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Introduction
This article is aimed particularly at the small (60mm) refracting
telescopes marketed by Tasco, but will also apply to many other small 'astronomical'
refractors sold in newspapers and popular mail order catalogues.
If you are new to the hobby and in particular if you
have bought one of these instruments for around £99. It could come as a shock to
read all the negative publicity that regularly appears in the astronomical press, mostly
in the readers letters sections, about your brand new telescope. But take heart, most of
what you read is just prejudice passed on from others, and unfair comparison with
instruments that cost a great deal more when new.
Above all do not be put off, with almost any
telescope it is possible to see a great deal more than with your eyes alone. All that is
really required to enjoy the hobby of amateur astronomy is a genuine love of nature, and a
warm coat! Any more than this could be called an obsession, and I should know.
The key to obtaining the best results is to
recognise the limitations of your telescope and to use it within these limits, that way
you will not be disappointed. What you have in these telescopes is a small budget
instrument, and it will perform as such. You may never be able to conduct any
'useful' science with such a small instrument, but by following the few tips below you
will at least realise its full potential.
A word on Magnification
Forget about the fancy advert that sold the telescope. In the real world there are
practical limits on the maximum (and minimum) magnification that can be used with any
telescope, if you hope to see anything at all that is!
There are several factors that decide the maximum
magnification that can be used with a given telescope. But as far as the instrument is
concerned the main ones are the quality of the optics and the diameter of the main lens or
mirror. A generally accepted figure based on the diameter of the main lens/mirror is
around 20x for every cm or 50x for every inch in diameter, under reasonable observing
conditions. For a 60mm telescope this gives about 120x magnification.
A good general rule for observing is to use the lowest
magnification that allows you to clearly observe the features you wish to see. It is far
better to have a small clear image than a large fuzzy one!
A few improvements
- If your telescope did not come with an equatorial
mount, and is not fitted with a stay (some are) then one of the first problems you will
find is that as you try to focus on an object the instrument see-saws up and down, and
tightening the large thumb screws simply locks the whole thing solid. This can be improved
by fitting a couple of washers under the heads of the thumb screws, and packing out the
space between the telescope tube and the mounting on both sides, with more washers.
- You will also have noticed that the tripod is, to be
polite 'a bit unsteady'. The easiest improvement here is to always use it with the legs
fully retracted, and to place a heavy weight on the accessory tray (I have seen a bag of
sand being suggested, this is not a good idea, as sand and optics do not mix). Better
still, you could add a tripod stabilizer, instructions for making this will be found in
another article. Reducing the magnification will also help a great deal.
- Next is a most annoying problem. Almost every time
you go to use the telescope, you will discover the finder has become misaligned.
Fortunately this is fairly easy to fix. Simply remove the finder from its mounting and
wrap some self adhesive copper or aluminium tape around it (PVC tape should also work
reasonably well) until it is a tight fit in its mounting. Refit the finder and adjust it
in the normal way.
- Unfortunately, for reasons best known to themselves
the manufactures choose to provide a set of accessories that do not allow you to get the
best out of your telescope. I'm afraid the only fix here is to spend a bit of money, but
it need not be much and you will be amazed at the difference it can make.
Firstly, the
Barlow lens, a good quality Barlow
lens can be an extremely useful accessory for a telescope, the one provided by Tasco is
realistically only fit for the bin. At best it should be placed at the back of a cupboard
and forgotten about. Using this will only lead to disappointment.
The other main accessories will include a couple of
eyepieces. If you look at the eyepieces you should find one or two letters along with a
number printed on them. The letter or letters denote the type of eyepiece you have, while
the number gives the focal length of the eyepiece. As far as the lettering is concerned
they will almost certainly be H, HM, R or SR these are basically the cheapest eyepieces
available, but to be fair they work reasonably well with these instruments. Should you
find the letter K then you have been lucky!
You should find that, one of the eyepieces will be
around 12.5mm and the other around 4 or 5mm focal length.
The secret to getting the optimum performance from
any telescope is to have a set of eyepieces ideally suited to that instrument, in order to
do this you will need to know the focal length of your telescope and the diameter of the
eyepiece barrel. The focal length of your telescope should be printed on the instrument
somewhere, often on the focuser, if not look in the documentation, you should find it is
around 800mm. The diameter of the eyepiece will need to be measured, measure the outside
diameter of the chromed barrel in inches, it will be either 1.25" or 0.96",
probably the latter.
For use with a 60mm telescope we recommend that you
obtain at least three eyepieces, one that gives 30 to 40X magnification, one that gives 60
to 80X and one that gives around 120 to 140X.
To find the magnification given by an eyepiece, you
simply divide the focal length of your telescope by the focal length of the eyepiece. For
a telescope with a focal length of 800mm an ideal choice of eyepieces would be 25mm (32X),
12.5mm (64X) and 6mm (133X) these focal lengths are all readily available.
You will probably already have the midrange
eyepiece, the 4 or 5mm eyepiece supplied with your telescope may be a little too powerful.
If your budget is limited then stick with the cheaper eyepieces (H, HM, R or SR), the 25mm
will be the dearer of the three but is probably the most useful, remember to buy the
correct diameter.
If you have the extra money to spare it is worth
upgrading to K (Kellner) eyepieces, these will produce noticeably better images.
We would not recommend looking at any of the more
expensive eyepieces, especially not if your telescope takes 0.96" eyepieces. Of
course if you intend to upgrade to a larger scope in the future, then it may be worth
getting the more expensive eyepieces now. But you should then bear in mind the
specifications of your possible upgrade before buying the eyepieces.
A note on the Sun
We make no apologies for repeating this warning
several times in the beginners section of our website.
Never look at the Sun directly though any optical
instrument unless it is correctly filtered or has been specifically designed for
that purpose.
Even through a telescope as small as 60mm looking
at the Sun will cause instant and irreparable damage to your eye.
This warning is particularly important here as some
of these telescopes come with a small 'solar filter' that screws into the eyepieces. If
you got one of these filters with your telescope throw it away now, do not be tempted to
use it. These filters are dangerous and do not constitute correct filtering, they
can crack during use allowing the unfiltered Sun though to your eye.
The other point to watch out for is that in theory
these small refractors should be ideal for projecting the Sun onto a screen, and this is
by far the safest way to observe the Sun. However you need to check your eyepieces first.
Look down the barrel of the eyepiece at the field stop, in most cases this will be just in
front of the first lens and about the same diameter, if it is made from metal (it may be
anodised or painted black) then all is well. If it is plastic as many of the cheaper ones
are then it cannot be used for solar projection, it will melt!
Finally, always keep the front lens of the finder
scope covered up, the finder will be damaged if you allow the Sun to shine into it.
Our view
A few years ago, I started looking at designing a
telescope for solar observation using a small CCD video camera. As part of the early
research I needed to obtain a selection of off the shelf object lenses for testing, the
obvious choice being those from small telescopes. At about the same time as I started
phoning around the various telescope manufacturers for samples of their lenses, a friend
from work mentioned he was selling his Tasco telescope for £20. At another time I would
not have been tempted, as I was somewhat less than open minded about Tasco scopes, but I
was desperate to begin experimenting and so became the proud owner of a Tasco telescope.
Here is not the place to go into details but, having completed several tests on the lens,
I was surprised by the results (the lens tested was the D=60 F=800 air spaced doublet used
in some models). Considering the price Tasco must pay for it, the lens was extremely good.
The lens was however quickly rejected, for reasons
other than quality, and returned to the telescope. Eventually I gave the telescope to my
son and it is still in regular use, though I suspect not just for astronomy.
With the few simple modifications above, and a good
eyepiece the telescope I tested was capable of very good results, in particular on a star
test the results were probably as good as I have seen with any 60mm production telescope,
but I have yet to test a 60mm Takahashi Fluorite!
If asked whether I would recommend buying a Tasco
telescope from new the answer would have to be no, if only because better
instruments are available for around the same price, provided you are willing to
search for them. If you have not yet bought a telescope and are trying to decide on one,
then the best advice we can give is to go to a reputable dealer that specialises in
astronomical instruments -
such as our own AstroShop
So is the criticism justified? Well yes,
not so much for the telescopes themselves but for some of the advertising that sells them.
Below is a quote direct from a mail order catalogue:
"Tasco 525 X 60 Telescope
525X magnification. 60mm objective lens.
Optimum usage up to 185X magnification." etc.
I can't see why the law does not protect against
this kind of advert. Whilst it cannot be claimed that the advertisement gives false
information, if you simply do the sums it will magnify 525X. The fact is the
instrument will be totally unusable for astronomy at this magnification, which amounts to
the same thing!
The worst of it is that, in order to support these
claims the instrument is sold with a set of accessories that do not allow it to operate
within its true optimum range. |