> Using device #0 Realtek RTL2838UHIDIR SN: 00000061
> Found Rafael Micro R820T tuner
> gain: name: LNA range: start 0 stop 0 step 0
> supported sample rates 250000-2560000 step 24000

OK very interesting .  First , try adding a new option when you run scope.py -S 250000 which should set the sample rate to 250K.  Seems it's unhappy with the default 320K.  When you're running scope.py manually you can add the -c NNN option where NNN is the offset in Hz.  If you know the PPM (error offset) you can multiply that by the frequency in MHz to get the offset NNN.  For example say your PPM error is 2.5 then 2.5 * 851 = 2127 so you would have -c 2127..... You may need to fiddle with the sign (so, also try -c -2127)

As far as adjustable gain - looks like you can't adjust the RF gain.  Is it adjustable when running osmocom_fft ?   Also you can try adding an option -o 50000 if there's a DC offset problem (usually shows up as a spectral line at the tuned frequency even when the antenna is disconnected)

so to sum up try ./scope.py --args "rtl=0" -f 851.012e6 -S 250000 -o 50000 -c 2127

Best

Max