Introduction:

Measurement	Cassegrain Focus	Newtonian Focus
Effective Aperture	212mm	212mm
Focal Length	2630mm	820mm
Focal Ratio	F12.4	F3.9
Diameter of secondary	Ø68mm	na
Diameter of diagonal	na	Ø72mm
Resolving power	0.55"	0.55"
Light gathering power	917x	917x
Limiting magnitude	13.4	13.4
Finder scope	7x50 6.3 deg.	7x50 6.3 deg.
Length of main tube	approx. 855.mm	approx.855.mm
Weight of main tube	approx..5kg	approx. 8.5kg

As is evident from the data, this scope can perform as a high resolution, high power cassegrain, which is ideal for planetary, double star, and lunar studies. With the swap of the secondary mirror, the scope can then act as a fast Newtonian offering fine wide field views of the skies. Using the provided field flattener, the Newtonian becomes an astrograph for film or CCD imaging. This is truly a versatile scope.

Arrival

• CN-212 optical tube with cassegrain secondary mirror, 1.25" adapter.
• 7x50 Finderscope.
• Metal primary dustcap.
• Metal 1.25" dustcap.
• Plastic clip-on Newtonian port dustcap.
• Newtonian diagonal secondary mirror assembly.
• Newtonian eyepiece adapters with field flattener housing.
• Collimating telescope assembly for the cassegrain mode.
• Collimation eyepiece and tube for Newtonian mode.
• Allen wrench and spanner for collimating the scope.
• User's manual, serial number and warranty sheet.

The telescope itself is like all Takahashi telescopes: It is a treasure to behold. The main tube is a glossy off-white finish, and the ends are the standard Takahashi green paint with a bit of texture to it. Looking into the tube shows the expected pitch black coating on all surfaces (but the mirrors!). There is a long baffle tube running through the center of the primary for cassegrain focus. This baffle is in turn baffled with rings along its length internally. The cassegrain secondary also has an extended baffle. The spider is a four-vane variety and is matte black on all surfaces. Focus is achieved by moving the primary mirror - in all modes. This can be an extreme reach when visual observing in Newtonian mode, but for CCD imagers, this is sometimes even more convenient. Mention should be made that at f/12.4, focus is not as critical as it is at f/3.9. The finderscope is the standard Takahashi 7x50mm scope which is permanenly attached to the main tube. Again, like other telescopes in the Takahashi line, the finderscope is used as a handle to lift and move the main tube out of the box and onto the mount. The optics of the finderscope are tremendous with a crosshair that does not block the exact center. A reticle illuminator assembly is available for the finderscope as an option.

The Takahashi mounting rings (Tube Holder) are standard Takahashi design with the textured Takahashi green paint. The hardware is all very solid and felt lined to prevent any tube damage. A single clamp holds the halves of the tube holder together. The other side is hinged. My mount is a Losmandy G-11, so I wanted to attach the tube holder to a Losmandy universal dovetail plate (DUP). This is easily done with a couple of cap screws (hex bolts) that came with the rings. You will want a washer or two to take up extra bolt length so as not to scratch the saddle on the G-11. All holes line up perfectly. No additional dilling nor adapters were necessary.

For Cassegrain focus, the secondary mirror is left in place as shipped and a 1.25" visual adapter is used for eyepieces and accessories. If one wants to collimate it, a collimation telescope is provided and makes collimation very easy during daylight hours. It focuses on the secondary mirror which has a small dot painted on its center. Fine tuning is then done at night on a real star with a normal eyepiece.

For Newtonian focus, the secondary mirror must be replaced with the provided diagonal secondary mirror. The alignment between mirror swaps is maintained by a notch/groove system on the mirror spindle and the spider. Collimation between mirror swaps is maintained well. Collimation of the Newtonian focus is a bit more difficult, with instructions in poorly translated English (from Japanese). A collimating eyepiece (a hole through the center of a 1.25" aluminum plug, and an extension tube with monofiliment crosshairs are provided for the task. After 30 minutes, collimation should be achieved by someone really paying attention to the process. Do it during daytime and with the cassegrain focus unplugged, so that light can enter from all sides of the tube assembly. Again, final tweaks to collimation are done at night with a real star and a real eyepiece.

First Light

First light was made using the telescope in Newtonian mode, since that will the be the mode primarily used at the Northwood Ridge Observatory for asteroid and variable star work. The secondary mirror was swapped from the cassegrain to the diagonal for Newtonian focus. It should be noted that collimation, according to the preliminary tests with the provided collimation tools, was spot-on. For Newtonian visual work, the provided field flattener is removed and an extension tube is added. The focus is far out from the telescope's body, thus requiring the extension tube. I used a 36mm Plossl and was astounded. The field was lovely, right to the edge. I just slewed the scope through the summer Milky Way and enjoyed the rolling motion. At such a low power, the secondary shadow was evident, but at higher powers it was not visible at all unless stars were placed out of focus. With the four vane spider holding the secondary mirror, diffraction spikes are visible on the brighter stars, like Vega. Even in Newtonian mode with its fast f/3.9 optics, the background was very dark with enough contrast to see stars throughout the field with bright objects in the same view. Diffraction rings were concentric, stable, and equal both in and out of focus. Focus was very tight: there is not a lot of leeway with f/3.9 optics. The scope is either in focus or not, and this is obvious at the eyepiece.

M-57 was the first deep sky object to be viewed. It was readily obvious and very bright. There was an obvious brightness to the center of the ring, which in many scopes appears as a dark space as of surrounding sky. This scope does indeed show nebulosity and contrast very very well. Focus shift is almost non-existant. See the CCD section below for this discussion.

M-67 was next. It was easily resolved to the core as a sparkling group of pinpoint stars. I was almost sad that I had not started this testing out in cassegrain mode. I am sure that it would be an awesome sight.

I then shifted over to M-27 and enjoyed a lovely view of the Hourglass in what looked like an almost 3-Dimensional view of the nebula floating in a sea of stars. It was a textbook image across the whole field of view. Coma is not visible (with my lousy eyesight), but I suspect that it is there. Why would Takahashi provide a field flattener for photography if there was no coma? For visual work, it is not an issue.

CCD Work

I started by achieving focus on Vega. It was certainly bright enough. I started out in binned 2x2 mode and quickly got to a close focus by watching the spider vanes merge. One thing is definitely true with this telescope: it is a light gatherer, and very little is wasted. In a one-second integration, the CCD was blooming on Vega. This does not happen on a C-8 at f/6.3. From this data point it became obvious that many fainter objects will be reachable while imaging.

From Vega, I moved down to the Ring Nebula, M-57. I was able to get a fine image with a one-second integration. Ok - too short, so I went to a minute. I was blooming stars in the field. This is good. M-27 was next, and, as the image shows, the detail is phenomenal in a 5 minute exposure.

I then moved onto M-67, then the Eagle Nebula. All the images will speak for themselves as to the quality of the optics in this telescope. Focus was interesting. It is very sensitive being an f/3.9 system. The focus knob is smooth and has a reasonable amount of resistance that gives a very solid feel. There is much less focus shift in this telescope than in any SCT I have ever used (since SCTs use the same method of moving the primary mirror, I am comparing the Takahashi to SCTs in this regard). In fact, there was so little shift that I was able to tweak the focus while the CCD was autoguiding, and the star did not leave the small 32x32 pixel guide area. That was pleasant! It should be noted though that for those of you who want to do film photography in Newtonian mode will need long arms so you can adjust focus at the cassegrain side of the tube while dealing with the knife-edge at the Newtonian eyepiece end of the tube. I am tall and have long arms, so it is not a problem. If you are shorter, and working on a ladder, then this could pose a dilema.

Overall Impressions and Conclusions:

Last Modified: 3/19/03 8:55p
This page:© Copyright 2005 by John A. Blackwell