Comet 21P: Giacobini–Zinner
Capture Equipment
Scope/Lens: Explore Scientific 102mm ED APO Triplet
Camera: Canon T3i (astro modified)
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 EOS, PHD2, Stellarium, Astrotortilla
Image details
Dates Captured: Sep 8, 2018
Processed: Mar 2019
Frames: 32 X 75 secs @ISO 3200
Calibration Frames: Bias, Flats, Darks
Integration: 40 mins
Location: Lake San Antonio (Fall Calstar 2018)
Processing SW: PixInsight; Adobe LightRoom
Astrobin: Image
Image Acquisition
Object Info:
Calstar 2018, was my first multi-day star party and a thoroughly enjoyable experience. My main imaging target was M31-The Great Andromeda Galaxy. Unlike other deep space objects with predictable appearance in the night skies, seeing/imaging comets is opportunistic. Comet 21P was fortuitously placed in the night sky at about 40° in Auriga around 4am just as M31 was getting low in the sky for meaningful imaging. Also providential was that 21P was near perihelion (at ~1a.u.)—when a comet’s tail is most prominent— and the closest to earth (0.39 a.u.) during this Calstar. I took my opportunity to image it my second night there. (Sep 8, 2018).
I had only ever observed two comets before: 41P/Tuttle–Giacobini–Kresák and C/2015 V2 (Johnson). Both seen as faint fuzzballs with no noticeable tail but clearly discernible motion against the background stars in the span of an hour. 21P however, at an estimated mag 6.5, showed a distinct tail seen through my trusty 10″ Dob. I was keen to see what an image of this object would look like.
21P is a Jupiter family comet with a period of about six and a half years and an estimated nucleus size of 2 kms. It was first observed/discovered over a century ago. It’s fascinating that solar wind and radiation create the enormous cometary coma and tail enabling seeing/imaging what is otherwise such a relatively minuscule object! The six and half year period also means that next time around (in 2025 CE), 21P will attain perihelion behind the Sun relative to earth.
To track the comet or stars?
Since comets move noticeably within the field of view of imaging, one approach is to track the comet itself. While preserving the comet nucleus, this will result in trailing stars which can be left as is or removed during stacking leaving a “pristine” view of the comet. The other technique is to track the stars but limit exposure length so that the comet nucleus is not smeared too much in the image while capturing the background stars. I chose the latter since a comet nucleus appears smudged in any case—going for 75 sec exposures at the higher than usual ISO of 3600. I took a couple of subs a few mins apart to gauge the speed of the comet relative to the background stars for appropriate faming. In hindsight I should have spent more time on this since my eventual framing was less than ideal for the duration of my imaging.
Image Processing
The 32 frames taken over an hour were processed in PixInsight (PI) using the method described in Warren Keller’s book. This is also described by Alejandro Tombolini here.
The method uses PixInsight’s CometAlignment process which has three parts to it:
- registering the images to the comet and integrating for a Comet Master image (below)
- registering a new set of the same images to the stars but with the comet removed from each; integrating for a Stars Master image
- Blending the two using PixelMath for the final integrated Comet and Stars image (below on hover)
For the blend itself I used this modified expression which adds a boosted comet image without the background star artifacts which are inevitable despite integration with aggressive pixel rejection:
Stars + ((Comet – Med(Comet))*6)
Stars + ((Comet – Med(Comet))*6)
You can see my Comet only image below and the blended version on mouseover (on tablet/mobile: tap inside-outside of image). These are auto-stretched but otherwise unprocessed.
The blended image was then processed using the usual suite of PI tools:
Noise Reduction (MMT) –> Stretch (Histogram Transformation) –> Color Saturation
using appropriate masks being the main steps.
21P the movie—now playing!
Seeing how the comet had traversed several arcminutes in the span of the hour, presented an opportunity to combine the images into an animation showing the motion. I registered all the images to one image. This set was then mildly processed (crop, NR, stretch, linear fit) using the Image Container and Process Container tools in PI. The resulting jpgs were combined to produce the animated gif below. I think it’s pretty cool to be able to see an object hurtling through space.
The red smudge on the right
An unplanned but pleasant addition to the image giving it greater character IMO, is the red paw print like smudge to the right of the comet— a part of the emission nebula known as Sh 2-235. There’s estimated hydrogen in these glowing clouds enough to form about half a million stars the mass of our Sun! There’s intense star formation occurring with the Spitzer telescope revealing 86 young stellar objects.
The faint vertical nebulosity seen between the comet and paw print is a part of Sh2 232 part of the same HII complex as the paw print located in the Perseus arm of our galaxy.