But rather than swapping to a shorter optical tube, how about "zooming out" with your existing 'scope?
Well, this is actually possible - by fitting a focal-reducer lens between the 'scope and camera. We've recently purchased one of these for use with our Atik Infinity camera system: http://www.telescopehouse.com/revelatio ... -1-25.html
This type is designed for use with small-sensor imaging systems (the Infinity has a 2/3"-diagonal CCD). A bigger lens would be requred for a DSLR imaging system: these are available, but are rather more expensive (>£100).
Here are some examples of our little Revelation Astro reducer in use with the Infinity camera, trained on a familiar local landmark: The reduction-factor of the lens depends on its spacing from the sensor. Fitting the lens right onto the front of the camera's nose-piece produced an f/5.6 system, but then adding a 15mm spacing-ring behind the reducer opened it up to f/4.2.
I found this on-line calculator from Wilmslow AS:
which helps in working out what can be achieved (the Revelation reducer-lens has a focal-length of around 95mm).
The "in-focus" parameter is a measure of how far the focus-adjustment needs to be retracted to achieve focus. The moving primary mirrors in SCTs & Maksutovs give them a larger focus-range, but refractors have a more limited accommodation, so less focal-reduction can be realised with them.
The benefits of using a focal-reducer include:
- the larger field-of-view makes it easier to locate your target. and allows larger targets to be imaged
- a brighter image allows shorter imaging-times
- the reduced image-scale means that tracking errors are less significant
Bear in mind that the ultimate image-quality with a focal-reduced 'scope will not be as good as with a telescope expressly designed for this shorter focal-length. But if you only have the one optical tube, using one of these lenses can give it a new lease of life to image a wider range of objects.