Introduction

Design

Moon Using Dobsonian

Challenges

Background

Conclusion

Resources Utilized

Large telescopes require active tracking to keep celestial objects within their fields of view for

even a few minutes because of the Earth’s rotation. We modified a Dobsonian telescope by

adding two stepper motors feedback-controlled by using a $35 Raspberry Pi and its $25 camera.

The computer measures the movement of the brightest stars between images, feeds back the

motion signal to the motors thereby keeping the telescope pointed at approximately a constant

field of view. Even though the Dobsonian mount requires variable tracking of both axes to

follow the circumpolar motion of the sky, we set out to explore whether simple feedback could

lock-on to the motion and be stable at all directions of the sky. By means of image analysis the

stepper motors will be controlled to move accordingly with the celestial body.

The Raspberry Pi has great potential astronomical applications. The Pi-cameras’ small sensor

results in diminutive field of view, which may work for lunar and planetary exploration, but is

not well suited for imaging larger, fainter celestial objects. Once manual long exposure control

is implemented, the Pi-camera on a small, wide-field telescope (finderscope) should be suitable

as a star sensor for a feedback-driven clock drive. Although a Raspberry Pi and Pi-camera have

orders of magnitude more transistors than a conventional Dobsonian clock drive, abstractly and

practically it is a much simpler system, requiring no calibration before each setup and use.

• http://www.raspberrypi.org/wp-content/uploads/2013/07/RaspiCam-Documentation.pdf

• http://www.raspberrypi.org/forums/viewtopic.php?f=37&t=7628

• http://www.scraptopower.co.uk/Raspberry-Pi/how-to-connect-stepper-motors-a-raspberry-pi

• http://www.raspberrypi-spy.co.uk/2012/07/stepper-motor-control-in-python/

• http://en.wikipedia.org/wiki/Dobsonian_telescope

• http://piscopeci2014.pbworks.com/w/page/73491050/FrontPage

• Stepper motors were kindly donated by Chuck McKinnon

Motorized Telescope Mount Using Raspberry Pi Juan Cervantes • Harel Chen • Kyle Clarke • Daniel Goede • Arvin Torosian • Brian Rasnow• Math 490

The idea behind the design of the mount consists of a fixed eyepiece for the Pi camera and two

stepper motors that try to mimic a GPS driven mount. Trying to mimic an equatorial mount is

to ease the use of following the Earths rotation and keeping track of celestial bodies. One of

the motors is set to move the base of the telescope and the other is to move the telescopes axis.

We discovered that our power supply is producing too much voltage which was then decided to

use inexpensive wiring which will cause resistance and voltage drop. This would prevent any

damage to our motors and to our Pi. Our stepper motors are controlled by Python code which is

connected through the GPIO on the Pi.

The first challenge we encountered was with the Pi camera. Exposure control in theory was a

great idea but the hardware limitation of the camera board did not allow for precise exposure

control, which then limited us to planetary objects. Firmware, board, software, hardware specs

were not accessible to achieve any type of different exposure effects other than the built in

functions. Given an opportunity to integrate a camera with custom exposure controls we will no

longer be limited to planetary objects which then can take long enough exposures to get images

of celestial objects. Due to this challenge we sought out a better alternative which is motion. In

essence the motion code will detect a difference through low resolution image analysis and

move the stepper motors accordingly to keep track of the celestial object.

Telescopes are often placed on expensive equatorial mounts, with one of its perpendicular axes

pointed at the celestial pole (near Polaris in the Little Dipper), driven by a “clock” motor

rotating one revolution every Siderial day (23h54min). Such a mount accurately tracks celestial

objects anywhere in the sky, but the angles of the axes are awkward, large counterweights are

necessary to balance the telescope, and achieving rigidity makes them heavy and expensive –

especially for large aperture telescopes. In the 1960’s John Dobson invented a vastly simpler

alt-azimuthal telescope mount providing Newtonian telescopes up to 36” with unprecedented

portability. These large telescopes require constant tracking on both axes, with the rate of

movement dependent on where in sky the telescope is pointed. Automatic tracking thus requires

considerable complexity -- the telescope must know its latitude and angles from the pole and

level to calculate each motor’s speed with trigonometry. This calibration typically takes 10-15

minutes on a Meade LX200GPS telescope. Given the low cost of the Raspberry Pi and camera,

we sought to test whether it could clock drive a Dobsonian. In particular, could image drift of a

bright star be fed back through the computer’s GPIO ports to altitude and azimuth motors

thereby holding a stable image, without requiring telescope calibration for level and north?

Moon Using Meade 14in LX200