Sh2-313
Abell 35, Flaming Crown Nebula
46’ x 30’ | 0.3”/px | 9216 × 6148 px
Hydra
RA 12h 53m Dec -22° 52’ | 180°
Sh2-313, also known as Abell 35, is a planetary nebula located in the constellation Hydra. Most Sharpless objects are emission nebulae, but eight of them are planetary nebulae. This one is notable for its large size relative to other planetary nebulae and unusual properties, which have led to debates about its classification. Initially considered a planetary nebula, recent measurements suggest it might be a Strömgren sphere, where the central binary star system ionizes the surrounding interstellar medium rather than being enveloped by ejected gas. The nebula has a low surface brightness. It spans approximately 10 arc-minutes across the sky and is estimated to be about 5.2 light-years in diameter. The distance to Sh2-313 is estimated to be between 1,200 to 1,500 light-years from Earth. The central star of Sh2-313 is a binary system consisting of a bright star (HIP 62905) and a very hot white dwarf companion. This binary system creates a dramatic parabolic bow-shock due to their rapid movement through the interstellar medium, energizing it and forming the distinctive shape of the nebula. Sh2-313 is also known as the Flaming Crown Nebula, reflecting its unique appearance.
source: Mistral
Data Acquisition
Data was collected over 10 nights, mostly during February 2025 using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered in narrowband using Ha, OIII and SII filters. For proper star colours, additional data was collected using short RGB exposures. With the target reaching a maximum altitude of only about 30°, images were collected from 2h before until 2h after passing the meridian. A total of approximately 18 hours of data was combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
| Sessions | Moon% | Moon° | Hum% | SQM | T°C | Frames | Exposure |
|---|---|---|---|---|---|---|---|
| 20250131 | 9 | 136 | 75 | 21.5 | 1 | 2 | 0h 20m |
| 20250201 | 17 | 150 | 70 | 21.5 | 4 | 8 | 1h 20m |
| 20250204 | 49 | 159 | 63 | 20.8 | 4 | 13 | 2h 10m |
| 20250205 | 60 | 147 | 70 | 20.4 | 0 | 13 | 2h 10m |
| 20250206 | 71 | 134 | 65 | 19.8 | 1 | 20 | 3h 20m |
| 20250208 | 88 | 107 | 80 | 18.0 | 4 | 30 | 1h 30m |
| 20250223 | 18 | 78 | 80 | 21.6 | 4 | 22 | 3h 40m |
| 20250224 | 13 | 91 | 80 | 21.6 | 4 | 12 | 2h 00m |
| 20250225 | 4 | 104 | 90 | 21.6 | 6 | 3 | 0h 30m |
| 20250324 | 23 | 120 | 75 | 21.5 | 5 | 10 | 1h 40m |
| Total | 133 | 18h 40m |
| Frames | Bin | Gain | Exp.(s) | Frames | Exposure |
|---|---|---|---|---|---|
| Ha | 1 | 2750 | 600 | 35 | 5h 50m |
| OIII | 1 | 2750 | 600 | 35 | 5h 50m |
| SII | 1 | 2750 | 600 | 33 | 5h 30m |
| Blue | 1 | 0 | 180 | 10 | 0h 30m |
| Red | 1 | 0 | 180 | 10 | 0h 30m |
| Green | 1 | 0 | 180 | 10 | 0h 30m |
| Total | 133 | 18h 40m |
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” Ha, OIII, SII (3nm) and R, G, B unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered, integrated and drizzled (2x) using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
Narrowband imaging using false colour palettes is always more of an art than a science. And in this case I found it particularly difficult to find a colour palette that was both pleasing for the eye, and illustrative of the three filters that were used. There are only 64 images currently published on Astrobin labelled Sh2-313, and the majority has been imaged in a HOO palette, so it is a bit difficult to know what to expect. Looking at the individual narrowband channels, distinctly different structures show up in each. A lot of the typical planetary nebula structure is visible in Ha. The characteristic bow-shock is mainly present in OIII. And the SII shows some specific highlights at the top area of the nebula, as well as in the central part. When experimenting with SHO, HOS and HOO palettes, there was never a satisfactory result. Pushing the Ha too much and the bow-shock in OIII would disappear. Pushing the OIII more and the overall structure of the nebula lost a lot of detail. Pushing the SII too much and the whole nebula becomes a super colourful Christmas-tree, not the most pleasing overall look. The palette I eventually went for was the Foraxx palette, in combination with a synthetic luminance created from Ha and OIII. Further enhanced by an SII image mapped to yellow, blended in with the rest. The processing therefore followed a bit of an unusual pattern. Let me explain some of the critical steps.
Starless images of each of the narrowband channels. A lot of structure in the Ha channel, the bow-shock primarily visible in OIII and intens SII signal at the top of the nebula as well as some highlights in the central part.
Rather than stretching each channel by hand, stretching was done using the Statistical Stretch script from SetiAstro. This allows all channels to be stretched to similar brightness levels. I found that this worked best with the stars still in the image. After stretching the stars were removed. Then the starless channels were combined using the Fornaxx script. Unfortunately, the script as once published by Paulyman Astro is no longer available. But it is primarily a blending formula that can be applied in PixelMath. The code for each of the colour channels in PixelMath looks as follows:
Red: (OIII^~OIII)*SII + ~(OIII^~OIII)*Ha
Green: ((OIII*Ha)^~(OIII*Ha))*Ha + ~((OIII*Ha)^~(OIII*Ha))*OIII
Blue: OIII
Normally this would be a point where NarrowbandNormalization would be applied to find the best final result. However, none of the combinations really worked well. It was particularly hard to find a balance between the detail of the Ha and the colour of the OIII. Therefore I decided to create a synthetic Luminance from the Ha and OIII. I simply took the brightest value of either the Ha or the OIII. In PixelMath this looks like:
RGB/K: iif(Ha>OIII,Ha,OIII)
This luminance was applied to the narrowband starless image. It was a bit too much, so for lightness a transfer function of 0.55 instead of 0.5 was used.
Now it was time to put some extra emphasis on the SII signal. For that I used the NBColourMapper script. The narrowband starless image was loaded as RGB and was not altered by the script. SII was mapped to a bright yellow colour (hue 61, saturation 1). That was way too much yellow, so this was toned down by lifting the midtones to 0.7. The result was a bit of an autumn-like colour in larger areas of the Ha and a bit of a yellowish/orange colour in some of the SII highlights. From here only the reds and the blues were a bit intensified, using colour-masks. Overall this created the final colours in the image.
After all the pushing and pulling, a very small reddish gradient in the background popped through, which could be effectively removed using AutomaticBackgroundExtraction.
The rest of the processing followed a standard processing workflow.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
