Takahashi Epsilon-210 F/3 Astrograph
by Richard Jacobs, M.D.
Over the last three years, my astronomy has migrated from visual observing to CCD imaging. The CCD has allowed me to reclaim my light-polluted back yard in Chandler for observing. It also allows me to keep a permanent record of what I see each night. While imaging deep space objects, I quickly gained an appreciation for the advantages that fast optical systems provide. Focal ratios below F/6 are naturally associated with wider field of views and shorter exposure times for the same image quality, all other things being equal. For this reason, it didn't take much to get me to try out the Takahashi Epsilon-210 F/3 astrograph.
The Epsilon-210 is an expensive Newtonian reflector for its mere eight inches of aperture. But I quickly recovered from the high sticker price when I received the new scope a mere 3 weeks after ordering it. The fit and finish of this telescope is simply perfection. Everything about the Epsilon-210 is solid and proclaims its high quality. The optical tube assembly is an unusually bright yellow color. Takahashi trims the scope with their signature gray-green endplates. My first impression was that this scope is built like naval artillery. The optical tube assembly is 750 mm long and just over 280 mm in diameter. And it weighs in at a hefty 35 pounds.
The next feature of the Epsilon-210 to capture my attention was its large, unusual helical focuser. The helical focuser is just over 6 inches wide and consists of three, spoked dials. Takahashi elected to use the helical focuser on this F/3 astrograph in order to facilitate obtaining the precise focus required of short focal ratio optical instruments. In fact, one complete revolution of the helical focuser advances the focus only one (1) millimeter. The dial used to focus the scope is calibrated and numbered. Each gradation on the focusing dial represents only 0.028 (28/1000) millimeters. A focus-locking dial is located below the focusing dial. Once a good focus is obtained, it can be permanently secured by tightening the locking dial. Closest to the optical tube assembly is the third and largest dial, which allows the user to orient the camera at any angle without disturbing the final focus. This astrograph can be used for visual observation, but it is clearly designed for photography.
The primary mirror aperture of the Epsilon-210 is 210 mm. It is figured as a hyperbola rather than the usual parabola. Its focal length is 628 mm. For this reason, the helical focuser contains a four-element corrector lens assembly that is self-contained in its own cell. This corrector lens assembly can be removed by loosening three small setscrews in its cell collar. The primary mirror end of the optical tube assembly is closed off by an adjustable vented plate. Thermal equilibration of the primary mirror can be expedited by opening the vents in the plate. The primary mirror is attached securely to its cell by its center, which is not aluminized. Adjusting a system of three collimation screw sets can make adjustments of the primary mirror.
The secondary mirror is a full 88 mm along its minor axis. Four spider vanes suspend the secondary mirror holder and mirror over the front aperture of the optical tube assembly. The secondary mirror can be rotated or telescoped up and down the long axis of the optical tube assembly after three setscrews are loosened in its attachment collar. A coarse secondary mirror collimation can be obtained with the primary mirror in this way. Adjusting the collimation screws in the head of the secondary mirror holder can make fine adjustments in secondary mirror collimation. Three "pull" screws and three "push" screws can be alternatively tightened or loosened to position the secondary mirror for final collimation. The entire secondary mirror assembly locks down very securely after collimation.
Cloudy monsoon weather has made extensive field-testing of the Epsilon-210 possible. The telescope saw "first light" from my back yard during a brief break in the clouds on the evening of August 20, 2000. With the proper adaptor, the SBIG ST-8e CCD easily attached to the helical focuser of the Epsilon-210. Focusing the CCD was somewhat of a logistical challenge as the entire camera had to be twisted around through several 360-degree revolutions to achieve a coarse focus. This is more complicated than it sounds as a parallel cord, auto guider cable, and power cord all connect to the camera itself. Once a coarse focus was achieved, a final focus was easily obtained by making small turns of the helical focuser. This was where the wisdom of this focusing configuration really paid off. And the helical focuser can be locked into position for future imaging sessions.
M15, the globular cluster in Pegasus, was chosen as the test subject. A quick calculation informed me that the Epsilon-210's short 630mm focal length would undersample images for my 9u X 9u pixels in the ST-8e. For this reason, I was anxious to see what a point source object would look like. The image of M15 was breathtaking and sharp as a tack. Resolution of the globular cluster was obtained down to its core. A fine image of M15 was obtained after only three 5-minute integrations.
My next test subject was the spiral galaxy in Pegasus, NGC 7331. The Epsilon-210 pulled in the entire galaxy group with only a very brief CCD integration. I was also very shocked to see Stephan's Quintet in the same field of view as NGC 7331! I began to image this amazing site when the monsoon clouds roared in to ruin both the image and my night.
Overall, I am very impressed with the Epsilon-210. Its quality is top notch. But it is expensive and much better adapted for photography than visual work. The Epsilon-210 is a little tricky to collimate and it is a fairly heavy optical tube assembly. For this reason, a relatively heavy-duty mount is recommended for this scope. In the final analysis, I am very glad I made this purchase and I am looking forward to using it - whenever and if ever the weather clears!