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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