An autoguider is a digital camera with a CCD or CMOS detector at the focus of the guidescope or, in the case of an off-axis guider, at the focus of the imaging instrument after some of its light has been diverted by a small prism.
The autoguider captures regular snapshots of the sky and a computer compares them to detect star drift caused by the apparent movement of stars across the night sky, issuing corrections to the mount via a cable attached to the autoguide port to keep the mount on track.
Why use an autoguider?
Today’s digital cameras possess awesome potential, empowering more people than ever to become astrophotographers.
But, while it’s easy to produce passable wide-field constellation portraits using nothing more than a DSLR, a fixed tripod and exposures of up to half a minute, capturing a detailed image of a typical deep-sky object is a bit more involved.
As a minimum, you’ll need to use much longer exposures and a lens with a focal length of at least 200mm.
Ideally, the camera needs to be attached to an equatorial mount that has been precisely polar aligned so that it accurately tracks the stars as they move against the night sky – otherwise you’ll see trailing in your images.
However, even the most expensive and precisely polar-aligned mount will exhibit unavoidable image trailing with long focal length lenses or long exposures.
For example, my 25-year-old Vixen GP mount is capable of unattended exposures with a 400mm lens – but only for exposures up to 3 minutes.
This limitation is due to the cumulative effects of minute imperfections in the drive system’s gears (termed ‘periodic error’), flexure in the camera and mount, and atmospheric refraction.
To maintain pin-point stars, we must resort to guiding the mount throughout the exposure.
Traditionally, astrophotographers would do this by carefully monitoring a star on the reticle of an illuminated eyepiece in a guidescope – a small scope mounted in parallel with the imaging instrument – making tiny drive corrections with a hand controller as and when required for the duration of the exposure.
Fortunately, today’s computerised equatorial mounts free us from such drudgery by possessing an autoguide port (sometimes called a CCD port) conforming to the de facto standard set by the Santa Barbara Instruments Group’s ST-4 autoguider nearly 25 years ago.
What’s in an autoguider?
An autoguiding system consists of the following parts:
- equatorial mount
- guidescope or off-axis guider
- autoguider (camera)
- guide software
- a link to the mount
The camera, computer and software are often integrated into one unit in standalone autoguiders such as the popular Celestron NexGuide or Sky-Watcher SynGuider.
Off-axis guiding has the advantage of eliminating a separate guidescope, thereby avoiding the potential for flexure and reducing load on the mount.
A popular alternative to a guidescope is a modified 50mm finder, particularly if your imaging scope has a focal length less than 1m.
If your budget doesn’t stretch to an all-in-one solution, you could use a cheap modified USB webcam as a guide camera.
Do bear in mind that both options require power, so a rechargeable 12V power pack capable of running all your equipment is essential.
Also, label your cables well, since you will be assembling and operating your equipment in the dark.
For extra help, head online – forums such as Stargazers Lounge and Cloudy Nights are frequented by experienced imagers willing to help to you create a working system that permits exposures of several minutes even with modest specification mounts – paving the way to capturing your perfect deep-sky image.
Can you autoguide an altaz mount?
While it is technically possible to autoguide some altaz mounts, you can still only synthesise long exposures, as you’ll need to use image editing software to combine a number of shorter exposures and remove the rotation between them.
Using a computerised equatorial Go-To mount with a built-in autoguiding port is much preferred.
As well as the autoguiding benefits, it will make locating target objects simpler, especially since many deep-sky subjects will be too faint to be seen visually.
Should I use a standalone autoguider?
Standalone autoguiders such as Celestron’s NexGuide or Sky-Watcher’s SynGuider (pictured above) combine a guide imager, computer and guide software into single hardware package, offering an attractive alternative to taking a laptop outside with you.
- Read our full SynGuider review
On the other hand, standalone systems typically possess small configuration screens and may require considerable tweaking to play nicely with your mount, though often require little intervention thereafter.
A self-built system employing a webcam, a GPUSB interface adaptor and a laptop running guiding software is potentially much easier to use – and cheaper, if you already own a laptop and a webcam.
Ade Ashford is an astronomer and science journalist. This article originally appeared in the November 2014 issue of BBC Sky at Night Magazine.