Caldwell 44 - Superman Galaxy
NGC7479 - Click here for full resolution
NGC 7479 (also known as Caldwell 44) is a barred spiral galaxy about 105 million light-years away in the constellation Pegasus. William Herschel discovered it in 1784. NGC 7479 is also recognised as a Seyfert galaxy and a LINER undergoing starburst activity not only on the nucleus and the outer arms, but also across the bar of the galaxy, where most of the stars were formed in the last 100 million years. Polarisation studies of this galaxy indicate that it recently underwent a minor merger and that it is unique in the radio continuum, with arms opening in a direction opposite to the optical arms. This feature, along with the asymmetrical arms of the galaxy and the intense star formation activity are attributed to a merger with a smaller galaxy.
source: wikipedia
NGC/IC:
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Object:
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Transit date:
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NGC 7479
Caldwell 44, Superman Galaxy
Galaxy
Pegasus
23h 04m 54s
+12º 19.0’
01 Oct
49º S
Conditions
NGC7479 is best visible in the Autumn season where it reaches decent altitudes above 40 degrees. Two attempts were made to image the object, both in September and October. Conditions were never really great with somewhat poor seeing and/or a near full moon. Nevertheless managed to get around 11h of exposure and finished the project with that.
Equipment
This was the first light for the new ZWO ASI533MM camera. Full details can be found in this blog. The camera combines modern sensor technology of low noise, no amp-glow and high dynamic range with a small form-factor. The square sensor is just a little bit smaller than the trusty ASI1600 and as such a good upgrade for somewhat smaller targets. NGC7497 is a small target, so a combination of the ASI533 with the TOA-130 is a perfect setup for it. Another first was the use of a new rolling pier, the Euro-EMC S130. Full details can be found in this blog. This pier makes it possible to roll out the complete rig, position it on a pre-determined spot, with almost no need to adjust polar alignment. A massive time-saver for the ‘mobile’ astrophotographer.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, FL67 flattener, Sesto Senso 2
10Micron GM1000HPS, EuroEMC S130 pier
ZWO ASI533MM Pro, cooled to -15 ºC
Astrodon 1.25” LRGB mounted, ZWO EFW 8-position
Unguided
Fitlet2, Linux Mint, Pegasus Ultimate Powerbox v2, Aurora Flatfield
KStars/Ekos 3.6.0, INDI Library 1.9.7, Mountwizzard4 2.2.7, PixInsight 1.8.9-1
Imaging
With the ASI533MM being a new camera in use, the required settings were still a bit trial and error. Typically for these new sensors, using them at Gain 0 for luminance works well, making use of the maximum full well depth. But this target was so faint, that during the second session, Gain 100 was used to capture as much as possible signal. Overall the ASI533MM behaved very well and appeared a perfect little camera for small targets like this.
This was also the first time a new focusing algorithm in Ekos was used, the Linear 1 Pass method. Unfortunately in this first iteration, backlash settings in driver vs. software could throw the algorithm a bit off, resulting in quite a few of the RGB images to be out of focus. They had to be discarded. The result is that of the 11h of imaging only 3.5h is RGB, and 7.5h is luminance. For such a small target this is not really a problem, as structure is the most critical part to pick up.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
2250 × 2250 px (5.1 MP)
1000 mm @ f/7.7
3.76 µm
0.78 arcsec/px
35' 27.3" x 35' 27.3"
-97 degrees
RA: 23º 04’ 56.338”
Dec: +12º 19’ 14.72”
Processing
Using the WeightedBatchPreProcessing script, all frames were calibrated using darks (50) and flats (25), registered and integrated to an L, R, G and B image. Rough edges were cropped away. R, G and B were combined into an RGB image using ChannelCombination.
Background of the RGB image was corrected using GraXpert and a green hue in the background was eliminated using SCNR. Colours were calibrated using SPCC before the image was sharpened up using BlurXTerminator (BXT). Star shapes were very good to begin with, so BXT was only needed for deconvolution. Stars were then removed before further processing using StarXTerminator (SXT). Unfortunately the flats had not properly corrected some dust motes in one of the colour channels, so a few blobs were still present. Now that the stars were removed, these blobs could easily be removed using the Clonestamp tool. The image was stretched to a non-linear version using several iterations of the GeneralysedHyperbolicStretch (GHS) tool in Colour mode, followed by a contrast and saturation enhancement with CurvesTransformation (CT). The significant stretching with GHS had left some serious noise in the image, which was removed using a strong application of NoiseXTerminator (NXT) at 0.9.
The luminance image followed a somewhat similar path with gradient removal using GraXpert, sharpening using BXT and stars removal using SXT. Stretching using GHS was a bit of a trial and error, trying to find a balance in brightening up the faint parts of the galaxy without overblowing the brighter parts of it. A final contrast enhancement was applied after stretching using CT. Noise reduction was applied using NXT at a somewhat milder value of 0.7 compared to the RGB image.
Both stars only images were stretched using a single GHS stretch to a level of brightness and size that felt in balance with the galaxy. The exact same stretch was applied to both luminance and RGB stars. They were then combined using LRGBCombination.
Moving the transfer functions away from their defaults of 0.5 can help to maintain colour when adding luminance to an RGB image. But be careful, as small changes have large effects
Both starless images were combined using LRGBCombination. The tool includes a section Transfer Functions, which allows for adjusting brightness and saturation. But be careful and use these sliders with care. A small adjustment already has a very big effect. Also keep in mind that the sliders work in the opposite direction than one would expect. To make the final image darker, increase the brightness slider. And if for more colour to come through, reduce the saturation slider. To maintain maximum colour in the final image, a value just above 0.5 for brightness and just below 0.5 for saturation gave nice results. The LRGB stars could now be added back in using Pixelmath.
Using BackgroundNeutralization in the ‘Target Background’ mode allows to set a uniform background level for all images, making them nicer to see side-by-side, for example on Astrobin.
When publishing my images on Astrobin as well as on my own website, it always strikes me how different the background values are between images. When looking at an individual image, it is hard to distinguish a background value of 0.07 vs 0.08, but when seen side by side, the differences can be huge. Therefore I have started to apply to my final images a background correction, making the target background value 0.07, using the BackgroundNeutralization tool in the ‘Target Background’ working mode. It does not really matter whether this is applied to the whole target image, or to a region of interest (preview). The upper limit can best be set just above the existing background level. The higher this value, the more brighter levels in the image will be affected. Also it is good to start with a background level that is somewhat in the range of the desired target level, for example using the CT tool. With a proper upper limit set, and a starting background not too far off the targeted value, this is a very quick adjustment with reproducible results, without affecting the overall image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
