Introduction.

An optical or imaging train is a way of describing the way the imaging equipment (hardware) is configured. That is, how a camera might be connected to a telescope including any adaptors, lenses, filters or any other mechanical or optical device that is fitted together in a certain way to achieve a given type of photography.

My Astrophotography Equipment

Cameras:

Canon EOS 400D Digital SLR
Phillips ToUCam Pro II 840K Long-Exposure modified webcam
Orion Starshoot Pro DSI III CCD colour imager
Orion Starshoot Autoguider

Optics:

Skywatcher Pro Series ED80 Apochromatic Refractor (F/7.5 Focal Ratio)
Skywatcher Black Diamond ED120 Apochromatic Refractor (F/7.5 Focal Ratio)

Previous telescope: Celestron Nexstar 11 GPS Schmidt Cassegrain Reflector (F/10 Focal Ratio), F/6.3 with focal reducer

Mount:

A.S.I.G.N. was initially fitted out with a wedge and fork-mount. Currently I am using an EQ6 Pro, German equatorial mount fixed to a permanent pier.

Methods of image capture:

Piggyback Camera and lens on top of aligned telescope
Prime focus photography via T-ring and camera adaptor through the telescope
Eyepiece projection (added eyepieces inside camera adaptor)
Video Capture via webcam through either telescope

Photography configurations

Camera and lens riding piggyback.

This is a method employed for using your camera and its own lens. The advantage here is that if you have a polar aligned telescope that can track the movement of the stars, then you can have long exposure photographs. This gives faint light more time to burn an image onto your film or chip, without suffering star trails.

All that is required is a bracket, to secure your cameraon top ofthe telescope, using your telescopes mountas a tracking platform only. This way you can practice on spectacular widefields, where you would only be able to expose for 30 seconds on a fixed tripod, now you can expose for ten minutes if you need to!

Prime focus photography.

Now you have practiced piggy back, its time to take the lens off the camera and use your telescope as a lens. This is done via a T-ring. A T-ring has a bayonet type fitting that is unique to your brand and model of camera. These are not available for all cameras, so make sure you can get one for the type of camera you want, before you buy it!

The other side of the T-ring has a fine thread that fits to the camera adaptor tube. This is just a hollow tube, that can be used on its own, or have an eyepiece secured inside it. (Thats another story, see below). The other end of the camera adaptor has a stepped-down tube, available in a couple of sizes, to fit the eyepiece holder of your particular telescope.

Once the T-ring and adaptor is fitted to your camera in place of a lens, all you do is pull out the eyepeice from your telescope, andslide the camera in.

To advance a little further on this, you can use a radial guider. This can be used in place of the camera adaptor for prime focus. The Radial guider is an off-axis setup. Simply put, it has the addition af a small prism that intrudes into the edge of the tube. This takes a tiny little edge out of the field of view (FOV).

You can then swivel your eyepiece and the prism inside, to pick up a guide star somewhere near your subject. You can then keep looking through this via an illuminated reticule eyepiece for the duration of your exposure, and make tiny adjustments to your telescope to keep this guide star centred. Once you press the shutter release, the flip mirror in your camera will not permit you to look through the camera anymore. For this reason, a radial guider is handy.

To focus your eyepiece in the guider, simply loosen the thumbscrews and slide the eyepiece in and out to get focus. This has nothing to do with the focus on the camera. Make sure you get that right first and lock it in place!

Getting closer-Photography with an eyepiece. (Eyepiece Projection)

The camera adaptor can be used for prime focus, basically just shooting through an empty tube to the objective lens (telescope). You can increase the magnification and make your target image look bigger by sliding different eyepieces into your camera adaptor. You can also use a Barlow lens, which increases your magnification by fixed factors of 2x, 3x, 5x etc.Optical QUALITY, not price, is the key factor in selecting these. Do yourself a favor, buy the very best you can afford.

These methodsmake the object look bigger, but make the field of view (FOV) smaller.They also make focus and trackinga lot more critical. This can have advantages and disadvantages. Sometimes its better to keep the image smaller but sharper.

The biggest thing to remember here is that it slows down your telescopes' light gathering ability and focal ratio, (speed)therefore making much longer exposures neccessary.

Shutter release cable (or remote cable).

Make sure you use a shutter release cable to prevent moving the telescope and camera in the image-taking sequence. If you don't have one, you can use the camera's delay timer. Alternatively you can hold up a black card in front of the lens or telescope, trip the shutter, wait for vibration to stop and take away the black card. Once the exposure time is up, hold the black card in front of the lens again, and release the shutter.