M74 - Phantom Galaxy
M74, also known as the Phantom Galaxy, is about 32 million light-years away from Earth. The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a grand design spiral galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. Its relatively large angular (that is, apparent) size and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study spiral arm structure and spiral density waves. It is estimated that M74 hosts about 100 billion stars.
source: Wikipedia
NGC/IC: NGC628
Other Names: Phantom Galaxy
Object: Spiral Galaxy
Constellation: Pisces
R.A.: 01h 36m 42s
Dec: +15º 47.0’
Transit date: 07 November
Transit Alt: 53º S
Sky-plots with a FoV of 50º (left) and 5º (right). Click to enlarge
Visibility
Images were all taken in the December/January period, but spread out over three years (2019, 2020 and 2022). All images were taken from the backyard in Groningen, The Netherlands (53.18, 6.54). Best time to observe this target is between November and February. Most of the images were taken at altitudes between 30 and 60 degrees.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout the session on 14 December, 2022 (right).
Weather
Weather conditions in the early two sessions (2019/2020) were pretty good. During the sessions in 2022 however, the weather was generally very humid, clouds rolled in on a regular basis and generally not very dark with SQM values in the high 18. Many frames had to be discarded as there was nothing to see, or default parameters like FWHM, # stars etc were much worse than the other images.
Imaging
M74 is a difficult target to image. From the Messier list of objects it probably has the lowest surface brightness. A very dark sky and/or a high aperture telescope would be best. For this image, neither was really the case. So the final image is limited in its detail due to these circumstances.
Photographing this target had started in 2019. Unfortunately it never came to sufficient frames for a full image. In later sessions in 2020, the target was too low on the horizon and during the following year other targets were worked on. When M74 was visible again in 2022, the majority of the frames used for this image were shot. There have been quite some changes in equipment used between 2019 and 2022, most notably a different camera. In 2019 imaging started with an ASI1600MM Pro while in 2022 the new ASI533MM Pro was used. Mount, telescope and filters remained the same.
The luminance images in 2019 were taken with a fairly long exposure of 5 minutes. The low brightness of the target would warrant a somewhat longer exposure, but it surely led to quite a number of clipped stars. When revisiting in 2022, it was decided to lower exposure to 3 minutes, resulting in less clipped stars, while relying on the very low noise-floor of the sensor to pick up sufficient detail.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, Sesto Senso 2
10Micron GM1000HPS, EuroEMC S130 pier
ZWO ASI1600MM Pro / ASI533MM Pro, cooled to -15 ºC
Astrodon 1.25” LRGB mounted, ZWO EFW 8-position
Unguided
Fitlet2, Linux Mint 20.04, Pegasus Ultimate Powerbox v2, Flip Flat, MBox
KStars/Ekos 3.6.2, INDI Library 1.9.9, Mountwizzard4 2.2.8, PixInsight 1.8.9-1
Geometry
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
2900 x 2900 px (8.4 MP)
1002 mm @ f/7.7
3.76 µm
0.774 arcsec/px
37’ x 37’
-13.4 degrees
RA: 1 36 41.467 - Dec: +15 47 05.40
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames and registered using the WeightedBatchPreprocessing script. Individual frames were normalized and scaled using the NormalizeScaleGradient script and integrated using NSG parameters. Unfortunately the luminance data from the 2022 sessions came out all bad. The flats had not calibrated out a series of dust spots in the system. It is difficult to pinpoint the exact cause of this calibration failure. Flats were taken at December 14, after the session. A few nights later, on December 17, a second set of flats was taken. When comparing the two master flats for the luminance filter, they were not very well aligned. So it leads to believe that maybe the filterwheel just is not very well calibrated and exhibits fluctuation in the individual filter positions. The calibration routine was run in Ekos and it remains to be seen if this solved the issue.
It was pretty difficult to correct the poorly calibrated luminance data, and many different routes were tried. The one that worked best is described here. It turned out that not using flats for the 2022 luminance data gave better results. This proves a point that many have made before: better not use a flat than using a bad flat. A bad flat can do a lot of harm to an image. Secondly, mixing the data from 2019 and 2020 in here actually helped, as they were not suffering from this flat-issue. As a third step, a DynamicBackgroundExtraction was done with lots and lots of sample point. The result was a much more complex background image, but definitely smoothed out some of the larger background structures from the dust bunnies. As a fourth step, the stars were removed, and, after cleaning up the background using the CloneStamp tool, put back in again. Finally, when combining the Luminance data with the RGB data, background was protected using a mask, so that the luminance enhanced signal, but did not introduce too much unwanted structures in the background. With all this together, the final result is better than expected when discovering the issue.
Fixing poorly correcting flats. The master flat (left) did not correct the luminance data at all well (middle). After very detailed background extraction (DBE), and Clonestamping a starless image, the final Luminance (right) was in good enough state to be used, especially with some background protection during the LRGBCombination process.
Apart from the flat correction issues described above, the rest of the processing was fairly straightforward, especially using some of the newer tools. Combining ASI1600MM luminance from 2019 with ASI533MM luminance from 2022 was very easy. The WeightedBatchProcessing script had to be fed with the proper Darks and Flats of course, but then it automatically crops and registers the ASI1600 images to a reference image from the ASI533. Since the 2022 luminance data followed an atypical path, both sets had to be manually combined using ImageIntegration. After DBE, the new BlurXTerminator tool was used, but with modest settings (stars 0.3, halos -0.15, non stellar 0.5). Using a combination of GeneralisedHyperbolicStretch (GHS) and HistogramTransformation the image was stretched into its non-linear stage. Contrast was enhanced a little bit, and noise was reduced using NoiseXTerminator (denoise 0.75, detail 0.25).
The color data did not have the flat-issue, and could be processed with a fairly standard process. Background extraction was not needed, but sharpening was applied using BlurXTerminator (stars 0.25, halos -0.15, non stellar 0.5). Colour was calibrated with the new SpectrophotometricColorCalibration tool. The colours came out really well, just a bit high in the green in the background, which was eliminated with a mild SCNR (amount 0.4). Stretching involved two stretches with GHS, the first using the ‘Colour’ mode rather than RGB. This retains the colours in a similar fashion as Arcsinhstretch does. Noise was reduced using NoiseXTerminator (denoise 0.7, detail 0.35) and contrast was enhanced using CurvesTransformaiton
It was now time to combine the Luminance into the RGB image. And due to some background protection in the RGB image, the galaxy brightened and sharpened up nicely, while the background stayed nice and evenly dark. The colour of the background was not completely neutral though. A few tiny adjustments using the black point slider in the HistogramTransformation tool were able to correct this very quickly.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
