Star Points for March, 2003; by Curtis Roelle|
A Digital Astrophotography Primer
Last month we discussed techniques for observing the moon. We went over how to discover what to observe and when to see it. This month we'll explore how you can obtain a photographic record of your observation by taking a digital picture of the moon. You may already have all the equipment you need to try your hand at "astrophotography" if you have a telescope and a digital camera.
Digital cameras are becoming more and more affordable. They are ranked by the resolution, or the number of picture elements (pixels) that make up a single image. You can buy a good quality 2 mega-pixel camera that is computer ready for under $200. All the software you need comes with the camera on CD ROM. High end digital cameras have 4 to 6 mega pixels and cost considerably more.
Nearly all digital cameras can also record short video sequences. The video capacity in seconds depends on the size of the camera's memory card. Memory cards may be upgraded and come in sizes from about 8 MB on up to at least 128 MB. They can be swapped in and out of the camera like a computer's floppy disks. Stores such as Walmart have digital cameras on display that customers can play with. This is a great way to check out the features of different cameras before buying.
Readers of this column have undoubtedly admired the beautiful astrophotos taken by the Hubble space telescope published in newspapers, magazines, and on the worldwide web. The exquisite detail and rich colors of distant objects are breathtaking. Wouldn't it be nice to take photographs like those yourself?
The reality is you cannot expect to compete with a two billion dollar telescope in earth orbit. You can spend thousands of dollars on equipment but cannot reproduce the Hubble images. This doesn't mean that astrophotgraphers aren't capable of making dramatic images in their own right. They can and do it all the time. But it requires the use of precision equipment, experience, patience, and realistic expectations.
Objects such as galaxies, nebulae, and star clusters are faint and the only way to image them is with a time exposure. Time exposure means that the camera shutter (whether it be mechanical or electronic) needs to stay open for a period of time. The amount of time may be seconds or it might be hours. Time exposures allow the photons of light to be collected or "integrated" on the film or electronic detector.
The problem with time exposures is while the shutter is open, the earth is rotating and the target object is moving. The result will be trailed stars not pin-pint dots. Time exposures require the use of a motorized telescope mounting that can compensate for the earth's rotation and track the target object.
Fortunately, telescopes equipped with such mountings are getting more affordable too. Computer controls and stepper motors are replacing finely machined and expensive geared mechanisms. For several hundred dollars a person can get a telescope that not only tracks but will "goto" an object with the push of buttons on a keypad.
If your telescope does not have a "clock drive" motor you are still in luck. As an alternative to time exposures all you need to do is find a bright object, like our friend the moon. A fraction of a second exposure is sufficient to render an image of the moon either on conventional film or electronically.
How do you take photographs through your telescope? There are several telescope and camera configurations astronomers use to image with telescopes. Some require using a camera with a removable lens.
One technique that does not require removing a camera's lens is the "afocal" method. Afocal images are made by placing the camera lens up to the eyepiece where your eye would normally be. Thus, the light passes through the telescope and its eyepiece and then through the camera's lens and onto its focal plane.
In order to focus and adjust the camera it must be a "through the lens" or TTL model. A TTL means that when you look through the camera's viewfinder you are looking out through the actual camera lens so you see exactly what the camera sees. If your camera only has a range viewfinder (usually a little window on the upper front corner of the camera) you won't be able to take pictures through your telescope unless you're very lucky.
The good news is that the video viewfinder displays on digital cameras are TTL. In other words the screen on a digital camera shows you exactly what the camera sees. Some digital cameras also come equipped with a range viewfinder as well as a video display. For astrophotography we are only interested in the video display only.
A couple of set up steps are needed before you begin. You must turn off the flash. Check your camera's manual to learn how to suppress the flash if you don't know how. If your camera has a setting for automatic white balance and brightness, use it. You can play around with the manual settings later once you've got the hang of it. Check to see that the video display is turned on.
You should begin with your telescope's lowest power eyepiece. Locate the moon and focus on it by looking through the eyepiece without the camera. The best time to photograph the moon is when it is NOT full. First or last quarter moons are perfect because of the rich detail that will be visible along the terminator.
Place the camera lens up to the eyepiece while being careful not to bump the telescope. Move it around until the image of the moon can be seen in the video viewfinder. If you can't see the moon in the video display put the camera down and look in the eyepiece again. Perhaps you bumped the telescope or the natural rotation of the earth has carried the moon out of the field. Readjust and try again if necessary.
Now, carefully, with your hands as steady as possible snap the picture. Be careful not to let the camera rotate or yaw while pressing the shutter. If your camera has an automatic timer you might consider using it if your pictures come out blurred by motion.
Check your work in the view finder. Digital cameras have a couple of advantages over conventional cameras. For one thing you get instant feedback. it's like having a Polaroid camera; you don't have to wait for the film to come back from the processing lab. Also, if you don't like the result, you can erase it and try again.
The disadvantage is that the viewing screens are small. An image may look sharp and clear on the small screen. However, on the computer screen the results might be a disappointing. Some improvements can be made using digital image manipulation software such as the popular Adobe Photoshop.
We gave our children digital cameras for Christmas. On a whim I took some shots of the moon through the observatory telescope. It wasn't perfect, there was a little bit of blurring around the northern limb. I cropped, rotated, and resized the original image with Adobe so that north was up. I have posted my first shot below.
If you give astrophotography a try I would very much enjoy seeing your results. You may e-mail them to StarPointsUSA@yahoo.com. If you don't mind I may add them to the web site.
Good luck using your digital camera together coupled with the telescope. You can shoot through binoculars too, if you mount them onto a sturdy tripod.
The author captured this image of the moon on February 11, 2003. It was shot using a FujiFilm FinePix 2650 digital camera with afocal projection through a 12.5" f/6 Newtonian telescope using a 40mm Televue Plössl eyepiece at 47x.
The image was rotated, croped, and resized using Adobe Photoshop. Click image to see the original raw digital image.
Click on abobe image to view the original raw image (341 KB).