Focusing a telescope seems like such a simple thing that, at first, it's hard to believe there is such a multitude of accessories to make focusing easier, faster, and more precise. There are entire companies that exist solely to build focusers for telescopes! Additional focusing accessories can be as simple as an improved focus knob and as complex as a wireless, temperature-compensating, autofocuser for CCD imaging.
Why Improve a Focuser?
There are several reasons to consider upgrading the focusing system on a telescope. There are two basic types of focusing systems on telescopes. Most Newtonian and refracting telescopes have rack-and-pinion focusers with a drawtube that slides in and out when the focus knobs are rotated. Most Schmidt-Cassegrain and Maksutov-Cassegrain telescopes have internal focusing systems in which the primary mirror moves when the focus knob is rotated. Each type of focuser has advantages and disadvantages.
The rack-and-pinion focuser is a very simple design, and it works well for most applications. However, inexpensive rack-and-pinion focusers can suffer from stiff movement, excessive play, and backlash. Stiff movement can result from the gear and teeth of the rack-and-pinion system being too tight, or from a poor quality grease used to lubricate the system. The tightness can often be adjusted and the grease replaced. Excessive play results from a poor fit between the drawtube and outer walls of the focuser. This can sometimes be adjusted, but in some cases the excessive wobble in the focuser can cause the image in the eyepiece to shift as the focus is racked in and out. Achieving precise focus can be almost impossible.
Backlash is the most common problem with rack-and-pinion focusers. It is inherent in the design of the focuser, so even good quality focusers still suffer somewhat from this effect (although much less than poor quality focusers). Backlash occurs because of the necessary gaps between the teeth in the rack-and-pinion gears. If there was no gap, the gears would bind. Too much of a gap, though, and precise focusing is lost because the focus knobs have to be turned farther to cause any movement of the drawtube.
Moveable Primary Mirrors
Schmidt-Cassegrain and Maksutov-Cassegrain telescopes usually focus by moving the primary mirror. This has several advantages. First, it keeps the moving components inside the telescope, so there is no change in eyepiece position as there would be with an external rack-and-pinion focuser. Also, it allows for a considerable focus range allowing a wide variety of accessories to be used, as well as allowing very close focusing for terrestrial observing. Finally, the distance between the mirrors on a Cassegrain-type telescope are optimized for infinity focus. By changing the distance between the mirrors as the telescope is focused closer for terrestrial viewing, the optical quality remains high.
The main drawback to moveable primary mirrors is image shift. The primary mirror is held at its center on a baffle tube that runs about halfway up the length of the telescope tube. The focus knob is typically located off to the right side of the eyepiece. The focuser operates by turning a threaded rod which pushes the mirror forward or pulls it back. Since this threaded rod is located off to the side of the mirror, but the mirror is held in its center, the mirror tends to tilt slightly when the focus knob direction is reversed. There is also typically some backlash associated with revering directions. For most viewing, a small amount of image shift is tolerable. But for high-magnification observing or planetary imaging with a small camera such as a webcam, image shift can be problematic.
Above: A Crayford style focuser, Starlight Instrument's Feathertouch Focuser
A poor quality rack-and-pinion focuser can be improved by replacing it with a better quality rack-and-pinion, but most observers upgrade to aCrayford style focuser. Crayfords eliminate some of the problems associated with standard rack-and-pinion focusers by doing away with the rack and the pinion (seems like an obvious solution). Instead of turning a gear, the knobs on a Crayford turn a roller that is in contact with a flat plate on the bottom of the drawtube. This eliminates backlash since there are no gears and no play. On the top side of the drawtube there are often small roller bearings. This provides a very smooth motion to the focuser.
Crayford focusers are often added to refractors and Newtonians. They can also be added to Cassegrain-type telescopes to eliminate mirror shift. Instead of focusing by moving the primary mirror, the mirror stays fixed and the focus is achieved by racking the focuser drawtube in and out.
Above: An accessory focuser for an SCT, the Feathertouch SCT MicroFocuser
Another option for SCTs and similar scopes is to enhance the standard focuser. This does not eliminate mirror shift but minimizes backlash and improves focus accuracy. It is also cheaper than adding a Crayford focuser. The Feathertouch SCT MicroFocuser replaces the original focus knob and adds a 10:1 fine focus adjustment.
Above: A typical motorized focuser, Celestron's Motofocus.
Another possibility is to add a motorized focuser to a telescope. Motorized focusers allow hands-free focusing which eliminates any vibrations transmitted to the scope. They also make remote focusing possible when imaging the sky. Many focusers can have motors attached directly to them. Alternatively, some aftermarket focusers, including rack-and-pinion and Crayford types are already motorized.
Above: Automatic focuser, Starizona's MicroTouch Wireless Autofocuser
The ultimate in accessory focusers is an autofocuser. This is used for CCD imaging and allows a computer to automatically focus the telescope. It is more precise and faster than focusing manually. Some autofocusers also include digital readouts and temperature compensation that refocuses the telescope as the temperature changes, keeping a perfect focus during the course of a night.