Diffraction Limited released new STX / STXL Firmware V2.54 on 2019-02-06 to improve the guide camera performance.

This version improves subframe images from the guide camera (Remote Guide Head, STX Guider, or FW8G-STXL filter wheel). Previously there was some level of gradient across subframe images. By eliminating this gradient you may find it possible to guide or AO track on fainter stars.

The new STX / STXL firmware version can be downloaded and installed using the SBIG Driver Checker.
Note: if your SBIG Driver Checker installation is more than two years old, please download and install the latest version rather than using the "Update" button. The newer version provides more reliable firmware installation on Windows Vista / 7 / 8 / 10.

You can get it here (or go to the website, look for your camera, download tab, and download the SBIGUDrv.
ftp://ftp.sbig.com/pub/SetupDriverChecker64.exe

If you need assistance, please go to the Diffraction Limited technical support forum:
http://forum.diffractionlimited.com/

Cheers
Colin