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NexStar 102SLT Telescope

9 September 2006

In February 2005, Celestron announced a new model line: the NexStar SLT.  The SLT would replace the NexStar GT line - the NexStar 80GT introduced me to the world of computerized telescopes so I was greatly interested in these new scopes.  As a beta tester for Celestron, I knew quite a bit about the design of the SLTs even before they were announced, but still, design and function are often two different things.  So, I looked over the SLT models and selected the NexStar 102SLT as my next telescope.

There was much to look forward to.  The SLTs include the latest firmware with features such as SkyAlign, Identify, Constellation Tour and more. 

NexStar 102SLT at a Glance

Optical System: achromatic refractor
Approximate Street Price: $420 (includes tripod)
Aperture: 102mm
Focal Length: 660mm
Focal Ratio: f/6.5
Supplied Eyepieces: 9mm (73x)
                                  25mm (26x)
Maximum Magnification: 200x
Maximum Field of View: 2.7
                       4.4 with optional 2" diagonal/eyepieces
Magnitude Limit: 11.7
Resolution Limit: 1.4 arcsecond
Finder: 1x power red-dot
Objects in HC Database: 37,981
Weight (includes tripod): 16 lbs. (7.3 kg)
Manufactured by: Celestron www.celestron.com

And with the user-upgradeable hand control, new features can be added as Celestron updates their NexStar firmware.  The tripod has been upgraded to steel legs.  The motor control firmware in the mount was completely rewritten, promising much improved tracking.  An Aux port on the mount allows motor control upgrades and the use of GPS receivers and other accessories.  The mount uses an industry-standard dovetail for attachment of the optical tube - thus a wide variety of tubes can be used on the mount.
 
Pluses
  • Latest NexStar firmware in a user-upgradeable hand control - all the same features as Celestron's top of the line models
  • SkyAlign is easy to use, even for a beginner
  • Very good main optics
  • Standard dovetail bracket allows use of other optical tubes
Minuses
  • Tripod is a little shaky for this weight of optical tube - a light touch is required when focusing
  • Tinting in window of red-dot finder is too heavy - only the very brightest stars are visible though the finder
  • Low quality eyepieces and diagonal hide the quality of the main optics - they will get you started, but plan on upgrading soon

I received my NexStar 102SLT in February 2006.  Everything comes very well packaged in a single large shipping box which contains another box within.  Inside of that box, individual components are further packaged in boxes, bags and padding to safeguard delicate items during transport.  The shipping box is a bit smaller than the US Postal Service maximum of 100 inches (length plus girth) - this is a plus for anyone in the US military as the 102SLT can be sent to FPO/APO addresses.

Setup
After unpacking, it took me about 5 minutes to assemble the scope.  A well-written setup guide with step-by-step photos ensures even a beginner will have their SLT together in just moments. 

The tripod comes first - open the legs and the accessory tray simply twists and snaps into place to lock the legs.  No tools and no wing nuts required and nothing to get lost in the grass at night.  To ensure success with alignment, the legs are then extended and adjusted to level the tripod with the built-in bubble level.
 

Next, the single fork arm mount is set into the cup-like tripod head.  The mount is secured to the tripod by hand tightening a single nut via a large plastic head.  Simple as tightening the lid on a jar.

Once the mount is assembled, the optical tube is placed in the dovetail clamp and a single bolt is hand-tightened.  The plastic head of the bolt is relatively small and may be difficult for some to tighten sufficiently.  I recommend you rock the optical tube back and forth


Securing the mount to the tripod.
click for larger image


CG-5/Vixen-standard dovetail
clamp.

click for larger image

while tightening this bolt - once I thought I had it good and tight and the optical tube nearly slipped out during normal use.  Part of the problem is that the metal dovetail clamp is smooth and doesn't really grip the dovetail with any significant friction.  I'm considering modifications to improve this situation, but, with sufficient care everything works as is.

Next, attach the red-dot finder and diagonal.  Finally, insert eight AA batteries in the internal battery holders or attach an external power source.  The AA batteries should be alkaline, rechargeable batteries do not provide sufficient voltage to run the scope reliably.
 

Even with alkaline batteries you can expect no more than about 4 hours of use - quite expensive if you use the telescope much at all.  For the SLT, I would recommend a 6-10 amphour rechargeable 12 volt battery.  Check at hobby and toy stores and you can generally find something that might even be light enough to Velcro to the front of the fork arm.  For other suggestions, see Odds and Ends and read the power source article.


Battery compartment and
bubble level.
click for larger image

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.

On my scope, the tension on the altitude axis was way too loose.  It was very easy to move the scope up and down by hand during normal use, thus losing alignment. A number of other folks have posted this on the NexStar and BabyNexStar Yahoo Groups.  Adjustment is very easy - the correct nut to tighten is the one on the inside of the fork arm.  To access it, remove the optical tube, completely unthread the dovetail clamp and remove the bolt.  The nut in the center of the axis can then be tightened as much as you would like - at least tight enough that significant force is


Altitude axis.
click for larger image

required to move the optical tube up/down by hand.  The nut can be seen in this photo behind the bolt running through the clamp.  The size is 13mm and is easily adjusted with a socket wrench.

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.


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