6″ Dall-Kirkham


As I appear to have gotten hold of a 600mm mirror blank, I’ve been considering what to do with it – my primary interests are photometry, spectroscopy, planetary imaging, and DSO imaging in that order. While I guess conventional wisdom would be to make a short focal length dob with this mirror, I would prefer to realize a dream I’ve had for a while, a scope with multiple Nasmyth foci for various roles. As such I’m contemplating making a Corrected Dall-KIrkham with a rotatable tertiary mirror so I can direct the focal plane to one of three foci – a spectroscope, a CCD package for photometry and DSO imaging, and a visual / planetary focus.   Fully robotic and enclosed in a geodescic dome observatory I’ve been drawing since I was 12 years old 🙂 I also have a 250mm “mystery mirror” thats perforated so I’ll probably build a CDK with that prior to attempting the 600mm.

However, I’ve only built newts in the past (13.1″ dob, ground, polished, and figured 8″ f10 and 9″ f5 mirrors, 16″ dob) so I’d like to try out a small 150mm f20 Dall-Kirkham along the lines of Matthew Paul’s project here. I have a couple of nice Pyrex 150mm mirror blanks on the shelf I inherited from somewhere so I have everything I need.  I’ve rough ground the primary to f4 already so I’m ready to bore the 32mm (1.25″) hole in the primary part way through the mirror 5mm short. I’m working on a fixed post mirror grinding machine big enough to handle the 600mm so I plan to mount my drill press head on the rails that the post mounts on and spin the disk as I drill to ensure the hole is centered.  I’m not bothering to do the small mirror on the machine since it’s just as fast to hand grind it.

As I understand the process, the primary is pretty normal except during figuring you’re only going about 70% of the way to a parabola, but the strokes and techniques are essentially the same?

I’ll be 3D printing most of the mechanical structure of the scope including a focuser with integral filter wheel, rotation, and focusing as well as secondary spider and mount, and the baffle and cell for the primary.  Once the optics are done I’m planning on mounting the scope on a standard commercial mount so this is just an OTA project.

I’m not finding much in the way of end to end recipes for building a Dall-Kirkham so I’ll be documenting my progress in a series of articles on my website gordtulloch.com and my local RASC newsletter (I’m the editor so I get to put ATM stuff in there!) so perhaps by the end of these projects I’ll have something to contribute to the community!

Optical Design

Obviously first of all I needed to understand what I was building. Fortunately, the excellent CassDesign2 software allowed me to enter some basic parameters and get a design for the optics that laid out exactly what I needed to do.