M61
Messier 61 Galaxy (slight crop) - Click here for full resolution
Messier 61 is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was first discovered by Barnaba Oriani on May 5, 1779, six days before Charles Messier discovered the same galaxy. Messier had observed it on the same night as Oriani but had mistaken it for a comet. Its distance has been estimated to be 45.61 million light years from the Milky Way Galaxy. It is a member of the M61 Group of galaxies, which is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster.
source: Wikipedia
NGC/IC:
Other Names:
Object:
Constellation:
R.A.:
Dec:
Transit date:
Transit Alt:
NGC4303
Swelling Spiral Galaxy
Galaxy
Virgo
12h 21m 54.9s
+04° 28′ 25″
14 April
57º S
Conditions
M61 is a typical target for the spring ‘Galaxy Season’ and reaches maximum altitudes of just under 60° at my location. Images were taken from the remote hosting site IC Astronomy in Spain across eight different nights in May 2024.
Equipment
The default rig at the observatory was used. This is built around 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 allowed automatic scheduling to find time-slots in the early hours of 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” RGB unmounted, Moravian filterwheel “L”, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.8.9-2
Imaging
Imaging was pretty straight-forward, with sufficient altitude of the object. M61 was captured using Luminance and R,G and B broadband filters. A total of 20h of data was acquired, of which 12h was RGB and 8h was Luminance. The amounts were chosen in such a way that a comparison could be made between RGB and LRGB recordings (see under Processing).
Resolution (original)
Focal length
Pixel size
Resolution
Field of View
Image center
9198 × 6156 px (56.6 MP)
2586 mm @ f/7.2
3.8 µm
0.30 arcsec/px
46' x 30’
RA: 12h 21m 56.199s
Dec: +4° 28 24.55”
Processing
All images were calibrated using Darks (50), Flats (25), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight.
Processing followed a pretty standard pattern, with creating an RGB image and a Luminance image separately, followed by combining them together into a final image. For both RGB and Luminance images, the steps were fairly similar. First they were cropped to eliminate the rough edges, and gradients removed using GraXpert. Then BlurXTerminator was used for deconvolution. Only for RGB colour was calibrated using SPCC. Only for the Luminance image the stars were removed, to have full control during stretching. Stretching was done using the SetiAstro script Statistical Stretch. Also GHS was tried, but Statistical Stretch gave a remarkably good and single click result, so I stayed with that. The saturation was enhanced in the RGB image and the image was a bit softened using convolution, to remove any hint of colour noise. In the luminance image, contrast was enhanced using both CurvesTransformation and LocalHistogramEqualization, before the stars were put back in with PixelMath.
RGB and Luminance were combined using LRGBCombination and saturation was enhanced a touch with CurvesTransformation. A mild noise reduction using NoiseXTerminator and setting the the background level to 0.07 using BackgroundNeutralization completed the processing.
The RGB image (left) has intenser colours and overall a flatter structure. For a bright object this can result in very nice images. But an LRGB image (right) definitely shows more detail in the fainter parts of the object and overall shows a much better signal-to-noise ratio.
From time to time the discussion on RGB vs LRGB imaging comes up on the fora. Some people would argue that when using the same amount of exposure, splitting this between 50% luminance and 50% RGB does not give better results than using the full amount of exposure time for RGB only. Some advocate creating a synthetic luminance from the R, G and B channels instead. Quite frankly, I have seen examples both ways in the past. Sometimes luminance greatly added detail to the final image. But sometimes luminance just washes out the colour and can even blur out a bit of detail in the process. When moving to a remote site, I wanted to do the comparison for myself. So for the M61 target, some extra processing was done.
One image was created by using 4h of each R, G and B channel, so a 12h exposed RGB image. Another image was created by using half of these R, G and B images, and instead adding 6h of Luminance exposure, to create a 12h exposed LRGB image, with 50% luminance and 50% RGB. A 50/50 balance between Luminance and RGB is generally considered the best balance. I tried to keep processing as much as possible the same, and the use of StatisticalStretch helped a lot in that. Unfortunately it cannot prevent that there are small brightness and colour differences between the two images. Above is the comparison for the M61 galaxy itself, and below some examples of smaller galaxies in the field of view.
Especially in the fainter, smaller galaxies in the image such as NGC3401 (left) and NGC4292 (right), the extra brightness helps to show more detail.
In short, the conclusion is that the LRGB image definitely wins. The brightness and ‘pop’ of the image is significantly higher than that of the RGB image. Stretching RGB to similar brightness levels is possible, but at the cost of significant amounts of noise. The additional brightness shows a lot more structure and detail in the fainter parts of the image, definitely a big plus.
But to be honest, the visual of the RGB images remains very appealing as well. Especially how detail remains visible in the center of the galaxy makes the overall image very pleasing to look at. Ultimately one might opt for different solutions in different situations. Very bright objects could be very well imaged in RGB and benefit from the flatter look and more intense colours, while fainter objects might benefit from the extra detail and clarity in the LRGB image. Going forward my personal plan is to go for LRGB imaging, even though some recent examples of RGB images came out pretty well.
Processing workflow (click to enlarge)
This image has been published on Astrobin and has received Top Pick nomination.
