M92- Globular Cluster
M92 - Globular Cluster. Click here for full resolution image.
M92, or NGC6341, is a globular cluster in the constellation of Hercules at a distance of 26,700 lightyears from earth. It is one of the brighter globular clusters in the northern hemisphere. Only M13, a globular cluster not too far away from M92, is brighter. The cluster can be seen with the naked eye.
The cluster is one of the oldest clusters known, with an estimated age of 11 billion years. Characteristic of such an old cluster is the low metallicity, or the low amount of elements other than Hydrogen and Helium. As an example, the amount of iron in the cluster is only about 0.5% of that found in the sun. Heavier elements than Helium typically form at the end of a star’s life-cycle. A low abundance of heavier elements therefore means it is made of material that has not yet undergone many star life-cycles.
In this old cluster, there are many RR Lyrae variable stars. These are stars that periodically change in brightness. That pulsating behaviour is probably initiated after shedding off a lot of weight as part of the red giant branch phase. RR Lyrae variable stars are typical for globular clusters.
Object
Visual Magnitude: 6.44
Apparent size: 14 arcmin
R.A.: 17h 17m 41.27s
Dec.: 43º 07’ 05.9”
Conditions
Astr. night: 22:34
Astr. dawn: 04:32
Moon: 68 %
Moon set: 13:00
M92 is particularly well visible during autumn season and reaches quite high altitudes. Imaging sessions were held on two consecutive days on 07 and 08 September 2018. While during the second part of the night the moon came out quite bright, it was at about a 128 degree angle, so did not interfere too much with the imaging. This image was shot during the early days of AstroWorldCreations, so not a lot of information around conditions was recorded.
Capturing
There is not a lot of detail in a star cluster other than the stars themselves. So a regular LRGB image is the most natural way to photograph this object. The most critical part is not too overexpose the individual stars. If stars are overexposed, they will loose all colour information in the final image. In addition, since it is a dense area of individual stars, it is important to keep proper separation between stars for the nicest image. For both of these reasons, the exposure duration of individual frames was kept relatively short at 60s. No differentiation was applied between exposure times of luminance vs colour channels. With hindsight, a somewhat shorter exposure of the luminance relative to the colour channels would probably have improved the overall sharpness and colour saturation of the image. Also, there is still some blending of stars in the center of the cluster, which could have improved by using an even shorter exposure length.
Framing was pretty straightforward. With a total size of 14 arcmin, the cluster fits easily right in the middle of the field of view of the TOA-130 and ASI1600 combination. A total of 2.4h of exposure has been collected, which for a cluster like M92 is enough to get a detailed and colourful image.
Technical details
Telescope
Mount
Camera
Sensor Temp.
Takahashi TOA-130 + 35 flattener
10Micron GM1000HPS
ZWO ASI1600MM Pro
-25 ºC
Exposures
Luminance
Red
Green
Blue
Total Exposure
57 x 60s @ Gain 139/21
30 x 60s @ Gain 139/21
28 x 60s @ Gain 139/21
27 x 60s @ Gain 139/21
2.4 h
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the BatchPreprocessing scrip. Originally 60 luminance frames and 30 colour frames for each colour were shot. Using the subframeselector tool in PixInsight, some poor frames were eliminated from the final processing. While this is a good tool to use when working with hundreds or even thousands of frames, it feels like a bit of a too mathematical tool to deal with a smaller sample set of several dozen frames. Alternatively analysing frames using the Blink tool has appeared to be equally effective to eliminate real bad outlier frames, while maintaining as many frames in the collection as possible.
This picture was shot during the early AstroWorldCreations days. At that time no detailed information was recorded on processing.
This image is published on Astrobin.
