CDK20 Focuser

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Revision as of 22:34, 23 December 2011 by John (Talk | contribs) (Wiring)

The CDK20 telescopes have Hedrick focusers that provide stable positioning of the camera and filter wheel assembly at the fixed focal plane of the telescope. The same type of focuser is used on the CDK125 telescope.

Mechanical System

The focuser on the CDK20N is shown below.

Cdk20n focuser motor attached.jpg

The camera is attached to a flanged tube centered on the optical axis of the telescope. The tube is constrained by roller bearings so that it cannot move laterally but can freely move in and out over a range of more than 30 mm with respect to the focal plane. One end of the leadscrew has a retainer that allows the leadscrew to rotate in a bearing attached to the focus tube. The leadscrew threads into a block that is attached to the focus tube housing. When the leadscrew rotates clockwise seen from the camera, it is pulled into the threaded block and the camera is translated toward the primary mirror. One turn of the screw advances the camera

A focus motor drives a bevel gear that engages a matching gear attached to the leadscrew that moves the camera. Since the screw has 32 threads per inch, one turn advances the camera focal plane 794 microns. The bevel gear and screw are clearly seen in this picture of the CDK20N Apogee camera.

Cdk20n apogee ifw focuser.jpg

When the motor assembly is removed the bevel gear may be rotated by hand to focus the telescope manually. At f/6.8 for the CDK20's, one turn of the focuser bevel gear will change the diameter of a stellar image by 794/6.8 = 117 microns. Since the Apogee cameras have 9 micron pixels, the change in the apparent image size is approximately 13 pixels per turn. In typical seeing we can adjust best focus for point spread functions of the order of 1 to 2 arcseconds, or 2 to 3 pixels. The focus can be optimized with final adjustments of the order of 1/8 of a turn.

Motor and Encoder

The CDK20 focuser motor is a DC servo with an integral optical encoder. A stock Celestron small telescope motor has been augmented with a machined plate to attach it to the focuser. The bevel gear, approximately 1/2 the diameter of the matching gear on the focuser, engages the focuser gear. There is a shaft extension that positions the motor gear. On the CDK20's the spacing may require shims. The motor assembly is shown below.

Cdk20 focuser motor.jpg

The motor gearing and encoder counts/turn are undocumented. We expect of the order of 500 counts per turn from the encoder, 10:1 reduction in the planetary gear, and 2:1 at the bevel so that of the order of 10,000 encoder counts correspond to one rotation of the focus screw. When the new motor electronics are installed we will document the calibration.


The encoder has a 4-position connector: ground, +5V, and encoder phases A and B. The 12 VDC motor has two leads. In Planewave's wiring the six wires are terminated in an RJ45 (8P6C) connector. The wiring of this connector on the CDK20 telescopes is different from the wiring on the CDK125 (and presumably also from the wiring on the CDK20's currently produced by Planewave). The focuser motor shown is the one from CDK20N. The red and black wires supplying motor power are connected to the RJ45 cable through a 2P connector, and the 4P connector is for the encoder.

Cdk20n focuser rj45 label.jpg