DESIGN JUSTIFICATION

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Aperture:

The local seeing can only support a 15 - 20 cm (6" - 8") aperture. However in order to be able to profit from an occasional night with perfect seeing it would be nice to have a 25 cm (10") diameter. It will usually be masked down to 15 or 20 cm. It can even be used as a 10 cm (4") unobstructed scope with an off-axis mask.

Focal length:

As long as possible. It must still be transportable, so I compromised on 200 cm (80"). For the full aperture that is f/8, but at the more used 15-20 cm (6-8") it will be f/10 or above.

Obstruction:

As small as possible. A circle of full illumination of 10 mm diameter is acceptable. Even though it could probably still be made smaller, I currently don't want to try for it. But since the secondary will be replaceable (intention) it could always be done later.

Spider:

The smallest possible diffraction spikes are produced by the thinnest possible vanes. Hence a wire spider. It might be argued that curved vanes don't produce spikes at all, but the total obscured area is bigger and the (worse) diffraction effects this causes are smeared out over the whole image, causing loss of contrast.

Focuser:

The helical focuser can bring the eyepiece closer to the tube than other designs. This gives the smallest possible secondary (obstruction). If the eyepieces are made parfocal, the reduced travel is not important. Besides, this scope will only be used visually.

Cell:

A full thickness mirror will be used. However in order to keep the figure as perfect as possible a 9 point floating support will be used.

Mount:

Alt/Az. I own one equatorial scope, and that's enough. I am convinced that equatorial mounts should only be used for stationary scopes. My first intention was to motorize the mount, but after reading "The Dobsonian Telescope" I decided not to do so.