ASTRO:
     type=emission nebula
     mag=(unk.)
     const=Cassiopeia
     dist=8,000 ly
IMAGE:
     location=Emerald Bay community, Lake Palestine, TX
     BrtlCls=4
     exposure=CMOS OSC; 83x300s (6.9h), G100
     palette=HOO
EQUIPMENT:
​     optics=ES102CF APO / Stellarvue 1.0x fld-fltnr, FL=714mm, f/7.0
     camera=ZWO ASI2600MC-Pro
     filter=Optolong L-eXtreme Duo NB
     mount=Celestron AVX
     guiding=Orion 60x240mm, ZWO ASi224MC
SOFTWARE:
     acquisition=Stellarium, NINA, PHD2
     processing=PixInsight (RCAstro), Photoshop (APF-R, WebSharp-Pro), LrC

Presented here is Sh2-173, the 173rd entry in American astronomer Stewart Sharpless' 2nd catalog of ionized glowing hydrogen gas.  Dr. Sharpless passed away in early 2013; but, he is most noted for his work in the mid-to-late 1950's on mapping the structure of the Milky Way Galaxy, and determining that the Milky Way is a barred spiral galaxy.  Using data obtained from the Mount Palomar Sky Survey which was completed in 1958, Dr. Sharpless found many of the H-II regions across the galaxy, and then determined their distances, and used that information to make his hypotheses on the structure of our home galaxy.  He released his first list of 142 nebulae in 1953 based on preliminary data from the Palomar Survey, but then subsequently released his 2nd list (Sh2) in 1959 containing those plus the remainder of 313 nebulae.  His list is used today by many amateur astro-imagers around the world to reference their images.

This object also shows-up as LBN 593.  A decade after Dr. Sharpless completed his meritorious work, American astronomer Beverly Lynds also performed extensive research on the bright nebulae of the Milky Way during the 1960's.  Using photographic data collected from several more modern observatories across North America, Dr. Lynds  developed her list of 1,125 "bright" emission nebulae (...known as Lynds' Bright Nebulae, or "LBN").  She assigned very accurate sky coordinates to each object listed, and recorded extensive data on the colors and relative brightness of each.  Her brightness scale from 1 to 6 is not used today; but, it led the way forward to a more robust and accurate understanding of intergalactic nebulosity and intensity of its light.   Dr. Lynds also developed a list of 1,802 "Dark" nebulae ("LDN") that serves as today's standard for referencing structures of dark nebulosity across our home galaxy.  Dr. Lynds passed away in late 2024.

This object's resemblance to a certain Broadway musical character has given it the nickname "Phantom of the Opera Nebula".  As with most such nebulae, Sh2-173 is associated with a star-forming region that will disappear over time as the stars born in and around it blow its dust and gases away with their powerful stellar winds.

The details visible in this image were captured using a dual-narrow-band (DNB) filter having 7nm band-pass characteristics.  This compares to the more restrictive 3nm width common in monochrome narrow-band filters.  Also, the DNB filter was mounted on a One-Shot-Color (OSC) camera; such cameras are known to have some amount of "cross-talk" across photographic color channels because of a RGB Bayer Matrix that is affixed on top of their sensors.  The Phantom is very faint.  Some nebulae show great detail when narrow-band filters are used; especially with the 3nm, monochrome variety.  But due to its faintness, this particular object just barely shows up when using monochrome equipment, even with extended integrated exposure times.  Both the wider band-pass of the OSC DNB filter, plus the cross-talk of the OSC Bayer Matrix - aid somewhat in 'easing' the struggle with faintness.   OSC's suffer from a minute amount of loss of detail when compared to their monochrome cousins; but on this targeted object, the gain in sensitivity makes the OSC version a preferred equipment configuration for capture.  The characteristic 'blue' color of OIII is missing from the image because the composition of the nebula is primarily Hɑ which shows up only as 'red'.

Meanwhile, enjoy this image while pondering upon the Music of the Night.

251113 - Sh2-173 "Phantom of the Opera" nebula