Galaxy NGC 55 is one of the major large bright southern galaxies and is roughly the same size and brightness as its more famous neighbor the great Sculptor galaxy NGC 253. Willasch and Slotegraaf in their Coffee-table book Pearls of the Southern Skies featured NGC 55 and called it the apt name "The String of Pearls".

NGC55 has been variously classified as an irregular galaxy; an S-shaped barred spiral galaxy; and as a galaxy having a structure similar to the Large Magellanic Cloud. It looks irregular because the brighter core is well off-centre and overall it is quite asymmetrical. But it is aesthetic beauty that elevates this galaxy into its own class. (On the other hand, I've seen similar looking objects appear on my car window after a flock of birds flew overhead.)

The addition of Hα data for this object is highly desirable given the stellar activity in the galaxy. The method described in "Enhance HII regions in Spiral galaxies" by Edoardo Luca Radice, where the broadband continuum spectrum from the R channel is scaled and subtracted from the narrowband Hα channel to obtain a better approximation to the Hα line spectrum, was used. This Hα line data was then incorporated into the R channel and an inferred Hβ line data incorporated into the B channel.


Additional Capture Info

This is my first Celestron RASA 8" complete project (and first time using PixInsight). This instrument, normally regarded as a wide-field astrograph, can actually operate at the seeing limited resolution when coupled with the tiny 2.4 μm pixel ZWO ASI183MM Pro — at a sweet spot of 1.246 arcsec/pixel. So it can do smaller DSO captures such as galaxies without compromise.

During capture every frame high dithering was used and during processing 2x drizzle was employed. For publishing here the drizzled images (0.623 arcsec/pixel) were resampled back to the original native resolution (1.246 arcsec/pixel). Although I have added a strongly compressed JPG of the drizzle version as Revision C, for interest.

Finally, due to the fast F ratio, the wavelength shift for the geometry of the RASA 8" theoretically averages Δλ = -4.70 nm across the aperture (assuming the substrate refractive index is 1.5); so using Astronomik Ha 1.25" 12 nm narrowband filter still easily passes the shifted and broadened Hα emissions.


Background Galaxies

The star field of the image is rich with galaxies (many hundreds). A single sharply-focussed subframe clearly delineates fainter pin-point background stars from small fuzzy galaxies. However, after stacking and processing these small fuzzy galaxies tend to become more stellar-like even with careful processing. A portion of the original drizzle image (0.623 arcsec/pixel) is shown above (so that you can have your Where's Wally/Waldo moment).

How many galaxies? The Principal Galaxies Catalogue, reveals there are approximately 400 "bright" PGC galaxies in the full image frame (Revision C) down to around magnitude 19.5. Each of these is easily found in the image and there are much more fainter again galaxies in the frame. All this leads to the thought as to what is so special about this region to have so many galaxies? The answer is nothing — as any deep image away from the Milky Way's galactic plane looks very similar.