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At first glance the equipment needed to successfully carry out CCD imaging seems incredibly complex with all manner of electronic gizmos and wires everywhere. While it is possible to image by setting up the equipment every night, life is certainly simpler if the equipment, or the majority of it, can be permanently set up and wired together in an observatory. For me, an observatory provides another vital function - it helps to mask neighborhood lighting. While general light pollution has to be endured, backyard lights shining directly into the telescope tube should be avoided. A classical dome helps with this, as just the slit is open (see Figure 9.3), and I can even reduce the opening further by using strategically placed plastic sheets. They don't have these problems on Mount Palomar!

Figure 9.2. David Ratledge at his observatory.

Figure 9.3. 16-inch telescope inside the observatory. A classical dome is ideal for blocking local light pollution. The lower computer controls the telescope while the upper laptop operates the CCD camera.

Figure 9.3. 16-inch telescope inside the observatory. A classical dome is ideal for blocking local light pollution. The lower computer controls the telescope while the upper laptop operates the CCD camera.

With my interest in the deep sky, a big fast Newtonian telescope was the obvious choice. It is also the easiest to home-build. It was a joint effort by a team of three: Brian Webber made the optics, Gerald Bramall made the tube assembly and I made the mount and electronics. A 16-inch (40cm) was chosen as a good practical size with a fast focal ratio of f/4.7. This translates to a nominal focal length of 75 inches (1900mm), which was a good match for the CCD I was likely to use (and afford). At this focal ratio, imaging would be quick and coma would not be too objectionable over the central 10mm. For bigger chips I would need a coma corrector, but that's a problem for the future. The tube assembly was designed with light pollution in mind and is of an aluminum-framed construction. It features a baffled design - the opening in each baffle increases away from the mirror matching the telescope's field of view - the idea being to trap internal reflections that would reduce contrast. The tube frame was finally covered in thin plastic to further reduce light entry. My pride and joy on the telescope is an 80mm-diameter electric Crayford focuser (see Figure 9.4). This is a dream to use with absolutely no backlash, and focusing to 1/1000 inch is straightforward!

With the light pollution that blights my site, virtually every object I image is invisible even in the 16-inch telescope. A flip mirror finder is therefore of no use in locating objects. The telescope is capable of imaging just about everything but I couldn't locate anything! I had therefore to build a computer GOTO mount to overcome this. This was based around Comsoft's PC-TCS software running on an old DOS PC. Big 48-volt stepper motors are employed to drive and effortlessly slew the telescope around. It was a relatively simple task to wire up all the components and make a hand control box. This has been a huge success and I can now acquire any object, visible or not. Also mounted on top of the main tube is a Meade 6-inch (150mm) Schmidt Newtonian, which is used for wider fields of view.

I have been somewhat less successful with CCD cameras. However, my original HiSIS22 camera based on a Kodak KAF400 chip (768 x 512) is still going strong after nine years. With its 9-micron pixels it is still my first choice for use on the 6-inch telescope for objects like comets and open clusters. The main telescope was designed with a Kodak KAF261 in mind. This 10mm square chip has 20-micron square pixels (512 x 512) and is a good match to the 75-inch (1900mm) focal length. After two troublesome cameras (from different manufacturers) I am now having better luck with a Finger Lakes Instruments MAXCAM camera (see Figure 9.4). This is a relatively compact camera, and results have been good. My camera was one of the first batch with a USB connection, which downloaded images in a leisurely 6 seconds. It was subsequently upgraded to a high-speed USB interface with downloads of less than 1 second! Focusing is now like having a video camera.

My final piece of kit is a filter holder. Initially I used a homemade filter system with twin wheels mounted close together. This permitted more combinations of filters than is possible with a single wheel. However, when two filters are used together, reflections between them can be a problem, especially if a bright star is present in the image. I replaced it with an Astronomik manual filter drawer and I now just use a single filter. To combat light pollution I generally image with a simple red filter in place. This not only passes the red part of the spectrum but

Figure 9.4. FLI

Maxcam CCD camera. Note the large Crayford focuser. Pencil marks on the side of the focuser might look crude but they indicate the approximate focus position.

Figure 9.4. FLI

Maxcam CCD camera. Note the large Crayford focuser. Pencil marks on the side of the focuser might look crude but they indicate the approximate focus position.

the (near) infrared as well, where CCD chips are still very sensitive. If the object is blue, then I use a Lumicon Deep-sky filter which, in addition to the red, also passes at the blue end of the spectrum.

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