In the first article in this series on deep-sky video astronomy, you learned about the capabilities and trade-offs of astro video cameras. These little devices effectively triple the aperture of a telescope, and provide electronically-assisted viewing of deep-sky objects for those who have trouble looking through an eyepiece, for urban stargazers who battle light pollution, or for those who wish to take color ‘snapshots’ through their telescope but who don’t want to fuss with the intricacies of astrophotography. Because the images from video cameras can be displayed on video monitors, video astronomy is also a great way to share images with large groups of people at public stargazing events. In this article, you get a look at the basics of how to connect an astronomical video camera. And you find out which telescopes and mounts work best with these little devices…
Connecting an Astronomical Video Camera
Astro videocams come with a detachable 1.25” nosepiece that slides into the focuser of your telescope. Take out the eyepiece, slide in the camera, lock it down, and that’s it.
Then it’s time to connect the cables. The camera needs power, and most cameras come with a cable to connect to a 12 VDC battery for field use or with an adapter to plug into AC wall current in your backyard or observatory. Plug in the camera and it turns on.
You also need a cable to carry the video signal to a monitor. At the back of the camera, there will be one or two analog video outputs such as “composite” or “S-video” to which you attach cables that run from the camera to a video monitor or TV. Some cameras have just one video output, and some have both. Plug one end of the cable into the camera, the other end into a video monitor, and you’re ready to go.
To view the camera output, you can use an old CRT video monitor, for example, though these are bulky and require AC power. Stargazers with access to AC and permanent observing set-ups have favored security monitors from Speco, for example, for video astronomy. Newer LCD monitors, which are more compact and can operate from batteries, work better for field use. Small 7” monitors from Benq, Haier, and Lilliput can work. Just make sure they have an input for S-video or composite to match the output of your camera.
You can also use an HD television with a video cam, but an additional converter is required to convert the video from the camera to an HDTV signal. Remember… the output from the camera is low resolution 640×480, so don’t expect Blu-ray quality video.
If you’re not inclined to use a monitor, you can take the video output and pass it through a “frame grabber” which converts the video into a digital signal that you can display on a computer. Such devices are available commercially and are often used for digitizing VHS and other old-style analog video. You can also get specialized frame grabbers intended especially for video astronomy. More about these and other accessories in a future article.
Once you connect power and a monitor to a camera, you have something like in the image below…
You may also need a third cable to control the camera using a hand-held controller or with a computer that runs a custom application made for the camera. But if you don’t want to haul a computer outside, some cameras have built-in controllers you can set with buttons on the back and which display the control menu on the monitor. We’ll cover camera settings in the next article.
(Images above courtesy of J. Huerkamp at Mallincam USA)
Telescopes and Mounts for Video Astronomy
And of course you need a telescope. Which telescope works best with the current crop of astronomical video cameras?
To keep a long story short, for most commonly-observed DSO’s, telescope with a focal length of about 400 mm to 1000 mm works very well with the small sensors in these cameras. If you have a scope with a longer focal length, such as an 8” f/10 Schmidt-Cassegrain, you can acquire a small lens called a focal reducer which mounts between the focuser and the camera. The lens effectively reduces the focal length of the telescope by a factor of 2x or 3x. This also reduces the f-number of the telescope by the same amount which results in brighter images of extended objects and faster imaging time.
You also need to make sure your telescope will focus an image with the camera. This is not usually a problem with Mak-Cass and Schmidt-Cass telescopes. Refractors usually have no problem focusing also, though sometimes a small and inexpensive extension tube is required to position the camera a little further away. Newtonians can be a problem. Without custom modification, many Newtonians do not have enough travel in the focuser to enable any type of video camera to come to a focus.
For newcomers to video astronomy, perhaps the best telescope is a small ED refractor with a focal ratio of f/6 to f/7 and an aperture of 80 mm to 90 mm. With an astro videocam, these small scopes will deliver excellent images of celestial objects with medium to large apparent size. Depending on your setup, you will get fields of view with a small refractor of about 0.3o to 0.8o This allows you to frame large and colorful emission nebulae like M42, M8, and M20, many open and globular star clusters, larger galaxies like the M31, M81 and M82, NGC 4565 and so on, larger planetary nebulae like M27, and of course the Moon. You will see all these objects and hundreds more with much more detail than you will ever see in an eyepiece.
Images of smaller planetary nebulae and more distant galaxies are a little too small with such short focal lengths. That’s when you want a larger telescope with longer focal lengths, though this will lead to very small fields-of-view which makes it trickier to find and track objects. In a perfect world, for both video astronomy and visual observing, you would have an 80 mm ED refractor and an 8” to 10” SCT and one or two focal reducers to keep the focal ratio low.
You will also need a motorized mount to track the sky when taking images of deep-sky objects with a videocam. Even for exposures of a few seconds, the Earth’s rotation will smear the image if the telescope mount is not tracking the stars.
But you don’t necessarily need an equatorial mount. You can track objects for up to 30 seconds with a motorized altazimuth mount with good results. In some cases, you can get away with tracking for 60 seconds. At that point, you notice the stars begin to elongate because of “field rotation”, the slight apparent rotation of the stars as they move across the sky. An equatorial tracks this naturally. An alt-az does not. But you can image a great many objects with a 30-60 second exposure on an astro videocam, especially if you have dark sky and do not require a light pollution filter to reduce the effects of a brightened sky background. Many experienced video astronomers use telescopes of 3” to 16” aperture only with motorized alt-azimuth mounts and they get very good results.
If you want to go after fainter objects, or if you need to use a light-pollution filter, you will often need longer exposures. In such cases, a solid equatorial mount with a motor drive will give you far sharper images. Also… with alt-az or equatorial mounts, “go-to’ control is a big advantage to help you find objects with the narrow field of view of most videocams.
In the next article in this series, you’ll get a look at the basic steps to control and configure a video camera and start taking images…