Experiment:
I restacked the SII with no darks, flats, and bias and there was no stripe. I then stacked them with the darks and the stripe was back. Eureka - the darks are somehow the problem! To be sure I restacked them again with only the flats and there was no stripe.
The confusing part is the same darks were also used for the Ha. If the darks were the issue, the Ha should also be affected.
Results:
Figure 1 - SII stacked with darks, flats, and bias
Figure 2 - SII stacked with no support frames
Figure 3 - SII stacked with flats only
Figure 4 - SII stacked with darks only
Figure 5 - OIII stacked with darks, flats, and bias
Figure 6 - Ha stacked with darks, flats, and bias
Equipment:
Camera: ZWO ASI1600MM-ProTelescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Focal Length: 805mm (644mm w/ FR)
f/7
Focal Reducer: 0.8x AstroTech Field Flatterner/Focal Reducer
Mount: Orion Sirius
Filter Wheel: ZWO
EFW 8 x 1.25"
Filter: Antlia Ha, OIII, SII; ZWO L, R, G, B
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope/ZWO 60mm Guidescope
Exposure: Ha 77 x 300, OIII 68 x 300, SII 71 x 300, R 78 x 90, G 48 x 90, B 49 x 90
Gain: 139
Sensor Temp: 0 C
