In order to learn more about how telescope mounts work, and to save a significant amount of money, Izak McGieson and I set out to design and program our own mount control electronics. We started this in June of 2011, after adding steppers to a very cheap Chinese mount carrying a 4.5" Newtonian. We completed the majority of the electronics and software on that mount, but after realizing the following December that the accuracy of our electronics was outdoing the accuracy of the mount, we decided to upgrade. We upgraded to a Japanese made Vixen GP-2, and also replaced the 4.5" Newtonian with an 80mm apochromatic refractor(Astro-Tech AT80EDT) with hopes of widefield astrophotography. Our mount and software are capable of closed loop GOTOs using plate solving to accurately place the telescope, and guided tracking for astrophotography. This post is about the mount electronics, you can find the post about mounting the motors here
. A post about the software will be written soon. With the exception of Astrometry for plate solving and Xephem as an object database, we wrote all of the software.
The mount control schematic. Click for a larger view.
The electronics are very simple. The bulk of the work is done by an Atmel ATMega644, communicating with the computer software over serial via an FTDI USB to Serial IC. The AVR has four main tasks; control the RA and DEC steppers for slewing, accurately control the RA motor for tracking and guiding, control the shutter of a Pentax dSLR for imaging, and control the shutter and amplifier on a webcam modified as a guider. Currently guiding is only done on Right Ascension, because the declination motor's steps are too large.
Both motors are unipolar stepper motors. Right Ascension uses a NEMA 23 stepper motor with a Pittman 10:1 reduction gearbox and runs at 5V(it is rated for 6V). Declination uses a smaller NEMA 17 stepper motor with no gearbox, and is run at 12V. We would have preferred to use the smaller NEMA 17 motors for each axis, but we only had a NEMA 23 size gearbox on hand. We experimented with microstepping the RA motor during tracking, but did not see a noticeable difference between that and half stepping with the gearbox. The motors can be driven in full step or half step mode, depending on the speed and accuracy required.
The software running on the AVR is written in C. It accepts commands from the computer to rotate an axis a given amount, or to control the shutter of one of the cameras. It does not keep track of where the telescope is pointing, this is handled by the software running on the computer. This was done so that we could achieve accurate pointing by blindly pointing where the object is expected to be, taking a short exposure, plate solving it to determine where the telescope is actually pointing, and repeating this process until the desired accuracy is achieved. We use the open source Astrometry
software for plate solving.
In addition to controlling the motors, the AVR also controls the shutters for two cameras. For the Pentax dSLR the external cable release port is used. This port is typically connected to a button or switch and operated by the user to prevent shaking during long exposures. The AVR emulates this with a solid state relay. This also electrically isolates the camera from the mount control electronics. The second camera is a long exposure modified Logitech webcam. We modified the camera as per Keith Wiley's instructions
(his instructions are based on Steve Chamber's modification of a different camera model). Instead of using buttons to control the shutter and amplifier, we connected those lines to the AVR so our autoguiding software could directly control the camera.
The electronics briefcase.
Power is provided to the electronics by a modified ATX power supply. The 12V rail from the power supply is used to actuate the relay that controls power to the declination motor, and the motor itself. The 5V rail is used to power the Right Ascension motor. A separate 5V rail is used to power the electronics, to prevent any fluctuations caused by the motor from disturbing the AVR or serial chip. This 5V is provided by a 7805 regulator, regulating the 12V rail down to 5V.
The only connections required when setting up the telescope are power, USB, and the rainbow ribbon cable seen to the right. The rainbow cable connects the electronics to the mount through another box. This box has two status LEDs, three switches, and connectors for the motors. The three switches control the laser finder, red lights to illuminate the setting circles, and a dew heater. This box is attached to a tripod leg with velcro.
The Ethernet port was at one time used to connect the laser, electronic focuser, and dew heater mounted on the 4.5" Newtonian. This port has since been replaced by a 7 pin DIN for the autoguiding camera, and a 2.5mm audio connector for the Pentax shutter control. We have not built an electronic focuser for the refractor, but plan to do so this summer.
As shown in the picture above, the electronics currently live on a breadboard. This was good during development, but during normal use can be frustrating as cables get pulled out. We plan on fabricating a circuit board this summer, and our goal is to fit all of the electronics in the small black box mounted on the tripod. This would greatly simplify setup and decrease size.