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Q: Can you please tell me what short-tube refractors
are good for? Can they be used for deep-sky and planetary observation?
What short- tube refractors would you recommend?
PHILIP BUGGY
A: Short tube refractors have limited benefits.
They are better suited to deep sky observations than a standard
refractor but not as good as a standard reflecting telescope.
If you are interested in deep sky observing, then
choose a reflecting telescope with a short focal length.
If you are interested in planetary observations
then go for a longer focal length refractor but if you have an interest
in both, then a medium length reflector would be your best choice.
Girl with telescope |
Q: I have bought a tasoro 600mm telescope from
Argos what can I expect to see from it? And what is the cheapest
camera for astrophotography?
LEE SAPSFORD
A: It is very hard to answer this question
without knowing how big the lens or mirror is on the telescope.
I imagine that the 600mm you refer to is the focal
length of the lens or mirror rather than its size. If you could
measure how far it is across the end of the tube then I can answer
the first part of your question.
Regarding astronomical photography, the best camera
to get to start out is a really basic SLR camera. These cameras
allow you to change the lens over.
Go to your local camera store and ask for a basic
second hand SLR that does not need batteries. When you take pictures
of the night sky, it can be really cold outside and the batteries
can often fail making the camera useless. For astronomy, the more
basic, the better.
Q: I am about to get my first telescope and
have a few questions. One is that I messed around with the one I
am thinking of getting and it is a long tube reflector with 25mm,
12.5mm and 4mm eyepieces.
It is the third from the cheapest one you can
buy at a department store. Do you think that is enough magnification
to see the rings on Saturn when the time of the year is right? To
see it not as a star but as the real thing?
If not could you please tell me if refractor
or reflector is better and what magnification I should get to best
see the planets.
ANGELA
A: Without knowing the exact model or specification
of the telescope in question, it is hard to be certain what the
resultant magnification of the system will be.
To work out the magnification, you take the focal
length of the telescope (this is typically stated on the side of
such telescopes on a sticker) and divide it by the focal length
of the eyepiece (such as 25mm or 4mm).
The best I can do is guess at the magnification
you will achieve with the 12.5mm eyepiece as being around 50x to
75x. This should be adequate to see the rings of Saturn.
The three eyepieces you would get will give you
a range of magnifications from the 25mm giving the lowest magnification
to the 4mm giving the highest magnification.
However, I suspect that the 4mm eyepiece will not
give a very bright and sharp image. Eyepieces of this focal length
need a very high quality telescope to be of use.
A final word, telescopes from department stores
can be of questionable quality. Often the lenses are made of plastic
rather than glass and can yield rather disappointing results.
Another option is to consider buying a reconditioned
telescope from a reputable dealer such as Telescope House in London.
They can be contacted on 0207 405 2156.
Q: I am thinking of building a refractor,using
a 75 or 100mm objective, but I can't find a supplier of surplus
or new lenses. Have you any suggestions?
CHRIS HELLON
A: Building a telescope can be a time consuming
yet rewarding task. Those who decide to build a telescope often
choose to build a reflecting telescope (using mirrors) rather than
a refracting telescope (using lenses).
There are a number of suppliers of materials for
reflecting telescopes but fewer who supplier quality lenses for
refracting telescope makers.
A good company for you to try is I.R.Poyser who
can be contacted on 01974 831471 or by post to Maes Gylfinir, Ystrad
Meurig, Ceredigion, SY25 6AX.
Q: How can I calculate the size of image that
appears in my eyepiece? For example, with a 1000mm focal length
refractor and a 10mm Plossl, the magnification is 100, and the true
field of view is about 0.5 degree.
If Mars is about 24 arc seconds, why don't I
get an image that is 2400 arc seconds, i.e. an image that more than
fills up the eyepiece? Instead, I get an image about the size of
a pea. How can I calculate the expected image size?
FRED ZIMMERMAN
A: It is not possible to determine the 'real'
image size of Mars when observed visually. It can be calculated
for photography or digital imaging however. The field of view of
your telescope is 0.5 degree (equal to 1,800 arc seconds).
Given that the angular size of Mars is typically
nearer 10 arc seconds then Mars will appear in your eyepiece to
be roughly 180x smaller than the field of view of your telescope.
In other words very small.
Do not get too engrossed with field of view when
considering size of an object when viewed. Magnification is the
key issue here.
If you are using a magnification of 100x for Mars,
you may want to try increasing the magnification to about 200x either
by using an eyepiece with a shorter focal length or by adding a
barlow lens into the telescope/eyepiece system.
Read more: What
are the best binoculars
to buy?»
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