Sh2-229 | C31 - Flaming Star Nebula

IC405 or Flaming Star Nebula - Click image for full resolution

IC405 (R.A.: 5h 17m 36.62s, Dec: +34º 17’ 23.7”) is an emission/reflection nebula in the constellation Auriga at a distance of 1,500 lightyears from Earth. The nebula is also known as the “Flaming Star Nebula'“, Caldwell 31 and Sh2-229. The nebula is brightly illuminated by the central star AE Aurigae. This star originally belonged to Orion’s belt, but a very close encounter with a neighbouring star 2.7 million years ago propelled it away from the Orion complex.

Planning

The Flaming Star Nebula is a typical winter-object, reaching highest altitudes between November and January. Images were taken on three consecutive nights in February with no interference of the moon.

*Visibility charts showing altitude at 22:00h throughout the year (left) and throughout the session on February 25, 2019 (right).*

Conditions

All images were taken from the backyard in Groningen, The Netherlands (53.18, 6.54), divided over 3 nights: February 25, 26 and 27, 2021. The advantage of three consecutive nights of clear skies is that equipment can stay out between sessions and alignment and calibration does not have to be repeated each day. During the third night, fog was severely hindering capturing any decent images during the second half of the night.

Capturing

The image was captured using the Takahashi FSQ-106 in combination with the ASI1600MM-Pro camera. Estimates of the Field of View of this combination, relative to the size of the object were a bit more optimistic than reality. This caused the framing of the object to be rather tight. Careful rotation of the camera was performed to get as much of the nebula in, while still keeping the bright stars 16, 17 and 18 Aur in the frame for a balanced composition.

Technical Details

Telescope
Mount
Camera
Filters
Accessoires
Software

Takahashi FSQ-106, Sesto Senso focuser
10Micron GM1000HPS, Berlebach Planet
ZWO ASI1600MM Pro, cooled to -25 ºC
Astrodon 1.25” H-alpha (5nm), LRGB mounted, ZWO EFW 8-position
Fitlet2, Flip Flat
KStars/Ekos, INDI Library, Mountwizzard, SkySafari, PixInsight

Frames

The image was captured using 5nm H-alpha filters and a set of LRGB filters. Below are the frames listed taken in each session that made it to the final image.

Image

By far the most signal of the nebula was visible in the H-alpha frames. In fact, even in the luminance frames, the signal of the nebula was very low and completely overtaken by the bright stars in the image. These bright stars ultimately were somewhat over-exposed. The part of the nebula that was well visible in the broadband filters was the area that lights up blue-is in the final image. With H-alpha showing the nebulosity the best, over 5h exposure was dedicated to this wavelength, or more than half of the total 9.4h of total exposure.

The final image has a resolution of 4316 x 3308 pixels, or 14.3 Megapixels. It covers a field of view of 1.77 degrees horizontally. The image would have benefited from a slightly larger field of view.

Processing

All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the WeighedBatchPreprocessing script in PixInsight.

First the H-alpha frames were processed. In a first step, an automated background extraction followed by a small crop was performed. The MMT-based noise-reduction technique nicely cleaned up the image. It was probably good to go at this stage, but just to crisp up the image a little bit more, a deconvolution routine was applied before stretching to the non-linear state. The overall image signal of the H-alpha was with 18,700 a lot brighter than the luminance signal at 10,200, so a LinearFit was applied to bring the two on the same level.

Next up was the luminance image. This largely followed the same process as the H-alpha image. Somehow the flats had not optimally corrected some vignetting, so multiple runs of the background extraction were run to correct this as good as possible. The image is very rich in stars, some of which are rather bright. Since the focus is on the nebula, the stars were shrunk a bit using morphological transformation. The structures in the nebula were enhanced somewhat using LocalHistogramEqualization. But the biggest issue with the luminance was an annoying blob right in the center of the frame. This was probably some dirt on the filter or so, and was not corrected with the flats. It could be more or less removed by very precise use of the clone stamp tool with a very small radius.

The base colour of the image, mostly focusing on the stars, was created through combining the RGB channels. Standard background extraction and crop were applied to the individual channels. The red channel had the lowest channel (mean 150), whereas green and blue were significantly higher (mean 200). A Linear Fit with Red as reference was applied before all three channels were combined using RGBCombination. Standard routines with Photometric Color Correction and Background Neutralisation were used to get the colours right. The small bit of green flair in the background could be corrected using the SCNR process. In an attempt to highlight the blueish colours around AE Aurigae, a small boost in colour saturation was applied. With the image still in linear state, this effect was only very subtle. Maintaining colour, especially in the blueish areas was also the reason that Arcsin Stretch was used to stretch the image to its non-linear stage. This was the first time this method was used and probably not applied in the best possible way, as little improvement could be observed relative to regular HistogramTransformation. The RGB image with the short overall exposure time was quite noisy. This was addressed by applying the MMT-based noise-reduction method, followed by a convolution. Adding the Luminance to the RGB using LRGBCombination brought back a good colourful image with reasonable detail and sharpness.

The final step was to add the H-alpha into the image. This was done using the NBRGBCombination Script. The red colour came out a bit too intense, so the saturation setting in the script was reduced ever so slightly to 0.46. From here it was a matter of finishing up to taste with some small curves adjustments and contrast enhancements. Final finishing touches were done in Photoshop. These involved mainly some local corrections around the ‘blob’ removal which still showed through a bit in the final image.

This image has been published on Astrobin.