NGC7789 - Caroline’s Rose
NGC7789 - Click here for full resolution
NGC 7789 is an open cluster in Cassiopeia that was discovered by Caroline Herschel in 1783. Her brother William Herschel included it in his catalog as H VI.30. This cluster is also known as the "White Rose" Cluster or "Caroline's Rose" Cluster because when seen visually, the loops of stars and dark lanes look like the swirling pattern of rose petals as seen from above. The cluster is also part of the Herschel 400 catalogue, a subset of the NGC catalogue with objects selected for amateur astronomers.
source: Wikipedia
NGC/IC: NGC7789
Other Names: Caroline’s Rose
Object: Open Cluster
Constellation: Cassiopeia
R.A.: 23h 58m 32s
Dec: +56º 49.5’
Transit date: 14 October
Transit Alt: 86º N
Sky-plots with a FoV of 50º (left) and 5º (right). Click to enlarge
Conditions
Images were taken on 09 October 2022 from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was almost full and at a 57º angle from NGC7789. Although the target is visible all year around, the best time to observe is between August and December. Most images were taken at altitudes between 60 and 85 degrees.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout one of the capture sessions (right).
Weather conditions for each session
Imaging
This was the first night out fully utilising the benefits of a new telescope pier, the EuroEMC S130. This pier has two sets of wheels. Two big air-filled tyres to roll the rig over uneven surfaces and absorbing any shocks. Three smaller pivoting wheels for easy manoeuvring on a flat surface. The whole rig was setup before the imaging session started. When rolled outside, the spikes were placed on precisely located little indents in the terrace. This location was determined during an earlier ‘getting started’ session. It turned out that polar alignment was spot on. The only thing that needed to happen was to run an automated 18-point mount model. The following morning the whole rig could be rolled back into the house again. Overall setup and breakdown time were 15 and 10 minutes respectively. This was a massive difference from the normal 45 and 30 minutes respectively. Pointing accuracy and tracking were also very good and seemed even better than normal. This indicates that the sturdiness of this pier is at least as good and perhaps even better than the previously used Berlebach Planet. The benefits of this pier seem so good that the plan is to write a blog about it.
Since the individual stars are key to the quality of this target, several test shots were made to ensure that blown out stars were kept to an absolute minimum. This resulted ultimately in an exposure time of only 60s and a gain of 0. This gain 0 reduces the signal, but also maximises the total full well depth. This then allowed for the colour filters to use double that exposure length and the ‘default’ gain setting of 100.
Clouds were supposed to roll in by midnight, but luckily it stayed clear for quite a bit longer than that, so after selecting the most optimal frames, there was still almost 5h worth of exposure time left in the final image.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, Sesto Senso 2
10Micron GM1000HPS, EuroEMC S130 pier
ZWO ASI533MM Pro, cooled to -15 ºC
Astrodon 1.25” LRGB mounted, ZWO EFW 8-position
Unguided
Fitlet2, Linux Mint 20.04, Pegasus Ultimate Powerbox v2, Flip Flat
KStars/Ekos 3.6.0, INDI Library 1.9.7, Mountwizzard4 2.2.7, openweathermap.org, PixInsight 1.8.9-1
Details of all frames captured during each session
Total exposure times for each filter and the whole image
Annotated image showing the deep sky objects, and some of the brighter stars
Processing
All frames were calibrated with Darks (50), Flats (25) and Dark Flats (50) and registered using the WeightedBatchPreprocessing script. Towards the end of the night some thin clouds deteriorated the signal. Subframeselector tool was used to select the best images. Integration of the selected frames was done manually using ImageIntegration using standard settings. All images were cropped a tiny bit along the edges, to clean some dithering artefacts.
The almost full moon had created a bit of a gradient in the Luminance image more so than the RGB images. This was easily removed by DynamicBackgroundExtraction though. Stretching had to be done carefully to not blow out the centers of the stars, but there was plenty of contrast in the image to play with, so it was not super critical. As a final touch contrast was a bit enhanced by applying an S-curve using CurvesTransformation.
The R, G and B channels were combined into an RGB image. Automated color calibration with PCC worked very well. There was a slight gradient visible, a bit more complex than in the luminance image. DynamicBackgroundExtraction was difficult to fine-tune to get this out, but a single run of AutomaticBackgroundExtraction, using low complexity (function degree 2) did a very good job of eliminating the gradient. Stretching such a colored star-only image is always a bit tricky. It is easy to wash out the colors. The GHS script has a special mode that preserves color, and was therefore used. It works somewhat similar to ArcsinhStretch, but is less prone to overcooking the image. A little contrast boost with CurvesTransformation did the rest.
Luminance was put into the RGB image and a slight color boost was applied using the saturation panel in CurvesTransformation. Overall the image was not very noisy at all, but it did clean up even more with a little bit of noise reduction applied using the ‘wonder tool’ NoiseXTerminator. A value of 0.5 was enough. The detail slider was kept at 0.14. Any higher value would quickly introduce some artefacts around the stars.
The final image had kept good color and enough detail. Overall happy with the result, given that the full moon was standing right next to it.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
