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.