NGC7635 - Bubble Nebula
NGC7635 - Bubble Nebula. Click here for a full resolution image
The Bubble Nebula, or NGC7635, is an emission nebula in the constellation of Cassiopeia. It is also known as Sharpless 162 and Caldwell 11. The distance to Earth is approximately 11,000 light years. It is in close proximity of the open cluster M52.
The nebula itself is a large molecular cloud, or HII region. In the centre of the nebula is a hot young central star (SAO 20575) that blasts out an enormous amount of glowing gas which bumps into the denser molecular cloud, resulting in the characteristic bubble in the centre of the nebula. That central star is rather hefty at around 10-20 solar masses.
The nebula is not very bright with a magnitude of around 11. But the characteristic shape of the nebula makes it a well appreciated object among astrophotographers.
Planning
Object
Visual Magnitude: 11.0
Apparent size: 15 x 8 arcmin
R.A.: 23h 21m 35.86s
Dec.: 61º 18’ 39.4”
Conditions
Astr. night: 18:37
Astr. dawn: 06:00
Moon: 68%
Moon set: 01:38
Humidity: 85-89 %
Pressure: 1028-1036 hPa
The Bubble nebula is a circumpolar object, and can be well photographed throughout most of the winter, but with best visibility in fall and early winter. The images were taken at two sessions quite far apart. The first set of 12 images for each channel was taken on November 17, 2018. In March 2019 it was decided to add some significant observation time. On March 29, 2019 the remaining images were shot for this object. During the March session, the altitude was quite a bit lower and with the object in the north it meant quite some glow from the city. Fortunately in narrowband imaging this is not too much of a problem. Weather conditions were fairly similar on both occasions, with humidity in the mid 80s and a fairly high pressure of 1028-1036 hPa. The moon was present, but did not give any problems because of the narrowband imaging.
Capturing
The images were captured in narrowband, using 5nm filters for H-alpha, OIII and SII. Because these filters block a lot of the incoming light, exposures typically are a bit longer than broadband imaging. Exposures of 300s were made, at a standard gain of 139 (unity gain).
Framing was fairly easy, the whole of the nebula easily fits into the field of view of the TOA-130 + ASI1600. During the session on March 30, 2019, for some reason the autofocus routine could not find a proper focus point. Therefore the focusing was manually set to the value appropriate for the outdoor temperature using the method described below. And with reasonable results.
The effect of temperature on focus is often related to the effect on the length of the tube of the telescope. But in a refractor, the temperature induced focus-shift is mostly related to the change in the refractive index of the glass of the main lens. One easy way to notice is that if it gets cooler, focus-draw needs to be pulled in. Whereas if it would be related to a shrinking tube, focus-draw should be pulled out.
Linear relation focus and temperature (FSQ-106)
Linear relation focus and temperature (TOA-130)
This temperature effect can be measured and modelled. Over time, focus positions (step-position of the auto-focuser) were recorded against ambient temperatures. A linear relationship is visible. The slope of the a linear fitted curve equals the # steps per ºC temperature swing. In order to keep the variation in data-points as low as possible, it is important to keep many variables constant. This includes settings on dew heater, placement of temperature probe, autofocus routines, etc. Given the variation this might not be a perfect focusing method, but if conditions can be kept similar, it could form a backup scenario in case autofocus fails. For the session on March 29, it certainly did.
Technical details
Telescope
Mount
Camera
Sensor Temp.
Takahashi TOA-130 + 35 flattener
10Micron GM1000HPS
ZWO ASI1600MM Pro
-30ºC
Captures
H-alpha (5nm)
OIII (5nm)
SII (5nm)
Total Exposure
37 x 300s @ Gain 139/21
38 x 300s @ Gain 139/21
33 x 300s @ Gain 139/21
9.0h
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the BatchPreprocessing script. Generally there are some overall differences in signal strength between the three filters, so a linear fit was applied to SII and OIII to make them comparable to the H-alpha. While it makes sense to keep overall levels somewhat comparable before combining, this process seems to carry the risk that colour balance is affected. In a fully artificial colour palette like the Hubble palette this is not a huge deal, but it might be nice to experiment with this a bit in future image processing. Noise reduction was applied using the MureDenoise script and the image was cropped to eliminate some rough edges that were caused by a tiny bit of field rotation.
Reviewing the results at 100% gave nice results, so processing was continued. However, during further processing and in the final image, there was a certain graininess visible that did not get any better during further noise reduction, stretching, etc. This image was processed shortly after the Rosette Nebula (NGC-2237), but it never gave the nice smooth atmosphere as the Rosette Nebula and always kept a bit of harshness. While preparing this website-post, the images were looked at again, but now at high magnification. It turned out that the individual filter stacks showed a very unusual noise pattern, with very harsh tonal transitions. One pixel could have and ADU of 350 and the next 770, while they were clearly part of an area with similar brightness. Probably this has caused the graininess in the final image. The question however is still where this strange noise came from. Because images were taken at sessions so far apart, a possible scenario could be that something went wrong during calibration and that calibration files have been used that fitted one set of images, but somehow did not really fit the other set. At this point this is all speculation, but certainly something to be investigated at a later point.
The individual filter stacks all show strange noise levels, akin to a very low bit-depth. Possibly caused during the calibration process.
Channels were combined using PixelMath, according to the Hubble palette. SII was mapped to Red, H-alpha to Green and OIII to Blue. At this stage it is difficult to judge colours, but a radial magenta-cast seemed to cover the edges of the frame. This was removed using a DynamicBackgroundExtraction. Stars had a strong magenta-colour, something quite common when using the Hubble palette. To correct that, the image was inverted , SCNR applied, and inverted back again to regular colours. For an impression of the effect of each of these steps, you can check out the processing of the Rosette Nebula. Then it was time to bring the image to the non-linear stage and stretch it using HistogramTransformation.
The next step was the art of creating a Hubble palette. This is done by performing a range of colour corrections, using specific colour masks targeting very specific areas of the nebula. The masks are generated with the script ‘ColorMask’, located under Scripts>Utilities. For this image, Magenta, Green, Yellow and Cyan masks were made. Each with the Mask blur set to 3. The steps were in line with the steps described for the Rosette Nebula. But this time, it was a lot more difficult to get a pleasing palette of colours. Stronger adjustments needed to be applied and in general the colours ended up with too much red and brown and not enough brighter blue and yellow colours. After several attempts the final result was accepted and further processing was applied. Possibly the noise-issue described above had an effect on the final colour rendition as well.
The rest of the processing was fairly similar to other images. A little bit more noise reduction was applied with MLT on the smaller scales. Layer 1 (3,0.33,3), Layer 2 (2,0.33,3) and Layer 3 (1,0.5,1) were targeted. DarkStructureEnhance was applied to create more local contrast. To focus more on the nebula and less on the stars, a MorphologicalTransformation was applied to shrink the stars (amount 0.5, size 11). Some sharpening was applied using UnsharpMask (StDev 3.0, amount 0.8) on only the brighter areas, using a range mask. As final touches contrast was enhanced by increasing the black point using HistogramTransformation and applying an S-shaped CurvesTransformation.
In summary the results were less pleasing than expected. The Bubble Nebula is an impressive object and this version is not the greatest. Possibly that has its root-cause in issues with the calibration. So hopefully that can be corrected in a future re-processing of the data.
This image is published on Astrobin.
