I have owned my STT-8300M for about a year. All of my narrowband images, mostly Ha and OIII, have looked fine but I have had a band of illumination that takes up the central ~2/3 of the FPA that is particularly noticeable in one of more filters of any LRBG sequence. No matter what I have tried -- flat box, sky flats, nearby vertical wall in twilight, Alnitak flip flat -- I have been unable to produce good LRBG flats, where good is defined as producing an acceptably flat calibrated light frame. The problem has not been a show-stopper for several reasons: I have been focused on narrowband imaging, I am not yet trying to make "pretty" color pictures and, when I do an LRGB sequence, I have been imaging from dark (21.5 - 22.6 mag/arcsec**2) sites and typically a single filter per night. What's changed...? I added the AO-8T recently, which brought about all kinds of new challenges. My old methodology was to use Sequence Generator Pro and PHD2 for automation and guiding; however, those tools cannot take advantage of the AO unit, so I switched to using Maxim DL manually for a few nights in order to shake out the AO unit. In another thread, I just posted a very successful first light image taken in H-alpha. I am now playing around with an evaluation copy of ACP in order to achieve the same level (or more!) of automation with MaximDL that I had with SGP. One of the things that I have been doing is running many auto-focus sequences just to exercise the software (ACP + Maxim DL) and hardware as well as to set up the various configuration files (e.g., filter focus offsets) in ACP. All of this recent activity under a bright (~17.4 mag/arcsec**2) sky. A few mornings ago -- notably, after many turns of the filter wheel during the evening -- I noticed the following in all of my bias and dark frames, another 36 x 4-minute sequence taken as dark, bias, dark, bias, ..., taken immediately after a few hours of RGBL auto-focus and other calibration runs. My filter wheel currently has seven filters and the eighth slot is completely empty. I am likely going to put a NII filter there as I am interested in planetary nebulae. FWIW, the order is: R, G, B, L, SII, Ha, OIII, empty. Final notes: The humidity that evening was near 100%, outside temp was ~12°C, and I ran at -20°C. I have regenerated the desiccant plug tonight but have not yet had a chance yet to power the camera back up and take some darks and bias from inside in a dark and dry (~40% humidity) room. Is it indeed persistent residual charge from the .940-um opto-sensor reaching the focal plane due to an empty filter slot, as described in the attached pdf? What else could it be, especially to show up persistently for ~2 hours in dark and bias frames? Thanks for your help!