This week, I spent most of my time advising Kenny on the assembly and testing of the mount electronics. Soldering the surface mount components proved to be unexpectedly challenging, and we had to get around multimeter reliability issues, inadequate desoldering equipment, and solder-related issues (Kenny had inadvertently ordered lead-free solder paste for use with the SMD components, which meant that he had to deal with the intrinsic difficulties of using lead-free vs. leaded solder as explained here: http://www.pcboardrework.com/difference-between-lead-and-lead-free-solder/). In addition to advising him on his difficulties, I also laid out procedures that he could apply to ensure the functionality of the mount electronics, ranging from the usage of the laser diodes intended for the test setup as makeshift current indicators for the mount’s unipolar motor driver PCB to the application of an ammeter (in conjunction with a large 10W resistor) for safely testing the NEMA 17 motor drivers’ H-bridges.
Given that the majority of my contribution is complete, I’m reasonably on schedule.
Tomorrow, we will conduct experiments to characterize the tradeoffs inherent in our design. These include the following:
- The choice to use a custom PCB and circuit to implement the camera mount’s unipolar motor driver vs a typical UL2803 Darlington array. This tradeoff will mainly be evaluated by measuring the amount of current each device can supply to the mount’s unipolar stepper motor with a fixed supply voltage of 5V, and the temperature the transistors reach after 30 minutes of operation (as measured by infrared thermometer).
- The decision to use a custom H-bridge circuit for driving our NEMA 17 stepper motors (used to implement panning and tilting of the camera mount) vs. a commercially available motor driver (such as the Easy Driver). This will be tested by measuring the H-bridge MOSFET temperatures after 30 minutes of powering 1 or 2 motor phases, and comparing their average to the temperature measurements listed on this site: http://www.schmalzhaus.com/EasyDriver/#Q14. A simple figure of merit will be calculated from the current each driver is able to supply, and the operating temperature.
- We decided to implement our camera mount using a barn-door-style compensator to track the rotation of the starscape, instead of a more elaborate and accurate equatorial type mount. This style of mount usually experiences a form of tracking error called tangent error, which renders its usage impractical for exposures lasting longer than 5-10 minutes with a 50 mm lens. We intend to examine the degree to which our software-based compensation ameliorates this error by making progressively long exposures with a 56 mm lens with and without said compensation until star trails are visible on the image. The presence of star trails will be determined by comparing long exposures of the starscape with comparatively shorter ones, and measuring the relative sizes of the stars on the images.