This page is being updated to reflect the current changes and upgrades in my setup. I am now using other alternatives such as a GPUSB or an Arduino as a means to establish communication between the telescope and the mount since the parallel port (the port used in the original version of this DIY) is now becoming obsolete.
Very long exposures requiring precise tracking needed for imaging deep-sky objects can be achieved through an advanced imaging technique called autoguiding. This article provides a brief introduction and how to build yourself an autoguider setup that delivers satisfying results.
Guided imaging simply involves active monitoring of the telescope’s tracking accuracy by observing a reference object (in this case, 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 this configuration is a setup involving an imaging telescope equipped with a finder scope on a tracking mount. Once an object to be imaged has been properly framed and focused, an 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 finder scope now performs the task of a guidescope. The idea is that, for as long as the guidescope’s crosshair is centered on to the guidestar, the telescope is tracking properly. Better sensitivity to tracking errors may be achieved through the use of more powerful (longer) dedicated guidescopes.
To achieve precise tracking, two conditions must be met: (1) a telescope’s mount must be perfectly polar-aligned, and (2) the telescopes is tracking at the correct rate, that is, the motor is rotating neither too fast nor too slow. If at least one of these two conditions is not satisfied, the guidestar would tend to move slowly away from its original position as viewed from the guidescope. This error in tracking is what is called drift.
Note that the north-south drift is corrected through proper polar alignment, while the east-west drift is corrected through guiding. East-west drift is inherent to all telescope mounts (even in most well-made and expensive ones) since unavoidable imperfections in the fabrication of the mechanical moving parts of the tracker, particularly its gears, may cause it to occasionally move faster or slower.
Guiding is used to be done manually, but in recent years, astrophotographers have developed a means of doing the guiding operation automatically, thus the term autoguiding. Instead of a person manually adjusting a hand controller to minimize drift (or keep a target centered), the process is done automatically by a computer. It is done by sending real-time images of the guidestar by means of a webcam mounted onto the guidescope, and have the computer analyze it for any signs of drift. The computer then sends appropriate signals to the mount if it detects that the tracking is a bit too fast or too slow. Basically what the computer does is to compare each and every photo of the guidestar to determine if it drifts, and then tries to compensate by adjusting the motor’s speed, through cables running from the computer to the mount. A typical autoguider setup with the key components is shown below:
The DIY Autoguider
This guide will help you interconnect the key components and devise a means for a computer to control your mount. It is also assumed that you already have the following:
- an imaging telescope
- an imaging camera (DSLR)
- a tracking mount
- a guidescope (a smaller telescope, about 1/3 of the length of the imaging scope)
- a guide camera (webcam)
- a means to piggy-back the guidescope to the imaging telescope (e.g., guide rings)
- a laptop computer
- a USB Guide Port interface (GPUSB) or a similar device (such as a DIY Arduino controller).
- It is assumed that you already have a working imaging setup: an imaging telescope with an imaging camera (DSLR) on a tracking mount. Check if your mount has an autoguider port. This port will make everything a lot easy for you as this port allows a computer to readily communicate with your mount. If it has an autoguider port, look for the pins for the RA+, RA-, and Ground. If your mount has no autoguider port, then you may need to resort on electronically tapping on to the left and right arrow keys in order for a computer to ‘talk’ to your mount–don’t worry, it is not difficult to do.
- Find a suitable guidescope with a focal length at least 1/3 of the main scope’s focal length (e.g., for an imaging scope with 500 mm focal length, the guidescope must have a focal length of at least 150 mm). The guidescope must be piggybacked on to the main imaging telescope (or placed side-by-side) in a manner that allows it to point up to some degree, to a direction slightly different to the main imaging scope, thereby searching for a nearby guidestar would not be much of a problem later on. This may be done easily by using mounting rings (like in the photo above). You also need a webcam and an appropriate adapter to attach it to your telescope. Any webcam will work, but some types of webcams work better than others. Some may even be modified (e.g. Logitech 4000) to detect even fainter guidestars. For now, just look for any webcam (preferably with a CCD sensor and not CMOS). Test your guidescope and guide camera and make sure that it could actually detect at least 3rd guidestars.
- You will need a GPUSB, a device that allows a computer to send guiding signals to a telescope mount. It has a cable that connects to a computer (through the USB port) and another cable to connect it to the mount (through what is called an ST-4 port). With a GPUSB, guiding software such as PHD2 or GuideMaster can now send commands to the mount, to move the telescope in any of the four directions: North (Dec+), South (Dec-), East (RA+), or West (RA-) during autoguiding operation. If you are comfortable working with microcontrollers, you can actually use an Arduino to establish communication between the mount and the telescope (the Arduino replaces the GPUSB–I will post a separate article on how to build one). In this tutotial, we will only deal with the East-West (RA) guiding, since the the North-South (Dec) error can be most effectively addressed through proper polar alignment.
Are you now ready to setup your own autoguider? Detailed instructions on how to interconnect the key components and the use of computer to control your mount will be discussed in my next post. If you have questions, feel free to leave a comment. Clear skies!
UPDATED (January 2016): This post has been updated to eliminate the need for a parallel port, which is now becoming obsolete.
© Anthony Urbano (Manila, Philippines)