IC59 and IC63 - Gamma Cassiopeia Nebulae
IC59 Gamma Cassiopeia Nebula - Click here for full resolution
IC 59 and IC 63 are faint reflection and emission nebula located in the northern constellation of Cassiopeia. Both nebulae are faint at apparent mag. +10, have extremely low surface brightness and surround bright variable star gamma Cas (γ Cas). IC 59 and IC 63 are 610 light-years distant. From our perspective, IC 59 is located on the northern side of gamma Cas and IC 63 to the northeast. Spatially the nebulae are roughly 3 light-years from gamma Cas, although IC 63 is slightly closer to the star. As a result, it appears mostly red due to a dominance of H-alpha emission, whereas IC 59 exhibits much less H-alpha emission and appears mostly blue due to dust reflected starlight.
source: freestarcharts
NGC/IC:
Other Names:
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Transit date:
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IC59 and IC63
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Nebulae
Cassiopeia
00h 57m 29s
+61º 08.6’
28 Oct
82º N
Conditions
IC59 and IC63 are visible all year round, with highest altitudes reached in the autumn. A first attempt to photograph IC59 and IC63 was made in September 2021, under near full moon conditions. In subsequent months the focus was on other objects, but in March 2022 there was an opportunity to complete the dataset on two consecutive nights. But since March is not the best time, imaging of this target was limited to the first few hours of the night.
Equipment
The two nebulae surrounding Gamma Cassiopeia make for a fairly large object in the sky. A wide-angle scope is the best option. For this image, the Takahashi FSQ-106 was used in combination with a full-frame camera, the ZWO ASI6200. The FoV of this combination is a bit too large, so image was cropped. The images were taken during one of the earlier sessions with the RainbowAstro RST-135E mount. At this point, guiding was not yet properly controlled, and switched back a few times from PHD2 to the guiding module in Ekos. But overall it was a great project to get to understand the mount a bit better.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FSQ-106, Sesto Senso 2
Rainbow Astro RST-135E, Berlebach Planet small
ZWO ASI6200MM Pro, cooled to -15 ºC
Chroma 2” RGB and 3nm Ha unmounted, ZWO EFW 7-position
William Optics Guidescope AC 50/200 RotoLock Red, ZWO ASI290MM
Fitlet2, Linux Mint, Powerbox Advance
KStars/Ekos 3.5.7, INDI Library 1.9.4, PixInsight 1.8.9-2
Imaging
Gamma Cassiopeia and its neighbouring nebulae are a tricky target to image. The star itself is super-bright and exposures should be controlled to not clip the core too much. Both gain 0 and gain 100 were used at start, but gain 0 was by far the better option. At gain 0, the full well depth of the IMX455 sensor is maximally used. but still significant clipping of the star nucleus occurred at 180s. Shorter exposures would have been possible, but especially IC59 has quite some blue colours as well, and this might not be captured in shorter exposures. Still, the blue in the nebulae did not come out too well. And that is probably not a surprise considering the total of 45 min exposure of the blue channel.
The other aspect of this target is that most nebulosity is only visible in Ha. Even in the red channel, almost no nebulosity is present. The Ha frames on the other hand show a lot of detail. Also here some experimenting was done, both with different gain settings as well as exposures. The final image was recorded as a short exposure RGB image for Gamma Cassiopeia, complemented with lots of Ha frames to capture the nebulosity.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
7266 × 4941 px (35.9 MP)
530 mm @ f/5.0
3.76 µm
1.46 arcsec/px
2º 56' 53.2" x 2º 0' 17.2"
-163 degrees
RA: 1º 00’ 07.212”
Dec: +60º 55’ 45.78”
Processing
With the images being taken during different sessions and with various different gain and exposure settings, calibration was a challenge. But the WeightedBatchPreProcessing (WBPP) script in PixInsight is a tremendous help. Throw all frames in it, and WBPP is able to sort out most of the combinations. The remaining combinations can be manually set. For image integration, different exposure lengths are easily combined. WBPP sorts things out by automatically applying different weighting factors. Ultimately all frames were calibrated using darks (50) and flats (25), registered and integrated to an R, G, B and Ha image.
The integrated R, G and B master images were cropped to adjust the framing. For aesthetic reasons almost half of the total image was cropped away. Background gradients were removed from individual images using GraXpert. BlurXTerminator (BXT) was used in ‘correct only’ mode on each of the images. Overall the star shapes were good, but the latest BXT has such an impressive effect on even the smallest aberrations, it is almost a waste not to use it.
Next step was to put the R, G and B images together using ImageCombination. The colours were adjusted by applying colour calibration using SpectroPhotometricColorCalibration (SPCC) followed by a mild SCNR to remove a slight green-cast from the background. BXT was applied a second time, but now to deconvolve the stars. For stretching, three different approaches were tried: GeneralisedHyperbolicStretch (GHS), MaskedStretch and HistogramTransformation (HT). Both GHS and MaskedStretch gave a very interesting result on gamma cassiopeia under normal stretching conditions. The nucleus of the star was condensed to a small circle and the remaining signal was left as a large halo around it. Not sure what caused this, perhaps the nucleus was too much clipped in the individual images? Anyway, it was almost impossible to find stretching parameters that did not give this effect. HT on the other hand was much friendlier to the star core, and allowed a nice stretching where gamma cassiopeia was obviously very bright, but in a more natural looking way. Saturation and contrast were then enhanced using CurvesTransformation (CT). A small magenta cast around Gamma Cassiopeia was removed by inverting the image, apply SCNR and inverting it back again. Some final noise reduction using NoiseXTerminator (NXT) completed the RGB image. Small hints of the nebula were present, but very faint.
Comparison of three stretch methods. Both GHS (left) and MaskedStretch (middle) caused the core of Gamma Cassiopeia to reduce to a little disc with a large halo around it. HT (right) on the other hand stretched the star in a much more natural looking way.
The nebula was to be added through the Ha channel. The integrated Ha image was cropped the same as the colour images and background gradients were removed with GraXpert. BXT was applied with emphasis on the nebulosity in the image. The stars were taken out with SXT and the starless image was stretched using GHS. For this image, GHS worked very well and allowed to add some contrast in the fainter areas of the nebula. Noise was removed using NXT. An area of interest was now the area around Gamma Cassiopeia. The red light of the bright star had caused a bit of a glow in the Ha image as well. One way of dealing with this is to use PixelMath to extract regular Red signal from the Ha signal to leave a much more pure Ha signal. Since the effect was so local, the glow was brushed away using the clonestamp tool.
Now it was time to add Ha to the RGB image. There are many different ways to do this, and the ‘Toolbox’ of scripts that comes installed with GraXpert, has two scripts for this exact purpose. CombineHaWithRGB did not work very well. The preview came out super bright and it was hard to find the proper settings. Perhaps this tool should be applied to linear images? The other one is called CombineRGBAndNarrowband. And this one worked a lot better. Separate controls for the blend amount, vibrance and black point allowed for some fairly decent control over the blending process. After a couple of tries I settled on a fairly dark blend where the brightness of the cores of the nebulae stand out well.
While BXT had given quite a bit of sharpness in the nebulae already, it felt that this could be enhanced a bit more. Best would be to do that without the stars. So stars were removed with SXT, and UnsharpMask was applied under the protection of a range-mask created from the starless Ha image. The sharpening introduced some noise again, which was removed one more time with NXT, before the stars were put back in. Final background levels were adjusted using BackgroundNeutralization.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
