An in-depth review of the Celestron NexStar 130 SLT

by Ian Morison, Jodrell Bank Observatory

130 SLT
The Celestron NexStar 130 SLT

A small sandbag adds stability

Image: Ian Morison

A low cost "go-to" scope that really works!

In the past low cost "go-to" scopes have never been quite as easy to set up as their manufacturers would perhaps have liked like us to believe.  As a result, I have never been too keen to recommend them to beginners.  However, in 2005, Celestron came up with a new method of implementing the initial alignment of a scope.  Called SkyAlign, it is wonderfully simple to use. It is available with their low-cost NexStar range making it possible for a beginner to acquire an easily used go-to telescope for a very reasonable sum of money. The NexStar range uses the same computerised mount and tripod on which is mounted a range of telescopes: achromatic refractors of 50, 80 and 102mm aperture and reflectors of 114 and 130mm.   (To give a 1000mm focal length in a short tube, a lens is incorporated into the 114mm design.) I chose to borrow and review the 130 SLT which uses a 130mm (5.1 inch), 650mm focal length Newtonian optical tube assembly.   This has a small, but worthwhile, increase in aperture over the previously available budget reflectors which came with 114mm diameters.   I would really recommend paying the 10% extra for the 130 mm over the 114mm reflector in the range.   The telescope was in its original packaging so was "off-the-shelf".

130 SLT
The Spider Assembly

You can also see the 2" focusser with 1.25 adaptor installed and the window of the red-dot finder.   The flash has made the inside of the tube look lighter than it really is!

Image: Ian Morison

The Optical Tube Assembly

I must honestly say that I was surprised and impressed at the f5 Newtonian tube assembly.   It seemed in several ways to be a real improvement on the 114mm Newtonians that I have seen.

1) It came with a thin steel vane spider.  The four arms of the spider support the secondary mirror.   They are made of thin steel tensioned by screws around the optical tube.   This is far, far better than the thick metal vanes that I have seen on several 114 mm Newtonians and less image degradation will result as a consequence.

2) The focuser came with adapters for both 1.25 inch eyepieces AND 2 inch eyepieces.   Though the scope comes with two 1.25 inch eyepieces it does mean that for wide-field views a 2 inch eyepiece of 30 to 40 mm focal length could be used.

3) It was provided with a "red-dot" finder.  As we will see below, the finder is hardly used except when initially aligning the scope.   A red dot finder appears to project a red dot on the sky which is aligned with the target star or planet.  For this purpose they are vastly better than the cheap 6 x 30 (which in practice are sometimes stopped down to nearer 10mm) straight through finders.   The finder has a quick release mount.

4) Reasonable looking quality 25mm and 9 mm eyepieces. The 25mm eyepiece gives a magnification of just x26 (650/25) and has a field of view of ~2 degrees. The inside of the barrel close to the field lens was a plastic retainer.   This is rather shiny and hence might reduce the contrast somewhat when viewing the Moon.  It would be easy to apply some mat black paint, perhaps mixed with a little sand, to give a rough surface.

The 9mm eyepiece gives a magnification of x72 (650/9) with a field of view of ~0.6 degrees - just nicely encompassing the Full Moon!   This equates to a apparent field of ~ 44 degrees - rather less than a Plossl.   Their performance will be described below.

So far, very good. Observing ones eye (reflected in the secondary and primary mirrors) through the eyepiece indicated that the collimation was very close to perfect (and has not had to be adjusted).  A small ring on the centre of the primary mirror helps to carry out collimation if necessary.

A few small points:

1) The mounting bracket for the finder was a little out of alignment and so one of the adjustments on the finder itself had to be at the extreme of its range to align it to the scope.   A tweak with a phillips screwdriver on the bracket screws fixed this.

2) I suspect that the secondary mirror was glued a little off its correct position - being a touch too far away from the eyepiece.  (It is meant to be a little further to that side, but not as much as I observed.  This would have rather little consequence though.)

3) The inside of the tube is, as it should be, matt black but I have seen "blacker" blacks.

We will see how the telescope performed in practice below.

The Altitude-Azimuth (Alt-Az) mount and tripod

An Alt-Az mount follows an object across the sky by using two motions: one in azimuth - so the telescope tracks a southerly star from east to west - and one in altitude (or elevation) - which lifts the scope as it follows the star to its highest elevation at transit (due south) and then lowers it down again.   The rates at which the two drive motors run will vary during this time.  For example, as an object passes transit, the altitude of the object does not change significantly.   This is in contrast to an equatorial mount when, apart from small corrections, only one drive is running.  One disadvantage of an Alt-Az mount is that the field of view rotates preventing the taking of long exposure astro-photographs.  But, to be honest, this is not a scope to hang large cameras on.   Having said that, the scope will support a barlow and astro imaging CCD camera as described below.

Due to the Alt-Az mount and the length of the optical tube assembly there is bound to be a region around the zenith (directly overhead) that cannot be accessed by the telescope.   The telescope will not attempt to drive to an elevation which might cause the bottom of the telescope tube to foul the mount or tripod.   The software in the computer contains an altitude slew limit of 65 degrees which I suspect is a little conservative.   I have seen a nice modification to the mount which interposes a plastic tube between the tripod and the base of the mount.   By lifting the mount away from the tripod, the highest elevation can be substantially increased.   This will require some do-it-yourself work, but could well be worth it.   Perhaps some accessory company might produce one as I suspect that there will be a lot of these scopes sold!

The mount uses a single fork rather than the two forks used in most Alt-Az mounts.   This obviously limits the weight that can be supported - to around 5kg.   More obviously, it does allow the tube, if pushed sideways, to move significantly more than if a dual fork were used.   But it always came back to where it was before so this is not really a problem and really, one should not touch the tube anyway except to adjust the focuser.   Having touched the focusser, the tube does "shimmer" a little - rapid, very small amplitude vibrations that take a few seconds to damp down.  I have not noticed on any other scope but then I have never used a lightweight single fork mount before.   However this really does not constitute much of a problem in practice.   I noticed this more when the scope was mounted on concrete rather than (very soggy) grass.   I suspect that the grass helped to damp things down somewhat.   One can buy damping pads of squigy plastic and these might well help.

The cover on the base of the mount opens to allow the insertion of 8 AA batteries in two packs of 4.   It is stated that one set should last for ~ 8 hours.   What pleased me greatly was that the mount appeared to work fine with a set of rechargeable NiMH batteries even though their nominal voltage is a little less.  There is a socket to provide an external 12 volts from a mains adapter, car lighter socket or external battery. (These are all additional accessories.) There is a recessed (good) on/off switch, the socket for the hand controller to plug in and an accessory port.  The hand controller/computer is Celestron's latest which allows for the software to be updated and, most importantly, includes the clever software that allows SkyAlign to work. The mount came already mated to its tubular tube stainless steel tripod.   This is certainly better than the aluminium tripods that have been used on this class of telescope before but it is still very light.   I have often advised owners to fill their aluminium tripods with sand both to increase stability and to improve the damping of the system.  The steel legs are riveted so it is not easy to fill them with sand so, instead, I placed a small sack of sand on the tripod spreader to give a similar result at least as regards stability.   A bubble level is incorporated into the base so the mount can be levelled but, as we will see, if SkyAlign is used, levelling the mount is not too critical.

A note for those who might wish to fit other tube assemblies to the mount.

The tube assembly attaches to the fork using the widely used "Vixen" standard wedge shaped plate.   In principle this would allow other optical tubes to be fitted provided they are not too heavy.   I have 4 telescopes with this type of mounting plate and only one would, in fact, fit.   The plate has to be slid into the clamping bracket from the side - not lifted in as is usually done on the Vixen GP clones.   Many of the plates do not have the correct profile along their entire length so cannot be slid in from the side as is required on the SLT mount.   One may thus need to remove some metal from the mounting plate before it could be used on the SLT.

First Light: Setting up and aligning the scope

The first chance of using the scope was an evening in March when, at first, high cloud meant that only first magnitude stars were easily visible.   I couldnít even see the three stars of Orionís belt!  Earlier in the day I had aligned the red dot finder with the optical axis of the telescope by observing the top of a distant street lamp.   The manual states that, with the power off, it is allowable to move the telescope tube by hand.   I set up the telescope by hand with the tube pointing roughly south at 45 degrees.  The tube can be moved in altitude relatively easily, but the clutch seems much tighter in azimuth and I have only ever moved it under motor control. I switched on the telescope and the controller asked me to choose a country and then a city (United Kingdom and Manchester).   It then asked me the date and time which I entered using the numeric keypad.   I was then asked which method of alignment I wished to use.   I chose "SkyAlign" as I believe that, in general, this is the one to use.

I was asked to use the red dot finder to point to a first bright object. (I use the word "object" as bright planets and even the Moon may be used.)   I chose an object high in the West (I knew it was the star Capella but I didn't need to know this.)   Using the red dot finder, four keypads are then used to drive the telescope up/down and clockwise/anticlockwise to move the tube.   When the red dot appeared over the star I pressed "Enter". You are then asked to centre it in the field of view of the telescope.   The computer automatically reduces the slewing speed at this point.   When centred in the field I pressed the "Align" pad.  I was then asked to slew the telescope to a second object and, in the same way, I aligned it on a very bright object low in the South (Sirius, as I suspect you might have guessed!).  One should try to get at least 90 degrees between the objects so this seemed a reasonable slew.   It's also best not to use objects in a straight line.   Through the murk I could see a bright object up to my left.  When I aligned on it and observed it in the low power eyepiece I realised it was the planet Saturn!   (I should have known that it was Saturn, but it was difficult to get ones bearings with so few stars visible.   This did prove that, just as Celestron say, planets can be used as well as stars and at least for this object I did not initially know what it was that I was aligning on - a great bonus for real beginners.)

The computer then sat for a while thinking and then came up with the magic words "Match Confirmed". It had worked first time out of the box!

130 SLT
The Hand Controller

Image: Ian Morison

That was it.   By pressing the "Undo" key I would have been told the three objects I had chosen: Capella, Sirius and Saturn.  But I was away. The numeric keys have letters or words on them relating to the various catalogues held in the computers memory. These are:

1) M: The Messier Catalogue of just over 100 objects. This is the one that most observers will initially use for seeking out "deep sky" Objects.

2) Cald: This is the Caldwell Catalogue named after Patrick Caldwell-Moore. It also contains just over 100 deep sky objects and includes objects in the southern hemisphere as well.

3) Planet: Lists the Moon and the planets visible in the sky at the time of observation.

4) NGC: Deep Sky Objects in the New General Catalogue.  Mostly Galaxies. Many of these will be rather faint for this sized telescope.   One that should be easily visible is NGC 2903, a nice 9th magnitude galaxy in Leo.   I am surprised that it was missed out by Messier and it is not included in the Caldwell catalogue either.

5) Star: The brightest stars in the SAO ( Smithsonian Astrophysical Observatory.) catalogue.

6) Tour: Takes you on a tour of the best objects in the sky at the time of observation.

7) List: Accesses a set of object lists such as named stars and named objects. (For example Sirius and the Dumbbell Nebula).  Selecting from the Constellation list gives the objects seen in each constellation.

The manual states that 4000 objects are included.  Some objects occur in two or more lists such as, for example, "The Eskimo Nebula", a planetary nebula which is in the list of named objects.  This is also Caldwell 39 and NGC 2392.

I hit the pad with the letter M on it and I was shown "Messier" with a three digit number to key in.   I keyed in 042- the Orion Nebula - and hit "Enter".  Down and clockwise it slewed.  I could not make out the nebula (remember I could not even see the three stars of Orion's belt with my eyes) but I could see the line of three stars and the 4 stars of the trapezium at its heart.  Their images were nice and crisp - my first intimation that the optics were fine.   As sometimes happens, when the sky transparency is worst, the "seeing" can be very good - that is, the atmosphere was very steady.   It was that night so making it excellent for testing the optics of the scope. I then had a quick look at the clusters in Auriga: M36, 37 and 38.   All appeared in the field of view though were barely visible through the haze.   Then up to M35 in Gemini.   Nicely centred in the field and more obvious than the Auriga clusters as this part of the sky was clearer.  Then across to Cancer to first, M44, the Beehive Cluster, and finally back to Saturn.  The image was very clean and focus was easy to find.  Again, indications of a good mirror and collimation. However the 9mm eyepiece (at x72) could not really give enough magnification to see Saturn at its best and so I added a Televue x2.5 powermate into the focusser to give x180.   During moments of very good seeing I could see the Cassini division and possibly a hint of a band around the surface of Saturn - by this time the skies were clearing somewhat.  The optics of this scope are clearly up to the mark!

So the first accessory that a purchaser should buy is a x2 Barlow.   Used with the 9mm eyepiece, this will give the effect of a 4.5mm eyepiece and be much easier to use than 4.5mm eyepiece as well as giving you an effective focal length of 12.5mm when used with the 25mm eyepiece.

My final move was back to Capella to do a little star testing - comparing the intra and extra-focal images.  There was a trace of spherical aberration but nothing to get excited about and I suspect that the mirror was nearer to 1/6th wave rather than the nominal standard of 1/4 wave.

Sometimes a mirror can have a good overall figure, but a rough surface. It is said that this reduces the contrast of the rings seen in the out of focus image.   They looked fine to me.  Overall I rate the the mirror as excellent - and definitely better than I was expecting in a budget scope.

SkyAlign

Lets have a look at the innovative method that this telescope can use for alignment.  The computer asks the observer to align the scope onto three bright objects.

Given the approximate location, the date and time and with a roughly level mount the computer "knows" the area of sky in which the three objects must be located.   This reduces the time for its calculations to determine which objects have been selected by the observer as it can eliminate many possibilities.   Again, given the date and time it knows the positions of the Moon and planets on the sky so that they can be used too.

By aligning on three objects it has three sets of azimuth and altitudes. From these, it can calculate the angle between each pair of objects.   All the objects must be greater in magnitude than 2.5 so from its catalogue of stars it will then first check which possible pairs of objects have been selected.   Given three such pairs, it is extremely unlikely that the correct set of three objects cannot be found.

The computer has made 6 measurements (3 azimuth and 3 Altitude) so it is then able to solve for the position of true north and the two numbers which define the tilt of the mount away from the horizontal.   It thus knows all that is needed to compute the azimuth and altitudes of other objects in its catalogue.   Brilliant!   I particularly like the fact that it allows you to select the objects.  I often try to observe when the sky is partially cloud covered.   One can simply choose the objects that one can see at the time provided they are reasonably far apart in the sky.

A little too clever?

Having set up the scope using SkyAlign a few times, there was one thing that I found just a bit too clever.   One initially slews to the objects using a fast slew rate.   Then, having got the object in the finder, it asks you to hit "enter" to carry out the final alignment at which point the slew rate drops to improve the accuracy.  Fine - except if you happen to hit "enter" by mistake whilst you are slewing onto the object.   This is quite easy to do as the enter key is just above the "Move up" key.   If this happens inadvertently one can only continue very, very slowly.  I think I would rather the software was not as clever and one had to change the rate in the usual fashion (by hitting the "rate" key followed by a number 1 to 9) to carry out the final alignment.

If you just take things gently there is no real problem.

Tracking with an Alt-Az mount.

As an Alt-Az mount requires the use of both motions to track it needs to be aligned before it can track.  Suppose (as happened to me one evening) that the Moon is popping in an out of clouds and there is very little sky visible so true alignment was not possible.   Here is a work-around that will give the scope an approximate tracking rate if you have a reasonable knowledge of the sky.   Enter the "Auto two star" align mode.  The computer will tell you to align on a specific star.  Move the telescope to a point in roughly the correct direction and align it as if you could see the star.   The telescope will then slew to a second star and ask you to centre and align it.   Do so without moving its position. The computer will be quite happy - even though it could be some way out.  But if you then go to the Moon it will at least attempt to track it across the sky.

Why bother to observe when much of the sky is cloud covered?

Because the clouds trap much of the light pollution.   Perhaps my best view ever of the Orion Nebula from a town location was when there were thick clouds scudding across the sky leaving gaps perhaps 30 degrees across.  The underside of the clouds were bright orange, but the sky seen through the gaps was really dark!

The Eyepieces

The first light observations described above showed that primary mirror was excellent.  On the next clear night I tried to assess the quality of the eyepieces.   Budget eyepieces will usually give a good image in the central part of the field of view but optical aberrations become apparent towards the edge of the field.   The most common is called lateral colour which means that a bright object will show some colour fringing near the field edge.   I compared the provided eyepieces with two excellent eyepieces.   For the 25mm I used a 24mm Celestron Ultima - a very high quality 5 element design - and for the 9mm, a 10mm Vixen Lanthanum which is also highly regarded.

The results for the 25mm were much as I had expected - some lateral colour and some drop off in image quality near the edge of the field, but fine in the centre.   Simple eyepieces can actually be better than more complex ones in the field centre as less glass is in the way and are thus often recommended for planetary observations. The celestron Ultima was fairly obviously better - but so it should be, costing ~ 1/3 the price of the whole telescope!   It would have been nice if Celestron bundled their 26mm E-Lux budget plossl with the scope.

However, I found it very hard to criticise the 9mm eyepiece.   Ok, it had a somewhat smaller field of view than the Vixen, but optically I couldn't really fault it.   It gave some really nice views of the first-quarter Moon which nicely filled the field of view.

Astro Imaging with the 130 SLT

On the night of Friday April 21st 2006, with reasonably good seeing but poor transparency, I attached a Televue 2.5 powermate and a Watec 120N camera to the telescope in an attempt to image Saturn and its moons.   (Note that the cables were hooked around the finder to prevent a downwards pull to the side of the telescope tube)   Having first viewed Saturn through the powermate and the 9mm eyepiece, I was very pleased to see that when the Watec was added, Saturn was still in the field, so the mount/tube is sturdier than perhaps I thought.   I took two images: the first with an exposure of 1/50th second to correctly expose Saturn.   On this image no satellites, not even Titan, were visible.  I then increased the exposure to 2.5 seconds.   This image turned Saturn into a burnt out oval area, but showed 6 satellites.   I then combined the two - a fair amount of cleaning up was required - to produce the composite image shown on the right of the second picture.   On the left of the image is a plot from the freeware programme "Cartes du Ciel" for around the time I took the images.   Several of the planetarium programs show Saturn's moons as one zooms in - but this was the only one to give reasonable agreement with what I photographed - so beware!

Well at least it shows that some astrophotography is possible with the 130 SLT!

Imaging System
Televue 2.5x Powermate and Watec 120N astrocam.

Image: Ian Morison

Saturn and Moons
Two images: left,Cartes du Ciel plot and right, Saturn plus Moons composite.

Images: Cartes du Ciel and Ian Morison

Additional Items in the package

Included with the telescope was a simplified "how to get going" 4 page colour brochure which I followed with no trouble.  The telescope manual gives greater depth but a beginner might not even need to refer to it.  There were also two CDs with software: The first was a "light" version of "The Sky" one of the most respected of the computer planetarium programs and worth about 30 pounds.   Impressive.

The second was a piece of software that would allow you to control the telescope from a laptop or computer given a suitable connecting cable which can be bought as an accessory.

Conclusion

Apart from a few, minor, caveats, this scope performed better than I ever expected.   The new SkyAlign alignment procedure has never failed to give a successful alignment first time and has always thereafter brought the chosen objects into the field of view of the 26mm eyepiece.  As I hope I have made clear, the optics are of such good performance that it will virtually always be the atmosphere that limits the image quality.

What more can one ask for?   I believe this and the other telescopes in the Nextstar range will be able to painlessly introduce many newcomers to the night sky.  They deserve to sell in large numbers and Celestron are to be congratulated not only in coming up with a brilliant way to align a computerised scope but providing this new software in a worthwhile aperture scope at a very reasonable price.   Well done!

Things to buy

As pointed out above, the first definite requirement is a Barlow lens to give the magnification necessary to view detail on the planets.

Then I would then be inclined to buy a 2" eyepiece for "rich field" scanning of star fields - and a wonderful view of the Pleiades.   The Moonfish 30mm 80 degree apparent field of view was very well reviewed recently.  I have one and am very happy with it.   The tube can support it.

Finally I would look out for a ~25mm plossl eyepiece - to replace the weaker of the two provided with the scope.

Reviewer's statement

The reviewer has no formal links with Celestron or its distributers, but greatfully acknowledges the loan of this telescope from David Hind's Ltd, the Celestron distributers for the UK.  The reviewer is Telescope Adviser for the UK Society for Popular Astronomy and co-author of two astronomy books for the Amateur.  He owns and uses the following telescopes:

102mm Takahashi Apochromatic Refractor.

127mm Alter 500 Maksutov.

150mm Helios Achromatic Refractor

150mm Meade Schmidt-Newtonian

200mm Orion Optics, UK, Newtonian Reflector

235mm Celestron Schmidt-Cassegrain.


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