The Dobsonian Telescopes of the Orion brand clearly rise above all others. Regardless of whether it being a Skyquest Classic or an Intelliscope, the Orion Dobsonians offer great value for the dollar. Over the years, however, rumors relating to a Go-To version of these scopes have circulated. This was due, in part, by the availability of such scopes outside the US. In April of 2010, Orion Telescopes and Binoculars announced the availability of the XTg series Go-To Dobsonians, models ranging from 8-12 inches in aperture and priced between $850 and $1,600.
Similar to all Dobsonian Telescopes, the Orion XTg requires some degree of assembly. It was initially assumed that drive motors would require mounting and setting the gear lash would be necessary for the scope to operate properly. When considering the XTg, however, all motors, encoders, and gears arrive pre-assembled and is easier to assemble than the Intelliscope models. Confident in assembling furniture from Ikea or turning screw drivers? If so, the Orion XTg should be no trouble whatsoever.
The XT8g on Paper
The optical tube assembly of the Orion XT8g is similar to the XT8 Classic -- with a focal length of 1200 mm and a focal ratio of f/6. The scope includes an EZ Finder II reflex finder comparable to the Classic models. Right angle finders, such as the 9x50 found on Intelliscope models, shouldn't be necessary for the XTg series. Also included is the two-speed Crayford focuser found on the XX12i and XX14i models.
Two eyepieces are supplied with the XT8g -- the 28 mm Orion Deep View and the 12.5 mm illuminated reticle eyepiece. The 28 mm Orion Deep View eyepiece features 20 mm of eye relief and a 56 degree apparent field. Yielding 43x magnification and approximately 1.3 degrees of true field, this eyepiece serves as a good finder eyepiece with, possibly, the lowest power usable with this telescope. While aligning the go-to function of the scope, Orion recommends using the 12.5 mm illuminated eyepiece.
One significant difference to this scope in comparison to the Intelliscope series, other than the motors, is the additional weight. Although Orion published the weights of the XT8g and the XT8 Intelliscope bases at 32.7 pounds and 21.3 pounds respectively, the XT8g -- the keypad and power cord included -- weighs approximately 39 pounds. In spite of this increase in weight, this scope is still easily manageable for most people.
Two Dobs in One!
While Orion advertises the XTg series as a go-to telescope brand, which it is, the telescopes can also be operated manually. Even when operated manually, these scopes will track whatever they are pointed at. Orion incorporated a clutch-free design, allowing the telescope to be pushed to any object desired. The scope will then track that specific target. Such features make this scope unique to most go-to telescopes, which require the drive motors or the releasing of the clutch to move. In the latter circumstance, altazimuth mounted scopes lose their ability to track. This is not the case with an Orion XTg, however. The optical encoders operate independently from the motors.
SynScan AZ Controller
The SynScan controller, found on Orion Atlas and Sirius mounts, supports altazimuth operations with the XTg scopes, as opposed to the equiatorial operations used by the Atlas and Sirius mounts. However, the software is functionally the same. Included with the controller is a massive database of 42,900 objects, including those in the Messier and Caldwell catalogs, 7,840 NCG objects, 5,386 IC objects, the moon, planets, and more. Although Pluto is not a planet, it is included in the controller as a planet. Pluto is really a dwarf planet.
The SynScan controller may be updated using an on-board serial port. Although a USB port would be desirable, most telescope mounts today still rely on serial communications. Unlike most other telescopes and/or mounts, which are required to be turned on while receiving updates, the SynScan controller is equipped with a built-in 12-volt DC power port, allowing software updates while disconnected from the mount. Anyone who has ever had to carry an entire telescope or mount to their desktop computer for a software update is sure to appreciate this feature.
The SynScan controller offers three tracking rates, sidereal, lunar and solar, as well as ten slew rates. These slew rates vary from 1x to 1000x sidereal rate, or a maximum of over four degrees per second. That latter number defines the maximum time it might take to slew between two objects if you had to go the long way around, which in this case is 90 seconds. The maximum slew rate of four degrees per second is actually middle of the road for today's go to telescopes and faster than anticipated for these scopes. This could explain why the base weight was higher than expected.
Initial Setup
The initial setup for the SynScan controller remains unaffected regardless of whether the operator plans to utilize the tracking mode or the full go-to mode of operation. Operators are first required to enter their observing location, in degrees and minutes of longitude and latitude. Next, the correct time zone, defined by the GMT, must be entered. This includes a range of -5 to -8, -5 being the US east coast and -8 being the west coast. The local date and time is then required, as well as answering a simple "yes" or "no" question regarding Daylight Savings Time. For future use, the observing location and time zone will be retained, although the date and local time will require input at every start up.
Once this initial information is provided, the SynScan AZ controller gives you the option to begin alignment. If you are the type that abhors reading user's manuals, it would not be obvious that choosing not to begin alignment will put the telescope into the tracking mode. What is also not obvious without reading the user's guide is that the telescope optical tube needs to be oriented in an initial position, specifically level and facing north, for the tracking mode to operate. It should have been a simple matter for Orion to include those instructions in the SynScan software, and, hopefully, they will incorporate this recommendation in their next software release.
Tracking Operation
At higher magnifications, the tracking function is most valuable. Initial tests were conducted while Jupiter was still almost due south in the late fall of 2010. From its initial standpoint, leveled and facing north, the scope was manually aligned and centered on Jupiter, using 200x magnification. Though a demanding test, this is also a predictable common use for the function. Any residual drift should be easily noticeable at that magnification, and sure enough, there was a visible drift. The drift was noticeably different than that observed when using the telescope on the same target in the full go-to mode.
To test the drift, a 17mm Nagler eyepiece, producing a true field of 1.15 degrees, was inserted into the scope. This large field of view provided an enhanced opportunity in measuring the residual drift. Instead of conducting multiple tests lasting a few seconds, the tests were performed over a period of minutes. A bright star in the northeast and Jupiter to the south were used during the tests. Following the tests, an elapsed time of 5-10 minutes were recorded before the objects drifted from the center to the edge of the field.
Several factors must be taken into consideration when interpreting these results. First, no extraordinary measures were taken when leveling the base or locating true north while initializing the telescope. Second, any residual drift would be more evident a long way from the celestial pole, which is exactly where Jupiter was at the time. All in all, the tracking function was found to be adequate for its intended purpose.
Go To Alignment and Operation
It was presumed that the familiar two star alignment feature to be included, which it was. However, the user's manual revealed a new feature. The trend in go to telescopes over the past decade has been to simplify the process such that less and less knowledge is required of the operator. The Orion SkyQuest XT8g is no exception to this rule.
Orion offers a simplified alignment method called the brightest star method. In theory, this method frees the user from being required to know the names of any alignment stars. One simply looks around and aims the telescope at the brightest object in the night sky, other than the moon, and tells the SynScan controller what direction that object is located in (e.g. northeast, east, southeast, etc.) If the wrong direction is selected, the SynScan controller will advise that no bright object was found.
Assuming for a moment that your sense of direction is pretty good, the SynScan controller identifies the brightest star in that area and then offers a choice (by name) for the seconds alignment star. The user has the option to accept that choice or to scroll to another. Once a choice is accepted, the telescope slews to the near vicinity of that star. Anyone unable to identify any stars by name must take it on faith that the telescope slewed to the vicinity of the correct star. The alignment method was tested using different initial objects to confirm that it works. During one test, Jupiter was intentionally selected, and the SynScan controller correctly identified the object as Jupiter.
Two Star Alignment
For those who know their way around the sky will probably opt for the traditional two-star alignment. A majority of the tests conducted with the Orion XT8g telescope were centered on testing the two-star alignment feature. The telescope is provided with a 12.5mm reticle eyepiece for alignment purposes. From the tests, it was noted that this eyepiece was helpful, but not necessary, in order to get a good alignment. In attempt to get as much error as possible, a 35mm TeleVue Panoptic eyepiece was used during the first alignment. It was noted that the telescope would not reach focus with this eyepiece if fully seated in the focuser.
On the first series of tests, no special effort was made to level the telescope base. The level was probably off by at least three degrees. The first alignment stars were Cappella and Rigel -- or to be more precise, Cappella was chosen manually, while Rigel was listed as a suggestion.
The procedures of the first round of tests were repeated, although the telescope base was carefully leveled, using a spirit level. Cappella and Rigel were used as alignment stars. One key difference in pointing accuracy was noticeable; leveling the base greatly improved the pointing accuracy -- greater than using the reticle eyepiece. Nevertheless, the sample scope missed two out of seven test targets.
Doing it right...
If you were to stop into Skies Unlimited and purchase any altazimuth mounted go-to telescope, you would probably get a lecture on the proper technique for aligning the telescope. That lecture goes something like this:
"Start by facing south. Look over your left shoulder (east) and pick a bright object, then use that as your first alignment star. Now look over your right shoulder (west) and pick another bright object. Make sure that this object is at a different elevation, either higher or lower than the first object. Use this object as your second alignment star. Choosing two stars on opposite sides of the meridian that are widely separated in both altitude and azimuth will yield the best results in terms of go-to accuracy."
The long and the short of this advice comes down to accurately locating the meridian. Choosing stars on opposite sides of the meridian makes this a matter of mathematical interpolation. Choosing two stars on the same side of the meridian makes this a matter of extrapolation. The former method is usually more accurate, and in some cases it is MUCH more accurate.
Following a brief review using Starry Night Software, Cappella and Rigel were confirmed to be west of the meridian on the first night of testing. This does beg the question "why did the SynScan controller recommend Rigel as the second star?" Based on the time and date of the tests, Deneb would have been a better alternative. The logic behind this is hard to understand. Hopefully, with future software releases, Orion will correct this issue.
The Final Test
Armed with a good understanding of what went wrong revisited this test early in the spring of 2011. This time around, Arcturus was selected as the first alignment star, followed by Cappella as the second alignment star. These stars were on opposite sides of the meridian, and much more widely separated. For these tests, a 17mm TeleVue Nagler was used, rather than the reticle eyepiece. Result: the scope was capable of hitting Messier objects in every direction. It also easily located Saturn. All testing was performed using the 17mm TeleVue Nagler which has a slightly smaller field of view than the supplied 28mm Deep View eyepiece. Most objects were not perfectly centered in the eyepiece, but a few were. The only object the telescope failed to hit during the follow-up test was the moon. Considering the telescope successfully hit Saturn, which was approximately twenty degrees away, this failure was rather unusual. Under normal circumstances, a miss such as this is connected to an error in the date and time. As the telescope successfully hit Saturn, however, the date and time error is ruled out.
Summary
In comparison to other mass market Dobsonians, the Orion XT8g was noted as an absolute joy to use. This telescope's ability to automatically track its target makes all the difference in the world. When properly aligned, the go-to performance was more than adequate. Using the go-to function will either allow a novice observer to see an object he or she might have difficulty finding, or allow the more seasoned observer to see many more objects in a limited amount of time.
Following a brief review using Starry Night Software, Cappella and Rigel were confirmed to be west of the meridian on the first night of testing. This does beg the question "why did the SynScan controller recommend Rigel as the second star?" Based on the time and date of the tests, Deneb would have been a better alternative. The logic behind this is hard to understand. Hopefully, with future software releases, Orion will correct this issue.
The Final Test
Armed with a good understanding of what went wrong revisited this test early in the spring of 2011. This time around, Arcturus was selected as the first alignment star, followed by Cappella as the second alignment star. These stars were on opposite sides of the meridian, and much more widely separated. For these tests, a 17mm TeleVue Nagler was used, rather than the reticle eyepiece. Result: the scope was capable of hitting Messier objects in every direction. It also easily located Saturn. All testing was performed using the 17mm TeleVue Nagler which has a slightly smaller field of view than the supplied 28mm Deep View eyepiece. Most objects were not perfectly centered in the eyepiece, but a few were. The only object the telescope failed to hit during the follow-up test was the moon. Considering the telescope successfully hit Saturn, which was approximately twenty degrees away, this failure was rather unusual. Under normal circumstances, a miss such as this is connected to an error in the date and time. As the telescope successfully hit Saturn, however, the date and time error is ruled out.
Summary
In comparison to other mass market Dobsonians, the Orion XT8g was noted as an absolute joy to use. This telescope's ability to automatically track its target makes all the difference in the world. When properly aligned, the go-to performance was more than adequate. Using the go-to function will either allow a novice observer to see an object he or she might have difficulty finding, or allow the more seasoned observer to see many more objects in a limited amount of time.
In spite of the many features and capacities of the telescope, Orion should consider changing the star selection algorithm, in order to prevent the problems that surfaced during the first test run. This correction would help to ensure that this telescope delivers the high-quality go-to performance that it is capable of.
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