Single AA input, variable voltage output, Night vision

I own a night vision device that uses an aperture on the lens to control light entering the unit.

The tube (light intensifier) always runs at max voltage and produces a lot of noise. When i use partly discharged batteries, with lower voltages, the noise is much less - greatly improving S/N (signal to noise ratio).

The unit uses a single AA battery, and gets 40 hours of battery life. So if the typical AA contains about 15000 Joules, that means it consumes about .104 Watts of power.

I would like to use a single AAA battery, and then put a voltage regulator with an adjustable potentiometer in the battery compartment to control the voltage the unit receives.

I will be using a single AAA rechargeable NiMH, 800 mA*Hr, 1.2 v battery.

You don't necessarily have to give me an exact answer, or application, i just want to know what i should research and maybe the answer so i can confirm my findings

You would be better off modifying the voltage step up circuit in the goggles so it's output voltage to the tube was adjustable. This would be more efficient. Without a schematic I can't tell you how to do this.
A simple 10Ω POT in series with the battery may work for you. To accurately calculate this resistance value we would need to know:
The battery voltage at which the SNR is acceptable.
The current draw at the above battery voltage input.