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A
beginner’s guide on understanding how your eyepiece works with your new
telescope
If you have
been following this motorhoming blog, you will be aware that I have just taken
up amateur astronomy. I bought a telescope and I am taking it with me on all
our forthcoming tours in Bryony, our Autosleepers 2019 Broadway EB.
Previous blog
posts about my new ‘astronomical’ learning can be found here
Why
take up amateur astronomy?
https://wherenexthun.blogspot.com/2021/01/the-joys-of-astronomy-is-it-hobby-we.html
How
to choose your first telescope as a beginner new to astronomy
https://wherenexthun.blogspot.com/2021/01/buying-your-first-telescope-what-do-you.html
The
importance of choosing the correct mount and tripod
In this blog
post, I look at eyepieces.
So, lets
start with how an eyepiece works. It takes the captured, focused light in your
optical tube assembly and magnifies the image that is seen by your eye. A good
eyepiece does this really effectively although as the light passes through it
some of it is lost. To try and reduce this light loss, manufacturers coat their
lenses.
My telescope
came with two eyepieces – a 25mm and a 10mm. My dealer suggested I replace the
10mm one (of OK-ish quality) with a better quality 12mm one straight away. It
seems that at some stage, most new astronomers will want to get better quality
eyepieces, sooner rather than later, in order to enhance their viewing
experiences.
Yet, there
are so many types out there, of such differing design and complexity, so how
does a beginner choose the best one for their budget and needs without wasting
time and money?
Back to my
dealer – he recommended adding a Barlow lens to my list of purchases,
explaining that it would immediately increase the number of lenses I had, from
two to four. I trusted him. I had no idea what a Barlow was so I just went
along with it.
Since then,
of course, I’ve been reading up on eyepieces and I’m getting some insight into
their importance. Below is a summary of what I think beginners need to know
about them. If you are an experienced astronomer and think I need extra
information, then please let me know in the comment box at the end so that I
can correct any unintentional mistakes.
Astronomers
on various forums recommended that as a beginner I start with three eyepieces:
·
A
low powered wide field of view piece
– for viewing large celestial objects and for finding objects in the sky before
switching to higher magnifications
·
A
medium powered piece
for getting more detail in objects such as close nebulae; for magnifying
smaller objects like galaxies and star clusters and for darkening the background
sky to gain more image contrast
·
A
high-powered piece
for seeing planetary details, resolving double stars and for observing small
deep space objects
Great advice,
but in order to do that, precisely what is it about eyepieces I need to
understand before parting with the cash?
Firstly, I
discovered that there are two barrel size eyepieces - 1 ¼” and 2”. This is the diameter of the
barrel that slides in to the focuser. Your telescope will be one or the other
or both and you need to know which! Take a look at the focuser unit on your
telescope and what it says in the accompanying documentation.
Secondly, the
function of an eyepiece is to magnify an image projected by the objective
or primary lens or mirror and the magnifying power is related to its focal
length. Remember from previous posts that eyepieces and telescopes are denoted
by their focal lengths. The two focal lengths give a magnification using the
calculation below:
Magnification
of EP = Focal length of telescope/focal length of eyepiece
e.g. Magnification EP = FL Ts
1200mm/ FL EP 25mm = 48x
Magnification EP = FL Ts
1200mm/ FL EP 12mm = 100x
Magnification EP = FL Ts
1200mm/FL EP 6mm – 200x
As you can see
from above, the shorter the focal length of the eyepiece – the higher the
magnification you will get.
Eyepieces come
in a huge range of focal lengths and it is tempting to just buy a few to
increase your range in your kit but beware, for not all telescopes will work
well at extreme ranges of magnification. There is the concept of useful
magnification, which I have mentioned in my previous blog posts. For my
telescope the useful magnification is up to 300x. Anything over this and images
will be degraded and fuzzy and I will have wasted money on unsuitable eyepieces.
You soon
become familiar with the relevant magnification capabilities of various
eyepieces – a 32mm or 25mm eyepiece will give a low magnification. A 12mm
or 5mm will give far higher
magnification.
As a rule of
thumb – the useful magnification on a telescope is x2 the aperture of your
telescope. So, if you have a 150mm
aperture telescope – then 2 x 150 = 300x. After this level of magnification,
the image you will be viewing will appear degraded and fuzzy
Tom Kerss did
an excellent webinar on using your telescope for the ‘Sky at Night’ magazine
and one of the points he made about magnification and eyepieces was this.
“As
a beginner don’t fall into the trap of buying eyepieces that would give us a
greater magnification than the ‘useful magnification’ limit of our telescope”.
He went on to point out that the UK atmosphere
has a high moisture content and often results in poor viewing. On a typical UK
viewing night – we will have viewing conditions best suited to no more that
200x magnification. On a really clear night, it will be 300x. Any eyepiece
which gives you an excess in magnification over this, is, in essence, wasted
money.
On an
observing session, astronomers always start with a lower magnification eyepiece
such as a 32mm and slowly build up to using a higher magnification such as a
10mm.
Below I have
suggested some focal length eyepiece categories and what they might be useful
for – based on internet research, reading of magazines such as ‘The Sky at
Night’ and thanks to help from various astronomy forums.
|
Focal length
range in mm |
Best uses |
|
3 - 6 |
High magnification
views – moon, planets, double stars (warning – will give too much magnification
if using longer focal length telescopes). Need steady viewing conditions.
Best on long FL telescopes and Schmidt Cassegrains |
|
7 - 13 |
Medium to high
magnification – good for galaxies, globular clusters, planetary nebulae,
wider double stars; for moderate magnification of lunar features and for
viewing planets on nights of poor seeing conditions. (7 – 10mm good for
shorter focal length telescopes; 10 – 13mm good across all focal lengths) |
|
13 - 18 |
Low to
medium magnification – great for all focal lengths - extended objects like
galaxies, larger open star clusters, and globular clusters on longer focal
length telescopes |
|
19 – 24 |
Lower
magnification – fast reflectors and refractors – excellent for extended
nebulae and larger galaxies; Good on longer focal length telescopes for wide
field and more extended objects – shorter FL’s – great mid-range
magnification |
|
24+ |
Lowest magnification
– sweeping large angular views of Milky Way in dark sky; for centring objects
in FOV before switching to higher magnifications - galaxy clusters, large open clusters |
|
25 – 30 |
For longer
FL telescopes – good for large nebulae and open clusters. For shorter FL –
fantastic for large objects like Orion Nebula, views of lunar disc, large
open clusters. Excellent locator eyepieces for all focal lengths |
|
30 – 40 |
Well suited
for shorter FL telescopes – extended views and large starry fields, nebula
with star fields etc. |
A couple of final
points about the concept of magnification – firstly, as beginners to astronomy,
we need to understand that increasing the magnification on a telescope is about
zooming in on an object to see less of it in the field of view. It is not about
increasing the brightness or detail of the object. Images of extended objects
like galaxies and nebulae will get far dimmer as you increase magnification;
double the magnification and the image becomes four times fainter. (In
contrast, images of stars at higher magnifications don’t get dimmer).
In addition,
the more you magnify an object, the shorter time it remains in your field of
view because the earth is spinning on its axis.
A third
concept to grapple with is that of Field of View! It is normally marked
on the body of the eyepiece along with its focal length.
The
apparent field of view (AFOV) of an eyepiece is the apparent angular width of
the sky presented to your eye (usually ranging from 40 to 100 degrees). A large
AFOV eyepiece shows you more sky for a particular magnification. Great if you
want to see extended celestial objects such as large galaxies and nebulae in a
single view. However, such eyepieces are normally very large and very
expensive.
Whilst the
AFOV of eyepieces typically ranges from 45 – 100 degrees, it is the eyepiece’s
true field of view that is important and involves another calculation.
The true
field of view (TFOV) is
AFOV/magnification
e.g.
50 degrees/54x = 0.93 degree true field of view (which is x2 the width of the
moon (it will look good but you wont be seeing any detail).
To put this
in context. My new telescope has a focal length of 750mm. If I use an eyepiece
of 25mm that gives me a magnification of 30x.
Now if I use
three eyepieces which individually have AFOV’s of 50d, 68d
and 82d respectively then using
the calculation above these eyepieces would give me TFOV of 1.6d, 2.2d
and 2.6d respectively. The eyepiece with the widest field of view shows
an area of sky about 0.5% more than the eyepiece with the narrowest view.
Further concepts
to understand include Eye Relief, Exit Pupil size and Focal
Ratio.
Eye Relief is about how far your eye must be
from the eyepiece in order to see the entire field of view; a maximum distance
you can place your eye from the top of the lens of the eyepiece. The bigger the
eye relief, the better the viewing experience and I’ve been told that 15 – 20mm
is generally best, particularly if you are a glasses wearer. This distance
means you are not jamming your eye right up to the eyepiece, which is often the
case when using short relief eyepieces of 5 – 8mm.
Exit Pupil
size is about the
diameter size of the image that comes out of the eyepiece, or to put it another
way, the diameter of the beam of light that projects from the eyepiece into the
entrance pupil of your eye.
A human eye’s
dark-adapted pupil copes with an image around 5 – 7mm. For an eyepiece of a
particular focal length to work well with my telescope I need to determine the
exit pupil. And yes, sorry, it is another calculation
Exit
Pupil = telescope
aperture/magnification (power)
As the power of the
telescope increases, the smaller my exit pupil becomes. My eye pupil can’t go
bigger than 7mm and not smaller than around 0.5mm. This means that I should
look to buy eyepieces that when combined with my telescope, provide an exit
pupil no larger than 6mm and no smaller than 2mm, for best effect.
E.g. If I use
an eyepiece of 32mm on my new telescope which has a focal length of 750mm and
an aperture of 150mm then the magnification is 23x. Using the calculation above
Aperture (150mm)/Magnification
(23x) = 6.5mm
Focal Ratio is
Focal
ratio (f/) = Focal
Length of Objective / Diameter of Objective
A small focal
ratio telescope is called a ‘faster’ telescope because it produces a brighter
image of extended objects and so are able to take a photograph in a quicker
time. Less than F/5 telescopes have greater field curvature so that when the
focuser is altered to bring stars to a focus in the centre of the FOV, stars at
the edge are slightly fuzzy. This means that to avoid this edge of field
fuzziness, you have to invest in high quality, premium eyepiece optics –
TeleVue, Nikon, Baader, Pentax are Brands that have been recommended to me.
‘Slower’
telescopes of greater than F/8 tend to be better for less expensive eyepieces
when it comes to getting a good focus across the entire FOV.
So, what
different types of eyepieces are there to consider?
Plossl
pieces
·
Wide
FOV (around 50 degrees)
·
Good
for both planetary and deep space object viewing
·
Short
eye relief especially with focal lengths <12mm
·
Constructed
with two back-to-back lens systems
·
Vary
in price from very cheap to very expensive
Radian
pieces
·
FOV
similar to Plossl above
·
Different
eye relief – much bigger – so great for glasses wearers
·
User
friendly
·
Good
for medium and higher magnifications to get plenty of planetary detail
·
6
or 7 lens elements that have very short focal lengths
Nagler
pieces
·
Very
large FOV e.g. 82 degrees
·
6
or 7 coated lens elements to increase the amount of light that travels through
eyepiece
·
Very
heavy
Orthoscopic
eyepieces
·
Effectively
replaced by Plossls, but still good eyepieces
·
Four
element optical system
·
Good
eye relief
·
FOV
40 – 50 degrees
·
Good
for observation of moon and planets
·
Slightly
different to an eyepiece but just as important are Barlow lenses.
A Barlow lens
or a Powermate doubles your eyepiece collection. It is placed in the focuser
before the eyepiece is added in and it will immediately make the eyepiece x2
longer than it is. E.g. an 8mm Ep becomes a 4mm one or a 32mm one becomes a
16mm one.
Depending on
the choice of Barlow you can increase magnification x2 or x3. It is a great way
to save money but extend your eyepiece collection.
Postscript update: We have been attending zoom sessions held by gostargoinglive. They have been excellent. One aimed for beginners with telescopes was most illuminating. The presenter called a barlow a 'gotcha' - one of those things sold but that aren't really useful to beginners. His advice was keep it in its box and just use the lowest magnification eyepieces you have at the start.
Some
tips given to me about choosing eyepieces
1. Don’t buy eyepieces that exceed the
maximum magnification for your telescope (remember maximum magnification rule
of thumb is 2 x your aperture).
2. When buying an eyepiece plan on 30x to
40x per inch of telescope aperture since that is what most amateur telescopes
cope with on average seeing nights in the UK.
3. Remember maximum magnification is
limited by your eye and exit pupil of an eyepiece.
4. If you are 40+ years old, your exit
pupil size is more likely to be 5 or 6mm which puts a limit on the maximum exit
pupil of any prospective eyepieces. Buy one of greater mm and the light doesn’t
enter the eye – its wasted.
5. On a reflecting telescope, a large
exit pupil may well cause the secondary mirror to become an obstruction in the field
of view.
6. Some forum members of various groups
suggested that an eyepiece giving an exit pupil diameter of 2-3mm provided
better visual experiences of many deep space objects.
7. You need to think about your observing
preferences before purchasing an eyepiece. Would you enjoy the huge AFOV of a
costly eyepiece e.g. 82 degrees or prefer to stay with a more restricted view
e.g. 68 degrees (pieces which are less expensive)?
8. Don’t forget to consider eye relief –
are you a glasses wearer? Will a longer eye relief be better for you?
9. If you are mainly interested in viewing
planets, double stars, planet nebulae, then eyepieces with smaller AFOV’s will
improve contrast and colour. Remember the ‘table’ I included earlier – have another
perusal of it.
10. Buy a range of eyepieces over time. A
range allows you to observe a greater number of different objects during any one-night
session.
11.Organise your eyepieces outside on an
observation by focal length order and remember to start an observing session
with a longer focal length eyepiece and work your way down to shorter focal
lengths until you reach the best magnification for the object you are viewing. Try
to get a tripod tray with holes in for your eyepieces or use a plastic food
container filled with cut out foam slots into which the eyepieces can fit.
12.Non-reflective coatings on your
eyepieces are a must.
They cut down reflections that would reduce the contrast and spoil the views. The
best ones are those which are ‘fully multi-coated’ or ‘multi-coated’. Avoid
those that are described as just ‘fully coated’ or ‘coated’. I read somewhere that if you fix a black cap
end to an eyepiece and then look down it, the darker the glass looks, the less
light is lost and the better the eyepiece is. Sadly, I can’t find the article I
read that on – poor referencing – my apologies.
13.You can increase the contrast by
ensuring the interior of your eyepiece is matt black in colour.
14. I’m told that a good starting eyepiece collection would include eyepieces of 6mm, 10mm, 15mm and 25mm – these will cover most observing situations. My Barlow purchase turns my two accompanying eyepieces into four – the 25mm becomes a 12.5mm; the 12mm becomes a 6mm. So, I’m almost there!
I hope that this has helped you better understand the importance of correct eyepiece choice when starting out in astronomy. As soon as I invest in my first additional purchase eyepiece, I will let you know how I do it and how I get on.
in the meantime, if you are an experienced motorhoming astronomer, do drop us some tips and advice in the comment box below. I would love to hear from you.
All of you take care out there in these strange times. Stay safe and well and remember 'take care out there'
Steve
references and acknowledgements:
https://www.skyatnightmagazine.com/
https://www.televue.com/engine/TV3b_page.asp?ID=202
I
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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