M81- Bode’s Galaxy
M81 - Bode’s Galaxy. Click here for full resolution image.
Galaxy M81, or Bode’s Galaxy, is a galaxy in the constellation Ursa Major. It is also registered as NGC 3031. This grand design spiral galaxy is about 12 million lightyears away from Earth. It has an active galactic nucleus and houses a massive black hole in the center the size of 70 million sun masses. The relatively large size and high brightness make it a rewarding galaxy for amateur astronomers to photograph.
M81 is often photographed together with its sibling galaxy M82, or cigar galaxy. They are both part of a large group of 34 galaxies, collectively referred to as the M81 group of galaxies. This group as well as the Local Group, to which our Milky Way belongs, are neighbours within the much larger Virgo Supercluster.
Planning
Object
Visual Magnitude: 6.8
Apparent size: 21.6 x 11.2 arcmin
R.A.: 09h 56m 18.03s
Dec.: 68º 58’ 44.3”
Conditions
Astr. night: 22:56
Astr. dawn: 04:08
Moon: 3%
Moon set: 22:12
Humidity: 94 %
Pressure: 1027 hPa
M81 is a circumpolar object and generally well visible all year around. January through April is probably the best period to photographing this galaxy, but really throughout the year it reaches high enough levels for proper photographing. The images here were shot on three consecutive nights on April 17, 18 and 19 2018 respectively. The first night humidity was quite high at 94%, but subsequent nights it dropped to 60%, making for a nice clear and dry sky. Due to high altitude, of the galaxy, the camera + filterwheel had a chance to hit the tripod, so it was necessary to be careful during transit time.
Capturing
This was the first time the system was used in its fully automated fashion throughout the night, using the local dedicated WiFi network and the Ekos Scheduler. Due to some inexperience with building the scheduler scripts, not all frames were shot using the originally intended gain settings. While an attempt was made to collect images with shorter exposure time to reveal more detail in the galaxy’s core, this effect was largely abolished because of the unintended higher gain setting. In the end all images were put together and corrected with their proper calibration frames. So it is a bit of a strange set of exposures, but it did work out in the end.
Framing was not particularly well planned, other than to make sure the galaxy was right in the middle of the frame. It resulted in the galaxy ‘standing on its side’. This was later rotated by 90 degrees, making the picture a bit more pleasing, but also loosing a bit of overall environment around the galaxy.
Technical details
Telescope
Mount
Camera
Sensor Temp.
Takahashi TOA-130 + 35 flattener
10Micron GM1000HPS
ZWO ASI1600MM Pro
-25ºC
Exposures
Luminance
Red
Green
Blue
H-alpha (5nm)
Total Exposure
42 x 120s @ Gain 10/10; 10 x 60s @ Gain 300/50
30 x 120s @ Gain 10/10; 10 x 60s @ Gain 300/50
25 x 120s @ Gain 10/10; 10 x 60s @ Gain 300/50
30 x 120s @ Gain 10/10; 10 x 60s @ Gain 300/50
25 x 180s @ Gain 300/50
6.2 h
Processing
All images were calibrated with Bias (40), Dark(22-30) and Flat (20) frames. registered and stacked using manual calibration routines. In the corners a few dark rings had not been completely corrected using the flat frames and were taken out by a small crop. All colour channels were then background corrected using a DynamicBackgroundExtraction step.
The luminance channel was deconvoluted (Global Dark 0.0090, 50 iterations). Noise reduction was applied using MultiscaleLinearTransform over 5 layers with threshold levels of 5, 4, 3, 2, 0 respectively and amounts of 0.8 and iterations of 1 for all of them. Then the channel was stretched to its non-linear stage.
The colour channel was created first by combining the R, G and B channels. There was quite a serious colour gradient visible, with a magenta hue on the left and a green hue on the right-side of the image. It took two background extraction steps to remove that. After subsequent background neutralisation and color calibration there was still a bit of a green hue present in the dark ares, which was removed using SCNR. Noise was reduced using MiltiscaleLinearTransform with the same settings as with the luminance channel. Then also the RGB image was ready for stretching to its non-linear state.
Luminance and RGB were combined and additional noise reduction was applied using TGVDenoise. This noise reduction is particularly good in removing high-frequency noise and gives a pretty smooth background. Obviously it is important to not make it unnaturally smooth. In this case settings of 5 (strength), 0.002 (edge protection) and 2 (smoothness) were giving good results. The galaxy has a very bright center and it is difficult to get a lot of detail in the spiral arms without loosing all the detail in the center. During capture it was attempted to get different exposures, but this failed due to the wrong gain settings. During post-processing another attempt was made to retain detail in the bright areas, using HDRMultiscaleTransform. This had a positive effect on the overall image.
The LRGB image was further finalised by applying steps such as contrast enhancement, enhancing sharpness and boosting the saturation. The image looked pretty good at this stage already, but there was still the H-alpha to be added.
The H-alpha channel followed a very comparable pathway as the luminance channel. During deconvolution it appeared difficult to not get worm-like artefacts in the background. Probably the luminance-mask was not sufficiently protecting the background. In later processing more success has been obtained by using a range-mask where it is easy to define a cut-off point for background levels such that there is 100% protection of the background. The same MultiscaleLinearTransform was applied as in the other channels to remove noise. After stretching the H-alpha channel to its non-linear stage, it was combined with the LRGB image to give the final picture, where pink little dots along the spiral arms indicate centers of active star formation.
As the original image was captured standing ‘on its side’, the final image was rotated. As this essentially crops in the image a bit, the final image is only 8 MP in total.
This image is published on Astrobin.
