M34
M34 - Click here for full resolution
M34 (NGC-1039 - R.A.: 2h 43m 29s, Dec: +42º 50’ 34”) is an open cluster in the constellation Perseus at a distance of 1,400 lightyear from Earth. It is a relatively small cluster, with approximately 400 stars. The 20 brightest stars makes for a fairly bright object with a total magnitude of 5.5. The cluster is relatively young at an age of approximately 200 million years.
Sky Plot (click to enlarge)
5º FoV + scope display (click to enlarge)
Conditions
Images were taken on 08 December 2021 from the backyard in Groningen, The Netherlands (53.18, 6.54). There was no interference from the Moon, which at 28 % illumination had already set by the time the images were taken. Visibility from this location is best during October - December. M34 was visible high across the sky, with altitudes between 60 and 80 degrees. The main goal of the observation session was to photograph Comet Leonard. But the comet would not rise until around 05:00h in the morning. This left time during the evening and the night to photograph another object, which was M34.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout the session on 08 December, 2021 (right).
Weather was generally good. A bit cold with temperature around 2 degrees Celsius. Humidity was fairly low for this location at 79%. Visibility was reasonably good with SQM values of 19.9 mag/arcsec2.
Capturing
The image was captured using the Takahashi FSQ-106 in combination with the QHY268c. This was a newly acquired camera for the observatory. It is a Once-Shot-Colour (OSC) camera with a 24MP APS-C-sized sensor. For most deep-sky objects, a monochrome camera is probably preferred, but there are situations where OSC cameras have their benefits. See for example the image of Comet Leonard. Also for bright clusters, such as M34, an OSC camera is a perfect tool, removing the complexity of exposing using different filters.
The QHY268c has a highly reflective coverglass in front of the sensor, without any blocking of UV or IR signal. Therefore a UV/IR blocking filter is advised to be added to the imaging train. An Astronomic L3 filter, was chosen. For UV/IR blocking filters it has some of the narrower bandpasses at 420 and 680nm.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FSQ-106, Sesto Senso 2
10Micron GM1000HPS, Berlebach Planet
QHY268c, cooled to -15 ºC
Astronomic L3 (2” mounted)
None
MacMini 2018 (MacOS 10.14.6), Pegasus UPBv2, Aurora Flat Field panel
KStars/Ekos 3.5.6, INDI Library 1.9.3, Mountwizzard4 2.1.5, SkySafari 7 Pro (iOS), openweathermap.org
Different exposures were tested. Given the brightness of the object, the session was started with 30s exposures. But this turned out fairly short. Exposures were switched to 120s and this is how the image was shot. For maximum dynamic range, the gain was set at 0.
Image
The image came out nicely bright and colourful. One of the attractions of M34 is its bright young blue stars in combination with some older bright red stars. The OSC camera captured these colours perfectly. Noise was generally low in individual frames and with a total of 184 frames stacked together, noise was well controlled. At an exposure time of 120s, even the brightest stars were for the most part not blown-out.
After a little crop to straighten the edges, the final image has a resolution of 5483 x 3656 pixels, or 20.0 Megapixels. It covers a field of view of 2.23 degrees horizontally. Due to rotation of the camera-angle-adapter, north in the picture is pointing to the left.
Every new camera requires some getting used to as far as settings and behaviour go. But already after two images (M34 and Comet Leonard), it became clear that imaging with the QHY268c is a pleasure. The workflow is quite a bit simpler, not having to plan different exposures for different filters, an equal amount of exposure per filter, etc. For dimmer and more challenging objects, the monochrome cameras will remain the main work horses, but the QHY268c is a worthy addition to the arsenal.
Annotated image showing other deep sky objects, stars brighter than mag. 9 and the image’s orientation.
Processing
Using the WeightedBatchPreprocessing, Flat frames (25) were calibrated with Bias (100) frames. Light frames were calibrated with Darks (50) and Flats (25) frames, debayered and registered. Image frames were normalized and scaled using the NormalizeScaleGradient (NSG) script and integrated using NSG parameters. During the early morning hours, M34 was setting over the North-Western horizon, where there is a lot of glow from the city. As a consequence, background signal went up significantly. The increasing light glow demonstrated itself also in the weighting parameters of NSG. For the final images, the 50th frame was taken as reference, and frames with weighting factors from 25% until 119% were included in the final stack. The NSG script allows writing the weighting factor as a pre-fix in the filename. This comes in handy when you want to decide at a later point which ones to include and which ones not.
In the stacked image, minor rough edges from the registration process were cropped out. A Dynamic Background Extraction was performed. The fewer grid points used, the better the results. When using a very dense grid pattern, the calculated background field became very blotchy, and only made the image look worse. Final DBE was done with a grid of 5 points per row. Colours were balanced using the Photometric Colour Calibration tool. There was a slight green-cast which was removed using SCNR at 0.5 intensity. Noise reduction using the MMT-method worked well, also on the OSC-image.
In an otherwise fairly plain image, it is important to keep the star colours vibrant. For stretching, the ArcSinhStretch tool is usually best in keeping colours in tact. But it also typically requires a second stage of modest stretching with the Histogram Transformation tool.
From here it was mainly processing to taste. Stars were shrunken a tiny bit using the Morphological Transformation tool. Curves Transformations were used to add a bit more contrast. And Colour Saturation of the stars was boosted using a luminance mask, while background Colour Saturation was reduced using the same mask in inverted mode.
This completed the final image. With the target being quite a bit smaller than the frame, the final published version of the image was cropped in a bit.
Processing an OSC image still feels a bit uncomfortable. It is yet another skill to develop, and more use cases will undoubtedly help to improve the final results. The simplicity of just having one colour image to work with is appealing on one hand. But it also feels like it is taking away some flexibility to work on individual aspects of an image. There’s probably lots of tips and tricks still to learn.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
