M45 - Pleiades
M45 - Click here for full resolution
M45, also known as Pleiades or the Seven Sisters is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of about 444 light years, it is among the nearest star clusters to Earth. It is also the nearest Messier object to Earth, and is the most obvious cluster to the naked eye in the night sky. It is also observed to house the reflection nebula NGC 1432, an HII region.
The cluster is dominated by hot blue luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from their formation, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.
source: Wikipedia
Messier:
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Transit date:
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M45
Pleiades, Seven Sisters
Open cluster
Taurus
03h 47m 23s
+24º 07’ 00”
12 Dec
61º S
Conditions
Images were taken during travel, from a dark location near Duns, Berwickshire, United Kingdom. SQM values were around 21.0 mag/arcsec2. M45 is a typical winter target with highest altitude in early December. Images were taken on four different nights from October 11 through to October 16, 2023. The moon was absent during this time. Temperatures ranged between 3-6 °C. It could be quite windy though, but mostly western wind, where the house offered some protection.
Equipment
The images were taken during travel, using the travel rig that was recently put together. A small addition that proved very practical was a laser pointer that is specifically designed for the RST-135(E) and is attached to the Polemaster connection. It allows very easy course polar alignment, after which the software-based polar alignment only needs the fine adjustments. The USB-PD solution worked flawlessly, with very reliable camera connections throughout the four imaging nights.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FS-60CB, 1.04x multi-flattener (370mm @ f/6.2), PrimaluceLab Esatto 2”
Rainbow Astro RST-135E, Gitzo GT5533S tripod
QHY268C, cooled to -15 ºC
Astronomik L3, 2” mounted, Baader filter drawer
Askar FMA180 with reducer (180mm @ f/4.5), ZWO ASI290MM
Fitlet2, Linux Mint 20.04, Pegasus Powerbox Advance
KStars/Ekos 3.6.5, INDI Library 2.0.2, PixInsight 1.8.9-2
Imaging
The western hemisphere was blocked for a large part by the house, so there was only little time available past the meridian. Since the mount has not been used a lot yet in fully automated ‘through the night’ session, I was not yet trusting an automatic meridian flip. Given the little benefit it would have, it was decided that imaging was stopped each night just before the meridian was reached. M45 was visible early in the evening, but still quite low on the horizon. Imaging was started at around 22:00h each night, when the target had reached an altitude of around 30°. With both these cut-offs, each night resulted in approximately 4-5h of data, making it a total of over 17h of images.
M45 is a typical RGB target, so just the Astronomic L3 UV/IRCut filter was used on the OSC camera. The target has a high dynamic range, so gain was set at 0, to maximise the camera’s dynamic range. Exposures were kept relatively short at 120s to not overexpose the 7 bright stars. Guiding worked generally well with RMS values in the 0.6 arcsec range. Dithering was done after each image. Due to the dark skies, good guiding, lack of clouds and generally well working equipment, almost no frames had to be rejected.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
10,000 × 6,667 px (66.7 MP)
370 mm @ f/6.2
3.76 µm
1.05 arcsec/px
2° 54' x 1° 56'
-13.4 degrees
RA: 3° 46’ 35.830”
Dec: +24° 12’ 33.09”
Processing
The relatively short exposure time (120s), in combination with the relatively long imaging time (17.2h) meant that there were 515 frames, or almost 27GB of data to process. And that is not included the Darks (50), Flat-Darks (50) and Flats (25). The full calibration process, including local normalisation was done in one run of the WeightedBatchPreProcessing script in PixInsight (WBPP). The short focal length of 370mm resulted in a pixel scale of over 2 arcsec/px and slightly pixelated stars. To improve resolution, a 2x drizzle was applied. And because there were so many frames to work with, I choose a drop shrink factor of 0.7, a bit lower than the default 0.9.
With all this loaded, it was no wonder that it took a while to process it all. On my Apple MacBook Pro with M2 Max chip and 64GB internal RAM, the total duration was 5h and 47 minutes to complete:
Full calibration run in WBPP takes its time, even on an Apple MacBook Pro with M2 Max processor.
The good news was that after all this calibration work on so many frames, the initial starting image was in a pretty good shape. Standard gradient removal with DBE and color calibration with SPCC made the image ready for deconvolution. Applying deconvolution on the very big stars gave some very strange artefacts. So I decided to protect them with a star mask for only the big stars. Even with the star mask, the star sharpening was set at a modest 0.15 and no halo interference. Overall the Astronomic L3 filter did not give much halo to begin with. To enhance the streaks of cloud structure a bit more, the non-stellar sharpening was set to 0.9. With automatic PSF selected there was not a big effect on the cloud structures. Better results were obtained by manually setting the PSF to a higher value than the FWHMEccentricity script gave as a solution.
There was not a whole lot of noise in the image, but whatever noise there was cleaned up nicely with NoiseXterminator with denoise (0.7) and detail (0.5) carefully dialed in. To get the image in a non-linear state, a number of GHS cycles were applied. A few final tweaks with CurvesTransformation concluded the processing of the image. A final crop to a reasonable framing resulted in the final image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
