If you have not already, please read the section on Basic CCD Accessories for some of the most common items used to make imaging easier. Below are some of the best accessories for advanced imaging.
If your CCD camera does not have built-in self-guiding (as do some of the SBIG and Starlight Xpress cameras), you will need a separate system for guiding the telescope in order to achieve longer exposures. Either a stand-alone autoguider or a second CCD camera may be used. Imaging is done with one CCD while the other guides the telescope. This has become much easier recently with the advent of inexpensive webcams and video systems that are capable of autoguiding.
In the past, especially with film photography, off-axis guiding was often necessary with many types of telescopes, but with the shorter exposures of modern CCD cameras and fast telescope systems it is possible to use separate guidescopes (see below) with most instruments.
These are useful for polar alignment, aligning a computerized telescope, and manually guiding long exposures. There are a variety available, but in general, a simple double-crosshair is the best choice.
Above: View through a typical double-crosshair eyepiece.
Crosshair eyepieces can be used to perform Declination Drift Alignment to achieve accurate polar alignment for long-exposure CCD imaging. See the section on Polar Alignment for more details.
The pointing accuracy of a computerized telescope depends on the accuracy with which you center the alignment stars in the eyepiece of the telescope. For CCD imaging, especially with a small CCD chip or with long focal-length telescope, precise pointing is critical. Using a crosshair eyepiece can aid in centering the alignment stars. For aligning a computerized scope, and for most polar alignment purposes, eyepieces where the position of the crosshairs is adjustable should be avoided.
Guiding can be done using a crosshair eyepiece in conjunction with either an off-axis guider or guidescope (see below). Often, however, the ease of use of an autoguider (above) is more advantageous, especially with the inexpensive systems now available.
For advanced imagers, a variety of specialized focusers are available. Most of these units are for use on Schmidt-Cassegrain telescopes, one of the most popular styles of telescope for CCD imaging, but others are available for refractors and Newtonians as well. Usually accessory focusers improve the ability to make fine focus adjustments or eliminate mirror shift from SCT scopes. On SCT scopes, it is possible to replace the original focus knob with a more sophisticated system (the Feathertouch SCT MicroFocuser), or to attach a Crayford style focuser (pictured above) to the rear cell of the focuser. Adding an external focuser to an SCT eliminates mirror shift, which can be an advantage especially at higher magnifications (i.e., planetary imaging).
Another accessory that is extremely helpful is a motorized autofocuser, such as the MicroTouch Wireless Autofocuser. Autofocusers integrate with CCD imaging software to automatically achieve pinpoint focus without user intervention. In addition to being easier than focusing manually, autofocusing can be more precise. Some autofocusers will also compensate for temperature changes during the night, allowing completely unattended imaging.
Guiding long exposures can be done several different ways. Perhaps the easiest is with a self-guiding CCD camera, such as those manufactured by SBIG. But many CCD cameras are not self-guiding and are therefore limited to short, unguided exposures unless another means of guiding can be found. The easiest way to guide a telescope is by using a separate guidescope. This is usually a small refractor mounted on top of the main telescope. Guidescopes are normally mounted using adjustable rings on a dovetail plate. The rings allow the guidescope to be pointed independently of the main telescope to facilitate the finding of guide stars, and the dovetail allows the guidescope to be balanced and easily removed for transport.
The advantage to guidescopes is that they are an order of magnitude easier to use than off-axis guiders (see below). However, the disadvantage is the possibility of flexure. Flexure occurs when the guidescope and main scope move slightly relative to one another; the amount of movement needed to ruin an image is very slight. Flexure is not normally a problem when the main scope is a refractor. Using a guidescope with an SCT is usually not recommended because the mirror in the SCT can move relative to the guidescope, but in truth, a surprising number of people have had success using guidescopes with SCTs, especially with the shorter exposures required with CCD imaging.
Tip: Flexure often occurs as a result of the autoguider shifting in the focuser of the guidescope. If there is only a single set screw holding the autoguider into the focuser of the guidescope, try drilling and tapping for a second set screw about 1/3 of the way (120 degrees) around the outside of the focuser to keep the camera from shifting under its own weight during an exposure. Many high-end refractors include compressing rings inside the focuser tube to prevent this problem, but most of the inexpensive refractors used for guiding do not.
To guide an SCT or similar telescope without having to worry about flexure, an off-axis guider can be used. An off-axis guider uses a small prism to pick off some of the light from the telescope and sends it to the autoguider (or crosshair eyepiece for manual guiding). Since the off-axis guider is using the main telescope optics, any shift in the mirror of the telescope is recorded by both the imaging camera and the autoguider, so there is no differential flexure. The disadvantage is that finding a guide star, especially with a small imaging CCD chip, can be very difficult. Despite the potential advantage of off-axis guiding, most imagers use a guide scope because of the far greater ease of use.