ETX 90 - The Purchase
Some years agoI was browsing through a local Sam's when I saw the ETX90-M below on sale for $250. I was flabbergasted. It didn't seem that long before when the ETX-90 had been selling for more like $500. I'd read such unbelievable reports about the incredible ETX 90 views, reports I wasn't sure I even believed. ButI thought for $250, I had to find out if it was all hype.ETX 90M
That time frame was before the computerized ETX models. The older models, styled somewhat like (though much more cheaply than) the well known Questar 3.5, had only a motorized polar axis. I didn't know what an M model was, but I decided that for $250 I was willing to find out. I found that it was a variation of the ETX 90 RA model, mainly coming with two eyepieces instead of one, but the two were Meade Modified Achromats (MA) and of lesser quality than the Plossl that came with the ETX RA model.
Mine came with a MA 25mm and a MA 9mm. These gave magnifications of 50x and 139x respectively. A good pair of magnifications for general use. The MA eyepieces are basically a form of Kellner eyepiece.
If you want one of these old ETX 90 RA models now, you can still find used ones on EBAY. You aren't likely to find mine there, as I consider it a keeper. In the meantime, Meade has moved on from that simple and elegant telescope. You can still get something I think is comparable, as handy, and at a similar price. I'm thinking of the Orion StarMax 90mm TableTop Telescope
. No clock drive with this simple design, but great optics, super portability and simplicity, just the features I love about my ETX 90.Another option is to get the Celestron NexStar 90SLT Mak Computerized Telescope
. Like the Meade ETX 90, it also is a 90mm Maksutov telescope on a motorized mount. But this one has the Celestron SLT altazimuth computerized mount, which operates much like the more modern computerized Meade ETX 90 version.And if it's an ETX you must have, you can go for the new computerized version, like the Meade Instruments ETX90 Observer Maksutov-Cassegrain Telescope
. The computer in this model is necessary because in altazimuth mode, both the vertical and horizontal axes must rotate to keep an object in view, and the rotational rates vary with position in the sky. Thus, a computer is necessary for keeping the two drive motors (azimuth and elevation) turning at the proper rates. It's the same fine telescope on an advanced mount, for less than the original sold when introduced, even without the computer. You can get up to speed on telescope mounts on my Telescope Mounts post.The computerized mount has an additional less obvious advantage over my old model of ETX, in addition to the computer assistance. That is, the new model sits in an altazimuth position, with azimuth and elevation axes. The advantage of this is all that parts of the sky are easily available. You can sit as comfortably viewing northern objects as you can southern objects. My older model, as illustrated in the above photo, is generally used in an equatorial configuration. This configuration works fine with the simple right ascension drive motor. And (for northern hemisphere observers), views of objects from an arc through the zenith to all objects south of that are easily available. But northerly objects put the eyepiece in an awkward position on the bottom side of the telescope. All fork mounts suffer that inconvenience.
When I examined my non-computerized ETX 90 purchase, I was happy to see that the ETX had many of the features of the venerable Questar, a telescope I'd ogled over periodically for nearly 50 years, but never found the cash to purchase. Of course, for the money, the Questar is a much finer piece of equipment. Sort of like comparing a Timex, a perfectly adequate watch, with a luxurious Rolex. The optics in the Meade ETX 90 are perfectly adequate (excellent in fact), but the rest of the telescope is much less refined that those of the Questar.
In my periodic impulses to obtain a Questar, I would eventually remind myself of the other fine (and larger) telescopes I could get for considerably less money.
None-the-less, Meade advertises that the ETX had precision optics equal to any telescope on the market. The incredible price was due to the heavy duty plastic construction on the ETX mount rather than the polished aluminum of the Questar, and a less extensive ensemble of features.
The summary version of this post is that I find the ETX optics to be of absolutely top quality, and the mount idiosyncratic, but usable. The image below of the lunar crater Copernicus was take with the ETX 90 RA, and demonstrates the optical quality. The shortcomings can be inexpensively overcome if one desires. Newer versions all have the same exemplary optics combined with computerized mounts. In this image, the excellent optics and conditions reveal the mountain peaks in Copernicus, and the rougher crater floor just above the central peaks.
The Features
My old model ETX, shown above, shares in common with the Questar a 3.5 inch Maksutov optical system, a clock drive on the RA axis, a fork mount with accessory table-top tripod legs, and a flip mirror with rear eyepiece/accessory port. It also has slow motion controls on both axes.
The diagram above points out many of the features, including the external view port, the RA adjustment knob and clamp, the DEC adjustment control and clamp, the focus knob, and the flip-mirror control. Also shown is a custom added slew control box, which gives fast-track and off controls that allow positioning in the RA axis without introducing backlash. Solar system object photography with the instrument would be difficult to impossible without the simple, custom control box.
The Questar, besides having all these mechanical features in far superior quality, has in addition a control to flip an internal Barlow lens into position, a control that switches the view to the built in finder system, and a back that can rotate left and right (from the image perspective) to allow the eyepiece to be more comfortably positioned.
The ETX, being a simpler and less expensive piece of gear, has no internal Barlow, a less handy drive system, and a small (barely useful) finder telescope. I often use an Orion Zoom eyepiece with my ETX, which largely compensates for not having a built in Barlow.
First Light
On the first evening that I had the ETX, I set it up and peered at Jupiter and the Galilean moons. I was pleasantly surprised to see the moons appear as perfectly defined tiny discs. I'd never seen the moons so well defined, having viewed them generally with Newtonian telescopes, which always added a few spikes around the moons. Jupiter's cloud bands showed clearly, and it was noteworthy that no color fringes were seen at the planets sharply defined limb. That was a bit disorienting at first, seeing refractor-like views but no color fringes.But, on this first experience, all was not euphoric. I encountered a couple of disappointments.
Disappointment one -- my big hands had trouble reaching in between the wide fork supports to manipulate the RA adjustment and clutch knob. The fork supports are much thicker than the trim aluminum ones of the Questar, and being plastic must be so to have the necessary strength.
Disappointment two - After only a few minutes of passable tracking, the clock drive seemed not to be working at all. I could put my ear to the base and hear the motor running, but the telescope was not tracking.
I scoured the manual and rechecked my procedures, and was left with only one conclusion, the clock drive was just not working properly.
I also found that the simple straight-through finder telescope was of little value. It was difficult to get into a position to peer through the finder, and it didn't focus sharply.
The Solutions
I found that the base was a very simple design. A small motor with a spur gear that drove a large heavy-duty (about 3 inch diameter) plastic gear that drove the RA axis. A simple clamp that locked the base to the gear was what engaged the drive. A circuit board acts as a voltage regulator to maintain a constant voltage on the DC clock drive motor for proper speed control.
Upon examination, I found that the motor in my unit worked properly, but the spur gear was slipping on the motor shaft. A drop of epoxy that covered the end of the shaft and the hub of the gear solved the problem nicely.
As to the finder, after struggling to use the one provided on my ETX I began to appreciate why Questar came up with their clever design. It's just very difficult to find room for any kind of finder on such a compact telescope.
To solve my finder problem, I replaced the finder with the Meade elbow-finder produced for the ETX-125, and it has worked better. It is still a bit clumsy in that the eyepiece of the elbow-finder gets in the way of viewing through my shorter eyepieces. One ETX owner suggested turning the elbow-finder so that the eyepiece of the finder sat about 45 degrees from the telescope eyepiece. I followed that advice and it reduced the interference problem. You can see the canted elbow finder in the above image.
To solve the problem of my clumsy big hands trying to work the RA controls, I concocted an on/off/fast-track control housed in a separate control box. Wires run from the inside of the fork mount base to the small hand-held control box (available from any electronics parts supply). The control box is visible at the bottom right of the above image. The control box is attached to the telescope base in a storage position with velcro when not in use. It is detached when in use, and then hand-held.
On the control box is one toggle switch and two momentary push button switches. The toggle switch is wired in series with the telescope's internal on/off switch. It simply extends the on/off switch to be more readily available than the original switch on the bottom of the base.
One of the momentary button switches is normally on, and is in series with the on/off switch. When it is pushed, it interrupts power to the motor, causing the Earth's rotation to make the viewed object "catch up". Think of it as a "slow" button. This procedure doesn't cause the backlash to reappear that occurs if I loosen the clutch to allow tracking adjustments.
The other momentary button is wired to allow battery power to bypass the regulating portion of the circuit board, so it applies full battery power to the motor. This causes the motor to run at about twice normal speed. Think of it as a "fast" slew speed. The combination of the two momentary switches gives a fast/slow slewing control to the RA axis. With this hand-held controller, the telescope is now quite convenient to use, and is one of my favorite instruments. I've been able to engage in some astrophotography with the unit now that precise pointing and clock drive control is available.
The computerized designs now used by Meade actually attack both of my trouble points, so I would have to say that I'd not hesitate to buy one of the newer units -- if my old one ever breaks. I must say that with all the plastic used in the telescope's design, I was concerned that it would not hold up. Now, over 10 years later, I can say that I've had no problems with my ETX 90 RA holding up.
The Views
In general, the high resolution views through the ETX are quite extraordinary. Images are very crisp, making the unit a fine lunar and planetary performer. There is no noticeable color fringing, and the stubby design of the telescope and mount delivers steady images. The above image of Mars, taken through the ETX in 2003, is a good example of the quality of images it provides. Check out the More ETX Images for more photo examples.
I generally view through the ETX at about 150 power, but on good nights on bright objects I can get to 200x. I can see the large-scale features on Mars, and of course the polar caps.
On Saturn, I can see the lighter color of the equatorial band, and can clearly see the Cassini division of the rings. I can see about 4 of Saturn's moons as tiny points of light.
On Jupiter I can see the dark bands nearest the equator, and an additional dark band in the Northern hemisphere. The Great Red Spot is visible as a light colored feature. I cannot really see much color on Jupiter with my aging eyes. Jupiter appears as a mostly black and white image.
The moon is a wonderful object with the ETX, especially with the fabricated hand-held controller. With a twist of the elevation control I can move up and down along the moon's terminator, and with the push of a button, slew left and right. Trolling around on the moon is a delight.
When comparing the ETX with the 6 inch telescopes, the ETX holds its own. On most nights, little more detail is seen on a planet or the moon with the bigger telescopes than with the ETX. But on good nights, as you would expect, the bigger telescopes will show more detail.
Star Observing with ETX 90
As to star observing, At first I didn't find this model of ETX all that useful. The aperture is big enough to show all the Messier objects for example, as well as hundreds of other objects. But with the combination of the rather narrow field of the Maksutov optics and the awkward finder, locating star objects is somewhat difficult. I'm sure the computerized versions help considerably in making the ETX a better star telescope.To address that problem with my instrument, I developed a program for my HP calculator to aid with finding stars and deep space objects telescopes that have setting circles. The calculator can hold up to a few thousand object coordinates, provides an easy alignment procedure, and presents a simple menu to select objects for view. The program accounts for Earth rate, so once the RA axis ring is set, it's good for the night (of course, some more expensive mounts have motorized RA setting circles that do this compensation automatically).
I've found that I can use this utility quite successfully with the ETX. By aligning the polar mounted telescope with Polaris and doing the utility's alignment procedure (which tells me where to set the RA axis) I find that the ETX makes a very nice instrument for star observing. The setting circles are plenty accurate for pointing the telescope. While it is true that for the diameter of the instrument, the ETX 90 has a rather large f ratio. Even so, with this diameter instrument the resulting field of view is very close to that of the venerable f/8 6 inch Newtonian of past, which was (and is) considered to be a good general purpose instrument.
With a 25mm Plossl eyepiece to aid in locating objects from the calculator supplied coordinates, I find that most times the object I seek using the calculator program as a starting point are within the field at first glance. To increase my chances, I've found that using my old 40mm MA Meade eyepiece works even better, as the view encompasses a larger area.
Now that I have a means of easily finding objects with the ETX, I can report that it's a surprisingly good instrument for viewing many star objects. It's a natural for double stars, and even most Messier and other objects show up very well. The instrument, though modest in size, gives pin point images and offers high contrast views, thanks to the telescope's internal baffling. It presents star fields with a very flat field, with stars in sharp focus to the edge of the field of view.
Since modifying the clock drive, I've tried a bit of photography through it. The drive isn't good enough for long term time exposures of star objects, but has worked well for photographing the moon and planets. The mount is still a little idiosyncratic sometimes. I often find that after I mount the telescope to the tripod wedge, I have to rotate the base to a position where the clock drive isn't jerky. I have yet to resolve that problem, but I can work around it with the simple rotation technique.
Recommendations
Would I recommend the ETX 90 RA? With some reservations, yes. Optically it is a wonder that is sure to please, but the clock drive setup needs some refinement to be useful. That said, it is clear that I'd recommend the newer models, like the Meade Instruments ETX90 Observer Maksutov-Cassegrain Telescope.The newer ETX 90 models solved my main complaint in an even more grand fashion by including drive motors on both axes and a computerized driving system. If your interest is a portable instrument to augment your equipment arsenal, I think you'd find the ETX90 a great supplement. If you don't want the computerized model, then keep your eyes on Ebay for an RA model and get a telescope with superb optics for a bargain.
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