Last updated: 15 September 2003
Subject: Review of ETX 90 RA for your site Sent: Tuesday, September 2, 2003 14:43:01 From: email@example.com (Euan Mason) Great web site, thanks. Here's another review for the site. Best regards, Euan -- ================================================================== Dr Euan G. Mason Silviculture, Modelling and Senior Lecturer Decision-support systems School of Forestry University of Canterbury New Zealand's professional Christchurch, New Zealand Forestry School Home page: http://www.forestry.ac.nz ------------------------------------------------------------------ "If in their geometrical conclusions two hypotheses coincide, nevertheless in physics each will have its own peculiar additional consequence." - Johannes Kepler ==================================================================
Meade ETX 90 RA A Review
I recently purchased a Meade ETX 90 RA, and Ive had time to put this superb little telescope through its paces.
It arrived well packed in a shaped compartment in a cardboard box that was itself packed in a cardboard box.
The finish is fabulous, and my first impression was that the scope was really cute. It's a Maksutov-Cassegrain design with a focal length of 1250 mm compressed into a tube 279 mm long. With a 90 mm aperture, it has a focal ratio of 13.8. It is fork mounted, and the base includes a right ascension drive driven by three AA batteries. Three small legs are included that provide an equatorial mount on a table top. A 26 mm Meade series 4000 super Plossl eyepiece comes with the scope.
The tube and base are aluminium and the forks are made of reinforced plastic. All appear to be sturdy. The optical tube assembly can be removed from the forks and mounted on a camera tripod; something that birders might wish to do. However, for birding you would need to purchase an optional 45 erecting prism which fits into the rear of the tube. The tube contains a flip mirror that allows either straight through viewing or viewing through the top-mounted eyepiece holder. According to the manual, the optics are slightly better without the erecting prism.
Installing the batteries involves removing three screws and detaching the base plate. This is not difficult, but it would be inconvenient if your batteries died while you were observing. It's best to use alkaline batteries, because, not only do they last longer, but they dont lose power when they get cold as normal batteries would. At this stage it's a good idea to flip the north/south hemisphere switch to south if you are in the southern hemisphere. The RA motor makes a very slight hum when it is switched on.
A small, 8x21 viewfinder needs to be installed in it's housing, and then you can sight it in and begin observing.
The first thing I noticed was that the adjustable declination leg only went has high as 42.5 latitude. In practice this is enough for Christchurch if you are only observing casually. An optional small leg can be purchased that allows polar alignment at higher latitudes. Apart from this concern, setting the machine up on a table in my front yard took just a few seconds.
The second thing that became obvious was that the finder scope is not terribly useful. It is mounted so that even a proboscis as small as mine prevents clear viewing, and getting under the scope to look through it means driving your head into the table top if the target is in the polar side of the sky.
The controls are easily accessed and work well. Focus is achieved by rotating a dial that moves the primary mirror. Like almost all such systems, there is a little bit of slop, meaning that the image is slightly shifted when one approaches focus from opposite directions. The declination slow-motion control is only useable when the drive is unlocked, so you have to rely on the drive alone to keep an object in view if you wish to use the motor. The declination, on the other hand, has a very effective slow motion control that works after the declination is locked.
The beauty of a Maksutov-Cassegrain design at this aperture is that all surfaces, including the corrector plate, are spherical, and so Meade has been able to achieve consistently high quality optics. All 90 mm ETXs that I have seen have wonderfully crisp optics, and mine is no exception. At 179x with a 7mm Pentax XL eyepiece, Mars polar caps were visible, and I could see dark markings on the surface. Star tests at high power showed almost perfectly circular diffraction rings. The small secondary obstruction and magnificent baffling of the OTA bring out as much contrast as I have ever seen in a reflector.
At this point I noticed that the drive motor wound to a halt. After disassembling the base, inspecting the gears, and carefully testing their operation, I have concluded that the motor drive doesnt work well with a very heavy eyepiece like the Pentax XL. The eyepiece is useable, but you need to track using the slow motion RA control. This makes a mockery of the manuals exhortation to use the motor drive for astrophotography when a camera is attached to the back with an optional T-adaptor. In addition, the drive on my ETX runs slightly too slowly, so that objects very gradually drift out of the field. It's also a feature of ETX RAs that when the drive lock is applied tracking takes up to 30 seconds to begin. One would have to conclude that rather too much cost-cutting went into the design of the drive. None-the-less, I soon learned to judge where to place a target in the field of the 26 mm eyepiece in order to get it centered when the drive kicked in.
Id already heard about the fine optics of Meade ETX 90s, so the biggest surprise came when I observed from our astronomical societys observatory at West Melton on a moonless evening. At 50x, observations of nearby faint fuzzies like e Carinae were simply stunning. The ETXs baffling meant that many of the subtle folds in the nebula were visible, and I couldnt believe this performance from a small aperture, high F-ratio scope. At 75x I could just discern the two bulbs of gas that emerged after the 1840 activity of this star. Truly this scope has extraordinary optics! Perhaps we should reassess the value of optical correction and baffling as opposed to aperture.
Like all amateur astronomers, Ive very quickly added to the accessories that came with the telescope. As a wearer of glasses, Im very sensitive to eyepiece eye relief, and the eye relief on the supplied 26 mm eyepiece is too small for me. Instead, I use a 25 mm Meade series 3000 eyepiece that has generous eye relief as my low power optic for the ETX. Ive successfully used a Celestron 2x shorty barlow lens with it, although the barrel cant be fully inserted into the focusser. I also use an old Vixen 12 mm Plossl that works well to give me 208 times when barlowed.
Recent purchases of a 4 mm Vixen LV and a 5 mm clone of the same design have allowed me to test the scope at higher magnifications. At 250x the image of Mars was magnificent in good seeing, with details on the surface clearly visible. At 312x the edge of the planet was resolved reasonably well, but the diffraction rings from a 90 mm aperture meant that only the polar cap could be seen and other, more subtle details were washed out. 250x is therefore the maximum useful magnification, and that is just over 70x per inch of aperture. What optics!
I chose a red dot pointer to supplement the pointing system. After some pondering, I obtained a small piece of 10 mm channel aluminium, and then spread it to 13 mm by forcing the shaft of a large screw driver into it. This created a perfect mounting bracket for the finder that could then be fixed to the existing finder bracket with permanent, double-sided foam tape. I can now point the scope by keeping both eyes open and placing the red dot on the target. In this configuration the finder is easily accessed, and is aligned every time it is re-attached without needing further adjustment.
I have enjoyed using the ETX in alt-az mode on a tripod, but to get the most from the drive a wedge was the next most obvious accessory. I made mine from two sections of plywood joined with hinges and fastened at the correct angle by slotted window catches on either side. A hole in the base provides easy access to the drive on/off switch. Ive used this wedge on a tripod and also on a permanent pier at West Melton. My new, simple design of a wooden tripod that is rock solid has made the scope fully portable and useable.
Finally, I purchased and adapted a hard plastic carry-on luggage case to transport the telescope. At NZ$95 (US$52) on sale, the case is very sturdy, and I inserted two sculpted pieces of foam. With the telescope, red-dot finder, legs, barlow, and two eyepieces on board, the case weighs about 7 kg. It's small enough to be carry-on luggage on an aircraft, but the scope would safely pass through baggage check-in inside it.
I purchased the ETX for US169.95. This price is available from a couple of sites in the USA (see, for example http://www.telescopes.net ), but the outlets will not send them outside of North America. The new ETX90 EC costs US$495, and I suspect the ETX90 RAs are being sold at less than cost, especially as the eyepiece alone is sold at US79.95.
In summary, I now have a very portable scope with the optical performance of a fine 3.5 refractor. The drive has some drawbacks, but it's functional. With extra accessories, especially the red-dot pointer and the wedge, it is wonderfully easy to use. For the price, it's better value than any other optical aid you can find from a retailer, in my opinion, but as this is probably a sellout to finish the line, it wont last long. If you are tempted, go ahead and buy one you wont regret it.
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