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Celestron NexStar 102SLT test review

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Celestron NexStar 102SLT test review
4 inch F6 refractor August 2010

By Ian Roberts, Johannesburg, South Africa

Figure 1.

I wanted a light, GOTO refractor scope of at least 4 inches aperture for casual use and possible activities at “star parties” where it could be set up easily after transportation on the back seat of the car.

My other scope, a wedge-mounted Meade LX200R 8”, is heavy, took a while to align properly, and did not lend itself to such activities.

The 102SLT was obtained used but basically NIB from an old man who had incorrectly set the time zone and site coordinates, so he undoubtedly saw little and the tracking must have been awful.

With the LX200R a lot of astrophotography and satellite tracking has been done, all under remote control via RS232 cable with USB extender cables for the cameras. These are a Meade DSI Pro 2 mono and a Canon 1000D DSLR. The LX200R has superb optics so this was immediately a yardstick for comparison.

Accordingly, I did not have high expectations from the entry level az/el mounted 102SLT which does not have exotic glass as used in expensive refractors and optical aberrations were anticipated.

Presentation and assembly

The scope looks snazzy and ultra mod and no one will laugh at you if you arrive with one of these.

Assembling the drive onto the tripod is a moment’s work, and the optical tube attaches to the drive using an industry standard Vixen-style dovetail mount, so other optics can be substituted on the drive base if required.

I was amazed how light the entire assembly is. The whole unit can be carried around if necessary.

However, the tripod is flimsy and prone to vibration, despite tightening up all nuts and bolts as necessary – to takes around 3-4 seconds for the scope to settle after a rap on a leg. Bumping it will cause loss of alignment as the tripod slides easily on a hard surface and the feet are hard plastic points.

I use a 7Ah sealed burglar alarm battery placed on the accessory tray to power the scope and provide some weight to the tripod and this does help. I do not use the 8 cell alkaline AA battery option as this only lasts a few hours and would prove too expensive.


A bubble level on the tripod enables the user to level it, but exact leveling is unnecessary.

There are various alignment modes: one star, two star, Sky Align, solar system.

Sky Align makes sense – after entering the site coordinates and time zone, which are retained by the scope after power down, the user simply sets the time, accuracy a dozen seconds or so seems OK, and date.

Then the red pointer LED on the peep sight is turned on, and the scope slewed to any 3 (unknown) widely separated bright objects: stars, solar system, even the moon, and each time pressing Align on the hand controller when the particular object has been centered in the eyepiece. The motors are quiet even at full speed. The user is reminded to turn off the LED as the peep sight/finderscope runs on its own 3V 2032-style lithium battery.

Thereafter nearly 38000 objects (Celestron’s advertising claims 4000) may be selected for GOTO with an accuracy stated to be 10 arcmins, but I found at times the error in DEC was at least double this. RA accuracy seemed better.

Regarding the finderscope, various users on the forums complain about its lack of performance. In my opinion mounting a replacement finderscope on the OTA (there is a mini dovetail mount holding the stock LED finderscope) defeats the object of the exercise. However if you decide to obtain something like a 7x50 make sure it has a right angle eyepiece so that you don’t have to crawl around on the ground to line up with the axis when at high elevations. In any event, at F6 the 102SLT’s field-of-view is large enough that a typical Sky Align GOTO will place the object in the main eyepiece.

At the Eyepiece

I could see no difference between the LX200R’s 1.25”star diagonal and Celestron Ultima eyepieces, and the supplied 1.25”diagonal and 9 (73x) and 25 (26x) mm eyepieces when used under light-polluted skies. However, in visual use under high power the collimation appeared to be out, although there was no sign of this when mounting cameras directly without the diagonal. This is a common problem with Chinese made diagonals, the Meade diagonal has the same problem – they are not centered properly in the optical plane.

When focusing and due to vibration, one has to let go of the focus knob to see what happened. The focus action is a bit stiff and play can be eliminated, there was none, by setting a pinch screw on top of the draw tube.

The scope retained focus for hours, even when slewing around with a Canon 1000D mounted, and showed no signs of slipping - nor did the motors, which have very good torque.

Objects are minute at the F6 (660mm nominal) focal distance, so the user is roped into using tiny eyepieces and Barlow lenses to obtain reasonable power. Separating Rigel from its companion, easy on the LX200R, is a challenge.

Objects are pin-point indicating a high Strehl ratio and under high power it’s possible to get solid views of 2-3 of Saturn’s moons. On the Trapezium (M42) the 4 main stars were widely separated at high power, but I could not see the 5th and 6th stars – easy on the LX200R.

Colour correction is indifferent – on a bright star each side of focus the colour is either green or red and focus may be set by mid-pointing this chromatic aberration. But in-focus it is far better than I expected.


Celestron released two firmware updates for the motor controller and the hand controller and these were installed using Internet access to the Celestron web site to download the new firmware. Subsequently these were installed via a home made 4-core RS232 cable from the PC’s DB9 serial port and plugged into the base of the hand controller which takes a 4-pin modular plug. This is recessed and a bit of a hassle to get in or out. Cable details on PC Control of NexStar Telescopes - NexStar Resource Site http://www.nexstarsite.com/PCControl.htm. Please also have a look around this site for details on using NexStar (the user manual in particular) and finding additional objects to view.

I found the hand controller easy and intuitive to use with good response and the software functionality made sense, other than Cordwrap which is on by default each time the scope is powered up (the manual states it is “Off” by default on the az/el mount!).

This lunatic function causes the scope to slew 360 degrees in RA rather than cross the local meridian, thereby wrapping the power cord, the hand controller cord, and the camera cable if in use around the drive base or tripod. Remember to turn it off after aligning the scope, specially if using long distance remote control via the RS232 line where the slew might not be observed..

Remote control via RS232 cable

Various software packages are suitable for GOTO functionality (and probably also guiding), including (ASCOM’s Unified Celestron Driver must be installed) Starry Night Pro 6, MaximDL, Stellarium or Celestron’s own NexRemote with built in driver and this works nicely.

The latest ASCOM driver allows the scope to slew when not in sidereal tracking mode by setting “AllowAllSlews” to “-1” in the ASCOM Profile Explorer, useful if one is tracking satellites.

When connected to the scope via the ASCOM driver, the user may elect to Sync the scope to the solved result after astrometry on an image – I use this function a lot during satellite tracking when wide slews in RA and DEC are used and pointing accuracy is then exact.

Also, one can insert RA and DEC values and institute a slew according to those values.

GOTO accuracy, sidereal tracking accuracy

The controller has user programmable values for anti-backlash and GOTO approach direction to enhance the GOTO ability, as well as a Calibrate GOTO function.

Sidereal tracking accuracy is good, far better than I had expected on an az/el mounted scope, see the images below - after an hour or so the target will probably still be in the 25mm eyepiece’s field of view.

Tracking rate can be selected to sidereal, lunar, solar or off (for terrestrial use or when tracking geostationary satellites).

Astrophotography and astrometry

The supplied eyepiece fitting accepts both 1.25” cameras and DSLR T-rings, enabling these devices to be used. Rather than reading my comments, judge the image quality for your self by having a look at some astro photos.

The sliced images below from the modified Canon 1000D (IR filter removed and not replaced), show:

  • Proxima - our nearest neighbour star, magnitude 11. Above Proxima the 3 stars are magnitude 13. All are in colour in the original image.
  • Pluto, magnitude 14, is illustrated in the next image; both Proxima and Pluto were confirmed astrometrically,
  • M20.

I found 30 second exposures quite feasible. In 10.1 M/pixel DSLR format, which is large, there is minimal coma around the outside of the field of view.  But there is field rotation of course due to the az/el tracking mode.


Figure 2. Proxima Centauri: seven 30 sec Canon 1000D DSLR colour images taken at F6 and stacked

Figure 3. Pluto: seven 30 sec Canon 1000D colour images taken at F6 and stacked

Figure 4. M20: several 30 sec Canon 1000D colour images taken at F6 and stacked. Resized version of the original image.

Bright stars in these images have a green tinge around them which must be looked for in the full sized images to see it. Annoying yes, but not that bad either.

When connected and under PC control, full telescope information is reported on the RS232 link and incorporated in FITs image headers, camera dependent, which is thus available for astrometry functions in software such as MaximDL, Pinpoint and Apex 2.


After some months of use, slewing to the right in RA became erratic with or without the weight of the OTA, with large speed variations.

I partially disassembled the drive and noted that the upper portion rides on three ½” PTFE buttons sliding on a well greased metal rail around the circumference of the base. How long these will last remains to be seen, especially since there is a gap there of about 2mm and grit can find its way onto the slide.

It will be a good idea to maintain hygienic conditions and regular maintenance around this slide. The metal RA and DEC gears should also be greased regularly.

I did not proceed further thinking rather wait for the RA drive to fail completely before messing around with it, but to date after around 8 months of robust use it still works and has never stopped completely. Also, despite these speed variations, alignment is not lost. After a while it turned out that the D.C. plug was not properly contacting the inner of the socket on the scope, causing a loss of voltage when drawing current for slewing. This plug should be a 5.5mm o/d, 2.1mm i/d plug, which can be difficult to find. The common 5.5, 2.5mm plug does not contact the socket’s inner pin at all. After attending to this, all is well again and RA slews are normal.

I tried a GSO 0.5 reducer in the nose of the DSI Pro 2 camera mounted directly without the diagonal, to provide a focal length of F3 (330mm). This was achieved; however images were severally mangled and only suitable for satellite tracking where image fidelity is irrelevant but good enough for the purpose.

After a slew and set to track at sidereal rate, the scope takes up to 30 seconds to settle down in RA. During this time objects slide right to left in RA in images until the RA “clock” has caught up, at which point the scope suddenly locks up and tracks at proper and accurate sidereal rate. This must be taken into account during imaging; otherwise images will be severely blurred and the user may conclude tracking is not too good. I suspect this problem could be solved by changing the RA approach direction from positive to negative, but have not tried it.

The scope cannot be slewed without a power source.


By and large, this scope has (greatly!) exceeded my expectations. It has abilities and features not anticipated when initially purchased and other than possibly the very similar Skywatcher/Synta 4” F5 refractor there is probably nothing as capable at the price.

It’s a sweet little thing to use, draws only 0.14A off 13V when tracking at sidereal rate and 0.43A peak, then 0.3A average, when full speed slewing both motors. So a 7Ah battery should last a couple of nights in normal use.

‘nuff said.

If you would like to comment or require further information, please contact me as below:

I wish you clear skies!

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