NGC884, NGC869
NGC884 and NGC869 Double Cluster - Click here for full resolution
NGC 884 (also known as χ Persei) is an open cluster located 7640 light years away in the constellation of Perseus. It is the easternmost of the Double Cluster with NGC 869. NGC 869 and 884 are often designated h and χ Persei, respectively. The cluster is about 14 million years old. Located in the Perseus OB1 association, both clusters are located physically close to one another, only a few hundred light years apart. The clusters were first recorded by Hipparchus, thus have been known since antiquity.
source: wikipedia
NGC/IC:
Other Names:
Object:
Constellation:
R.A.:
Dec:
Transit date:
Transit Alt:
NGC884, NGC869
-
Cluster
Perseus
02h 22.0m
+57° 08”
28 Nov
70º N
Conditions
NGC884 and NGC869 are visible all year around, with peak altitudes towards the end of the year. At the time of imaging it was much lower on the horizon for most of the night, with altitudes ranging from 21° until just over 50°. Images were taken from the backyard in Groningen, The Netherlands on two nights in early March 2022.
Equipment
A smaller setup than regular was used, designed to work as either a second rig to increase usage of clear nights, or as mobile rig to take out to a dark site. In this case it was used as a second rig. So while the main rig was collecting data on the Iris Nebula, this second rig was capturing an extra target. The FSQ-106 was paired with the QHY268c, an APS-c sized OSC camera, which gave more than enough field of view. The mobile nature of the setup is based on the very light and small RainbowAstro RST-135E, which is a harmonic drive mount of only 3.4 kg.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FSQ-106, Sesto Senso 2
Rainbow Astro RST-135E, Berlebach Planet small
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 3.5.7, INDI Library 1.9.4, PHD2 2.6.11, MW4 2.2.0, PixInsight 1.8.9-2
Imaging
Imaging was pretty straight-forward. NGC884 and NGC869 were captured using the Astronomik L3 filter, which is essential a luminance filter for the OSC camera, with cut-offs on both the UV and IR side of the spectrum. A total of 10h of data was acquired over both nights combined.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Image center
6252 × 4176 px (26.1 MP)
530 mm @ f/5.0
3.8 µm
1.46 arcsec/px
2d 32' x 1d 41'
RA: 2h 22m 31.769s
Dec: +57° 08’ 59.79”
Processing
All images were calibrated using Darks (50), and Flats (25), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight. As is often the case in OSC images, the first integrated images often shows colour artefacts in the form of aberrations with one side of the stars having a blue hue and the opposite side having a red hue. This is caused by atmospheric dispersion and not automatically corrected in an OSC image. In a separate blog, you can find all the details of where this aberration comes from and how to handle it. So here just a quick summary.
First step is to separate the Red, Green and Blue channel from the integrated RGB image. The next step is the alignment of the Red and Blue channels to the Green channel. In this case I chose to use the DynamicAlignment tool for that. Next step was to correct the star shapes for each of the three channels using BXT with the option ‘correct only’ checked. After those two treatments, an RGB image could be created with almost not aberration.
The original image (left) shows serious aberrations. Separating the colour channels and aligning Red and Blue to Green (middle) improves a lot. Applying BXT ‘correct only’ (right) almost completely eliminates any aberration.
From here the processing was pretty straightforward. Gradients from the RGB image were removed with GraXpert. Colours were calibrated using SPCC. I did some testing in using different white reference settings in SPCC, as I typically find the images quite ‘warm’. I found that using S0 galaxy as a white reference gave a slightly cooler result, which appealed to me a bit more. After that a regular stretch with HistogramTransformation brought the image to its non-linear state. Contrast and saturation were slightly enhanced using CurvesTransformation and noise reduced with NXT. The full image had a wider field of view than necessary, and the double cluster was a bit off-centre. Both were corrected with a DynamicCrop. A final adjustment of background to a standardized level of 0.07 completed the processing.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
