CODY Astronomical Society

 

The Meade ETX 90mm Maksutov Cassegrain telescope

by Gordon Lloyd                                     etx90ec.gif (8680 bytes)

I will not write here a full description of this telescope because it is described in the Astronomy Magazines. I spent some time viewing the Moon through it and the Society's Zeiss 150mm refractor alternately, on the same nights. Obviously, the Zeiss telescope is a superb instrument and has a massive, rigid, professional mounting, but I was impressed with the relatively tiny Meade ETX.

To make a fairer comparison, I sometimes used the 100mm reduced-aperture "stop" available on the Zeiss telescope to make it more comparable with the Meade’s 90mm aperture. On Nov 28th, when I did the main comparison, the steadiness of the atmosphere was "good", and for a few seconds, now and again, "very good". At full (150mm) aperture, when the atmosphere was steady for a few seconds, the Zeiss gave its expected, slightly better resolution. The rest of the time, I could see no difference - except for the slightly dimmer image in the Meade. With the Zeiss refractor stopped-down to 100mm, there was no significant difference between them.

There is a slight, internal reflection in the Meade when viewing the extremely bright Moon which is a very small, dim image of the aperture of the telescope with its central obstruction. It really doesn’t matter and probably wouldn’t be noticed by someone less fussy than me!

The only significant drawbacks of the Meade are mechanical. Fundamentally, almost any telescope with a build-standard less massive and engineered than that of the expensive Zeiss Refractor will vibrate when touched. Amateur astronomers live with this fact! However, good design will minimise this. I made a "Wedge" for the Meade ETX and, with less success, adding a "Sight". The Wedge allows the Meade telescope to be set at the correct angle on a tripod to become an Equatorial Mount. If the axis is then aligned with the Celestial Pole (within about 1 of the Pole Star) the Meade’s built-in electric Declination Drive works well and without vibration to track stars well enough for visual use. As with any Sidereal Rate drive, the Moon drifts across its own diameter in about an hour. The Society's telescope has an "Average Moon" tracking rate available. The "Sight" is a simple tube mounted as a Finder. I made this because there is a basic design fault with the Meade ETX. This is that the Finder, as well as being less than ideal, cannot be used with the telescope pointed at higher Declination angles. Therefore, the telescope itself cannot be readily aimed at the Pole Star to align its mounting! This is because, with only about a 1 Field of View, it is very difficult to find the Pole Star without use of a Finder! My tube Sight is therefore mounted so that it can be used when the finder cannot be reached. The problem using it is that it is hard to see if ones view of the Pole Star is actually coming down the tube!

Despite these problems, the Meade 90mm ETX is a worthwhile, very compact, highly portable, small telescope. Its optical resolution is superb for viewing the Moon, Planets, Double Stars, Clusters and some brighter galaxies etc. Its short, closed tube prevents the "Tube Current" problems with longer tubes, particularly if they are open at the top. A Lens Hood should however be made and used. The Meade ETX can even focus down to a few metres to become a "Long-range Microscopic" of astonishing power! Its shortish focal length unfortunately requires high-power eyepieces and/or a barlow lens for the high magnification which its resolution justifies on good nights. The thread on the focusing screw is too coarse for easy adjustment. A second port allows a camera to be attached, with a flip-mirror to the eyepiece for alignment and focusing. The limitations of the Meade are its somewhat wobbly mounting, its irritating mechanical "fiddlyness" in use and its basically small light-gathering power for "Deep-space" viewing of dim objects. However, at around a basic 750 pounds sterling (with three eyepieces), and easily portable in a modest carry-bag, the Meade ETX is at least as good a bet as others in that price range. However, for "Serious" work, (but far less portability) one requires 150mm aperture, a more solid mounting, an adjustable drive, a better focus system and smoother controls.