M97 - Owl Nebula
M97 - Click here for full resolution
M97, or the Owl Nebula, is a planetary nebula approximately 2,030 light years away in the constellation Ursa Major. It was formed from the outflow of material from the stellar wind of the central star as it evolved along the asymptotic giant branch (a massive red giant star). The nebula is arranged in three concentric shells, with the outermost shell being about 20–30% larger than the inner shell.
source: Wikipedia
NGC/IC:
Other Names:
Object:
Constellation:
R.A.:
Dec:
Transit date:
Transit Alt:
NGC 3587
Owl Nebula
Planetary Nebula
Ursa Major
11h 14m 48s
+55º 01’ 08”
03 Apr
88º N
Conditions
The images were taken on two consecutive nights from the backyard in Groningen, the Netherlands. From this location, M97 is circumpolar, so visible all year around. But in late winter, early spring, it is at its peak and visible most of the night around the northern sky. During the two sessions the moon was almost full and only around 50° separated from M97. So it was hardly dark with SQM values around 18.0 mag/arcsec².
Equipment
Planetary nebulae are typically small in size and M97 is no exception. Therefore the TOA-130 (FL=1000mm) in combination with the small sensor of the ASI533 is more than enough to capture the nebula with room to spare. The target can be very well imaged using broadband LRGB filters. But imaging with just H-alpha and OIII filters is also a feasible option. Given the strong interference of the moon, the latter option was chosen.
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” Ha/OIII (5nm) mounted, ZWO EFW 8-position
Unguided
Fitlet3, Linux Mint 21.1, Pegasus Ultimate Powerbox v2, Aurora Flatfield, Pegasus Astro Flatmaster 150MBox
KStars/Ekos 3.6.3, INDI Library 2.0.0, Mountwizzard4 3.0.1, PixInsight 1.8.9-2
Imaging
Default settings for imaging with the 5nm H-alpha and OIII narrowband filters on this setup are 5-minute exposures at gain 100. Default settings appeared sufficient with good visibility of the nebula in each of the filters. At the start of the first session some thin clouds were interfering, but after that, and the following night, imaging went successful. In total 156 usable frames were shot, totalling 13h of exposure.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
2800 × 2100 px (5.9 MP)
1000 mm @ f/7.7
3.76 µm
0.77 arcsec/px
0º 36' 5.8" x 0º 27’ 4.4”
158.8 degrees
RA: 11° 14’ 46.959”
Dec: +55° 01’ 01.72”
Processing
All frames were calibrated using Darks (5), Flats (25/filter) and Flat-Darks (50), followed by registration and integration, all using the WeightedBatchPreProcessing script in PixInsight. The OIII master had a much higher signal value than the H⍺ master. LinearFit was used to adjust for the difference. An HOO image was created using ChannelCombination. The resulting image was cloned so that one imaged could be processed for the stars, and the other for the nebula.
Stars: Getting star colour correct in an HOO image is almost impossible. But there are a few things you can do to get at least a bit closer than just white stars. First is to apply SpectroPhotometricColorCalibration. This adjusts white balance to have the best overall match of each stars’ colour from the database with the corresponding star in the image. To maintain colour during stretching, a combination of ArcSinghStretch and HistogramTransformation was applied. There are a few stars located within the nebula that can easily be obscured when boosting the nebula. To make sure they remained visible in the final image, they were stretched a little bit further, by means of a mask generated using the GAME script, essentially covering the nebula. A saturation boost with CurvesTransformation completed the processing of the stars. StarXTerminator (SXT) extracted the stars to create a Stars-only image.
Nebula: First step was to remove the stars using SXT. The image looked pretty clean with very little gradients. Still a DynamicBackgroundExtraction was applied to remove any residual gradient present. The nebula does show some structure, but it can be easily obscured by too much stretching. BlurXTerminator turned out to be a very good instrument to enhance and sharpen any existing structure. With the stars removed, the PSF value could not be measured automatically. Instead, the strength was set to 0.7, and the PSF diameter was adjusted to have a maximal effect without creating artefacts. Stretching was tried both using the GeneralisedHyperbolicStretch (GHS), as well as the more traditional HistogramTransformation (HT). In comparing the two results the HT version appeared more pleasing than the GHS version. Theoretically this is different than what one would expect, so this was probably more a function of operator skills than anything else. Now came a very tricky part. At this stage, the regular HOO image had a very teal-like look to it. And the red areas of H⍺ around the edges were largely overwhelmed by the OIII signal. Bill Blanshan has developed a very nice tool to properly balance the colours in narrowband images, the NarrowbandNormalisation script. This gives a lot of control to the final look of various colour palettes.
Finding a pleasing colour palette using the NarrowbandNormalization (NN) script. The initial result (left image) of the HOO channel combination resulted in a very teal-looking image, with not much detail in the Halpha outer layers. Applying the NN script in its default HOO-setting, using Halpha for lightness gives a much better overall blue hue to the nebula, but the Halpha areas became a bit yellow. Interestingly, selecting the HSO palette and boosting with the OIII and SII sliders resulted in an image that had a nice blue center area of the nebula, while bringing out nice red colours in the Halpha outer areas. Using Halpha for lightness still meant a bit of a dim image, but that could later be boosted with regular CurvesTransformation.
With the colours now in place, an extra boost was given to brighten the image up a bit, using CurvesTransformation. A final step of mild noise reduction using NoiseXTerminator created the final Nebula-only image.
Nebula-only and Stars-only were now put together using Pixelmath. The object is pretty small compared to the Field of View, but other than cropping from a square to a 4:3 image ratio, the final composition was left as is. Only some finishing touches, mainly contrast enhancing, using CurvesTransformation were applied. When exporting the image and looking at the final JPEG it occurred to me that the stars in comparison to such a small target were a bit more bloated than desired. So BXT was used to only adjust the stars and make them a bit tighter. This resulted in the final image as shown above.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
