Caldwell 45
Caldwell 45 | NGC5248 - Click here for full resolution
NGC 5248 (also known as Caldwell 45) is a compact intermediate spiral galaxy about 59 million light-years away in the constellation Boötes. It is a member of the NGC 5248 Group of galaxies, itself one of the Virgo III Groups strung out to the east of the Virgo Supercluster of galaxies.
source: wikipedia
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NGC5248
Caldwell 45
Galaxy
Boötes
13h 37m 32.0s
+08° 53′ 07″
25 Apri
46º S
Conditions
Caldwell 45 is visible in spring time, during galaxy season. Images were taken over five night from February until early April 2023 from the backyard in Groningen, The Netherlands.
Equipment
The target was pretty small, so the largest telescope available at that time (Takahashi TOA-130) was coupled with the smallest cooled astro-camera available (ZWO ASI533MM Pro). In preparation for a larger setup, the 10Micron GM2000 mount had already arrived and was used for this setup. At the same time a second rig was operated using the GM1000 mount
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, FL67 flattener, Pegasus Astro Motor Focus kit v2
10Micron GM2000HPS, EuroEMC S130 pier
ZWO ASI533MM Pro, cooled to -15 ºC
Astrodon 1.25” LRGB mounted, ZWO EFW 8-position
Unguided
MacMini (M1), MacOS Ventura, Pegasus Ultimate Powerbox v2
KStars/Ekos 3.6.3, INDI Library 2.0.0, Mountwizzard4 3.0.0, 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
3008 × 3008 px (9.0 MP)
1002 mm @ f/7.7
3.78 µm
0.77 arcsec/px
38' x 38'
RA: 13h 37m 32.340s
Dec: +8° 53’ 07.25”
Processing
All images were calibrated using Darks (50), and Flats (25), registered and integrated using the WeightedBatchPreProcessing (WBPP) script in PixInsight. As this was an old dataset, it was a bit challenging to find the proper flat files with the correct images. Overall the flats did not fully correct the luminance stack, resulting in some background structure that could not easily be eliminated in processing.
The processing followed a fairly standard pattern. R, G and B channel were combined, gradient removed (GraXpert), calibrated (SPCC), deconvolved (BXT) and noise reduced (NXT). Before stretching (GHS), the stars were removed to create the stars image that would later be added back. The luminance channel followed the same pattern excluding the colour calibration. Here noise was removed after stretching. There was quite a bit of noise in the image, and while NXT did a great job, it could not completely eliminate the noise. Luminance was combined with the RGB image using ImageBlend in Colour mode. RGB was used as the blending image. Black-point was raised by 0.1, Highlights reduced by 0.1 and midtones set to 0.4. This gave a colourful look, better than typical LRGB combination. Luminance stars were discarded, only RGB stars were used. They were stretched using StarStretch at strength 6 and colour boost 1. ImageBlend was used again to put the stars back in, in screen mode.
A nice trick from Adam Block was used to enhance the blues in the image and add a bit of yellow to some of the reds. This was done using CurvesTransformation in the Lab channel. The trick is to set three fixed points in the curve, one at 0.12/0.12, one at 0.5/0.5 and one at 0.87/0.87. Lowering the lower half of the graph now will enhance the blues, while lifting the top half of the graph enhances the yellows. The results looked pretty natural, and is something that can greatly enhance an image without increasing the saturation overall. I’ve added an icon to the desktop that can just be dragged onto the image to apply the effect.
Final step in processing was the adjustment of the background level to 0.07 to align with the other images. The final image is far from clean. It still has a bit of noise in it, and the background is a bit uneven. But given the status of the old files and the calibration files at hand, this is probably the best that can be done. There is no problem with keeping files for a long time before you process them. I’ve completed several datasets like that in recent months. But it is probably good practise to at least calibrate the frames shortly after obtaining them, so that extra/new calibration frames can be shot when needed.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
