While this seems like a straightforward procedure, there are some tricks to assembling all the imaging accessories which will help you avoid problems during an imaging session.
Mounting the CCD Camera
There are quite a few different ways to attach a CCD camera to a telescope. It mostly depends on what you wish to image, but also on the type of CCD you have and the type of telescope being used. For deep-sky imaging, the usual procedure is to mount the CCD at the prime focus location, where an eyepiece would usually be placed (the rear of a refractor or Cassegrain, and in the focuser at the front of a Newtonian). Often a focal-reducer is used to shorten the exposure time and widen the field of view. For planetary and lunar imaging, more magnification is desired, so a Barlow lens or eyepiece projection adapter is used. For extremely wide-field imaging, a CCD may be mounted piggyback, riding atop the telescope with a camera lens attached. Celestron’s Fastar telescopes offer another option by allowing the CCD to be attached in place of the secondary mirror for an ultra-fast, wide-field imaging system.
No matter where you are mounting the camera, there are a few important tips which are universal:
It can be helpful to keep the CCD camera square to the axes of the telescope mount. This makes doing the initial mount alignment easier.
Always keep track of the cables. There can be a lot of wires running from an imaging setup. CCD cameras, focus motors, dew heaters, telescope control cables, computer control link, plus all of the associated power cords can make for a mess! Be sure everything is free from tangling. It is critical to tie the cables off to avoid extra weight on the telescope and mount while making sure there is enough slack in the wires for the telescope to freely point around the sky.
Make sure the camera is attached snugly. Even the tiniest amounts of play between camera and scope can result in flexure which can ruin an image. This especially becomes a problem as you add more and more accessories. If your CCD weighs 5 pounds and is attached to a color wheel, adaptive optics unit, focal reducer, Crayford focuser, and all associated adapters and extenders, there are a lot of opportunities for flexure to arise. This type of system can work, but be sure that everything is snug. That said, don’t over tighten things as they may be hard to get apart later, especially if the temperature drops and the metal shrinks.
Tip: Some focusers have only one set screw for holding a CCD camera in place. Flexure can arise even if this screw is tight. A pair of screws offset at about a 120-degree angle, or a clamp ring system, is a much better choice and will avoid flexure problems.
One of the most common CCD imaging accessories is a focal reducer. This is because one of the most important factors in deep-sky CCD imaging is focal ratio. A faster (smaller) focal ratio yields shorter exposure times, which is always a plus. Take for example the popular Schmidt-Cassegrain telescope. This is an extremely versatile type of telescope, and while its f/10 focal ratio is ideal for viewing, a faster focal ratio is usually desired for imaging. A focal reducer provides a wider field of view and a faster imaging system. Some focal reducers have a specific “backfocus distance,” the spacing between the reducer lens and the CCD sensor. Make sure the adapters you have allow you to achieve this ideal spacing.
Most SCTs have focal reducers which thread on to the rear cell of the telescope and convert them to f/6.3 or f/3.3. These reducers are not always directly adaptable to the 2″ nosepieces on some CCDs. Also, in the case of a telescope using a Crayford-style focuser — which is also a common CCD imaging accessory — the reducer is not usually then threaded onto the telescope.
The best solution is to use a 2″ camera adapter, intended for mounting a 35mm camera to a 2″ focuser. By taking the camera adapter apart, as shown below, and placing the focal reducer in between the two halves, a simple adapter can be made to attach the CCD to the focuser.
Above: A 2″ prime focus camera adapter (left) and an f/6.3 focal reducer
Above: By taking the camera adapter apart and mounting the focal reducer between the two halves, the reducer can be made to fit to a 2″ focuser
Again, it is important to try to avoid flexure with this system since the camera is now much farther back from the rear of the telescope.