M44 - Beehives Cluster
M44 - Beehives Custer - Click here for full resolution
M44, also known as Beehive Cluster or Praesepe (R.A.: 8h 41m 39.42s, Dec: +19º 35’ 20.6”) is an open cluster in the constellation Cancer. It is one of the closest clusters, at a distance of 610 lightyears from Earth. Already in ancient times this cluster had various names. The Greeks referred to it as Phatne, which translates in Latin to Praesepe and in English to manger, or hive. The Greeks and Romans saw the stars Gamma and Delta Cancri as donkeys eating from this hive.
The cluster contains several red giants and white dwarfs, as well as a lot of main sequence stars. This makes for a very colourful cluster. The cluster contains several double stars and in the background, a number of distant galaxies can be observed.
Sky Plot (click to enlarge)
5º FoV + scope display (click to enlarge)
Conditions
Images were taken on February 11, 2022 from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was out full at 80% illumination, but the brightness of the target allowed decent images. Visibility from this location is best in wintertime. At the time of observation, M44 was visible over the southern horizon, with altitudes between 30 and 60 degrees.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout the session on February 11, 2022 (right).
Weather was good, nice crisp slightly freezing weather with clear skies. Humidity was fairly low for this location at 81%. With the almost full moon out, SQM values remained at 18.9 mag/arcsec2.
Capturing
The image was captured using the Takahashi TOA-130 in combination with the ASI6200MM-Pro camera. This combination provided a field of view with a width of around 2 degrees, large enough to capture the widely spread cluster.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130 + FL67 flattener, Sesto Senso 2
10Micron GM1000HPS, Berlebach Planet
ZWO ASI6200MM Pro, cooled to -15 ºC
Chroma 2” LRGB unmounted, ZWO EFW 7-position
Unguided
MacMini 2018 (MacOS 10.14.6), Pegasus Ultimate Powerbox v2, Flip Flat
KStars/Ekos 3.5.6, INDI Library 1.9.3, Mountwizzard4 2.1.2, SkySafari 6.8.2, openweathermap.org
The image was captured using L,R,G,B filters. Gain was set at 0, to use the maximum full-well depth of the camera, to minimise blown-out highlights. At first, 120s luminance exposures were taken. Upon closer inspection, some star-cores were still blown-out. Therefore exposure of the remaining luminance frames was limited to 60s. Below are the frames listed taken that made it to the final image.
Image
A star-only image usually does not require a very long overall exposure time. The 6h total exposure here were more than sufficient to get a nice low-noise image of the cluster, with nice pin-point stars and sufficient colour. As a general rule-of-thumb combined RGB exposure was kept similar to the luminance exposure.
The Beehive Cluster area contains many other objects. This includes several double stars and various galaxies. In the below image some of them are magnified from the original image
Several objects within the Beehive Cluster, including the double stars STF1254 and ENG37, as well as the galaxies NGC2647, NGC2624 and IC2388.
The final image did not require any crop, so the final resolution of 9576 x 6388 pixels, or 61.2 Megapixels represents the full resolution of the camera. It covers a field of view of 2.06 degrees horizontally and 1.37 degrees vertically.
Annotated image showing other deep sky objects, stars brighter then mag. 9 and the image’s orientation.
Processing
All frames were calibrated and registered using the WeightedBatchPreprocessing script. Flat frames were calibrated with Bias (100) frames, light frames were calibrated with Dark (50) and Flat (25) frames. Lights were normalized and scaled using the NormalizeScaleGradient script and integrated using NSG parameters.
Processing followed a pretty typical path for LRGB images and did not pose too many problems. Background modelization was done slightly different than normal. Individual sample points were placed, rather than the typical grid. This allowed a bit more control. By repeating the process three times with excessive stretches in between, the background could be made very homogeneous.
The noise-reduction, especially in the luminance worked very well, with almost no noise left.
Stretching took place in two stages. First a mild stretch using ArcsinhStretch. This maintains best the colours of the stars. In a second round, final stretching was achieved using regular HistogramTransformation. In general the stretching of the image was kept modest, to keep the stars nicely sharp and colourful.
For finishing touch, only the saturation was worked on a bit more. With background masked out, saturation of the stars was enhanced, and with the same mask inverted, saturation of background was reduced to eliminate a very faint hint of red in the background.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
