IC1805 - Heart Nebula — AstroWorldCreations

The Heart Nebula (IC805) - Click here for full resolution

Nebula IC1805 is a large emission nebula, located in the constellation Cassiopeia, about 7,500 lightyears from Earth. The nebula Is very rich of glowing ionised Hydrogen and darker dust lanes. The outer rims of the nebula have a very characteristic shape, giving it’s nickname Heart Nebula.

In the middle of the nebula, there is a very bright area with its own catalogue number of NGC896, which was the first part of the nebula discovered by William Hershel in 1787. Driving the large clouds of hydrogen into its current shape is a small open star cluster in that bright area, known as s Collinder 26 or Melotte 15. Some of the stars in this cluster are 50 times the brightness of our sun.

This is the first image that was captured using the newly acquired ASI6200 camera. For a more detailed technical description of the first light experience with this camera, see this blog post.

Planning

IC1396a is a circumpolar object, so above the horizon all year around. Throughout the whole winter season it is reaching quite high above the horizon, so it is not very critical when to observe. Images were taken during two separate imaging session, that were quite far apart. The first was on 07 November 2020, when the camera had just been received. Then the weather conditions remained very poorly for a long time. The second set of images were acquired on 13 January 2021. The moon was absent on both nights, but since this is narrow-band imaging, that is not too relevant. On 13January, there were some clouds passing by, which resulted in some frames to be rejected. But much worse was that around 1AM in the morning, sleet came out and the session had to be aborted. The telescope was brought indoors quickly to not get damaged by the sleet. This resulted in the loss of about 5 hours of valuable imaging time.

Capturing

The image is captured in 3nm narrow-band, using Ha, SII and OIII wavelengths. During initial testing it was found that exposures of 300s at gain 100 typically lead to proper exposures with little to no blown out highlights and a decent amount of detail.

While the ASI6200 is a full-frame camera, the total field of view was still not good enough to capture the whole nebula. So rotation was chosen carefully to make sure that the essential shape of the nebula was right in the center of the frame. Since this was the first time the ASI6200 was used, quite a bit of experimentation went on. This resulted in variations in settings between frames. For the final image, all images were pooled together. For details on differences in settings and how this affected the image, see the separate blog on this.

Technical details

Exposures

Ha (3nm)
OIII (3nm)
SII (3nm)
Total Exposure

31 x 300s @ Gain 100/21
26 x 300s @ Gain 100/21
25 x 300s @ Gain 100/21
6.8h

Processing

Bias, Dark and Flat frames were all collected at the right length and for the appropriate gain and temperature settings. However, when calibrating the light frames, a strange type of noise started to appear, making the integrated image look very bad. Upon closer inspection it turned out that some of the light frames had background signal that was lower than the corresponding bias and/or dark frames (!). Some settings must have been different between the calibration frames and the light frames. The FITS-headers were identical, so it must be something more subtle than that. In the blog on the first-light experiences with the ASI6200, this topic is discussed in more detail. The image was now processed without any calibration files. Given the very clean nature of the ASI6200 files, this was worth a try.

All frames were registered and integrated per filter. It turned out that between the two imaging sessions, there was a bit of rotation of the camera. Normally this would be cropped out at this stage, but since this was the first 62MP ASI6200 image, it was decided to keep all the pixels in and see how it would end up. To both OIII and SII, a linear fit was applied to the H-alpha channel, resulting in three colour channels ready for further processing. Both OIII and SII signals were very weak and had to be stretched a lot, creating quite a bit of noise, especially since they had not been calibrated.

The three narrow-band channels were then combined into a Hubble palette, with SII mapped to Red, Ha mapped to Green and OIII mapped to Blue. Some basic edits were performed before stretching, such as an automatic Background Extraction, a Background Neutralisation and a Color Calibration. Then the color image was stretched using Histogram Transformation.

First the magenta rings around the stars were removed by inverting the image, applying SCNR and inverting back. This method is quite effective. But it removed also a lot of magenta in the rest of the image. Perhaps next time it would be better to do this process under a star-mask. Next up was the creation of the typical Hubble Palette look. With Colormask, four color masks were created (Cyan, Green, Magenta and Yellow, all blur 3), and using CurvesTransformation, the colours in the image were adjusted to taste. The weak OIII and SII signals were making it difficult to get very intense colours, but after some back and forth, the colours looked about right. The image was very noisy though. The lack of calibration and the high stretch of the OIII and SII channels showed their marks. To deal with that, a modest convolution was applied to create smooth tone-curves across the image. While this definitely helped to get rid of some of the noise, a proper noise reduction, perhaps even using the MMT tool on the individual channels, would have been better. For now processing was continued and while the convolution created a very soft image, that would be corrected when adding the Luminance.

For luminance, a clone of the H-alpha channel was used. Noise reduction was applied using the MMT-method, which worked out really easy and very effective. This was then followed by deconvolution, with protection of the background using a range-mask. The PSF image was created using the PSFImage script, and was very quick. After stretching and adding a little bit of contrast, an attempt was made to correct the triangular boxes that were the result of different rotation settings between sessions. Stars were separated from the background using the StarNet process. Then the artefacts in the background were corrected using the clone-stamp. And finally the stars were put back in. This worked quite well.

The Luminance and the SHO image were combined. This is always a very critical and often rewarding moment as this is getting very close to the final result. The image came out allright when zoomed out. But upon closer inspection, there was a lot of noise present, still the consequence of no calibration and no noise-reduction applied to the colour image whatsoever. A pretty heavy TGVDenoise was applied. An UnsharpMask was applied to add some additional sharpness to the image.

Some final touches were made using the Curves tool. In Affinity Photo some subtle vibrance, brightness and HSL adjustments were made. Also some of the artefacts from rotation differences between sessions, that were removed in the Luminance image, had come back through the colour image. So ultimately in Affinity Photo the image was slightly cropped to eliminate this.

As a first image of the ASI6200, it came out allright. However, there are several areas for improvement. The various settings between frames, used while experimenting with the camera don’t make for the best input images possible. It certainly prevented the use of regular calibration routines. With properly calibrated files, a more systematic noise reduction from the start of the processing would probably have helped to improve the image.

This image has been published on Astrobin and has received Top Pick nomination.

IC1805 - Heart Nebula

Planning

Capturing

Technical details

Exposures

Processing

M35 - Open Cluster

IC1396a - Elephant’s Trunk