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.
much to look forward to. The SLTs include the latest firmware with
features such as SkyAlign, Identify, Constellation Tour and more.
102SLT at a Glance
Optical System: achromatic refractor
Approximate Street Price: $420 (includes tripod)
Focal Length: 660mm
Focal Ratio: f/6.5
Supplied Eyepieces: 9mm (73x)
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
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.
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
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.
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
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
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
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
click for larger image
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
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.
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
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
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
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
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
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
- 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.
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.