Takahashi Epsilon 180 ED Hyperbolic Astrograph

The Takahashi Epsilon 180 ED is an extremely fast f/2.8 astrograph with a hyperbolic primary and a refractive corrector element built into the focuser to yield a 44mm flat field image circle, large enough to cover a 35mm sized CCD chip. With a tube that is only 22.4" long and 9.13" in diameter, this is a heavy little telescope at 22 pounds.

As pictured above, the E180 is held with a pair of Takahashi mounting rings on a custom plate attached to a 13.5" long Losmandy dovetail plate. The SBIG STL-11000M camera
mounted to the telescope is in perfect balance with this configuration, not requiring additional counterweights as see on some amateur setups.


Focus on the Epsilon 180 is extremely critical to get tight, round stars across the entire field of view. Based on the Critical Focus Zone equation, CFS = fr squared x 2.2, taken from the Wodaski book entitled The New CCD Astronomy, critical focus range is approximately 17.248 microns or 0.00068 inches. The smallest step size I could achieve with a Robo-Focus unit coupled directly to the focuser shaft of the Epsilon 180 translates into approximately 0.001 inch of drawtube motion, which is outside the resolution needed based on the Critical Focus Zone equation. To overcome that problem I designed and built a 1: 2.86 reduction unit to bring down the minimum drawtube displacement to 0.00035 inches, well within the theoretical focus zone.

The Robo-Focus reduction unit consists of a custom made Robo-Focus bracket, two timing pulleys and a timing belt. The pulleys and belts were purchased on line from SDP/SI (http://www.sdp-si.com/) for about $18.00 total. Parts list as follows:

Timing pulley P/N A6A16-060DF2508, aluminum alloy, 2 flanges w/hub, 1.528" pitch dia., 0.250" bore

Timing pulley P/N A6A16-021DF2508, aluminum alloy, 2 flanges w/hub, 0.535 " pitch dia., 0.250" bore

Timing belt, single-sided, 0.080" MXL pitch, 1/4" wide, urethane/kevlar materials, 7.76" pitch length

The Robo-Focus motor and Tak 8mm coupler kit were purchased from Technical Innovations. The custom Robo-Focus bracket was machined by me in my home shop. By removing the locking knob on the focuser, the Robo-Focus bracket was attached to the Takahashi R&P focuser by means of a M8 hex socket head bolt threaded into the factory tapped hole.