Below are the most common accessories used by beginning CCD imagers to make life under the stars a little easier.
Dew Shields and Heaters
Moisture forming on the front lens of a Schmidt-Cassegrain telescope can easily ruin a CCD imaging session (or even visual observation, for that matter). A dew shield fits onto the front of the telescope preventing dew from forming on the lens. Even in a dry climate dew shields can be handy. They work well as lens shades to block stray light and they can help balance a rear-heavy telescope. For HyperStar imaging, with the camera at the front of the telescope, Astrozap makes notched dew shields that allow cables from the camera to easily be routed out of the shield.
If the moisture level is high enough, even the dew shield will not be enough and a dew heater is required. A dew heater consists of a strip that wraps around the outside of the scope and heats the corrector plate to just above ambient temperature, preventing dew from forming.
Speed and field of view are critical to CCD imaging. Since the objects being imaged are faint, exposure times can be long, and any way of shortening them is always a benefit. Also, many of the objects being imaged are large and a wider field of view is a definite advantage. Focal reducers are lenses which shorten the focal length of a telescope and at the same time decrease the focal ratio. A smaller focal ratio mean shorter exposures and faster imaging.
Focal reducers are usually rated by a compression factor, usually something like 0.63x. This means that the focal length and focal ratio are shortened to 63% of their original values. Often, if intended for a specific telescope, this value is simply given as the resulting focal ratio. For example, since most Schmidt-Cassegrain telescopes have a focal ratio of f/10, 0.63x reducers are called f/6.3 reducers. However, this reducer used on an f/11 telescope will give an f/7 focal ratio.
Most Schmidt-Cassegrain telescopes have options for 0.63x or 0.33x (f/6.3 and f/3.3) focal reducers. Many refractors and other types of telescopes have 0.75x reducers available. Other types of reducers are available, 0.5x and 0.4x being other common values for compression factors. The ultimate in focal reduction for a Schmidt-Cassegrain telescope is Starizona’s HyperStar lens, which converts the telescope from f/10 to f/2, making the telescope 25 times faster photographically! The field of view also becomes 5 times wider than the native telescope.
Above: Focal reducers come in a variety of types. Some thread into the nose of a CCD, some attach to the rear of a scope, and some even replace the secondary mirror on a SCT.
Focusing is one of the trickiest tasks facing a CCD imager, but it need not be too difficult. The use of a motorized focuser is often helpful in that it eliminates the vibration induced by the imager’s hand touching the telescope. Also, since you need to watch the computer screen during the focusing process, it is sometimes helpful to be a few feet away from the telescope; motorized focusers allow you to be some distance away from the telescope, watching the computer screen and focusing without having to run back and forth from computer to scope between each focus adjustment.
For the ultimate in ease of use, an autofocuser will do all the work for you. Autofocusers interface with the computer and use the output from the CCD camera to determine the best focus point. They automatically drive the motorized focuser and achieve precise focus faster than could be done by hand. Often, autofocusers will incorporate features such as a digital readout for repeatable focusing using different filters or optical setups, and temperature compensating software that will change focus based on the known expansion or contraction of the optical system during the course of the night.
Above: Control box for Starizona’s MicroTouch autofocuser.
Polar Alignment Scopes
CCD imaging requires accurate polar alignment to eliminate field rotation and to minimize tracking errors. The easiest way to get a precise alignment is to start by using a polar alignment scope. This is a device which gives the offset from Polaris to true celestial north (there is a difference of about 3/4 of a degree). How the device works exactly will depend of the type of telescope you have. Most German-equatorial mounted telescopes have a polar scope which aims through the polar (right ascension) axis of the mount. Fork-mounted telescopes have polar scopes which attach to one of the fork arms of the mount. Some telescopes have finderscopes which are integrated polar scopes.
Most computerized telescopes have polar alignment routines built into their software which allow for easy polar alignment as well. In some cases this eliminates the need for a polar scope. But using a polar scope gets you much closer to begin with, making the computer routine more precise.
Software and Apps
Planetarium programs like TheSkyX or Starry Night allow you to see exactly what objects are where at any given time, from any given location. Information such as brightness and size are right at your fingertips. Also, field of view (FOV) indicators are available in some programs which display a rectangle showing exactly what the CCD camera will pick up, making framing objects a simple task. This is essential when using an off-axis guider in order to find an appropriate guide star. Controlling computerized telescopes is also a possibility.
Above: Field of view indicator in TheSkyX software. This is set up to show the field of the main CCD, the guide camera, and an annulus showing possible guide stars that could be used by rotating the camera.
Tip: If you are using a German-equatorial mount, crossing the meridian while imaging an object can be a problem. Software programs can tell you exactly when an object will cross the meridian. By knowing this you can easily determine how long you have to image an object, or if you have enough time to obtain your desired exposure.
A very important step in setting up a telescope for CCD imaging which is often overlooked is tying off loose cables. When all is said and done and your telescope is ready to image there will be a vast multitude, a plethora, an array, a congregation of cables hanging off it. It will resemble an accurate rendering of the human vascular system, a scale model of the Amazon River’s tributaries, or a map of the L.A. freeway system…. We’re talkin’ about a lot of cables. And it is essential to tie them off properly in order to prevent tracking problems. This is the number-two source for tracking problems (right after not balancing properly).
Velcro cable ties, plastic cable routing hooks and cable tubing are all useful ways of keeping cables where they belong. The important thing is to keep them from catching on knobs and other protrusions from the mount. Watch your cables as the scope slews all the way across the sky and make sure they won’t hook on the counterweight shaft of a German equatorial mount, etc.
Above: There are 7 cables attached to this CCD camera. In a typical imaging setup there might also be cords for motorized focusers, dew heaters, telescope interface, and additional accessories. These cables are all potentially image-ruining if not properly tied off.
Above: Cables tied off to the telescope mount with Velcro straps.
Above: Velco–the perfect way to tie off cables. $7 worth of Velcro can mean the difference between good and bad pictures with $10,000 worth of imaging equipment.