this approach remains open to debate…
The final image in HOO-Mode3:
