The advancement of affordable CCD cameras has been a real boost to amateur astronomers who can afford them. And just when it appeared things could not get better, another, much more affordable camera has come along to help the rest of us. The little, inconspicuous webcam, has finally put digital planetary imaging within reach of every amateur astronomer today! Of note, the recent close apparition of Mars was the most imaged apparition of that planet ever, due mainly to the availability of quality webcams. The astronomy world has never seen anything like it! Since a webcam is within affordable reach of almost every amateur today, we will discuss webcam imaging in some detail.
Webcams, when first introduced, were intended for use with computers, to send live communication over the Internet. We could talk with and see our friends and relatives at the same time. The webcam produces live, streaming video images. It wasn't long until some industrious amateurs figured out how to use webcams for astronomy to capture images of bright objects, namely planets. Today, many webcam manufacturers provide software to allow webcams to be used to capture images through a telescope. My personal favorite is the Phillips ToUCam Pro webcam (Fig. 9.9). This camera can be obtained with the proper software and a screw in barrel so the camera can be inserted into the eyepiece holder of a telescope (Fig. 9.10).
The really wonderful news about webcams is that they produce a color image and are extremely affordable, especially when compared to CCD cameras. Mine cost less than $300, including the software and eyepiece barrel! And they are plenty sensitive enough for planetary imaging.
Webcams use a small CCD chip, about 1/4 in. or smaller to collect an image. This image is an avi. file. With the software provided, this stream of images is broken
down into individual frames, and the frames are counted. A shutter speed can be specified for the exposure, and gain and contrast can also be adjusted for the camera. The gain and exposure time controls the brightness of the image being captured. I most often use a gain setting of 50-75% and a shutter speed of 1/25 to 1/50 s. With this setup, even my 8-in. telescope has produced surprisingly detailed images of Jupiter.
With the webcam hooked up to my laptop computer (Fig. 9.11), hundreds of frames can be captured in just a few seconds. It is not unusual for me to capture 500 frames in 30-40 s. Webcams can even be used on really large (Fig. 9.12) telescopes!
The individual raw images reveal a surprising amount of detail, but need to be processed to really bring out the full potential in the final image. The frames must be aligned and stacked. Fortunately, software is available to do just that, and the best part is, the software is free!
There is a variety of software available that will align and stack images. The one I prefer is called Registax. This is amazing software, written by Dutch amateur astronomer Cor Berrevoets, and then offered by him to the world for free . What a wonderful gift to the world of astronomy! The software can be obtained from the maker over the Internet and comes with complete instructions. It is very easy to use. The website is http://aberrator.astronomy.net/registax/.
To take images with a webcam, set up your telescope as you would for visual observing. While I have seen webcams used successfully with dobsonian mounted reflectors that did not track with the Earth's rotation, I prefer to use an equatori-ally mounted telescope. I also perform a reasonable polar alignment. However, I do not take as much time aligning as I would for a one-hour deep sky exposure.
A reasonably close alignment is good enough. A webcam can record images that can be stacked with a less than perfectly aligned telescope. However, I prefer a good polar alignment because it will reduce declination drift, allowing the telescope to keep the image centered without frequent adjustment. I prefer the image stay in the center of the field of view at least 10 min before I have to center again. This way, the aligning and stacking program does not have to work so hard making the alignment. Remember, we are trying to capture images that are as crisp as possible. So, exercise a little patience here in setting up the telescope. It is also important to setup the telescope far enough in advance of the imaging run so the telescope can settle down to the outside air temperature. Air currents inside the telescope tube will degrade the images you capture. I often set up my telescope outside prior to having dinner. That usually allows enough time.
Using one of my telescopes, an 8-in., f/10 Schmidt-cassegrain, let's go through a typical imaging run. I plug the webcam into my laptop that also runs the imaging software. After setting up the telescope so it can settle down, and polar aligning it, I locate Jupiter in the telescope field of view with a low power eyepiece of about 80x. Then I insert the webcam. Usually, the image will be out of focus. Patience is needed here since the out of focus image will appear faint on the computer screen against the background sky and may be difficult to make out. With a little practice, you will learn which direction to adjust the focus to begin bringing the image into focus on the screen. As soon as I have an image of Jupiter on my computer screen, I use the slow-motion controls on my telescope to finish centering the image. Now I finish focusing. This can be tricky, because touching the telescope to adjust the focus also causes the image to dance on the computer screen. Once I am near focus, I begin
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