Camera and CCD Mounts and Accessories

To get the most out of your camera equipment, it is necessary to have the proper supporting equipment. The devices used for 35-mm photography and for CCD imaging are very different from each other so we'll discuss them separately. In photography, 35-mm cameras cannot be directly connected to a telescope so adapter equipment will be necessary in order to make the mechanical connection. The minimum that you will need is a T-ring and a T-adapter. The T-ring is a metal ring with two threads. One screws into your camera and provides the interface for the T-adapter which screws into the other end. At the far end of the T-adapter is a universal thread ring that will mate to the rear cell of a Schmidt-Cassegrain telescope in place of its visual back. These two devices allow the camera to be mounted at the prime focus of the telescope and allow for photography at the telescopes normal f-ratio (for me, f/10). Photography using this type of setup is therefore called prime focus imaging. This type of setup allows nice, short-exposure images of the entire Moon or Sun, reasonable length images of star clusters or other bright objects in the deep sky and the shortest possible exposure time for any other type of object, unless a telecompressor is used. For imaging the planets or small objects, the magnification is generally inadequate to show any detail. For small targets, a different technique must be employed. We can gain extra magnification by introducing an eyepiece into the light path. This will require moving the camera several inches farther away from the telescope using an empty metal tube called a "tele-extender." After inserting an eyepiece directly into the visual back of the telescope, the tele-extender then screws over the outside of the visual back. The other end of the tele-extender screws directly into the T-ring on the camera. This has the effect of dramatically increasing the focal ratio of the telescope, sometimes into triple digits! Tele-extension photography can be extremely challenging because with so much power, it can be difficult to focus the telescope and like with the CCD imager, field of view is agonizingly small. This can make it difficult to even find your target, never mind focus on it and shoot. If you get good at it though, the results are well worth it.

Long-exposure astrophotography requires that the telescope be carefully guided throughout all the time the shutter is opened. If the target is allowed to drift, the result will be a badly streaked image. Modern clock drives are very good at tracking objects, but no motor drive is perfect so you will always need to correct in right ascension. With an extremely accurate polar alignment and a perfectly level telescope, you can eliminate the need to correct in declination. Still, very few of us are that good or that lucky so some minor declination correction will almost always be required as well. To make these corrections, you will first need a control device. This is a simple four-button (up, down, left, right) hand-held box that connects via a telephone-style jack to your drive base. It controls the right ascension correction by speeding up or slowing down the motor in response to your commands. For declination corrections, commands are sent to an external motor that inputs the north-south correction. The motor, normally purchased separately, connects to a power port on the telescope drive base. Using these motors draws a lot of power from your telescope battery, so be prepared to plug into either a larger DC battery or some source of AC power with a DC inverter. Most telescopes come with equipment to allow you to do the latter while DC/DC adapters (to plug into your car's cigarette lighter) are readily available.

Now that you have the means to make corrections, you need to be able to see what you are doing. With the camera shutter open, there is no means to look through the telescope to see the target. There are two ways around this problem. The old fashioned way is to mount a guide telescope on top of the main telescope.

This is usually a 2-3 inch refractor with a high-power eyepiece. The guide scope is mounted precisely in line with the main telescope so that by keeping the target object centered in the guide scope, you also keep it centered in the main scope. The advantage of this apparatus is that you can use the target itself to guide the telescope. This is a huge advantage if you are imaging a moving object, such as a close-by comet. The problem with the guide telescope is that if you are photographing a faint deep-sky object in your main telescope, it may prove too faint to see in the guide scope.

Most owners of Schmidt-Cassegrain telescopes prefer to use an off-axis guider for controlling the telescope. The off-axis guider connects directly to the rear cell of the telescope. Most of the light passes directly through the guider body to the camera. A small amount of light is redirected by a prism to an eyepiece at a 90-degree angle to the light path. This allows the observer to see a small amount of the field of view and pick a target to guide on. The advantage of the off-axis guider is that you can take advantage of the full light gathering power of your telescope to find a guide star. The problem with this system is that you cannot see the actual object you are trying to photograph. The prism does not extend that far into the light path. You must aim and focus the camera carefully, then hope that the off-axis guider is able to view a star onto which you can focus, center and guide the telescope.

A CCD camera adds to the aim, guide and focus difficulties because you cannot view through it at all. Attaining a proper focus is a difficult process of trial and error. This is very frustrating because image quality is very dependent upon a very precise focus. The range of focus position is literally only a few hundred microns wide. To help aim and focus, many amateurs now use a device called a "flip mirror." The flip mirror uses a fixed mirror that directs light to an eyepiece that allows the viewer to see exactly what the camera will see. When the target is centered in the eyepiece, the mirror is then retracted out of the way allowing the light to go to the camera. Once the camera is focused once, you can bring your eyepiece to focus. Then all you will need to do is center and focus in the flip mirror, and you will have a point and shoot CCD setup. The flip mirror can also be used for 35-mm photography provided it is large enough to allow the entire field of view to reach the camera. If the field of view is smaller than the camera frame (due to the obstruction of the flip mirror) then you will see the circular edge of the field of view on the frame, a phenomenon called vignetting. I use the larger of two flip mirror models offered by Meade. Some more modern CCD cameras will allow you to view real time video. This makes aiming and focusing easier, but it only works with the brightest of objects.

When you buy any photographic accessory, make absolutely certain that it is of sound construction so as to avoid any mechanical flexing caused by the weight of the camera. If the camera or CCD is allowed to bend the mount by even a few millimeters, then the light path will no longer fall into the center of the camera's field of view. This was a key selling point in selecting the Meade flip mirror. Other devices that I tried caused the camera to flex slightly off the telescope's optical centerline. T-rings and adapters and tele-extenders are less than $40 each but the flip mirror and illuminated reticule eyepiece can set you back close to $400 if you go for larger models.

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Digital Cameras For Beginners

Digital Cameras For Beginners

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