CDK17 – Final Assembly

Those who follow the thread on CloudyNights have probably seen most of the pictures in this post already but here’s the final write up on the design and construction of my 17″ CDK.

I received the optics about 9 months ago now and kept them packed pretty much until now. I knew I wasn’t ready with the observatory/mount yet so I figured they would probably take less dust that way.

I prepared the cell for the primary mirror by adding Delrin blocks under all triangles and then tightened all collimation screws (pull ones) so that all the triangles would sit perfectly flat. I made some more Delrin blocks to support the mirror and give the right spacing between the weld nuts and the back of the mirror. In my case I went with 1.5mm spacing roughly. I then put a blob of RTV on each of the weld nuts and just dropped the mirror in place.

CDK-17-final-assy-3The mirror cell has 6 lateral supports which have 2 set screws (with soft Nylon tips). This lateral supports made the centering of the mirror pretty easy. Very little adjustment was required to have it perfectly centered. The centering is done on the OD per manufacturer specifications if you were wondering. I let the RTV cure for about one week before final assembly of the structure. I looked up some reference dimensions in my 3D models/drawings and adjusted the collimation screws so the mirror would sit at the right height above the back plate. And I obviously removed all of the Delrin blocks which were no longer needed. I then adjusted each of the 12 set screws to make contact with the sides of the mirror. The final step was to screw in the 17″ aperture mask atop the 6 lateral supports. It gives a clean 17″ aperture and adds some strength to all 6 supports.

CDK-17-final-assy-2I then assembled the corrector lenses in my baffle assembly. The latter is composed of 4 different parts. The first one is just a spacer that screws onto the back plate and spaces the corrector lenses at the right height above the back plate. The next is the corrector lens housing itself. It has several inner shoulders on which both lenses rest onto. Since both lenses have the same OD, the first lens is installed from the front of the housing and the second lens from the rear. Both lenses are retained with a very thin threaded rings with two notches to allow for tightening of the ring. The third and fourth parts are just baffles, I made them out of two parts for practical reasons during machining. All these parts just screw in each other with fine custom threads. Once all together the whole tube (which from the outside very much looks like a 80mm refractor) is just screwed onto the back plate.

CDK-17-final-assy-4With the primary cell ready, I installed some of the electronics on the side of the telescope, replaced the original socket cap screws by some nicer button head sockets. I changed the carbon fiber tubes and connectors for larger ones. I didn’t like the ball head connectors I had which allowed for rotation of the tubes around their main axis.

CDK-17-final-assy-5I followed the same steps with the secondary mirror. The housing/baffle for the secondary was re machined in two parts instead of just one. The original design didn’t allow for removal of the Delrin blocks once the mirror is glued. I made some collimation screws out of M5 threaded rods and threaded knobs. I made those long enough so that they protrude just a little bit off the spider.

CDK-17-final-assy-6At this point I am roughly 6 days after the primary was glued. I was lucky enough to have both of my friends Dan and Scott to help me install the structure on the back plate to lock everything in place. We then took the scope to the observatory to install it on the mount just to realize I was short of a weight on the counterweight side! We stared at it for a while and then took it off. And luckily both of them were able to come back on the next week-end to reinstall it, this time for good.

CDK-17-final-assy-7Below is a short timelapse of the first week-end install (and uninstall!) that Dan took while we were working on it!

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