MEADE ETX-90EC, CELESTRON NEXSTAR 5 GUEST COMMENTS
Subject: NexStar / ETX-90 comparison From: firstname.lastname@example.org (Autostar Software Review Project) ETX-90 and NexStar 5: A Comparison What follows is an attempt to compare the ETX-90EC and NexStar 5 telescopes. Readers looking for a clearcut up or down vote for either telescope will be disappointed. There are, however some points which will, I believe, prove instructive. It's important to understand that comparisons of what either scope can or can't see are almost meaningless. The ETX-90's overall diameter compared to the NexStar's diameter puts it at a disadvantage as far as light grasp is concerned. Additionally, the ETX-90 is a Maksutov-Cassegrain design (deeply curved compensator lens with the secondary mirror being a reflective film deposited on the inside of the lens) while the NexStar 5 is a Schmidt-Cassegrain (nearly flat compensator lens with a slightly larger mechanical secondary mirror that has to be adjusted occasionally). It's also worth noting the ETX-90 is an f/13.8 system while the NexStar is an f/10 system. For the same magnifcation, each system will give the same scene brightness but the faster scope favors astrophotography; Meade specfically says the ETX-90 is not intended for astrophotography. In terms of subjective impressions, both scopes produce sharp enjoyable images at magnifications typically used for viewing objects ranging from open globular clusters to planet disks. Even more importantly, these scopes deliver good images in locations with significant light pollution. I've experienced some mirror shift with the ETX-90 but the NexStar's mirror shift is imperceptable. Manually focusing the NexStar takes a little patience because, of course, turning the knob means moving the OTA. Nonetheless, the vibrations stop quickly and achieving a sharp focus is easy. The NexStar focus shaft could use a bit of drag; it's motion is almost too loose. Most of my observing is from my front yard, with street lights on either side of where I set up my telescopes. I use the shadow of the telephone pole holding the closer light (perhaps 25 feet away) to screen out the light but the other light, 65-70 yards away, goes unscreened. Throw passing cars into the mix of light sources and any real dark adapatation is limited at best. Still, it's possible to resolve close binaries and observe significant detail in M42, the Great Orion Nebula, without resorting to nebula filters. Standing side by side, the Meade Deluxe Field Tripod and Celestron's NexStar Tripod look remarkably similar but there are several important differences. It's likely that the same Taiwanese company supplies the tripods; the leg extrusions and accessory hardware are almost identical. The legs on the NexStar are a couple of inches longer and the adjustment clamps use knobs with two paddles on the edge while Meade's tripod has three paddles sticking out of its knobs; hardly anything to get excited about. Moving up the legs, however, a major difference shows up. Both tripods have struts that join at a triangular plate under the center of the tripod's head. Meade's tripod has simple metal stampings for the struts and a thin center plate that bends too readily. Celestron uses thicker plastic struts that meet at a much stiffer plastic center plate. Meade has a round accessory tray with several holes for 1 1/4" and .965" eyepieces. A threaded stud in the center of tray sticks down through the center plate and another threaded three-bladed knob clamps the tray in place. Except where the plastic around the thread insert presses against the plate, there is no other stiffening for the struts or center plate. Celestron's triangular tray also bolts to the center plate with a three-bladed knob for a nut and another with a threaded stud acting as bolt and nut. Three additional bolt and nut sets secure the tray roughly midway down the length of each strut. The accessory tray is located in four different places, making the tripod assembly much stiffer than the shorter Meade tripod. Interestingly, there are slotted holes in the Meade tripod struts. Mounting a tray similar to that used on the Celestron tripod should be easy and a good project for an evening when the skys are clouded over. I used a 5 pound scuba weight in the Meade tray in an effort to load up the structure and stiffen it. That was effective but the weight takes up too much of the tray's area. Hanging the weight under the tray is not recommended as any swaying will adversely affect observing by making the tripod sway slightly. In my experience, Meade's Autostar software (including the loader, Autostar Updater, as well as Autostar, the firmware for the telescope) has a somewhat checkered history. See the discussions in Weasner's Mighty ETX Site for the details. What matters here is that in the past few months, there have been significant changes in the software for all ETX's and the LX-90 (which also uses the Autostar system). These changes included Autostar Uploader A2.3 which was almost unusable because it crashed any system it ran on and didn't run on Windows NT/2000 systems. Problems have also been reported with equitorial mount tracking rates in right ascension (RA). Celestron's NexStar system hasn't been fault-free either. Among other problems has been an occasional controller lockup that left the NexStar scope continually turning in azimuth on its base, problems with the RS-232 (serial port) link, and other less frequent problems. While not a bug, per se, early versions of NexStar did not include the Moon as "select by name" object but did and still do list the Sun as an object. It should be noted, however, that the scope can be slewed to the Moon and a lunar (as opposed to sidereal) tracking rate selected; NexStar could and can track the Moon. Getting there was the only real problem. NexStar also allows pointing the scope at the Sun although there is a warning message which must be acknowledged before motion will begin. As of mid-February 2001, Celestron does not have any software updates on its web site while Meade does. Nor does Celestron list any specific software or firmware update policy. When one stops to consider the work being done by the dedicated computer systems, that there aren't more problems is the real surprise. Each system maintains an extensive database of objects and converts the object's location in telescope pointing information while maintaining, accurately, the time of day and handling operator key presses for data and telescope motion. Autostar is designed to handle a range of telescopes and uses the LX-200 command set. It's quite versatile, being able to select stars, NGC, IC, and Messier objects as well as asteroids, comets, and satellites. The guided tour function enhances the ETX's usefulness even further. With it, even a novice observer can view a wide variety of objects without knowing their location; it's all in the Autostar's databases. The handbox, in addition to controlling the telescope, can also accept commands from an observing program that can send and receive the LX-200 command set (e.g., SkyMap, Deep Sky 2000). It also can be used to control the Meade Electrical Focuser. The focuser handbox gives two rates while using the focuser through the Autostar handbox gives four speeds. In general, the slowest speed is still too fast but it can be used by briefly moving the motor and checking focus. The NexStar's controller, although capable of holding a large number of objects, is not as versatile as the Autostar. There is no support for comets or asteroids (except to allow either RA & dec or alt-az coordinates to be input) and no way to track a satellite other than perhaps a geostationarry satellite. There is a tour function but it extracts a number of objects visible in a given month; there is presently no way to script a tour. Several sources have reported that stars can only be selected, as a target, by entering a number which is not an SAO or other known catalog number. That's not quite accurate. There is a "stars by name" function which will access approximately 40 stars by name (e.g., Rigel, Vega). The full list of over 10,000 stars does require a look in a directory, available on line as either an Excel spreadsheet or Adobe Acrobat .PDF file. The spreadsheet makes it possible to sort the catalog by RA, name, and so on. It is a cumbersome arrangement but many common stars can be accessed by name. The basic task of either controller is to point the telescope accurately at an object. How do the two systems stack up? I think the edge goes to NexStar but not by much. For either telescope, the initial setup is all-important. Although the idea sounds excessive, use a bubble level to level the telescope's barrel after leveling the tripod. Practical experience shows this is a crucial part of establishing the scope's frame of reference. It's also essential to get the telescope pointed at true north before starting alignment. Here it's more a matter of speeding up the alignment process but accurate orientation does make a difference. When turning the scope towards north, move the whole tripod. Don't turn just the base. The Autostar and NexStar handboxes are similar in appearance but there are a couple of operational features in the Autostar handbox worth noting. First, the brightness and contrast of the LCD display can be varied in the Autostar but not in NexStar. Curiously, there is no way to dim the lighting in the buttons. In the NexStar handbox it's all or nothing. The display can be turned on and off as can the buttons but there is no dimming or contrast control for the display. Pressing any button on the Autostar handbox stops any slewing immediately. With earlier versions of NexStar firmware there is no way to stop slewing once it begins on some versions of the software. With version 184.108.40.206, however, pressing direction keys does stop slewing. It should be noted, however, that there are no obstacles to NexStar scope motion whereas the ETX OTA can only depress so far before hitting the base. The lack of a test for elevation can produce the curious sight of the NexStar slewing to point below the horizon to begin tracking an object. Although the NexStar handbox doesn't report an object being below the horizon, pressing the INFO button shows the object's current azimuth as the first piece of information. Passing up negative values avoids seeing the NexStar try to go into what one reviewer calls "X-ray mode". There is no mount for the Autostar handbox while the arm for the NexStar OTA has a recess specifically built to hold the handbox. Engaging the hooks for the NexStar handbox , however, is often a frustrating process. A few patches of Velcro (tm) hook and loop fastener material on the back of the handbox and around the base work well with the Autostar handbox. The Autostar handbox includes a small LED flashlight, handy for reading the setting circles on the forks and base. There is nothing similar with the NexStar handbox. I use a ScopeTronix "red dot" zero power sight to confirm the ETX's position when setting up the alignment. Once the dot is close to the target, I check in the finder scope and then move to the eyepiece to finish the alignment. If enough care is taken, the results pay off handsomely. The scope will actually point at a target when the motion stops. The NexStar alignment process is the subject of some misconceptions. First and foremost, it is possible to move the OTA with the direction buttons before doing an alignment, contrary to some comments I've read. Tracking, however, does not happen until an alignment has been done. Use the direction buttons to move the OTA into the right place (and again use a bubble level on the OTA). Because there is no motion stop (the NexStar can rotate though 360 degrees of azimuth repeatedly if not in "cord wrap" mode), it's OK to use the direction keys to aim the OTA. By contrast, the ETX can only turn so far before internal wiring is twisted too much. The firmware, however, takes this into account so long as the OTA is placed in the proper place at start-up. Unlike the ETX, the NexStar can update its alignment stars as observing goes along. This ability should be underlined because it's a feature that Autostar should have, too. Here's how alignment updates are done. If the scope was aligned on Rigel and Pollux and things don't center up well when looking to the southwest, it's possible to point at Aldebaran (use "Star by Name" to select the star and Enter to move to the star) and then press the Align button. NexStar asks which star's alignment should be removed (I'd remove Rigel), tells the observer to center Aldebaran in the eyepiece and then press Align again to confirm the change. Some people think Autostar's SYNC function is similar but it only works over a very limited area while NexStar's alignment update can be used anywhere there's a star known to the internal database. Moerover, in Meade's web site's FAQ section, using SYNC repeatedly is specifically discouraged. The ability to update alignment on the fly is, I think, part of why NexStar's pointing accuracy is consistently slightly higher than Autostar. As errors begin to show up, a star in the area of interest can be used to update alignment. Autostar "remembers" information about the slack in the gear train between the two positioning motors and the OTA or telescope base. Any time the firmware is updated, the program must be "introduced" to the motors by the Train function. While the overall procedure is documented in the ETX manual, one vital point is missed. It is *crucial* that the OTA be elevated to about 45 degrees. Training with the OTA nearly level will cause a problem called Snap-back Slewing. The symptoms are unmistakable. Attempts to position an object in the field of view are thwarted by an unrequested move away from the target. The scope seems to be snapping back to its last known position. Training takes into account the weight of the OTA as well as slack in the elevation gear train. Training with the OTA close to horizontal simply doesn't expose the gear trains to the same loads seen when the telescope is elevated. Again the tripod must be level and the OTA must be leveled before raising it. NexStar doesn't require training. Just level it, point it at north, and do the alignment to get going. Does this make the ETX a worse telescope than the NexStar? No, only different. The NexStar motor and gear drive system is less complex but uses different, and more expensive, components. Both telescopes can have the amount of of anti-backlash tension (the motors back up to take slack out of the gear trains) set by the user. One place that I do think the NexStar clearly excels over the ETX is the base and mounting fork design. Plastic is not a bad construction material but Meade simply hasn't made the forks and their attachment to the base rigid enough. It's possible to apply relatively light pressure at the objective end of the ETA and see the forks twist. Doing the same thing to the end of the NexStar's (longer) OTA gives a bit of twisting in the tripod but no clear deflection. Although the single arm seems inherently weaker than the two used with the ETX, the aluminum casting is simply far stiffer. This pays off better resistance to light breezes and brief contact with the OTA while observing. In the best of all possible combinations, Meade optics and the Autostar controller on a NexStar mount and tripod would be my choice. Anything else means considering one's needs and requirements carefully to make a decision. If a feature-rich controller with the ability to track comets or satellites is important, the ETX is the clear choice. If the need is for a solid mount without doing after-market modification, the NexStar is the clear choice. Either way, though, today's telescope buyers have a choice of riches that even a few years ago might have seemed impossible. # 30 # (c), 2001, Richard B. Emerson, All rights reserved.
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