In my first post about astronomy and motorhoming, I shared my fascination with the heavens above and you can access it here at
https://wherenexthun.blogspot.com/2021/01/the-joys-of-astronomy-is-it-hobby-we.html
In this second post, I share with you what I have learnt so far about choosing a beginner’s telescope. I must emphasise at the start that I am new to astronomy and am literally beginning a personal journey of discovery. I am not a font of knowledge but I hope that my new learning encourages some of you to consider taking up the hobby.
When my new telescope arrives, it will be only the second telescope I have ever owned in my fifty-eight years on this planet (the first was a birthday present when I was eight). And, truthfully, I cannot begin to describe how excited I am about this.
But,
why am I buying one now?
Well, simply
put, we have a motorhome and at some stage this year we will be touring across
the UK and hopefully next year down through Europe when the pandemic eases
sufficiently for us to do so safely.
There are going to be countless evenings on campsites or wild place
camps, where we can star gaze away from light pollution.
“And, what
could be possibly more fun than setting up your telescope near your motorhome
at 10pm, for an evening of stargazing in the skies above?”
Spiral
galaxies, globular clusters, double stars, the Milky Way, constellations,
comets, shooting stars, the planets, the moons of Jupiter, the rings of Saturn,
deep space objects, the craters of our own moon? There is soooooo much to
discover and explore.
“At 1am,
pausing from my tour of the night sky, I will screw my canon DSLR onto the
eyepiece and start some astrophotography; a new skill set being learned –
trying to catch colourful shots of the Orion nebulae”
A telescope
will bring all the small things I can see above me in the night sky much
closer. “What differences will I see, what surprises will lie at the heart
of those twinkling colours of light, some of which has travelled millions of
light years to reach my retina? And, just as exciting, what objects will
suddenly appear in my eyepiece, which have been invisible to my naked eye thus
far?”
I really
can’t wait to find out. Like millions of amateur astronomers before me, I will
be following not only in their footsteps, but also those of Galileo, who was first
to turn his spyglass telescope to the night sky some four hundred years ago.
Unlike him, on a clear night with calm air above we can today, point telescopes skywards and use our mobile phone apps to explore far distant enthralling star systems, constellations and nebulae that were beyond his telescope. The vastness of space, how enticing.
Buying
my first telescope – a beginner’s perspective
It has taken four weeks of research, emails, posting on astronomy forums and talking with sales staff at a local astronomy supplier to finally arrive at my choice of beginner telescope -
https://www.firstlightoptics.com/az-goto/sky-watcher-star-discovery-150i.html .
I will be
honest from the start - it is a compromise and even now I am not 100% sure it
is the right choice - but it will see me through a few years until I have
developed my specific astronomical interests and skills. And then, if need be I
will sell it and buy another one.
I arrived at
this particular telescope as a result of having my initial requirements and
expectations sifted, sorted, refined and reassembled into a more realistic set
of aspirations and desires. And, this was in reality, my first ‘painful’ lesson
- learning that there was no single telescope that would do what I wanted.
Choosing a telescope,
it seems, requires some degree of compromise.
Below are the
aspirations I had for my ‘second’ telescope:
·
Highly
portable, easy to store and assemble
·
Easily
transportable in car, motorhome and on occasions a large rucksack (so that I
could hike to one or two more inaccessible tors on Dartmoor).
·
Be
used in the back garden here in the suburbs of Plymouth
·
Come
with a computerized ‘GOTO’ data base of planets and Deep Space objects
(DSO’s)
·
Would
allow me to develop beginning skills in astrophotography imaging and as well as
just basic observing (and that I could use my existing Canon 800D DSLR with it)
·
Would
give me good views and images of the moon and planets as well as many deep
space objects, millions of light years away
Well, such a versatile and exquisite telescope doesn’t seem to exist and, in my beginner’s excited delirium, I’ve been somewhat overambitious. The very generous, knowledgeable and good-humoured members of the ‘Stargazing Lounge’ forum (https://stargazerslounge.com/ ) helped me rationalise my ‘great expectations’ down into more realistic propositions.
Helpful
questions posed were:
- ·
How
much was I willing to spend?
- ·
How
portable did the telescope actually need to be for what I wanted to do?
- ·
How
important was the astrophotography bit to me in the first few years of learning
astronomy?
- ·
And
most importantly, what did I hope for and expect to get from starting astronomy?
From the
start, forum members made it clear to me that how the night sky would look
through my telescope would depend on what type of telescope I chose. It seems
that the type of telescope determines what kind of things we can look at through
it and so we all need to decide which astronomical objects are the ones we are
most interested in viewing so that we can ensure we buy the correct type.
So, I set off
to read and research as much as my poor little brain could accommodate about
the three types of telescope commonly available for beginners. But, before
going through the three types, I share what I have learned about the basics of
how all telescopes work.
How a telescope works - the simple version
A telescope
is an optical tube assembly (OTA) which collects light and then brings it to a
focus at an eyepiece. Essentially, light enters the OTA, and through a series
of mirrors and/or lenses, the light is reduced down into a cone whose apex
reaches a focus point. Here at this point, the light is now brighter as more
light photons have been packed more densely into a smaller area; and an image
appears at this point. If you put an eyepiece at this point on a focuser, then
you should see the image collected.
So, now let’s
look at each type of telescope (which have their own advantages and disadvantages).
A beginner to astronomy needs to know what the qualities of each type are
BEFORE deciding on what kind they might want to investigate further.
1. Refractor telescopes
Telescopes
that use lenses to capture light are called Refractor telescopes.
The light within them is bent (refracted) by the large lens to a focus
called the focal point. Putting an eyepiece at the focal point
allows you to see the image. Achromat lens systems involving two lenses
are not colour dependent and bring all coloured light to a focus. However, Apochromatic
lens systems involving four pieces of glass, are more expensive and give a far
superior image quality and focus.
Advantages
of a refractor
telescope are:
·
They
are the best performing telescopes £ for £ spent
·
They
need least maintenance due to their rugged construction with a sealed tube - so
no dust collecting
·
That
they don’t need collimation (lens alignment)
·
Lightweight
and so are better ‘grab and go’ telescopes
·
Better
at image contrasting because no light is blocked as it passes through the
lenses. This is much better for observing planets and double stars where good
image contrast is required to be able to see finer details
·
Good
for seeing the solar system, bright deep space objects (DSO’s), planets and
double stars
·
Often
shorter tubes and so are more easily transported – good portability
Disadvantages of a refractor telescope are:
·
They
are expensive compared to other telescope types
·
That
they have a long optical tube with the eyepiece at the bottom of the tube –
requires some bending or a very tall tripod
·
That
they need a long cool down period (the time needed for a telescope to adjust
from being inside a warm house or vehicle to being at the outside temperature
and ready for viewing through)
1. Reflective telescopes
In a Reflective telescope light is reflected from a primary mirror to a smaller mirror that sits in front of it (this is called the secondary mirror). Hence it is a mirror rather than a lens system. The larger primary mirror should be a curved parabolic one. The smaller secondary one should be flatter. The light is then further bent through an eye piece. A reflecting telescope where the light is bounced off the secondary mirror sides through a hole near the top of the telescope optical tube is called a Newtonian telescope.
Advantages of reflecting telescopes are:
·
That
they give the best size per £ ratio and so are often the least expensive
telescope type
·
That
they are best for viewing fainter distant objects such as galaxies and nebulae
·
That
they have no excess colour – so no colour fringes around objects viewed
·
The
easiest to adjust and modify due to a simple design
·
That
they can be viewed from the top of the tube thus allowing for shorter tripods
and a more comfortable viewing stance
Disadvantages
of reflecting
telescopes are:
·
That
they are sensitive to bumping and so need regular collimation of the primary
and secondary mirrors
·
That
they suffer from the ‘coma effect’ – a defect that causes stars at the very
edge of the field of view to look long and thin like comets
·
To
do with being open tubes - great dust collectors if you are not careful (which can
affect viewing quality and experience)
·
That
they fully invert and flip any image to what it is in reality – basically you
will see part of the moon surface upside down and flipped left to right – which
could be pretty confusing, if you weren’t aware of that
1. Catadioptric telescopes
Advantages of Catadioptric telescopes are:
·
That
they have the most compact design, making them excellent ‘grab and go’ telescopes
·
That
they have a long focal length in a very compact tube and are excellent for all
near and deep space viewing except for distant deep space objects
·
That
as a short tube, they are easier to mount on a tripod
·
There
is no dust problem as they are a sealed tube
·
That
manufacturers tend to sell them as complete systems with accessories included
Disadvantages are:
·
That
they are fairly costly telescopes
·
Ones
that require periodic collimation
·
That
as closed tubes, they often have the longest ‘cooling down’ period of all
telescope types, when taken out of the house and onto the observation site –
basically you can’t use them immediately – they take time to adjust to the
outside temperatures
Prospective
‘beginner’ astronomers also need to have a basic understanding of how a
telescope functions before making a purchasing decision. There are
some key concepts to understand, which can help in any decision-making on
purchasing a suitable beginner’s telescope.
All telescope
types share four common measurements that help us understand how a
telescope will perform, irrespective of what optical arrangements it has.
The four
concepts are:
Aperture
size
Focal Length
Focal Ratio
Useful Magnification
Understanding
what these concepts mean enables you to make informed choices as to what you
can and cannot do with a particular telescope.
Key to
absolutely everything seems to be a telescope’s light gathering ability.
This is
called its Aperture – (the diameter of the primary mirror or lens
in a telescope and how much ‘light grasp’ of light photons it can collect).
·
The
bigger the aperture, the more light grasp there is.
·
The
more light coming in, the brighter the object seen and the more contrast detail
seen in it.
·
Also,
the more light that can be gathered, the fainter the celestial bodies you will
be able to see.
Here, the
maxim ‘bigger is better’ holds is true.
The amount
of light a telescope gathers is directly proportional to the area of the
aperture. A 6” or
150mm diameter aperture telescope will gather 4x more light than a 3” or 75mm
one.
When I had
got my head around this, most advice seemed to be pointing me in the direction
of a minimum 3” or 75mm aperture. 4” (100mm) would be better still and if I
really wanted a good viewing experience, a 6” (150mm) or a whopping 8” (200mm)
diameter beginner’s telescope would be ideal.
Of course, just
remember, the bigger you go, the less portability you have!
(Please
note at this point that aperture alone isn’t enough. You need high quality
optics and mechanics in the telescope tube assembly and on its tripod mounting
system. Similarly, the quality of the mount system is critical. Flimsy mounts,
sensitive to the slightest ground vibrations will give a frustrating viewing
experience. Too heavy and awkward to carry, store and set up, or too
complicated to use, then the greater the likelihood that the telescope and its
mount will be left unused in a cupboard).
As I already outlined, in any telescope, the primary mirror or lens bends the light coming through the aperture and converges it down to a small bright image called the focal point. The distance the light travels between the aperture and the focal point is known as the Focal Length and its measured in millimetres.
Why is knowing the focal length of your telescope so important?
Firstly, focal length helps determines magnification (which
I will explain a little later on) and secondly, it gives you an indication
of what field of view you can expect to see in the eyepiece of the
telescope.
Focal length
is essentially the length of the actual telescope and a short focal length
will give a wide field of view and so is better suited to observing large areas
of the night sky and for star hopping – although beware - any objects in that
wider FOV will appear small.
Conversely, a
long focal length will give a narrow field of view (smaller area of sky) and is
ideal for planetary close-ups and for seeing fine contrasting details on them.
Any objects in a narrow FOV will appear larger. (An advantage of a longer focal
length is that you can use eyepieces with something called ‘longer eye relief’
(the ideal distance your eye should be from the lens of an eyepiece) and this
is great news if you are a glasses wearer).
The third
concept to understand is about Focal Ratio. It is also known as
the f/number and it describes the relationship between the focal length and the
aperture of a telescope.
It comes with
a calculation!
FR = Focal Ratio
FL = Focal Length
FR
= FL of telescope/ aperture (all figures being in mm)
e.g. a telescope with a FL of 900mm and an aperture
of 130mm –
900/130 =
f/6.92 FR
So, what
does the focal ratio tell us?
Larger
f/numbers imply higher magnification with any given eyepiece and a narrower
field of view.
Conversely, smaller F/numbers imply a lower magnification and a wider field of
view.
(I should say
at this point that I also discovered that f/numbers can be described as ‘fast’
or ‘slow’ in relation to how a telescope will work when used for
astrophotography. Do I understand this? No, I don’t! At this point I was losing
the will to live and I just couldn’t get my head around this particular set of
ideas – sorry!)
A few other
quick points about f/numbers:
·
FR’s
for refractor telescopes are typically f/12 to f/16;
·
for
Newtonian reflectors they are typically f/4 to f/7
·
and
for catadioptric reflectors they would be f/8 to f12
·
lower
f/number (lower FR) telescopes can suffer from field distortion called
‘coma’ (stars near the edge of the field in your eyepiece turn from sharp
points into blurry streaks)
·
Lower
f/number telescopes also require more expensive eyepieces with shorter
focal lengths to get higher magnifications to observe the planets or double
stars.
The last
concept to grapple with is that of Useful Magnification.
Every
eyepiece that comes with your telescope has a focal length and the relationship
between the eyepiece focal length and the telescope focal length gives you the
magnification (or power) of your particular telescope.
The eyepiece
is basically just a magnifying glass that allows you to put your eye closer to
the image so that you can see it better. Basically, the eyepiece takes a sample
of the image coming in through your aperture.
Again, to find
the useful magnification of your telescope – there is another simple
calculation
M = FL of telescope/FL of eyepiece
e.g. telescope FL 1200mm/FL eyepiece 20mm = 60x
magnification of the image you are seeing in the eyepiece.
However, that
same telescope of 1200mm focal length – if you put on a 10mm eyepiece, immediately
things change – 1200mm/10mm = 120x magnification.
Counter
intuitively in my opinion, the smaller the FL of your eyepiece, the closer
you can get, the more the image is magnified and the finer details of visible
objects can be seen. Aperture, by the way, is completely
irrelevant at this point.
However, it
is also worth realising that the view through a 100-mm f/12 telescope using a
20mm focal length eyepiece is identical to the view through a 100-mm f/6 telescope
of equal optical quality when you use a 10mm focal length eyepiece. Confusing isn’t it, but worth persevering
with.
So, why is
knowing the useful magnification of your chosen telescope so important?
Well, unlike
aperture (where bigger is better), more magnification isn’t actually always
better! The image formed by our telescope’s primary lens or mirror is never
perfect and so there is a limit as to how big you can magnify it before not
seeing anything new on it. (If you look at a newspaper photo with a magnifying
glass, for example, all you see are the dots making up the photo image – at a
bigger size. You don’t see any more detail in the dots!) Bigger
magnification doesn’t always mean more detail seen!
There is a
useful rule of thumb to help you work out the useful magnification of your
particular telescope. The theoretical useful limit is x2 the aperture in mm.
e.g. a 150mm
aperture has a useful magnification limit of 300x magnification.
So, I was
warned to be very careful not to be seduced by telescopes offering a
magnification of say 600x
on a 150mm aperture – absolutely meaningless and useless! Beyond 300x, all I would
get is a closer, fuzzier and dimmer view of what it is I was looking at!
Remember,
it is the eyepiece that essentially does the magnification in tandem with the
focal length of your telescope.
I might buy a telescope of a certain focal length that gives an image of the
full moon that is 10mm in diameter. A longer focal length telescope would give
me a larger image; a shorter focal length one a smaller image. By adding an eye piece to the telescope, I
could then magnify that original image given. A larger focal length telescope will
give me potentially a greater magnification but a smaller field of view.
So where
does this leave us all, other than possibly confused?
If you want
to see the craters on the moon or the cloud bands of Jupiter, you want a
telescope with a fairly long focal length.
If you want a good wide angled view of star clusters and star fields, then a shorter focal length telescope would be better.
Thanks for
staying to the end of this blog post. Top marks for fortitude, perseverance and
mental toughness. I am assuming that if you have read this far, you must be
interested in the possibilities and fun that might open up when taking a
telescope with you on your motorhome travels.
In my next
blog post on the subject of telescopes, I will explore what beginners need to
know about mounts and eyepieces before choosing a telescope suitable for their
needs.
I will post the link to this blog post at the end of this particular post, so you can always book mark this one. You could also search our blog for ‘astronomy’ and the post will appear in the search list.
As always,
have fun, stay safe and remember ‘take care out there’ when on your motorhome
travels.
Steve
(and Maggie,
who, poor soul, had the unenviable job of proof reading this blog post.
She is
currently lying down recuperating with intravenously applied restorative G and
T’s).
Reference sites and acknowledgements:
https://www.skyatnightmagazine.com/
https://www.saga.co.uk/magazine/home-garden/craft-hobbies/hobbies/beginners-guide-to-stargazing
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Hi, we always look forward to hearing your comments, tips and thoughts. Drop us a line or two below. Take care now. Steve and Maggie