SH2-173 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. This is a 9.5 hour exposure done on September 30, October 1 and 2, with 3.5 hours of hydrogen alpha data. The imaging session was under the control of CCDAutopilot 5. Click the image for a larger view.