In the first article in this short series on video astronomy, you learned about the capabilities of the latest generation of astronomy video cameras. In the second installment, you saw the basics of how to connect one of these little cameras and how to match it to a telescope and mount. In this third installment, you look at the key settings for an astronomy video camera, and you get a few tips to help you take your first image.
A look at the globular star cluster M71 in Sagitta and the planetary nebula M27 in Vulpecula using a Mallincam Extreme astronomical video camera
The settings of these video astronomy cameras are usually controlled in one of three ways. Most camera have 4-5 buttons on the back which allow you to call up a menu on the video screen and configure the many settings directly. Some cameras will work with an optional wired or wireless remote controller so you can configure settings and initiate imaging without touching the camera itself. Again, you see the settings pop up on the video screen. And some cameras can be controlled through the USB interface of a computer with a specialized program that runs on a PC (or in some cases, a Mac). It may come with the camera, or it may be installed from a third-party website as free or paid software. It depends on the camera.
When you first start using a video cam it’s a little intimidating at first, and some of the terminology is quite different than DSLR cameras, for example. Cameras from each manufacturer are a little different, so you need to consult the manual. But there are four main types of setting to consider. The terminology is slightly different for each camera, so it’s not possible to give you a user manual here, but these are what you need to look for.
Automatic Light Control. Most cameras let you set “Automatic Light Control”, or ALC, which is something like a shutter control or exposure setting. For the Moon, filtered Sun, or for terrestrial objects, you turn on ALC and choose a shutter speed of 1/1000 of a second or 1/100 of a second, for example. At such speeds, you see a continual stream of images on your monitor at a video rate of 15 or 30 frames per second, with each image captured at the set shutter speed. If you need to detect fainter objects such as a star or bright DSO, then you turn ALC off and use a “SENSE UP” feature which adds (or integrates) multiple video frames (from 2 to 128 frames) together into a single image. This gets you integration times of up to a few seconds, which is enough to give you a view similar to what you see visually through an eyepiece. Again, the view continually updates itself and sends a stream of images to the monitor.
To get the really faint stuff, you turn the shutter off, set the SENSE UP to maximum, and turn on the electronic gain of the camera using the Automatic Gain Control (AGC).
Gain. Gain, which is set by AGC, is similar to ISO on a DSLR camera. A higher value of AGC gets you more sensitivity and short integration times at the expense of more image noise. The noise looks like a graininess or false flecks of color in the image. For most faint objects, you choose an AGC of 2 to 4, though some cameras go as high as AGC of 8. In any case, keep gain as low as possible to get a good image. When using gain for fainter DSO’s you can set the integration time from a few seconds to many tens of minutes (depending on the camera). Higher gain and longer exposures also lead to more “amplifier glow” which appears as a grey-green glow in one corner of the image. It comes from the heat generated by an on-chip amplifier. It is inherent in all video cameras.
Image Processing. Most cameras allow on-camera control of brightness, contrast, white balance, “gamma” which is a form of luminance, sharpness, and so on. There is room for you to experiment with these settings to help you get the best image for a particular celestial object.
Cooling. Higher-end video cameras include electronic coolers on the chip to reduce noise. This is especially important if you use the camera on warm nights. Some cameras give you a little control over how aggressively you cool the chip, which has an effect on the image quality and the power consumption of the camera.
Let’s put it all together and review the general steps you need to take to set up for a night of video astronomy:
- Set up and align the telescope and mount with an eyepiece. As you get more adept with the camera, you can try aligning using the camera itself. But at first, use an eyepiece to align the mount to the celestial poles and the go-to computer to the current sky configuration.
- Once aligned, replace the eyepiece with the camera. Power-up the camera and connect the video and control cables to the camera and the video monitor or computer. If you are using a computer to capture the images, you’ll need extra hardware (called a frame grabber) to convert analog video to digital information.
- Use your focuser to bring the camera into focus on bright star. The easiest way to do this is to use a Bahtinov mask, a full-aperture mask with a pattern cut into it. Put the mask over the front of your telescope, set the camera for integration times of a few seconds or less, and adjust the focuser until the diffraction pattern of the star on the screen looks right (see image below).
If you don’t have a Bahtinov mask, focus on a bright star or better yet, a bright cluster of stars. As you come toward focus, look for dimmer stars to appear in the field. A good indication that you are close to focus is when no more dim stars appear, but none disappear either.
Now you’re ready to point the telescope at a bright DSO. Start with a bright star cluster or nebula to make sure everything works. Or try the Moon with the appropriate camera settings. Once you get good images, you can move to fainter objects.
Using an astronomical video camera sounds easy, and it usually is. But it takes practice to learn each camera and telescope configuration and to get things all set up and aligned. If you can get an image of a single DSO on your first night, you have done well.