An off-axis guider is a device that goes in the optical system just before the camera. It has a small pick-off mirror or prism that diverts some of the light from a star to an eyepiece or autoguider.

An autoguider is set up to work through an 80mm piggyback guidescope. It makes corrections to the mount's movements to adjust for inaccuracies in the gears and for atmospheric effects to produce precise tracking.

A piggyback guidescope is a second telescope that rides on top of the main scope. A star's position is monitored in it, and corrections are made when the star moves off of a cross-hair in the eyepiece to bring it back to its exact starting position.

Off-axis guiders work very well because they use the light from a star being imaged by the main telescope that is taking the picture. This automatically corrects for problems such as flexure in the imaging system, as well as any possible mirror slop or movement. But an off-axis guider can be difficult to use because sometimes it is hard to find a guidestar at a convenient position in the field being imaged.

Piggyback guidescopes are much easier to use, but such imaging systems can suffer from flexure, or movement, between the guidescope and the imaging scope, especially if one of the scopes has a mirror in it.

Guiding allows for longer exposures and corrects for errors in tracking caused by periodic error and atmospheric effects. Guiding will not correct for errors in polar alignment. A scope can be accurately guided when the polar alignment is not correct, but the resulting image will suffer from field rotation around the guide star.

Manual Guiding and Autoguiding

An autoguider uses a small CCD chip to take a picture of the star's location, and then another picture, one after another, and then calculates how much the star has moved between exposures and how much mount needs to be moved to bring the star back to its original position. Autoguiders are much more accurate than manual guiding, but require the additional cost of a guidescope, rings, guiding eyepiece, autoguider, computer, and additional power necessary to run them.

Some autoguiders have a computer built in to them so they don't need a separate computer to work in the field. These are called stand-alone autoguiders, and they are more expensive than an autoguider that uses a separate computer. The cost of an autoguider and laptop to run it are comparable to the cost of a stand-alone autoguider.

A less expensive method of guiding is to do it manually, but it is difficult and tedious. In this method, a guidestar is placed on the cross-hairs of a high-power guiding eyepiece, and when it begins to drift off the exact center of the cross-hairs, it is moved back manually by hand by pressing the correct direction button on the hand controller of the mount.

Guiding and Autoguiding - The Bottom Line Tracking means to move the telescope via the mount to compensate for the Earth's rotation. Guiding means to add corrections to tracking to correct for inaccuracies in the drive gears and other atmospheric effects. Manual guiding is done by eye and hand. Autoguiding uses a computer to issue these corrections. Once you get into advanced astrophotography at longer focal lengths, you will want to guide your photos. Autoguiding relieves the astrophotographer of the boredom and tediousness of manual guiding.