1. Long exposures require tracking.

2. Dobbies can't track because they have altaz mounts [0].

3. But short exposures don't require tracking. Therefore a Dobbie might work for short exposures iff you can lock down the altaz axes and sufficiently reduce vibration.

[0] It is possible -- and fairly common -- to track with altaz mounts. Both axes require coordinated, computer-controlled stepper motors. In addition you need a third motor to rotate the tube. This was not possible before digitally-controlled motors. So some Dobbies do track but that kind of defeats the purpose of a Dobbie as a cheap light bucket anybody can afford.

In contrast tracking with an equatorial mount only needs a single motor and that motor doesn't need to be digitally-controlled.