Arp78
Arp78 - Click here for full resolution
Arp78, also known as NGC 772 or the Fiddlehead Galaxy, is an unbarred spiral galaxy approximately 106 million light-years away in the constellation Aries. It was discovered on 29 November 1785 by German-British astronomer William Herschel. At around 200,000 light years in diameter, Arp78 is somewhat larger than the Milky Way Galaxy, and is surrounded by several satellite galaxies – including the dwarf elliptical, NGC 770 – whose tidal forces on the larger galaxy have likely caused the emergence of a single elongated outer spiral arm that is much more developed and stronger than the others arms. Halton Arp includes NGC 772 in his Atlas of Peculiar Galaxies as Arp 78, where it is described as a "Spiral galaxy with a small high-surface brightness companion". Three supernovae have been observed in NGC 772.
source: wikipedia
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NGC772
Arp78, Fiddlehead Galaxy
Galaxy
Aries
01h 59m 20s
+19° 00.0′
22 November
71º S
Conditions
Arp78 is best visible in the Autumn time, when peak altitudes reach 71° in the South. The current image was taken a bit early in its cycle. Images were shot over six nights throughout the months of September 2024 from the remote observatory at IC Astronomy in Oria, Spain.
Equipment
The default rig at the observatory was used. The core of this rig is a Planewave CDK-14 telescope on a 10Micron GM2000 mount, coupled to a Moravian C3-61000 Pro full-frame camera. The RoboTarget module in Voyager Advanced automated the process to find optimal time-slots during astronomical night.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14, Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro, cooled to -10 ºC
Chroma 2” LRGB unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.8.9-3
Imaging
Arp78 is a typical broadband object, best recorded using an LRGB imaging scheme. Originally it was planned to collect 20h of exposure data. But one of the sessions had a lot of rejected images, due to lots of clouds. These rejected images were not corrected back in the Voyager software. So Voyager recorded a 100% of intended exposures, based on which I started processing. All in all just over 17h of exposure had been collected. The field of view of the rig was a bit too large for this relatively small target, so the final image has been cropped to focus on the galaxy itself.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Image center
6500 × 4350 px (28 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
32' x 21'
RA: 01h 59m 19.434s
Dec: +19° 00’ 28.84”
Processing
All images were calibrated using Darks (50), Flats (50) and FlatDarks (50), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight. The start of the processing was the same for all stacked images. After cropping away the stacking artefacts, gradients were removed with GraXpert and deconvolution was applied using BlurXTerminator (BXT).
The luminance image now had the stars removed using StarXTerminator (SXT) and carefully stretched using multiple runs of GHS. Contrast in the stretched image was further enhanced using LocalHistogramEqualization with a small kernel radius (64). The image did not look particularly noisy, but some noise removal using NoiseXTerminator definitely helped making it look very smooth. A final touch of sharpness was added using UnSharpmask.
The R, G and B channels were combined using ChannelCombination and colour calibrated using SPCC. Then the stars were taken out with SXT. The starless image was stretched using GHS in the colour mode. The saturation was not too high, so CurvesTransformation was used to add a bit of colour intensity. Noise was removed using NXT. The starless luminance image was now added using LRGBCombination. This is always a very satisfying moment in the processing. Two moderately interesting images together transform into a new nice and crisp image.
The RGB stars were stretched using the automated Star Stretch script from SetiAstro and colour was further boosted with CurvesTransformation. The image was smoothened with Convolution before the Luminance stars were added. The Luminance stars were also stretched using the Star Stretch script and added to the RGB stars using LRGBCombination.
The stars were added to the starless LRGB image using the CombineImages tool. A bit of remaining noise was removed using NXT. The background was adjusted to 0.07 to be in line with all my other images. As mentioned above, the field of view of the OTA and camera combination is quite large for this rather small target. So the final image is approximately a 70% crop of the original image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
