About a year ago (November 2011), I started constructing a home-built autoguider, a setup astrophotographers use in imaging galaxies, nebula, and many other deep-space stuff. The setup is no different from what is used by observatories world wide, except that this one was built entirely from scratch. Feel free to browse the details of the project here.
This part of the DIY guide focuses on the actual guiding operation and the drift-alignment method for precise polar alignment. We begin by first assembling the telescope along with the guidescope. We also attach the imaging and the guiding cameras and connect all the necessary cables leading to and from the computer.
Now that we have already devised a simple contraption that allows a computer to convert guiding commands into light pulses, our next task is to devise a way for a telescope mount to “read” these pulses and translate it into actual east-west movement. This part of the DIY guide will describe the wiring diagrams that will enable any computer to talk to any type of telescope mount (i.e., with or without an autoguider port).
Guiding commands from the computer are sent through a port called ‘LPT1‘, or the parallel port (or sometimes called printer port). It is a kind of interface that allows a simple way for a computer to communicate with other devices. We will try to convert these ‘commands‘ into a form that can be easily interpreted by your telescope mount. The simplest way to do that is to convert the commands into light pulses using Light-Emitting Diodes (or LEDs). These light pulses in turn will be used to drive what is called a ‘light activated switch‘ that we will connect directly to the autoguider port or hand controller. In this DIY guide, we will focus first on how a computer (with the use of the guiding software called GuideMaster) can generate light pulses, by connecting LEDs to the computer’s parallel port.
The parallel port is mounted on a socket called DB25F(F stands for ‘female socket’) or DB25M(M stands for ‘male socket’). It has 25 pins (1 to 13 top row, 14 to 25 bottom row). For this project, we are only interested in pins 4, 5, and 25 (other pins will be utilized however in future upgrades). Shown below is a photo of my laptop’s parallel port.
Very long exposures requiring precise tracking needed for imaging deep-sky objects may now be achieved through an advanced imaging technique called autoguiding. This article provides a brief introduction and how one could construct a do-it-yourself guider that delivers equally satisfying results for a fraction of the cost of ultra expensive commercially available counterpart.
Guided imaging simply involves active monitoring of the telescope’s tracking accuracy by observing a reference object (any bright star) and making the necessary adjustments to nudge the telescope to the east or to the west so that the reference object remains stationary for the whole duration of an exposure. The simplest example is a setup involving an imaging telescope with (equipped with a finderscope) on a tracking mount. After the object to be imaged has been properly framed and focused, the imager adjusts the finderscope and centers its cross hair to the brightest star in its field. This bright star now serves as the reference object called the guide star and the finderscope now performs the task of a guidescope. The idea is that, for as long as the guidescope’s crosshair is centered on the guidestar, the imager knows that the telescope is tracking properly. To achieve better sensitivity to drift, more powerful dedicated guidescopes may be used.