Sh2-236 - Tadpoles Nebula
IC410 (Tadpoles Nebula) - Click here for full resolution
The Tadpoles nebula (IC410) is an emission nebula in the constellation Auriga. At the heart is a star cluster (NGC1893) that illuminates the surrounding gas clouds from which the young blue stars have just been formed. Characteristic are the two curly structure which give the nebula its nickname Tadpoles Nebula. They are clumps of gas and dust as part of the star formation. The high radiation pressure from the star cluster causes some of the gas to be pushed blown away into space, causing the curly tails. The very Hydrogen-rich area is also an object in the Sharpless catalogue, and known as Sh2-236. IC410 is part of a much larger nebulous area, that also incorporates the Flaming Star Nebula.
NGC/IC: IC410 - NGC1893 - Sh2-236
Other Names: Tadpoles Nebula
Object: Emission Nebula Galaxy
Constellation: Auriga
R.A.: 05h 24m 5s
Dec: +33º 32.3’
Transit date: 20 February
Transit Alt: 70º S
Sky-plots with a FoV of 50º (left) and 5º (right). Click to enlarge
Conditions
Images were taken on four nights in November 2022 from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was very bright during the first three nights, but absent on the last occasion. Best time to observe this target is between November and January. Most of the images were taken at altitudes between 30 and 70 degrees.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout the session on 25 November, 2022 (right).
Weather conditions at each of the sessions were as follows:
Capturing
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, Sesto Senso 2
10Micron GM1000HPS, EuroEMC S130 pier
ZWO ASI533MM Pro, cooled to -15 ºC
Astrodon 1.25” SHO (5nm) and RGB mounted, ZWO EFW 8-position
Unguided
Fitlet2, Linux Mint 20.04, Pegasus Ultimate Powerbox v2, Flip Flat
KStars/Ekos 3.6.1, INDI Library 1.9.8, Mountwizzard4 2.2.7, openweathermap.org, PixInsight 1.8.9-1
Frames
Exposure
Geometry
Annotated image showing the deep sky objects, and some of the brighter stars
Processing
All frames were calibrated with Dark (50), Flat (25) and Dark-Flat (25) frames and registered using the WeightedBatchPreprocessing script. Image frames were normalized and scaled using the NormalizeScaleGradient script and integrated using NSG parameters. The NSG script has seen an update since last use and allows for much finer control of which subframes to include in the final stack. There are two criteria, Clouds (Transmission) and Light pollution (Weight) that all subframes are ranked on compared to a reference frame. Using sliders for both settings, there is a graphical representation of which frames will be rejected under which setting. Very comparable to the SubframeSelector tool. NSG does not look at star morphology, such as eccentricity due to tracking errors etc. So it is still good practise to closely analyse star shapes, for example using Blink or SubframeSelector. Other than that, the NSG tool seems to be a great selection tool. For this image I did a comparison with regular SubframeSelector + stacking using PSF weights, and the NSG images came out with better detail and more contrast.
Narrowband (SHO) image
The images were taken over four nights, so there were some changes in geometry. A crop on all filters straightened that out. The flats for the SII image had not very well corrected for one of the sessions, which resulted in a slight reversed vignet on the final stack. This was eliminated using DynamicBackgroundExtraction. The other images were so full of nebulosity that it was hard to identify any gradients at all, so no DBE was applied. Then the individual filter-stacks were stretched. The GeneralisedHyperbolicStretch script (nowadays also available as regular process) is a very promising new tool for that, but for some reason I have not mastered it well enough to get great images out of it. It is very easy to ‘over-cook’ an image, resulting in a very grainy ugly result. Therefore I now use a mix of tools. The first very rough stretch is done by a simple HistogramTransformation stretch of the midpoint, just enough to see the structure in the image appearing. Then GHS is used, and the symmetry point is set to the signal strength from the faint nebulosity. Using quite a bit of local intensity allows now to really bring in the contrast in the nebulosity, much better than regular HT. As a final step HT is used to set the black-point and overall brightness, typically with the histogram peaking at 17.5%.
The narrowband image was assembled using the Hubble-palette, mapping SII to the Red, H-alpha to the Green and OIII to the blue channel. To have maximum flexibility in working on the nebula, the stars were eliminated using StarXTerminator (RC Astro). To get the colours right in the starless SHO image, a new PixelMath script from Bill Blanshan was used. Default settings were applied, but SCurve was set to 1, to prevent dark areas from becoming noisy. The resulting image had quite some green in it left, which was eliminated using another script from Bill Blanshan, the ModifiedSCNR script. Mild noise reduction (0.6) was applied using NoiseXTerminator.
The blue and yellow/brown tones were brightened up using CurvesTransformation. For each colour, a mask was created using the ColorMask script. The bandwidth of hue values was taken from the image. With the Readout Data of the cursor set to ‘CIE L c h’, clicking on the image shows a popup screen, with a lot of parameters, including the hue (h) value. As a final touch-up of the image, DarkStructureEnhance was used to accentuate the darker areas in the nebulosity just a little bit more, thereby enhancing the overall contrast. And finally some finishing touches with the CurvesTransformation tool completed the (starless) SHO image
Broadband (stars) image
Missing data in Red and Green data (left), resulting in Blue stars (middle), replaced by stars from the SHO image (right).
The R, G and B channels were captured purely to add native star colours back into the SHO image to the end. For that purpose, short exposures and few frames are usually enough. When stacking the frames, it became clear that the R and G image lacked a significant portion of the FoV, even after the crop from the SHO image applied. The night when Red and Green images were acquired was not great at all (all Blue images failed due to clouds coming in), and most likely the telescope had not been properly aligned to the target. In hindsight it was lucky that the Blue images were taken at a later time, as it made correcting the error easier. In the below image, the lack of data on the left hand side of the image in the Red channel can be clearly seen. Since blue had all data, combining the three channels into one image then resulted in all blue stars on the left. The stars that were extracted from the SHO image were used to replace the blue stars. This was done with a PixelMath trick learned from Adam Block. He uses this technique to eliminate color blips (single pixels that have an intense red, green or blue color). The PixelMath expression is as follows:
iif( $T[2] > 2* ( $T[0] + $T[1] ), SHO_Stars , $T )In which $T[0] is the red-value of a pixel, $T[1] the green-value and $T[2] the blue-value. So the script replaces every pixel where the blue value is more than two times the sum of red and green with the corresponding pixel from the SHO stars image. The factor two is a bit arbitrary. I used 1 first, but that gave some unwanted replacements at small edges of some very blue stars. The result looks reasonably good. Obviously the stars on the left hand side of the image have less intense and/or accurate colours. But for the whole image, this is not a very big problem.
Final image
The final image was constructed by simply adding the SHO starless image with the RGB stars image using PixelMath. The resulting image was taken into Photoshop where a little bit of dodging and burning gave a little bit more pop to the image. The tadpoles themselves had become quite bright. Using a brush, some highlight reduction was applied to bring their brightness back to be more fitting to the rest of the image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
