Having Nikon D750 and 200-500 f/5.6 E VR lens, i faced myself with a simple challenge - can it be done building a tracker from simple e-bay (cheap) components?

And here it is. Image of the light more than 2 million years back in time. The light emitted from Andromeda galaxy.

Done with 28 exposures of 30 s, finally stacked with DeepSkyStacker. 

The key component is not a D750 sensor, but a home build star-tracker (see below) keeping pixels in the fixed direction in space for about 30s. Without a tracker, light from the same star would move over 350 pixels during such a 30-s exposure. Impossible to fix within Photoshop. Once pixels can follow the direction, the sensitivity of the sensor is more than high enough to create nice image.

Tracker has been developed in several steps. Starting with simple barn-door-tracker and ending with tracker based on equatorial mount geometry, home-made gimbal-like holder and linear rail driven by 12V bipolar stepper with 1:51 gear and microstepper driver controlled with Arduino and my own software, which control the nonlinear step time to transform linear step into angular one at constant angular velocity.