Sh2-174 - Valentine Rose
Sh2-174 is a planetary nebula in the constellation Cepheus, about 1800 lightyears away. It has not always been considered a planetary nebula, as it has been regarded emission nebula as well. But based on radio imaging it is now considered a planetary nebula, with a white dwarf visible quite offset to the right of the nebula. This asymmetry is caused by the interaction of the nebula with a large cloud of hydrogen. This interaction causes the intense H-alpha signal on the left. With its bright H-alpha signal, this planetary nebula is a bit of a unique object in the Sharpless catalogue of HII regions. It is also the most northern object in the catalogue with a declination of 81º.
source: Internet
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n.a.
Valentine Rose
Planetary Nebula
Cepheus
23h 48m 05.59s
+81º 04’ 19.7”
11 October
62º N
Conditions
For most of the sessions, the moon was illuminated 75-100%. And the target was in the Northern sky, which from the observatory is right above the glow of the city of Groningen. So far from optimal conditions. But taking narrow-band images should help combat these challenges. The target remained quite high in the sky for most of the night, so that helped. In fact, the good visibility made this a very nice target to combine with other targets that were shot during the same sessions, but which were not visible throughout the night.
Equipment
The diameter of Sh2-261 is approximately 15 arcmin, so a perfect target for the 40 arcmin field of view of the combination TOA-130 with ASI533MM. During the session of 08 December, a new meteo sensor was introduced, the Pegasus Astro Uranus. This is also a GPS receiver and unfortunately that caught me a bit by surprise. The session started normally, but after the meridian flip, the system had difficulties pointing to the right target again. It turned out that Ekos (now version 3.6.2) had automatically switched to ‘GPS’ mode and was not properly communicating this timing to the mount. After this error had been discovered, the system was reset, a new pointing model was built and all worked flawlessly again.
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” SHO (5nm) mounted, ZWO EFW 8-position
Unguided
Fitlet2, Linux Mint 20.04, Pegasus Ultimate Powerbox v2, MBox
KStars/Ekos 3.6.1, INDI Library 1.9.8, Mountwizzard4 2.2.7, PixInsight 1.8.9-1
Imaging
Sh2-174 is a target that is specifically visible in H-alpha and OIII. Unfortunately it is also a very faint target, with little structure on individual subs. I went for ‘default’ narrow-band exposures of 300s, but possibly it would have been even better to use 600s exposures. As mentioned before, this was not the main target for the sessions, but over three sessions a total of 12.6h of data could be collected.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
3008 × 3008 px (9.0 MP)
1000 mm @ f/7.7
3.76 µm
0.774 arcsec/px
38.8’ x 38.8’
-12 degrees
RA: 23º 46’ 51.787”
Dec: +80º 56’ 01.67”
Processing
All frames were calibrated with Dark (50), Flat (25) and Flat-Dark (50) frames, registered and integrated using the WeightedBatchPreProcessing (WBPP) script. Unfortunately there was a mix-up on offset values used between sessions. Both offsets 5 and 10 had been used by mistake. The result was that additional darks had to be taken and that the calibration process got a bit more complicated. But nothing that WBPP could not handle. I did get the impression though that the low offset created a bit of extra noise in the final stacks, because they looked noisier than normal. On the other hand, this is a very faint target, and during some sessions there was quite some moonlight present. So the noise could also just have been the result of a faint target under unfavourable conditions.
The first step in the processing was the removal of stars. With the excellent results of the AI-based tool StarXTerminator, working on starless images for most of the processing has become the 'go-to’ method now for narrowband images. Both H-alpha and OIII were stretched individually, to keep maximum control over the optimal stretching process for each channel. The Generalised Hyperbolic Stretch script/process in PixInsight allows for a lot of fine-tuning, so optimising per channel makes sense. The two channels were now combined by simply mapping H-alpha to Red, and OIII to Green and Blue. Another option would have been to use the Foraxx palette, as explained by Paulyman Astro in his YouTube movie. However, when doing that, in this case the Blue came out as a kind of Cobalt-blue, whereas the classical HOO palette gave a more teal-like colour. These things are personal preference of course, and for this target I chose the HOO palette.
There is not a whole lot of detail in this very fain nebula, and noise reduction was pretty much the only further processing that was done before the stars were put back in. Interestingly, the stars generated by the Foraxx PixelMath script were more natural looking. Both the red and blue stars had a nicer, more natural colour than stars in an HOO palette. So the ‘Foraxx stars’ were put back into the nebula, using the screening function. Just a little contrast enhancement using CurvesTransformation was all that was applied for the final image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
