Range, pixel size and cycle time, they are your controlling factors.

Also remember scientific instruments are much different critters than tactical instruments.

I am not discounting contrast, what is the ambient temperature of the area of space you are attempting to scan? As it varies from location to location. Also look to the first line of this response, what is the pixel size of your bogey, a single flare in a single pixel in a single scan.

As for rules for sensors I use a variation of CT's detection ranges coupled to the simplified computer rules from High Guard and Mayday.

As for long distance scientific electronic survey last I remember is using either Scouts book and/or DGP's Grand Survey.

Evyn