M51 | Arp 85
NGC 5194, NGC 5195
32’ x 24’ | 0.3”/px | 6500 × 4875 px
Canes Venatici
RA 13h 29m Dec +47° 12’ | 89°
Messier 51, also known as NGC 5194 and more famously known as the Whirlpool Galaxy, is one of the most striking and well-known galaxies in the night sky. Nestled in the small northern constellation Canes Venatici, it lies approximately 23 million light-years away from Earth and presents itself as a grand spiral structure, almost like a celestial pinwheel caught mid-twirl. The galaxy was discovered in 1773 by Charles Messier. M51 quickly stood out due to its brightness and form, but it wasn't until the 1840s, when Lord Rosse observed it with his massive 72-inch telescope, that its spiral nature was first revealed—making it the very first galaxy recognised as having spiral arms. What makes the Whirlpool Galaxy especially captivating is not just its structure, but its dynamic relationship with a nearby companion galaxy, NGC 5195. The smaller galaxy appears to be interacting with M51, tugging at its arms and distorting its otherwise symmetric spiral pattern. This gravitational dance has triggered waves of star formation in the Whirlpool's arms, lighting them up with clusters of hot, young, blue stars.
source: ChatGPT
Data Acquisition
Data was collected over 16 nights during April and early May 2025, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using standrad LRGB filters and to highlight the areas of active star formation, enhanced with Ha signal. M51 was one of the first objects that I photographed when I started back in 2018. It is still one of my most favourite deep sky object. So after 7 years of astrophotography, with many improvements in hardware, software and skills, I wanted to give this one a second try. And this time make it a very long exposure. So a total of more than 61 hours of data was combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
| Sessions | Moon% | Moon° | Hum% | SQM | T°C | Frames | Exposure |
|---|---|---|---|---|---|---|---|
| 20250405 | 62 | 68 | 65 | 19.1 | 8 | 31 | 1h 57m |
| 20250406 | 72 | 62 | 70 | 18.5 | 9 | 23 | 3h 50m |
| 20250407 | 80 | 56 | 50 | 18.2 | 8 | 43 | 7h 10m |
| 20250408 | 88 | 52 | 55 | 18.0 | 10 | 16 | 2h 12m |
| 20250417 | 77 | 91 | 75 | 20.9 | 9 | 49 | 2h 53m |
| 20250418 | 68 | 99 | 75 | 21.5 | 10 | 28 | 1h 46m |
| 20250419 | 59 | 106 | 60 | 21.4 | 7 | 28 | 1h 24m |
| 20250420 | 48 | 113 | 70 | 21.3 | 8 | 64 | 4h 20m |
| 20250421 | 38 | 124 | 60 | 21.6 | 10 | 11 | 0h 39m |
| 20250422 | 28 | 125 | 75 | 21.6 | 9 | 55 | 3h 45m |
| 20250423 | 18 | 129 | 75 | 21.5 | 10 | 120 | 7h 24m |
| 20250425 | 4 | 128 | 50 | 21.5 | 15 | 120 | 7h 08m |
| 20250426 | 1 | 123 | 55 | 21.6 | 10 | 120 | 7h 16m |
| 20250429 | 9 | 98 | 85 | 21.6 | 9 | 61 | 4h 11m |
| 20250502 | 36 | 64 | 60 | 21.0 | 12 | 68 | 4h 18m |
| 20250503 | 46 | 64 | 70 | 20.7 | 10 | 11 | 0h 55m |
| Total | 848 | 61h 08m |
| Frames | Bin | Gain | Exp.(s) | Frames | Exposure |
|---|---|---|---|---|---|
| Lum | 1 | 0 | 180 | 481 | 24h 03m |
| Red | 1 | 0 | 300 | 97 | 8h 05m |
| Green | 1 | 0 | 300 | 96 | 8h 00m |
| Blue | 1 | 0 | 300 | 96 | 8h 00m |
| Ha | 1 | 2750 | 600 | 78 | 13h 00m |
| Total | 848 | 61h 08m |
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” LRGB, Ha (3nm) unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert, CosmicPhotons and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered and integrated using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
The Ha signal was captured to highlight the areas of active star formation. These are these typically pink little ‘tangled knots’ that are spread across the spiral arms of NGC 5194. One way to do that is to just overlay the Ha information onto the Red channel with PixelMath or a tool such as NGRGB Combination. In an early image of M33, I followed this process. But a much better way to do this is by Continuum Subtraction. And that was used for this image.
Continuum Subtraction
The concept behind Continuum Subtraction is to extract from the Ha image only the signal that originates from Hydrogen. The Ha bandwidth is included in the regular bandpass of a standard red filter. So besides signal from Ha, it will also capture ‘normal’ red light. This ‘normal’ red light is considered the ‘continuum’ signal as it covers a wide bandwidth. Subtracting these signals is essentially a mathematical challenge, which can be done using PixelMath. But these days there are several scripts that can do this step automatically. Here I used the ContinuumSubtraction Utility from SetiAstro. This tool works on linear images and offers different options for the base images to work on, and whether they include stars or not. I found that the starless images gave the best results. So I loaded a starless Ha and Red image into the tool (after gradient removal and BXT). The tool produced a stretched Ha-only image. This is essentially a few strings of tangled knots along the spiral arms.
The next step is to add the Ha image into the main fully processed LRGB image. I used ImageBlend from CosmicPhotons to do so. The LRGB image was taken as base image and the Ha as blend image. The blending was done with filter type ‘colour’, which allows the selection of a particular hue you want the overlay to look like. I chose a hue of 341 with a saturation of 0.7. This gives a nice subtle pink-ish result, but not as intense pink as you sometimes see in these images. Blending mode was set to screen. By sliding the mid-tone slider the intensity of the Ha signal could be set to a pleasing blend with the rest of the image.
A continuum subtracted Ha image is created by subtracting the Red channel (left image) from the Ha channel (center image). The result is an image that only shows the pure Ha signal (right image), essentially a few strings of ‘tangled kots’ along the spiral arms.
Adding Ha signal to the LRGB image enhances the image by highlighting areas of active star formation as subtle pink-ish glowing ‘knots’ along the spiral arms.
Apart from the continuum subtraction, the main image was fully processed in the presence of the stars. SXT pulled a lot of non-star signal from the heart of the galaxy, something that would have been very difficult to correct. It turned out that processing with the stars present was not a problem at all. The stretching method used (SmartStretch scriplet in the PixelMathGUI script) does a great job in preventing the center of stars to clip. And with the Saturation protection on, it also keeps colour in both galaxy and stars very natural.
In the RGB image the reds in NGC 5195 were enhanced by selectively increasing saturation around the red hues. A more general saturation boost would have made the overall galaxy too blue.
The Luminance channel was enhanced twice using LocalHistogramEqualization, once using a pretty large kernel radius of 274 to enhance the larger structures, and once using a much smaller kernel radius of 64 to enhance the small structures. As always I apply these LHE adjustments in minimal amounts (0.1-0.2).
The rest of the processing followed a pretty standard processing workflow, as shown in the overview below.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
