Pirich's Full Review: Celestron NexStar 102 SLT (200 x 102mm) Telescope
The NexStar SLT series telescopes are a second generation of the smallest of the NexStar mounts, previously the NexStar GT series. These include a light weight tripod with stainless steel legs and a small computer driven mount to track the stars in altitude-azimuth (alt-az) mode. The mechanics of this variant have several fixes to annoying problems on the earlier NexStar series, though these are not trouble free as the computer alignment system can be difficult (if not impossible) to use if the voltage on the power source drops. More general information on choosing a telescope can be found in my article on Picking a Telescope.
Background
Celestron made a complete line of computer guided telescopes the center of their product line startign around 1999. The models came out a few at a time, but within a few years the lowest to the highest end scopes they sold had computer guided versions. The main unusual feature of these is the heavy reliance on using computer guidance systems to align a telescope and have it track objects in the sky when it was not equatorially aligned. This is, for all practical purposes, a visual use only mode since objects will appear to turn as they go across the sky (if you watch the moon when it is half full, notice how the lit half is pointed up from the horizon as it rises, it is edge on when the moon is at its highest point in the sky, and then is turned around again to point into the ground as it sets, so you have seein it turn 180 degrees as it went through the sky).
Good examples of this first generation of NexStar series Scopes are the NexStar 80GT, the larger NexStar 5i, and the largest size, the NexStar 8 GPS. These scopes had one major flaw in common: They violated a patent by Meade Corporation for an alignment method based on pointing the optical tube north, leveling it, and then pointing at two stars. So, Celestron ended up paying Meade a settlement on every copy of this version of NexStar sold.
So, this had to stop, and a new set of NexStars appeared with what appeared to be a neato (and perhapes far easier) alignment method to replace the old one. OK, so I can hear people asking "Huh?" Simply put, getting a computerized telescope sited to start guiding to objects and tracking them is very different from just setting up an equatorial mount. The telescope can't see- instead, it has an internal map of the sky stored, which based on knowing where it is, how it is pointed, and what time it is, let it predict where objects are in the sky.
To make this clearer, I will start with what one would do if they were just using an old-fashined equatorial mount. first, you would walk outside and put the mount down with its hub pointed at the pole (near Polaris in the Northern hemisphere, near nothing in particular in the Southern). This alignment allows a clock drive to push this one axis around so the scope will follow something as it rises, crosses the sky, and sets. With this underlying correction for the earth's spin, the telescope will follow over 99% of the objects in the sky if you just point to them. The trick without a computer is either you need to know where objects are, and can point to them, or you need to use a star map to find them.
Celestron's new guidance technology is called SkyAlign, is supposed to do away with knowing these things. You take the telescope and give it its location and the local time, then look for three bright stars in different parts of the sky, point to them, and the telescope is aligned, without you even needing to know what any stars are named or where they are.
Description and Usage
The new NexStar 102 SLT is a 4" diameter refractor with an f/6.6 prescription. Tripod on this telescope has polished stainless steel legs with an adjustment knob half way up. The tray is a small and triangular with just enough room for the telescope's front lens cap to drop into it. At the top of the tripod, there is a small bubble level indicator. In all sincerity, I do not know why it is there other than to overcome setups on rough ground.
The mount itself is updated from the older version with two significant improvements. First, it has an on/off switch at the bottom of the scope arm with a small red light to let you know the scope is on. Like the earlier small NexStars, the power inlet is still a separate battery pack with a plug in the bottom of the fork arm. The second major improvement is the telescope now mounts using a standard Vixen dovetail, which means the optical tube just clamps in the top of the mount and can be instantly removed to break down for transportation or storage. In terms of putting another optical tube there (I was instantly tempted), this feature has a strange plastic boot which comes in to take up space around the optical tube, though it does not help to support it. So, you would either need to plan to put something smaller there, like the excellent AT-66ED, or do some major surgery on the plastic boot. As it turns out, I ended up concluding the performance of this mount is NOT good enough to justify getting one of these just to do this, though it looked like a good idea on paper. Note, the dovetail section on the telescope is just barely large enough to fit into the dovetail clamp and has NO provision for locking devices to keep the scope from accidentally falling out, so make sure the clamping knob is tight every time you use the scope, since handling/ movement can loosen it.
The telescope has a 2" rack and pinion focuser, with essentially the same amount of focus travel as the Celestron 80ED. the front end of this telescope is all plastic, with what appears to be a slide-out dew/glare shield which DOES NOT MOVE. Weird, but there it is. I didn't have my large dial caliper along to see if the barrel diameter is a little small and this might be a make-up section. The telescope has minimal glare protection, so it is fairly easy to find the Moon in the sky by looking at where the field looks brighter in the eyepiece. With that said, the color correction is surprisingly good, with only a thin layer of lime green on the edge of the moon and a small amount of purple around the bright white star Sirius. All in all, I would have to reccomend the optical tube as a fine home viewing telescope.
The computer driven mount package turns out to be a very different story, unfortunately. The scope boots up instantly when turned on, and the handset asks you to get started on the alignment. There is some time waiting for the instructions to slowly scroll by. First, you need to tell it where it is, and it has a US and international list. I selected the US, then it has a list of states, and I selected Arizona, which is followed by a fairly extensive city list, and I set it on Tucson. Then it wanted the time. It appeared to be about 9:15 PM, so I put in 9:15 and it asked for AM or PM, and I set PM. The first option is SkyAlign, which I was given to believe would let me point to anything. So, I put it on Capella, and like previous NexStars it asks to put it in the finder before the eyepiece and hit ENTER. So, doing this I noticed the finder is very reflective with dark greenish tint compared to the original Red Dot Finder. What it ends up looking like is the star is a green dot you merge the red dot with. But since the interior is reflective, if there is any light where you are, the reflection of your own face is very distracting. I hit align after centering the star in the eyepiece, and then drove it over to Sirius. Here is where I noticed this mount is quite a bit slower than the original small NexStars, which would zip around at 5 degrees per second. This appeared to be around 3 degrees per second, which is noticably slower. After Sirius, I went to the Moon and centered it. Then came something new: ALIGN FAILURE. I started over and tried a second time with the same objects.
Another observer there suggested the Moon might be the problem, so I turned it off, turned it back on again, and tried with Polaris (the north star), Capella, and Sirius, and again had alignment failure. Twenty minutes of playing with alignment had gone by and two experienced observers had already failed. This time I turned it off and unplugged the power to completely reset it. We tried improving the time setting and a third opinion on maybe stars near the pole being bad came up. We took the time from a GPS unit and punched it in as accurately as we could, which showed the original time I put in was close to 5 minutes off. I then used Arcturus, Capella, and Sirius, the three most obvious stars in the sky at the time. The telescope crunched a biit longer on the new set of data, and I started to feel hope of getting a good alignment. Then the display showed ALIGN FAILURE. So, that was that. I have a Master's degree in mechanical engineering, 25 years of observing experience, 6 years of using computer guided scopes, and I still can't make the supposedly idiot-proof version of NexStar alignment work, even with the help of a professional astronomer.
I had a chance to do more investigation as to why this was happening, especially when two siblings of this telescope were so cooperative. As it turned out, the reson for this was we were trying to run the telescope off of a lithium-ion gel pack battery, which produces a slightly low voltage. Although a NexStar 6SE runs off of this source with no complaint, the difference was enough to crash the alignment performance. Apparently the larger Schmidt Cassegrain type NexStars have better power management systems and are resistant to this problem. So, run it off of an AC adapter if you can.
Plan B for alignment is to use one of the other alignment modes this telescope allows, in this case, the two star alignment. Unlike the usual methods I had seen previous NexStar series telescopes use, this one just presents a list of named stars and it is 100% up to you to know where they are in the sky. So, off down the list of stars like Mintaka (no idea), Castor (um, Gemini, next to Pollux, no idea which is which) dubeh (no clue), Capella- yep, that one is obvious, so drive the scope there. When this was done for Sirius as well, the telescope was ready to go. Needless to say, this came at the point of 45 minutes into using it, when I am usually at the end of a quick observing session and about to go inside.
This is where the going gets odd- it was already necessary to keep moving the telescope around, and picking up the battery pack and controller to keep the cords from getting wrapped up in the tripod. When doing some observing tests, we decided to try M81, a beautiful spiral galaxy, and M82, the chaotic collision of two galaxies right near it. Since the moon was out, all I could do is detect *something* was there, but not what. But in going to M82, the mount surprised me by doing a 360 degree rotation back to the nearby object. What was even stranger is it did it again when asked to re-approach the same object! This weird behavior also turns out to be a symptom of not enough voltage in the power supply. So, if you are mobile, something like a Power Tank or a Car adapter is probably a necessary item.
Since Saturn was high in the sky, I tried hitting the Planets button to get a look at it, and instead it showed the Moon and blinked when I tried to go down the list to get Saturn. Instead of warning about slew limits, it was instead just refusing to acknowledge the planet existed. Since there was no leg in the way, I manually drove the scope to Saturn. Here is where another shortcoming compared to the older NexStar 80GT showed up; the eyepiece is so low to the ground, you have to kneel to see, even with the tripod fully extended. The image of the planet through the scope was nice and sharp, and the mount did track once I had it pointed at the right spot. But it just seemed like the novice user would have no idea what was wrong and thus miss seeing one of the most beautiful objects known.
In short, this mount appears to embody everything one could fear about a computer drive system. If everything is perfect, it works. If not, it is able to produce a long list of apparently unrelated problems with no explanation from a simple problem like a fading battery. The mount does correct a couple of shortcomings in the original version, and the telescope itself is solid in its own right for such an inexpensive instrument. The control system and its quirky response to power just look like an enduring excercise in making sure you go out with a checklist, and would seem to be the opposite of anything you would want parents trying to show their kids something neat to have to deal with.
If you should decide to adapt another scope to this mount, you will need to get a Vixen type dovetail bar. Dovetail bars are available from Orion (www.telescope.com- search on Dovetail) and more detail is in my review of the Orion Dovetail.
Conclusion
All I can say is I am profoundly disappointed in this product. The NexStar GT series which came before was simple to use, and more forgiving if everything you were doing wasn't perfect. This version is the opposite of that, where the first requirement is perfect power. from where I stand, this is reason to re-think my conversion to deciding computer guided scopes are a good idea. At $479, at least the optical tube is decent, though something about this size and weight mounting the vastly superior C80 ED Refractor would seem to be a better investment. The finder is not going to be useful away from this mount, and the mount is partly compromised by the power sensitivity problem and a silly plastic boot to make putting another scope on it difficult. The tripod is too low to the ground, so what we are left with is a mediocre mount, driven entirely by the saving grace of a functional telescope.
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