M51 Arp85 - Whirlpool Galaxy
M51 - Whirlpool Galaxy. Click here for full resolution image.
M51 (NGC5194; R.A.: 13h 30m 45.40s, Dec: +47º 05’ 16.5”) is a galaxy in the constellation Canes Venatici at a distance of 28 million lightyear from Earth. The galaxy is better known as the Whirlpool galaxy, due to its unique swirly looks. This swirl is caused by an interaction with its neighbour galaxy, NGC5195. Final confirmation of such interaction was only recently obtained from radioastronomical observations. According to current hypothesis, about 500 to 600 million years ago NGC5195 passed through M51’s disk from behind, to make another crossing about 50 to 100 million years ago to the position we can now see it.
The Whirlpool galaxy is the dominant member of a small group of galaxies, that included M63, NGC5023 and NGC5229. M51 is of approximately similar size as our Milky Way and the Andromeda Galaxy and has an angular diameter of 11.2 arcmin. Total weight is estimated at around 160 billion solar masses.
In 2019, a supernova-like event took place. Within a short period of time, a star became visible in one of the spiral arms of M51. While originally thought to be a supernova, it turned out to be an Intermediate-Luminosity Red Transient (ILRT). This is an event where a lot of energy is emitted in a short time due to the close interaction of two stars in a double-star system. The image as described here was from 2018. Additional frames were captured in 2019, and indeed, AT2019abn as this event was named, could be observed. See here for further details.
Sky Plot (click to enlarge)
5º FoV + scope display (click to enlarge)
Planning
With a declination of 47 degrees, M51 is visible throughout most of the year in the northern hemisphere. Highest altitudes are reached in late winter and early spring. This image was taken in the early days of the observatory and one of the first ‘real’ image taken with a dedicated astrocamera with several hours of total exposure. The attractive shape of the galaxy and it’s relatively bright appearance at magnitude 7.9 made this a perfect target at this stage. The other pictures shot during this time were M81 or Bode’s galaxy and the Leo Triplet. Pictures were taken on four nights on April 19, May 5, May 6 and May 7, 2018.
Visibility charts showing altitude at 22:00h throughout the year (left) and throughout the session on April 19, 2018 (right).
Conditions
All images were taken from the backyard in Groningen, The Netherlands (53.18, 6.54), divided over 4 nights. At this point in time, little information was collected on observation conditions. Temperatures were around 11 ºC during the first session and around 4-7 ºC during the last three sessions.
A lot of effort was spent on getting used to the equipment and the software. And several mistakes were made, not having all the settings right. So inconsistent gain settings between images, missed meridian flips and a rendezvous-vous between the filterwheel and the tripod were just some of the hurdles that were taken during these early days.
Capturing
The image was captured using the Takahashi TOA-130 in combination with the ASI1600MM-Pro camera. At this time, exposure and gain settings were still very much unknowns. Frames were shot using Red, Green, Blue and Luminance broadband filters. To highlight the areas of intensive star formation, an extra set of frames was shot using a H-alpha narrowband filter. Partly due to some errors in the settings, 120s frames were shot @Gain 0 and 300 for the broadband filters and 180s frames @Gain300 were shot using the H-alpha filter.
Framing was not particularly well chosen and rotation was ultimately just wherever the system was. The galaxy stands a bit uncomfortably on its side, rather than in a more pleasing horizontal or diagonal orientation. As a consequence, north is pretty much pointing to the bottom of the image.
Technical Details
Telescope
Mount
Camera
Filters
Accessoires
Software
Takahashi TOA-130 + 35 flattener, Sesto Senso 2
10Micron GM1000HPS, Berlebach Planet
ZWO ASI1600MM Pro, cooled to -25 ºC
Astrodon 1.25” LRGB Gen2 E-series and H-alpha 5nm mounted filters, ZWO EFW 8-position
Fitlet2 (Linux Mint 18.04)
KStars/Ekos, INDI Library, SkySafari
Frames
The image was captured using HaLRGB filters. Below are the frames listed taken in each session that made it to the final image.
Image
In total 9.2h of exposure made it to the final image. Over 4h for the luminance and about 1h for each of the colours and the H-alpha. This was significantly more than anything that had been shot until then.
Even though the composition was not very well chosen, it was decided not to rotate and crop the image more than strictly necessary. Almost the full resolution of the camera was used. The final image has a resolution of 4586 x 3422 pixels, or 15.7 Megapixels. It covers a field of view of 1.02 degrees horizontally.
Annotated image showing other deep sky objects, stars brighter than mag. 11 and the image’s orientation.
Processing
All frames were calibrated with Bias, Dark and Flat frames, registered and stacked, mostly using manual processing. This was one of the earlier images that was fully processed in PixInsight. No record was kept on the exact processes applied.
The resulting image looked satisfactory at first, but over after processing other images, it became clear that the processing of M51 has been done pretty poorly, with way too much red in the image and overall not a good colour balance. Since there is a decent underlying dataset available, and with the additional luminance captures in 2019, hunting for AT2019abn, this dataset would deserve a completely new processing. That would probably be something to do when all the other unprocessed datasets are completed.
This image has not been published on Astrobin.
