Nigel,
There are a few issues to be aware of with focal digiscoping, I do this occasionally on my scopes too. You didn't say what your camera is, and I am guessing it is possibly a small automatic compact ?
To get the light from your scope into the camera it must pass through the aperture (iris) in the camera lens. Secondly, consider what happens for light entering the scope off-axis at the edge of the field of view - see the animation under "A Basic Telescope" halfway down this page
http://www.jayandwanda.com/digiscope...s/pupils3.html
- the iris of the camera lens must be placed at the eye-point of the eyepiece., otherwise the off-axis light will not pass through the iris to reach the camera sensor.
So:
1. Set the camera so the lens iris is wide open, as wide as it will go. On my LX5, I have manual control and can set the lens to f/2. Use a rule or pair of callipers to get some idea of the apparent diameter of the iris, in mm. It also varies with the zoom setting. For example, on the LX5, with the lens set to f/2 and the zoom at its widest, the iris is 2.6mm and zooming in to the maximum focal length the iris increases to about 10mm.
Note: If your camera is fully automatic and doesn't let you set the aperture manually, it will be a huge problem and quite likely almost impossible to control.
2. The exit pupil from the eyepiece should be no larger than the iris diameter. This means the magnification required is m = D/i where D is the aperture of the telescope, and i is the aperture of the iris.
The eyepiece focal length required is then f = F/m where F is the focal length of the telescope.
Alternatively f = i x F/D
On your Dob, 42mm will be way too long for sure and quite possibly even 13mm is too long, depending on your camera.
3. You need to place the camera carefully so the lens iris is at the eyepoint of the eyepiece. This should fill the field of view in the camera. If it is too close to the eyepiece, or too far back, the field of view will vignette badly (i.e. the edges will be cut-off).
4. Focussing. Again, a camera with manual focus control is important. You will be lucky to achieve much with autofocus cameras.
Put the camera focus into manual.
Start by looking through the eyepiece and focus visually.
Attach the camera.
Watching the image in the camera, use the eyepiece focusser to get the image focussed in the camera as best you can. If this is reasonably good but not perfect, you can then let the camera's autofocus do its thing.
5. Exposure control
In normal use camera has just three ways to control the exposure - by setting the ISO (sensor sensitivity), shutter speed (exposure duration) or by varying the lens aperture (which alters the image brightness). However, in digiscoping you must use the maximum lens aperture to avoid vignetting, so only adjust the ISO and shutter speed to achieve the desired exposure. The working focal ratio of the overall combination of telescope + eyepiece + camera is
fr = FR x l/f
where FR is the focal ratio of the telescope, l is the focal length of the camera lens, and f is the focal length of the eyepiece.
6. Image stabilisation.
On some cameras IS will work as usual, typically those with optical in-lens OIS (e.g. Panasonic). With other cameras - usually those with IBIS - it may respond incorrectly and makes matters worse rather than improving the image. Try shooting with it on and off to see which is better.