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12.5" Dall-Kirkham f/20 Optical Tube Assembly

A simple and practical telescope designed for high resolution work.


On February 1, 2009 I drove down the southern New Jersey, where, arriving late in the afternoon (and losing the light for really good photographs), the first of the 12.5" Dall-Kirkham OTAs was delivered and tested on the sky. After a very cold winter season it was gratifying to have at least a relatively  warm evening. The sky was perfectly clear though high altitude turbulence would limit any deep testing of the optics. None the less, a number of close and not so close doubles, as well as a first quarter moon, were looked at; Castor, Rigel, the usual. Only the triple Iota Cassiopeiae offered a chance for some close scrutiny with the AB unequal pair at 2.5 arc sec. At 270X the two stars were clearly and cleanly separated and colors distinct in moments of steadiness. Moving the power up to over 600 was difficult owning to the turbulence but things were hanging together nicely. The simple baffle tube, with my own super non-reflective interior completely suppresses stray reflections without the need for field-vignetting baffle stops.



Rear view showing focuser, collimation screws and slewing handles. The full Surrier tube structure is made entirely of aluminum solidly welded and bolted together and attractively powder coated in semi gloss black for the ends parts and silver vein texture for the struts and center square. A powder coated surface is very tough and particularly resistant to the effects of bumps abrasion. The entire OTA weighs only about 55 lbs. Note finder scope mounting bracket at upper left of back plate and dovetail mount on top to accommodate accessories.



Front end shot shows the double curved spider and generally light and simple nose unique to this telescope; no heavy spider vanes or large obstruction out there. This spider produces less wavefront degradation than if a Maksutov type corrector were used, assuming losses through typical design and fabrication errors - imagine the additional weight and cost of a 12.5" Mak corrector, yet here we have only two simple mirrors. Careful scrutiny could detect no spikes or other diffractive abnormalities even on very bright stars such as Capella. And careful design work and proportioning of optical system suppresses coma to within the diffraction limit and an unobservable level even at the edge of the field. Central obstruction is only 24% of primary diameter. 

As I have often advised, this is not a general purpose all around observing telescope but for some it's the telescope they have long been waiting for, a true planetary Dall-Kirkham.

As things progress I'll be posting more extensive photos as well as additional technical detail.

More to come ...