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Video Astronomy
by
Craig Zerbe

 

What kind of video camera do I need?

Almost any video camera can be used for the Sun (with a proper filter), Moon, Venus, Mars, Jupiter and Saturn.

What video cameras generally work best?

Many surveillance cameras are designed for low light environments and are black and white. These cameras contain no tape recording hardware and must feed their signal to a monitor, VCR or camcorder. The kinds of surveillance cameras of interest for astronomy are those rated at less than 3 lux. A lux rating less than 0.05 lux would be best. Note: video cameras featuring a lux value of 0, indicating that they can record in total darkness, are of no use for astronomy. This feature is used in conjunction with an infrared light and has a range of about 10 feet. You need to know the lux rating of the camera under "normal" conditions. One problem encountered on most camcorders is the inability to control the exposure. If the camera exposure is too much or two little where is little you can do about it. Some of the security cameras or cameras refitted specifically for astronomy give the user manual control over exposure length and gain functions.  These are particularly helpful when imaging a planet against a dark sy, for an automatic exposure will try to compensate for the "black" sections, washing out the image of the planet.  Thus, I consider some method of exposure control to be fairly important for astronomy imaging.

Do I need a lens for the surveillance camera?

No, most people using a surveillance camera couple the camera directly to the telescope.

Do I need a special telescope for video astrophotography?

Almost any telescope will do, though one that has a tracking motor makes imaging easier. As the video chip generally covers a very small field of view, it will be necessary to constantly adjust the telescope to keep it on target. Larger telescopes have an advantage as well. Their greater light gathering ability will allow for more magnification and during a night of excellent seeing and quality optics, superior resolution. On an "average" night a scope with an aperture greater than 6" is unlikely to make an appreciable difference.

Should I use filters?

If you are using a refractor that is not an APO, using a green filter may help to minimize the chromatic aberration. Some wavelengths are less effected by atmospheric distortion that others. A red or infrared filter may help. Red can also be used to improve contrast. Filters can also be used when imaging the planets to bring out particular features. The disadvantage of filters is that they may dim the image so much that your camera is no longer able to properly expose the image.


How do I hold the video camera to the telescope?

Hold the video camera up to the eyepiece (afocal). This can be a little awkward and tiring. Is also can introduce image distortion and light loss if the light path is not kept perpendicular to the lens.
Set the video camera on a tripod and aim it into the eyepiece. Again, if the light path is not kept straight, this will produce image distortion and light loss.
One manufacturer, Stratton Video Brackets, sells a flexible but expensive bracket that you mount on your telescope. You can then to attach any video camera to it and then slide the video camera up/down and out/in to line it up with your eyepiece (see photograph at bottom of this page)



Some of the available surveillance cameras can be fitted with a 1 " nosepiece, which would allow you to slide it onto to the telescope instead of using an eyepiece (usually referred to as a prime focus method). Some of the cameras modified for astronomy purposes are sold with the 1 " nosepiece as standard equipment. The cameras modified for this purpose are usually a pound or less, so they do not constitute a balance issue for the telescope.
Some manufacturers also sell a sleeve that fits over an existing eyepiece (eyepiece projection) and then fit the camera's nosepiece into it.


How do I vary magnification?

If you are using a video camera that has its own lens you will image afocal (putting the camera up to the eyepiece).
Adjust magnification by changing the eyepiece or adding a barlow lens.
Use the zoom on the camera itself, but limit yourself to the maximum optical zoom.
If the video camera has no lens then the camera is coupled directly to the telescope (this is called prime focus);
Increase magnification with a barlow lens. If a barlow lens is used, the magnification can be further adjusted by sliding the camera (within limits) closer or farther away from the back of the barlow.
With a special adapter it is also possible to use eyepiece projection (imaging through the eyepiece).

What video format is best?

For video camera formats in use by the "general public", digital camcorders are best, followed by Super-VHS, Hi-8, VHS, 8mm and CVHS. VHS, 8mm and CVHS have about half the resolution of the other video formats. While the format is important, an 8mm camcorder with a high quality lens may do better than a digital camera with a cheap lens. Take note of the image below to see a comparison of 3 different recording methods. In this example the same camera (Astrovid 2000) was used to image a $20 bill with the output directed to the various recorders.



What features are important?

For capturing still images to your computer, the ability to review frames one at a time is very helpful,
Some of the more expensive digital camcorders have a progressive scan mode. Most video cameras use interlacing where the even lines are imaged and then the odd lines. A 1/60th of second separates these two captures which can cause a blurring of the images. In progressive scan the entire image is captured at one time.
Low lux value (sensitive to low light). Black and White video cameras are usually more sensitive than color ones. Because the moon is a relatively bright target, a low lux value is not as critical as it would be in imaging the planets. If very high magnification is employed (increasing the
effective focal ratio of your telescope) even your lunar imaging will benefit from sensitive camera.

What quality videotape should I use?

Use the best you can find. Cheaper videotapes generally have a thinner backing making them more subject to stretching and jamming. Working at low light levels tends to accentuate the worst in tapes. Higher quality tapes generally have low noise levels, and produce a more stable image. Studies also show that using blank tapes (instead of recording over previously recorded tape) makes a difference as well. Super VHS tape is compatible with standard VHS tape and is generally of superior quality to VHS tapes.

How do I get my video images into my computer?

For composite video output (i.e. not digital) a frame grabber is the most economical way to go. You can purchase frame grabbers, like Snappy, for less than $100. They are slow and only capture one image frame at a time.

There are other cards that will allow you to capture all the video images to your computer. They are generally expensive, take lots of memory and storage, and often save the frames in a compressed format where some image loss may occur.

For digital recorders, a Firewire computer card is used (these cost $100+ depending on the software included). This will allow you to digitally transfer some or all of your images to the computer. If you capture more than a few minutes of video, the storage requirement on your computer can be very considerable. Some software packages have a feature to allow you to review frame by frame and then save only the frames you select. Because this process is entirely digital, no information loss or degradation will occur.

How do I set up a system using a VCR?

Typically you send the signal from the camera at the telescope to the VCR video-in terminal. At the VCR video-out you can connect a monitor to see the image being recorded.  Do make sure that the VCR can capture the resolution that your camera produces.  A camera that has high resolution (SVHS) should be matched to a SVHS video recorder. If it is used with a regular VHS recorder, a significant amount of the resolution will be lost

Can I record from a surveillance camera to my camcorder?

Yes, this can be a very convenient method. The surveillance camera signal goes directly to the camcorder's video-in terminal. The image can be seen in the viewfinder or a monitor can be connected to the camcorder's video-in terminal. Note: not all camcorders have a video in capability. On a digital camcorder, it must have an analogue-in capability (many digital camcorders do not have this feature). In the future some surveillance cameras may have a digital out.  Finally, it would be worth while to check the 'noise' produced by the recording elements of the camcorder (the S/N ratio), and make sure that it is comparable to a stand alone video recorder. If it is much lower, than a significant amount of noise may be introduced to your images.

Why process my images in the computer?

Capturing a raw video image to the computer facilitates the making of still images. These images can be further adjusted for brightness, contrast, sharpness, and color balance. You may also want to combine them into a mosaic, or stack frames to reduce image noise.  The image (below) illustrates an example of what processing can do. On the bottom, I stacked 5 images and applied a high pass Kernel filter.  This sharpens the detail by increasing the contrast between light and dark transitions. The top image is one of the five stacked images with no processing dome. The images are their original size. The smallest craters visible are about 3km. While you should be able to see the smallest details in either image, they stand out more in the processed image.

How do I process my images with the computer?

There are a number of programs available for processing photos. Some programs specifically intended for processing CCD astronomy images are CCDSoft by Software Bisque, and MaxIm Dl. Others like SuperFix, MegaFix and Adobe Photoshop also contain most if not all the features needed. Even some of the inexpensive "consumer" picture packages for enhancing family photos have many useful features.

What kinds of digital processing are done?

Listed below are some of the processes employed most often. Some imaging software may use simple labels like "sharpen" or "contrast" on a particular feature, but behind the scenes it is most likely performing one of the following procedures.


Kernel filters perform low and high pass filtering (smoothing and sharpening). A kernel filter applies a kernel matrix to every pixel in the image. The kernel contains multiplication factors to be applied to the pixel and those surrounding it. Once all the values have been multiplied, the pixel is replaced with the sum of the products. Some Kernel filters are optimized to eliminate "dead" or "hot" pixels. Kernel filters are relatively simple, and some software programs allow you to customize your own formulas.

FFT filters perform a controlled low and high pass filtering (smoothing and sharpening) using a Butterworth algorithm. The image is processed using a Fast Fourier Transform, the filter is applied, and then the image is converted back using an inverse FFT.

Unsharp masking is used to sharpen an image. It subtracts a low-pass filtered (smoothed) version of an image from itself. The low-pass filtered version is called the mask.

Screen Stretching is used to manipulate the overall dark and light contrast of an image. The bright areas might be toned down to allow details to show that would otherwise appear washed out. Changing the contrast range of an image can help emphasize the details you want to bring out.

Stacking is a method of laying multiple images on top of each other and combining them into a single image. There is an amount of distortion (referred to as "noise") inherent in a video images. Much of the noise is random in nature. By stacking images the "noise" level is decreased. Four stacked images have 1/2 the noise level of a single image. Less noise makes for a smoother image. It also allows for better filter processing. Filters used to sharpen an image tend to amplify the noise in an image as well. Some users may stack over 50 images. Stacking does have a downside, as it can lead to a loss of detail.

There are also a number of unique filter algorithms used especially for astronomy, Maximum Entropy, Lucy Richardson, and Digital Development to name a few. Some of these are geared towards compensating for the effects of imperfect atmospheric "seeing" and can have dramatic results.

What is the best computer file format for retaining the most information in captured images?

TIFF 16 Bit preserves the maximum information, TIFF 8 Bit is subject to some compression. JPEG is fine for saving processed images, but not for working with them. Avoid saving a JPEG file more than once, the quality will degrade through recompression each time. With the Snappy, saving to TIFF is best.

Wouldn't I be better off using a high quality still camera?

A camera does have some advantages over video or CCD imaging.

35mm film covers a much larger area than video chips do.

Film grain generally has a finer resolution than the pixels on a video chip.

Video has more imaging "noise."

A video camera has some advantages over film cameras

Atmospheric turbulence are the single biggest obstacle to taking high resolution lunar images. With a camera you might "click" at just that one great moment of steady seeing (good luck, you're going to need it). With video you are taking 30 images a second. In 10 minutes, that's 18,000 images. It would take 500 roles of film to take the same number of images. It is very easy to focus the video camera since you are getting real time feedback.

You can adjust the exposure setting real time.

500 roles of film plus development $4000, 1 high quality videotape $10.


Will I be able to image the Apollo lunar module?

No, The smallest details that have been captured using earth based imaging systems are about 200 meters across. Amateurs using moderate sized telescopes (9-12 inch aperture) have captured objects smaller than 1km (about the size of a football stadium). It takes a night of steady seeing and quality optics to achieve these results.

What setup do you use?

My telescope is a Celestron 11" SCT mounted on a Losmandy G11. I use an Astrovid 2000 video camera. This camera allows for manual control of the exposure, gain and gamma. I send the signal to a Sony digital camcorder and a small 9" television. I capture single frames to the computer over a Firewire. I use PhotoDV by Digital Origin to control the Firewire image capturing. MaxIm DL to process the images. Before I owned the digital camera I used a SuperVHS recorder and Snappy frame grabber. See my site at http://www.AstroImaging.com for specific examples of image processing,

How much of a factor is air turbulence?

Below is an example of 6 consecutive images captured on video. This all occurred in less than a second. This is a typical example of how much the seeing changes from one moment to the next even though each shot is separated by just 1/30th of a second. If a CCD or film camera had been used the odds were not in favor of getting a good image. The seeing on the night that these images were taken was "very" average.  Only one of the six images has reasonable good detail.

Where is a good resource for finding out more?

The Video Astronomy Discussion Group is an excellent source of information and video related links. You may also email me CTZerbe@aol.com with additional questions.

Sky and Telescope published the book "Video Astronomy" by Steve Massey,

You may also email questions to me. (Craig Zerbe).

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