M20: The Trifid Nebula
Capture Equipment
Scope/Lens: Explore Scientific 102mm ED APO Triplet
Camera: QHY247C (CMOS OSC)
Mount: Orion Atlas Pro AZ/EQ-G
GuideScope: Orion 50mm Deluxe Guidescope
Guide Camera: Orion Starshoot Autoguider
Flattener: Hotech Field Flattener
Mount and Capture SW: ASCOM EQMOD; Backyard Sequence Generator Pro, PHD2, Stellarium, Astrotortilla
Image details
Dates Captured: Aug 1, Aug 31, 2019
Processed: May 2020
Frames: 35 X 300 secs; 33X180: gain 2200
Calibration Frames: Bias, Flats, Darks
Integration: 4.5 hours
Location: Pinnacles National Park, E
Processing SW: PixInsight; Adobe LightRoom
Astrobin: Image
Object background
The Trifid Nebula (Messier 20, M20) as the name suggests, is a combination of three nebulae types: an emission nebula (the red from the Ha), a reflection nebula (the blue light scattered in the same phenomenon that results in blue skies on earth) and a dark nebula (the dark lanes within the emission nebula).
At a distance of ~5000 ly and a radius of 22 ly, the nebula is a young star forming region in our galactic neighborhood, bright enough to be easily visible in small scopes. My observing logs tell me that I first observed M20 in Sept of 2016 through my 10″ Dob: with the lobes and dark lanes discernible especially with an OIII filter (my sketch from the night is included below). But deep sky objects rarely show much color through scopes since the faint light is insufficient to trigger the color receptors in our eyes. As a result, the Trifid is more an astrophotographer’s object than an observer’s.
This region of the sky also harbors the bright open cluster Messier 21 seen at the 7 o’ clock position in the image above.
Image acquisition
After working with an astro modded DSLR, I decided to take the plunge in Jul 2019 buying a dedicated and cooled astro camera: the QHY247C CMOS OSC. I wasn’t quite ready to go all the way to narrow band just yet but desired the full spectrum, cooling and low read noise.
Galaxies are my fav imaging (and observing) targets. But Aug is nebula season. Messier 20 with its 3 types in one object + colors was a natural choice to test out the new camera.
The data in this image was acquired over two nights in Aug 2019 from my go to dark sky location, Pinnacles National Park. These nights were also used to capture images of the Helix nebula available in my astrophoto gallery. Since this was a test of the camera, I tried a few different exposure duration and gain settings.
These were mostly clear nights but an astrophotography outing is seldom without the proverbial fly in the ointment. In this case, it was guiding. My set up is tuned to get me less than 1 arc sec RMS error but on these nights it was greater than 2 arc secs. I never was able to figure out the cause, though I suspect incorrect scope/equipment balancing on the mount. My image, as a result is not as sharp as it could have been, making this a less than ideal test for the camera.
Processing notes
Image stacking and processing was done in PixInsight following the standard processing steps I have come to use. I Bayer drizzled during integration with a scale of 2. Finally after all processing, I rescaled back to original resolution.
This region of the sky in the general direction of the center of the galaxy is densely packed with stars to the point where it detracts from the object of interest. To help the nebula stand out, I applied Adam Block’s star de-emphasis technique. This has become a regular part of my astro processing toolkit. I find this to be a more effective way in reducing stars than others avail in PixInsight (I am yet to try Starnet++).
The image below shows the before and on-hover, the after of star de-emphasis application.
Up Close:
Overall I am pleased with the image of this cosmic rose: considering that this was effectively first light for a new camera and hence entailed a degree of experimentation with setting, not to mention the guiding challenges I encountered. But I’m also keenly aware of the great room for improvement and some immediate things I need to fix for when I am able to engage with astrophotography again.
2 thoughts on “M20: The Trifid Nebula”
What was you gain and offset setting for each corresponding sub length?
Gain for the 180 sec exposures was the camera default 1228; for the 300 sec it was unity gain of 2200; Offset as far as I can remember was the default.