M94- Croc’s Eye Galaxy
M94 - Croc’s Eye Galaxy. Click here for full resolution image.
The spiral galaxy M94, also known as NGC4736, is located in the constellation Canes Venatici. It has a typical and somewhat unusual form in that it actually contains two rings. The inner ring is an oval spiral structure, while the outer ring consists of a complex structure of spiral arms. The two ring structures give the galaxy a typical look and the nickname Croc’s Eye Galaxy, or sometimes Cat’s Eye Galaxy.
There is still a lot to be learned about the unusual two-ring structure. The historic explanation for the origin of the two rings included some sort of interaction with a satellite galaxy or some passing star system. Recent research suggests that that is not the case. When studying the galaxy in different wavelengths, both in UV and IR, it appears that the structure is quite different, depending on wavelength chosen. In the UV spectrum, the outer ring turns out to be actual a set of two spiral arms. It could well be that the galaxy is transitioning from one quasi-stable state to another.
The inner oval is bright at magnitude 8.9, and has very active star formation processes going on. The outer ring looks less active, and typically shows up in visible light as a faint band of luminosity. But in fact, it makes up for 25% of the galaxies stellar mass and 10% of star production.
Planning
Object
Visual Magnitude: 7.88
Apparent size: 7.7 x 6.7 arcmin
R.A.: 12h 51m 50.56s
Dec: +41º 00’ 34.5”
Conditions
Astronomical night: 20:52
Astronomical dawn: 04:25
Moon: 0.8%
Moon set: 17:40
Humidity: 47-73%
Pressure: 1025-1035 hPa
M94 is a typical galaxy to observe during ‘Galaxy season’, at the end of the winter, early spring. Frames for the image here were captured on four subsequent days, on March 23, 24, 26 and 27, 2020. There was almost a week of clear skies, so the telescope could stay out in the garden, such that starting and ending an imaging session was a lot faster and more efficient. On March 24, some high clouds passed by, creating some variation in background signal in some of the frames.
On the same days, also M82 was imaged. With M94 passing the southern skies and M82 the northern skies, between the two targets it was possible to make maximal use of the whole night while navigating between the trees. Also during this time some images of comet C/2019 Y4 Atlas were made.
Capturing
The image was captured using LRGB broadband filters. Exposures of 180s at unity-gain seem to be a sweet-spot for this telescope/camera setup, so was applied for this image. The most difficult part of this galaxy is to get the faint second ring sufficiently exposed. With these settings that worked. The core of the galaxy seemed a bit overexposed though, so an additional set of 60s luminance exposures were made to possibly pull out a bit more detail in the center.
Technical details
Telescope
Mount
Camera
Sensor Temp.
Takahashi TOA-130 + FL67 flattener
10Micron GM1000HPS
ZWO ASI1600MM Pro
-25ºC
Exposures
Luminance
Luminance
Red
Green
Blue
Total Exposure
135 x 180s @ Gain 139/21
60 x 60s @ Gain 139/21
47 x 180s @ Gain 139/21
48 x 180s @ Gain 139/21
43 x 180s @ Gain 139/21
14.6h
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the BatchPreprocessing script. As the center of the galaxy is quite bright compared to the rest, a set of 60s exposures was made, next to a set of 180s exposures. These were first combined using HDRComposition. A modest improvement of the details in the center could be achieved while maintaining overall brightness and detail of the image.
The center of the 180s exposures is clipped. An HDR composite with 60s exposures adds a modest amount of detail back.
This time, for some reason, the flats did a poor job in correcting the images properly. In all filters, there was some weird vignetting going on that was both visible in the flat frames and the light frames, but somehow did not even each other out. This could be easily corrected by cropping the image in a bit. M94 is small enough to allow for a bit of a crop on the original image. Another problem with the flats were two faint rings in the luminance data. These were also visible in the flats, but slightly differently positioned, so that in the flat-field correction the problem did not go away. After a lot of testing it turned out that these were two dust particles of reasonable size on the luminance filter. When the flats were taken at the end of the night, it was done at a different focus distance. Because the dust particles were so far away from the sensor plane, they gave a large circle of shadow, extremely sensitive to even small differences in focus. Since this was only visible in the luminance frames, it was decided to just move on, be careful with final stretching and see how much would be left in the final image. It turned out that after the whole processing, very little, if any, was still visible.
Dust particles on the luminance filter, far away from the sensor surface result in vague shadows on the image.
Noise reduction was applied to all channels using the MultiscaleMedianTransform tool, as described here. This is one of the first times this method was applied, and it worked out quite nicely. Dialing in the amount of noise reduction works by changing the stretching of the inverted clone masks. After noise reduction, regular deconvolution was applied to the luminance channel to pull out a little bit more detail from the galaxy. The R,G and B channels were combined. Some basic color correction was applied using the HistogramTransformation and the CurveTransformation tools. Then both the luminance and the RGB image were ready to move on to the non-linear state.
From here it went quite the regular route for galaxies with first a LocalHistogramEqualization. Settings were Kernel radius 24, Contras Limit 1.2 and amount 1.0. LHE is a great tool to emphasise structure for example in spiral arms. But you can overdo it very quickly. Especially the contrast limit is a sensitive parameter. In this case anything at 1.4 or above gave very unnatural contrasts, so it was left at a modest 1.2.
At the end of each processing there are the little finishing touches. In this case it was decided to rotate 90º clockwise. With the galaxy ‘standing up’ it does not look much like an eye, but in a somewhat horizontal/diagonal orientation, the eye is a lot more visible.
This image has been published on Astrobin and has received top pick nomination.
