Review of Celestron 130 SLT

This is a 130 mm f/5 Newtonian reflector on a motorised GO TO mount with tripod. It is a lightweight scope, which is a good or bad thing depending on your point of view. As a take-anywhere fairly compact scope with an aperture large enough for viewing a good range of objects, it is fine. But it is not a rugged and versatile workhorse which will allow you to take detailed photographs of the sky and planets.

In summary, many beginners want a GO TO telescope which will show them some of the sights in the sky without costing them a huge sum, and this telescope is ideal for that purpose. Its optical performance is adequate but not outstanding, and it is more suited to deep-sky observing than close-up planetary viewing. The GO TO system works pretty well, and is much easier to use than the equivalent Meade Autostar.

Setting up
The unit comes in three basic sections: the tripod; the GO TO head; and the telescope tube (also known as an OTA or optical tube assembly. It comes with two eyepieces (25mm (26x) and 9mm (62x)) and a red dot finder.

The tripod is ready-assembled out of the box, and has a double-hinged centreplate so that the legs fold up without requiring you to remove lots of wing nuts which then get lost in the grass. There is a central screw to take the GO TO head, which is also ready-assembled and is easy to fit. Then the telescope fits onto a dovetail fitting on the head. I have not yet found out whether this is a universal dovetail that will take other telescopes.

The setup is very simple and the only problem I experienced was trying to locate the scope into the dovetail in the dark and making sure the knurled knob was tight enough to hold the telescope. The knob is a good size, however, and easy to get a purchase on.

Power supply
You can use either eight AA batteries or an external power supply. On the test model that I borrowed from David Hinds Ltd one of the little leads linking the two plastic battery compartments had broken off, so I used the 12V power box that I use for my Meade LX90 (a commercial unit for starting cars from Maplin, for which I made my own connecting lead). The jackplug that fits the Meade is slightly too small for the Celestron, which occasionally resulted in interrupted supply. This is a problem with all these scopes -- if the external power supply becomes momentarily disconnected, you lose all the alignment even if you also have batteries inside the compartment. This is an issue which I'm sure could easily be addressed by the manufacturers -- would be nice if the internal batteries took over if the external power becomes disconnected, with a warning bleep.

I don't know how long a fresh set of internal batteries will last as I didn't use them, but I would imagine it is measured in a few hours rather than many hours. A set of eight AA alkaline batteries costs say £4 even if you buy them in bulk and cheaply, so we are probably talking about at least £1 an hour running costs. This applies to all GO TO telescopes of course, not just this one. However, while you can just about use an ETX without the motors, with difficulty, the mount of the SLT is very hard to turn manually (in fact I couldn't turn it at all in azimuth) so the telescope is virtually useless once the batteries run down.

How it works
Because Meade have patented the idea of aligning a GO TO by levelling it and pointing north to give the telescope a starting point, Celestron have come up with the idea of using any three stars. This in my view makes it easier to set up than an ETX. Every star party I have been to, someone comes along who has had an ETX for months if not years, asking for help in setting it up. And this is meant to be a beginners' telescope that does it all for you! There are other major faults with the ETX software which Meade have not addressed after years of production.

I did not bother to even look at the little levelling bubble on the SLT tripod, but in theory I should have done so. I don't think a small mislevelling should make any difference anyway because the telescope has to learn where it is from the sky itself. Only if it is way out of level, or you give it wrong data about where it is and what time it is, will it get things wrong.

The SLT system needs to be pointed at any three objects brighter than mag 2.5 to learn what part of the sky it is viewing. That's it. You don't even need to know whether they are stars or planets. There are not all that many objects in the sky this bright, and the chance of there being two sets of three objects which are exactly in the same relationship to ane another is very small. For example, if this is 21.00 on 14 Feb in Chipping Sodbury, there are only 50 stars plus maybe a couple of planets brighter than mag 2.5 spread across the entire sky. Pick any three of these objects at random and they form a unique triangle on the heavens. Pick any other three and the triangle is a different shape. Then all the software has to do is to work out which set of three objects matches exactly the triangle you have pointed it at.

That's why the initial levelling is not crucial, except that if it is way out, or the date and time are wrong, the telescope will be trying to match the orientation of your measured triangle to a false model of the sky.

In use
The system starts up quicker than an ETX. On comes the display, you just press enter and it asks you to confirm your observing site, giving the last one you chose. You need to give it a city within 50 miles, so I chose London. I have not tried the custom site facility, but I have no doubt there is one.

One drawback I have found (and this is based on just a couple of nights of use, with no batteries in the scope so it may not apply if there are), is that I found that it did not keep track of the time, and had remembered only the previous night's date and time. So I had to reset these by pressing Undo when the site location was displayed, even if I was realigning the same night.

Having done that, it tells you to centre (OK, center) the first object. You pick a bright star and slow round to it using the motors. Switch on the red dot finder (having aligned it with the telescope properly, right? -- absolutely essential with any telescope). Line up the star with the red dot (I find this tricky, but it's a matter of practice). Press enter. Now you look through the eyepiece (25 mm, essential) and if the finder is properly aligned the star will be within the field of view. Now you centre it within the main eyepiece. And though it is obvious, Celestron have worked out that you need a different rate of slew at each stage, unlike Meade, who have baffled every user by not giving you fast slew for the initial stages, so everyone thinks their motors are not working.

One of the buttons on the handset needed a good hard press -- a possible sign of problems over a period of time. But it was easy enough to centre the star. Now you press Align and it asks you to find another star, ideally some distance away. The slew rate is a bit slow, but the motor is fairly quiet as a result and if you have tetchy neighbours this is a good thing. Do the same again, then find a third star. Once you have done all three, the telescope thinks for a few seconds (a rotating bar on the dsiplay tells you this -- there is no beep) and then you can start to find objects.

You do this either by choosing named keys (eg Planets) or going through a list. I won't go into details here because all GO TOs have similar systems, but I did find it easier to navigate than the ETX keypad. One oddity was that pressing the Star button asks you to enter an SAO number rather than scroll through a list of named stars. Another is that the named stars are weird in the extreme -- names I've never heard of. I can't believe that Celestron think they have to choose different names for the stars from Meade, but this seems to be the case as there are so many oddities in there. Pointless, in my view, and makes things more difficult on a system which otherwise works so well.

So how well does it find objects? After a few teething problems, which I think were due to not resetting the time as mentioned above, I found that I could set it up quickly on any three objects, including planets, and get good GO TO within the field of view. Some alignments were better than others, and after a while things did start to drift off, but on the whole I was pleased. I'd have to spend a lot more time using it to give a proper verdict. I haven't checked how to synchronise on a known star so as to make it more accurate when finding a nearby object, but I imagine this is possible. I did find out how to replace one of my alignment stars with another nearby one, but this didn't seem to improve accuracy. Maybe I replaced the wrong star. More work needed here.

Note added later: Two-star alignment is particularly quick. Choose this and it suggests a star, as with the ETX, though unlike the ETX it may choose a fairly faint star, such as Albireo. Having centred on this, it then slews to its second star. On the two occasions when I tried this, its slew was far more accurate than I get with my LX90. The whole procedure from switch-on takes a couple of minutes.

It seemed to track the objects it had found well enough, but I have yet to do a long duration test on this. [See below .]

Optically, there is a good deal of coma at the edge of the field of view, but then this is an f/5 instrument and the eyepieces are basic (probably Kellners). I have not compared eyepieces yet. My view of Jupiter was not brilliant using a 9 mm eyepiece (a Celestron Orthoscopic of my own) but it was not terrible for an f/5 instrument. I could see a fair bit of detail but there was a bit of scattered light around it. I didn't get the chance to investigate this further. If you want to study and photograph planets, this is probably not the ideal telescope for you anyway -- no f/5 Newtonian is. I took a quick look at the double-double and could see both stars in each double, but I want to inspect this at higher power. Star images on-axis looked OK, and racking in and out of focus did not reveal anything untoward with the out-of-focus star images. The collimation seemed OK judging by out-of-focus images, but I did not examine it critically.

The focuser is not good -- though rack and pinion, it has a lot of image shift in it when moving in and out of focus. It does, however, take 2-inch fitting eyepieces, which is unusual for a cheap scope. But of course a big wide-field Nagler would weigh and cost as much as the telescope itself! Because the telescope is lightweight, the image jitters a lot during focusing, and though the night when I tested was windless, I can see this being a problem on gusty nights.

Note added 22 May: I have now taken some Toucam images of Jupiter with the telescope. Main problems were the telescope shaking in gusts of wind and the difficulty of  focus, particularly as the image shift took the image of Jupiter from one side of the field of view to the other at f/20 (approx) and it took ages to settle down as well. But despite all this I got a pretty acceptable image. The telescope tracked Jupiter well for several minutes at a time, and I just had to make minor corrections to keep it in the frame.

Jupiter photographed at 22h10m UT on 21 May 2006. Celestron 130 SLT,
using Celestron Ultima 2x Barow and extension tube. 400 frames stacked in Registax.

This review is based on literally a couple of hours of testing on three nights when conditions were not brilliant, and the clouds quickly learned that I was observing and rolled in.

At £279 in the UK, the telescope is much cheaper than a comparable ETX. It is also less fiddly, but optically it is not as good. A good ETX Maksutov, well aligned, can give brilliant images, but we are talking about chalk and cheese -- a Mak is f/14 and the 130 SLT is f/5. And as there is no way you can collimate an ETX, you are stuck with it unless you send it back to Telescope House. But as a beginner's scope, it knocks spots off any ETX in my view, as it so much easier to set up. Meade are sniffy in their ads about the idea of choosing three stars, but in practice it works so much better than their very ropey Level North Technology which they are so pleased with -- in my experience this doesn't work at all well.

The 130 SLT is not as compact as an ETX, and this could be an issue if you want to travel by plane with one. But it will pack into a suitcase pretty well, so this should not be a big issue. I would love to use one in a good dark sky somewhere farther south!

Robin Scagell
17 May 2006

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