Sh2-173 (also known as Mayer 1) is an emission nebula in the constellation of Cassiopeia. The red nebulosity is so faint that even an extended exposure through standard filters does not reveal it, but a hydrogen alpha filter will bring it out. Physically this is a H II region with a diameter of 77 light years, located at a distance of about 8800 light years. Sh2-173 is also nicknamed “Phantom of the Opera”: indeed this name can be derived from its resemblance to a theatrical ghost mask.
Telescope: 16″ f3.75 Dream Scope
Camera: FLI ML16803
Mount: ASA DDM85
Exposure: 8 hours (40x300s Ha + 32x300s L + 3x8x300s RGB)
Date: September 2019 – July 2020
Location: Southern Alps, France