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NexStar 102SLT Telescope

9 September 2006

Mechanics
The first thing I noticed was that the scope seemed to vibrate quite a bit when touched.  Most of the bolts on the tripod needed tightened.  The hex bolts used to attach the legs to the tripod head were particularly loose (5mm hex wrench required) and contributed the most to the instability.  After tightening everything, performance was acceptable though this optical tube is definitely at the limits of the weight capacity of the SLT mount with this tripod.  After a rap to the optical tube, vibrations settle within 2 or 3 seconds.  Most of the vibration is caused by the tripod; a heavier tripod would allow better performance.  Note that although the legs are now steel, the bracket at the top of each leg is plastic; this definitely contributes to vibration.  The complication though is that unique single-bolt "cup" attachment.  No other tripod on the market uses such an arrangement and a custom adapter would need to be fashioned.

The new alignment routines do work just as advertised.  SkyAlign is as simple as it gets for a newcomer to astronomy.  Simply point the scope at three bright celestial objects (even planets and the Moon), center each in the eyepiece, and the NexStar hand control is ready to go.  There are some tips that can help increase the pointing accuracy and ensure a good alignment - visit Manuals to download my "Hand Control Version 4 User's Guide".  Once aligned, GoTo and tracking were excellent, night after night.

The SLT's motor control firmware is definitely a great improvement over the GT models.  Besides the tracking improvements, response to the direction buttons is much smoother.  Backlash compensation works much better contributing much to the responsiveness.  With the GT, it was necessary to be slow and methodical when using the arrow buttons for manually slewing, if you got in a hurry, the mount would often simply ignore you.  With the SLT, every time I press an arrow button, the scope responds just as I expect it should.  Part of this improved responsiveness can also be attributed to a new bearing arrangement on both axes - smoother motion is an obvious result.

Another improvement is noise level.  The SLT is much quieter than the GT, both during high speed slewing and when tracking.  In fact, the SLT is nearly silent when tracking.

Optics
I was a little bit hesitant about the 102SLT.  Celestron's previous 102mm short-tube refractor had a spotty reputation.  It seems that an f/5 achromatic refractor of this aperture is likely to suffer optically.  This version though is f/6.5 - potentially much easier to manufacture with good optical quality. 

And Celestron has indeed delivered.  My 102SLT has excellent optics as determined both with a star test and actual use.  Intra- and extra-focus images of a star are virtually identical with no significant optical flaws noted, save one - chromatic aberration.  Of course, this is to be expected with an achromatic refractor.  The false color is not over-powering in most cases, but, significant improvements can be made with a minus violet filter.  I have installed a Baader Fringe-Killer filter (see Equipment Reviews) in the front of the diagonal and simply leave it there.

Looking down into the front of the scope through the fully multi-coated optics, I found three baffles.  I didn't have any eyepieces that would push the scope to its 4.4° maximum field of view, so I leave it to another reviewer to see if these baffles are correctly proportioned to ensure no vignetting.  The internal blackening of the optical tube was uniform and darker than I've seen on other introductory refractors.  I expected high-contrast views.

Jupiter exhibits multiple bands in this scope and color is easily visible, particular with the Fringe-Killer filter.  Stars are pinpoint and show their true colors, one of the things I like best about a moderate aperture refractor.  All objects are shown against a jet-black background, indicative of excellent contrast.

Now, all of the observations above were made with a high-quality prism diagonal and Celestron Ultima and Axiom eyepieces.  The supplied diagonal and eyepieces are okay to get started, but they hide the quality of the optical tube.  Plan on replacing them.  Nonetheless, I think Celestron definitely saved money in the right way by using these inexpensive accessories rather than cheapening the main optics or the mount itself.

The red-dot finder is another matter.  The tint on the window (required to reflect the red dot back to your eye) is much too dark.  It is nearly unusable.  In fact, I found it very difficult to see anything other than the very brightest stars through this finder.  Fortunately, the finder's dovetail mount is a very common type - so replacement with another finder is a simple matter.  Any of the Orion products work straight out of the box.  The only positive thing I can say about this finder is that it will ensure beginners select nothing but bright objects during alignment.

The focuser tube on the 102SLT is a two inch model; the supplied diagonal and eyepieces are 1.25".  A 2" to 1.25" adapter is supplied.  This adapter is threaded for a T-ring to allow simple attachment of SLR cameras.  This would be one heck of a birder scope!

The cast aluminum focuser is just like other Synta focusers I've seen, for example, it is essentially identical (though larger) to the f/5 80mm model sold as the NexStar 80 GT and the Orion short-tube 80.  Tension and stability on the focuser tube is achieved by a long nylon block at the top of the focuser.  This block can be adjusted by two recessed hex screws.   Look for two tiny holes - on in front and the other in back of the large chrome knob on the top of the focuser - and use a 1.5mm hex wrench to adjust.  On my scope they were a little loose and the focuser tube wobbled quite a bit.  I eliminated the wobble by tightened the hex screws just enough to remove the slop while still allowing easy movement.  The large chrome knob on the top of the focuser is a focus lock in the event you are imaging with a heavy camera. Update - newer 102SLTs are shipping with a lower quality plastic focuser - unfortunate as stability and smoothness suffer.

And Other Features...
One of the nicest things about Celestron's entry level GoTo scopes is that they have basically all the same features as their bigger cousins. Other than imaging-specific features (EQ alignment and PEC), everything found in a NexStar GPS/CPC is here.  And with those features, use of a NexStar is a joy.  Commonly used catalogs (Messier, NGC, Planets and more) are all available with a single button press.  For the most part, you will never use the full menu system during normal operations. Simply said, the GoTo system stays out of your way to allow you to enjoy the night sky.  Additional power features like Constellation Tour and Identify make the scope a good educational tool.  To learn about all recent features, visit Manuals to download my "Hand Control Version 4 User's Guide".

The mount (motor control or MC) and hand control (HC) are fully upgradeable by the user.  Details of the firmware versions available for the MC and HC can be found in Firmware Versions where you will also find links for the step-by-step process to perform those upgrades.  At this time, the SLTs are shipping with HC firmware version 4.03 and MC version 5.09.  I updated my HC to version 4.10 and then 4.12 (current version when I wrote this) with no fuss.  MC version 5.09 was at the time of this article still the most recent.

The SLT has two ports - Aux and Hand Control.  Actually they are both identical; in other words, the hand control can be connected to either port.  The second port is specifically designed to allow the use of accessories like a GPS module or other "smart" accessories.  Currently that consists of four pieces of gear:

• Celestron's Auxiliary Port Accessory Kit (part number 93965) - this provides two additional Aux ports and a PC Port.  The PC Port is one way to upgrade the motor control firmware and can also be used to run the scope without using the hand control by using NexRemote (see below).
• Celestron's CN-16 GPS module - provides accurate date, time and location information to the hand control.
• Earthshine Technologies' StarDate GPS Adapter (more here) - allows use of most handheld GPS receivers to provide accurate date, time and location information to the hand control.
• Belsico's Skyan Wireless Controller (see review here) - a simple wireless control for manually slewing the scope and controlling a motorized focuser.

Out of the box, and after executing Factory Settings, you will likely find the altitude GoTo Approach is set to negative.  I recommend positive as most of the SLTs are back-heavy. Additionally, with GoTo Approach set to negative, the scope is more likely to hit a tripod leg during a GoTo.  After changing the setting to positive, be sure to use the right and down button to center alignment stars in the eyepiece.

The NexStar SLT series is fully compatible with Celestron's NexRemote software.  NexRemote provides full remote operation of the telescope via a Windows PC.  Also, the SLT is compatible with almost all astronomy software on the market - select NexStar GPS or CPC if there is no SLT-specific setting.  See PC Control for more details on NexRemote, general PC/PDA control and a list of programs compatible with SLT telescopes.

Conclusion
The NexStar SLT line is a solid value in the beginner to intermediate telescope market.  In its price range, I really feel there is no other computerized GoTo telescope that comes close to comparing.  If you are looking for a good telescope to get started in astronomy or a smaller second scope, a NexStar SLT telescopes will likely be a perfect fit.

Copyright Michael Swanson     Contact the webmaster: swanson.michael@usa.net