Currently, the best planetary images are being taken with webcams. The techniques that these inexpensive cameras allow make the capable of achieving better planetary images than even the most expensive CCD cameras. For details on using these cameras, visit the Webcam page. Many of the accessories listed here are useful for webcam imaging as well as CCD imaging of the planets.
Planetary Imaging with CCD Cameras
Because the planets are relatively bright (at least the ones which are interesting to image) and therefore require shorter exposure times than deep-sky objects, it seems that they should be pretty easy to image. Unfortunately, the opposite is true: imaging planets can be quite challenging. This is mainly due to the fact that the conditions must be just right to get the best images. Due to the small apparent size of the planets it is necessary to employ special equipment to effectively increase the magnification of the telescope. This also increases any distortions introduced by Earth’s atmosphere or by vibrations or tracking errors in the mount. Below is some of the equipment used by planetary CCD imagers. See the section on Planetary Imaging for tips on getting the best shots.
More Power, Scotty!
One of the biggest surprises to new amateur astronomers is that high magnifications are usually used to look at the relatively nearby planets, while the far-more-distant deep-sky objects actually require low magnification. When trying to image nebulas or galaxies, CCD imagers often try to get the widest field of view possible in order to capture the entire object. For planetary imaging we need to do exactly the opposite: we need more power!
What is needed is image scale. The planets appear very tiny and you will want to enlarge them enough to cover a sufficient portion of the CCD chip. There are a number of ways to increase the effective focal length of a telescope and thus enlarge the image scale. See the Planetary Imaging section for details on which setups are appropriate for different situations as well as tips on how to select the best match for your telescope and CCD.
Barlow & Friends
Placing a Barlow lens between the telescope and CCD camera is a good way to increase the power of your scope. These work just as they would for visual observing. Most Barlows double the effective focal length of a telescope, thus doubling the image scale. This works well for telescopes with fairly long focal lengths, such as 8″ or larger SCTs, and for a larger target like Jupiter. However, for small planets like Mars, or for scopes with shorter focal lengths such refractors or Newtonians, more magnification is required. 3x Barlows exist but many are of questionable quality (there are exceptions, of course, such as the TeleVue 3x Barlow). Also, there are 4x and 5x Powermates made by TeleVue which can significantly increase focal length on shorter scopes. Meade also makes a similar line of higher power Barlow-type lenses.
Eyepiece projection adapters allow you to shoot images through an eyepiece which increases the effective focal length of the telescope. There are both fixed and adjustable adapters. The adjustable ones allow the distance from the eyepiece to the CCD chip to be changed. This adjustment affects the image scale – the greater the distance between eyepiece and CCD, the greater the image scale. The effective focal length is also a function of the eyepiece being used, just as magnification is when observing visually.
Above: Eyepiece projection adapters work by holding an eyepiece between the telescope and CCD. Some adapters allow the user to adjust the distance from the eyepiece to the CCD chip which affects the magnification factor.
Above: An eyepiece projection adapter attached to a CCD camera. Note the original nosepiece from the CCD had been removed.
Determining Image Scale
Increasing focal length also changes the telescope’s focal ratio (affecting exposure times). Using the new focal ratio you can determine the pixel scale of your setup and the size of a given target in your image. (See the Field of View Calculator section.)
Using a Barlow or Powermate, you can simply multiply the telescope’s focal length and focal ratio by the magnification factor. For example, using a 2x Barlow on an 8″ f/10 SCT, the new focal length is 4060mm and the new focal ratio is f/20. Using a 4x Powermate with a 5″ f/6 refractor results in a focal length of 3120mm and a focal ratio of f/24.
Determining the magnification of an eyepiece projection adapter can be a bit trickier. You need to know the distance from the eyepiece to the CCD chip (or at least come up with a reasonably accurate estimate). Then using the following formula you can determine the magnification factor. Then multiply this by the focal length and ratio to determine the new values.
Magnification Factor = (Distance from Eyepiece to CCD / Eyepiece Focal Length) – 1
For example, using a 12mm eyepiece and an eyepiece projection adapter providing a distance from eyepiece to CCD of 90mm, the magnification factor is 6.5. Using this setup on an 8″ SCT gives a focal length of 13,200mm and a focal ratio of f/65. While this sounds extreme compared to typical telescope focal lengths, Jupiter would still fill just over half of a typical small CCD chip at this focal length!