Triangulum Galaxy – M33, September 9, 2023

The Triangulum Galaxy, or Messier 33 (M33), is a spiral galaxy 2.73 million light-years from Earth in the constellation Triangulum. With a diameter of over 61,000 light-years, it is the third-largest member, yet the smallest spiral galaxy, of the Local Group of galaxies, behind the Andromeda Galaxy (152,000 ly) and the Milky Way (87,000 ly).  M33 is probably a satellite of the Andromeda Galaxy, or on its rebound into the latter, due to their interactions, velocities, and proximity to one another in the night sky.M33 subtends an angle of 71 x 42 minutes of arc (moa).  The full Moon, by comparison, is 31 moa in diameter. 

The Triangulum Galaxy is roughly 2 full Moons long and over 1 wide.  It would be quite an astounding sight in the night sky, if our eyes sensitive enough. Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen by some people with the fully dark-adapted naked eye.  It is the most distant permanent entity visible without magnification, being about 50% further away than the Andromeda Galaxy (M31).  It is a diffuse, or extended, object rather than a starlike point, even without magnification, because of its physical extent.  It is one of the reference objects of the Bortle Dark-Sky Scale with an effective visual magnitude of 6.6.  This means that a precondition for visibility is that the observer can see stars at least as faint as magnitude 6.6, which is fainter than many people are able to see, even at a very dark site.

The Triangulum Galaxy is a difficult object to image well, due to its low magnitude being spread over a relatively large area (low photon flux), similar to M101, the Pinwheel Galaxy.  Three hours of total exposure, at least in Bortle 7 coastal skies, is barely enough to create a respectable image.  Six to nine hours total is more likely required. This image is also the result of processing a 32-bit TIFF file from Astro Pixel Processor as far as possible in Photoshop.  Certain functions do not work with 32-bit images, such as Select Color Range, Select and Mask, etc.  However, most of the functions and add-ins I use up front work well in 32-bit mode:  Levels, Curves, etc.; NoiseXterminator, StarXterminator, and Hasta La Vista Green.  I’ve also started using NXT early in the processing workflow, with good results.  StarXterminator does a g great job of removing stars for nebula processing, and works with 32-bit images, whereas StarNet2++ only works with 16-bit TIFF files.

Equipment and Exposure Information· 

Optical Train:  Astro-Tech AT70ED telescope (336mm efl/f4,8), IDAS LPS-D3 filter, ATR8 0.8x field flattener/focal reducer, Ha-modified Canon T3i

Eposure Information:  ISO 800;  89x 120 second exposures for 178 minutes total exposure (2hrs 58 minutes) in the Bortle 7+ skies of Point Loma, San Diego, California·       

Processed in Astro Pixel Processor using 3x Master Darks, 3x Master Bias and 2x Bad Pixel Maps·       

Guide Scope and Camera: ZWO 30mm f/4 with ASI-120MM-mini camera·       

Capture software: ASCOM, Carte du Ciel, ASTAP, O’Telescope Backyard EOS 3.2.3 (BYEOS), PHD2, Polemaster camera and software·       

Processing software: Astro Pixel Processor 2.0.0-beta26, Photoshop 2024, Light Room Classic, Deep Sky Colors Hasta La Vista Green (HLVG), R-C Astro NoiseXterminator 1.1.3 AI2, R-C Astro StarXterminator 2.0.6 AI11