20 inch
Next I made the telescope with the 20-inch f/4.0 Orion Optics mirror set. Nowadays this is almost considered to be a 'slow' mirror, because very fast mirrors (f<4.0) have become customary these days. But when I ordered the 20-inch f/4.0 mirror, this was considered very fast. Up to then, 20-inch f/5.0 mirrors were the norm, but that definitely requires a ladder and that has always been a no-go for me. Still, the dimensions felt like a shock when placing it next to the 12 inch. It has been said many times before, but one thing is very true. Being accustomed to the 12-inch, every aspect of a 20-inch seems to become huge and heavy!

With the Argo Navis and ServoCAT systems installed, its a very accurate telescope in terms of pointing accuracy and tracking, with actually no systematic error that can be defined by TPAS.

Furthermore, compared with the 12-inch, the views with the 20-inch are of another order. ~2.8 times more aperture indeed shows! Views of for instance galaxies can be spectacular beautiful.

This picture accompanied an article by Jan van Gastel about the history of Dobsonian telescopes (Zenit, January 2009).

After I made the 16 inch telescope I decided to rebuild the 20 inch along the same lines. I made the telescope lighter and somewhat smaller. Because of the use of side wheels and the joint assembly of trusses plus secondary cage, the telescope is now as easy to handle as the 16-inch. In fact, the rebuild telescope looks more like the 16-inch than the original 20-inch.

click for larger picture

But, I sold this telescope after I decided to build a binoscope.

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