Measure a very steep slope cut on and cut off spectrum of a bandpass filter

I have a problem measuring a very steep slope band pass filter, the cut on and cut off rise from OD6 to 90% in less than 4nm, how can I adjust the equipment measure correctly

I want below:

• reduce the slit width to minimum can I adjust this?
• reduce the spot size can I reduce more than "reduce"
• the noise is very high, how can I do that by not touching the SBW

thanks,

Khor