Symmetrical Grid Pattern in STF8300M Flat Fields

To Whom it May Concern

I have been taking flats with narrowband filters with LED lightbox. Symmetrical Grid patterns appear in the flats that dont go away with dark reduction or with stacking.

Steps Taken:

SOFTWARE

1. Updated drivers
2. Connected to camera with CCDOPS

HARDWARE

1. Bought new USB cable for STF8300 connection
2. Tried different computer usb ports
3. Removed Reducer and shot flats
4. Flipped mirrored surface of HA & SII filters in the filter wheel
5. Disconnected PC from mains power while taking flats

LIGHTBOX

1. Tungsten lamp also tried instead of LED

PHOTOGRAPHY

1. 30-40% ADU ideal exposure observed

I have attached a screenshot below of what I see.

Shot in CCDOPS, then reduced in CCDSoft.
Flat Dark reduced then stacked via median combine ~5 flat frames

Any feedback or troubleshoot ideas would be thoroughly appreciated

With my Regards

Robert Hamlin