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And welcome.
Welcome to 'UnderSouthWest Skies'. Thanks for stopping by. I hope you find the experience worthwhile. This blog post has been written from my own 'beginner perspective' after doing lots of reading on various basic astronomy and astrophotography topics of interest to beginners. In getting my head around it all, I may have got odd bits slightly wrong; but, like you, I am on a leaning journey, so please forgive any unintentional errors. Also, just because I have gone down a particular road regarding equipment, it doesn't mean that I have chosen the 'CORRECT' route!
Meanwhile, clear skies to you. Take care and thanks for visiting Steve aka PlymouthAstroBoy
My beginners guide to eyepieces for your new telescope
My new telescope – the skywatcher Star Adventurer 150i wifi GOTO - came with two eyepieces (a 25mm and 10mm) and almost the first thing my dealer (First Light Optics) recommended was to replace them. This was good advice. Oh my! The qualitative difference in my subsequent observation sessions was immense. Better quality eyepieces led to a much better astronomy viewing experience, believe me!
So, in this post I am going to try and explain why this
is.
With so many types and designs of eyepieces, I will also try
and help you navigate this minefield so that you use your budget wisely by distilling
and simplifying what it is we beginners NEED to know about
eyepieces.
As always, a caveat –
I am still not fully conversant with all the concepts involved (focal lengths,
magnifications etc). Therefore, I may have made some silly errors below, for
which I plead your forgiveness! If you are an experienced astronomer and
think I need extra information OR need to correct some, then please let me know
in the comment box at the end so that I can right any of these unintentional
mistakes – and thank you!
Here are the questions then that I will try and answer in
this blog post:
·
What is an eyepiece?
·
How many eyepieces does a beginner need?
·
What are the basic ideas I need to understand
about eyepieces?
o What
is barrel size?
o What
is field of view?
o What
is focal length and magnification?
o What
is eye relief?
o What
is pupil exit size?
·
What different types of eyepieces are there
and what are they best used for?
·
How should we care for our eyepieces?
·
Are there any tips we should follow when
choosing our first eyepieces?
Despite my best efforts at simplifying what I have read on
these topics, this will be a long read, for which I apologise now. And, to
thank those of you, who manage to persevere and reach the end. True hero’s and
thank you for your time and effort.
“What is an eyepiece?“
Nothing like starting with the basics then is there Steve?
An eyepiece takes captured focused light in your telescope
optical tube assembly (OTA) and magnifies this image that is then seen by your
eye. This needs to be effective if you are going to get a great view of a deep
sky object; for as light passes through the eyepiece, a little of it is taken
away by lenses etc. To reduce this loss and prevent unnecessary reflections,
eyepiece makers will coat the lenses with various substances and so these
coatings are an important factor in your choice of eyepiece. “Fully
multi-coated” are best, followed by “Multi-coated”. Emphasis is on the word “Multi”!! Black matt
internal barrels are a good thing as is dark glass – means less reflections and
loss of light. Without the right eyepiece, most telescopes won’t deliver their
full performances. Different eyepieces offer differing levels of clarity,
magnification and field of view; thus type can really change how deep sky
objects appear.
So, we really do need to assimilate and understand the
different types, options and concepts associated with eyepieces to get the best
visual observation experience.
“How many eyepieces should a beginner get Steve?”
Experienced astronomers on various forums I perused suggested
three starter pieces:
·
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
It seems that astronomers on an observing session, always
start with a lower magnification eyepiece such as a 32mm and slowly build up to
using a higher magnification such as a 10mm.
Before we delve in deeper, I feel I should say here, that when
you buy your new first eyepieces, think of them as a long-term investment and
try and buy the best quality your budget will allow. Quite often, astronomers
will invest in a new telescope but try and keep and use their existing eyepiece
collection! Worth remembering!
“What basic ‘concepts’ about eyepieces do I need to
understand before buying any new ones?"
1.
Barrel diameter is an important first
concept. There are two basic 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! It should be written somewhere on your
telescope focuser unit. Barrel diameters under an inch are probably from
cheaper telescope packages. Avoid them. Decent ‘beginner’ barrel sizes will
start at 1.25”; by far the most common barrel diameter. The practical upper
limit on focal lengths for this barrel size is about 32 mm.
2.
Focal length and magnification – concept
2! I found this initially tricky to grasp, so hang on in. The focal length of
an eyepiece determines the level of magnification it will give. The focal
length of an eyepiece is written on its barrel. Your eyepiece magnifies an
image projected by your telescope’s primary lens or mirror. But, how much will
it magnify by? We can do a calculation to work it out! Written somewhere on
your eyepiece will be a number in mm. This is the focal length of your
eyepiece. By using the focal length of your telescope and of your eyepiece you
can work out the magnification level of
your eyepiece. The calculation is: Magnification of EP = Focal length of
telescope/focal length of eyepiece
e.g. Magnification EP = FL Ts
1200mm/ FL EP 25mm = 48x OR Magnification
EP = FL Ts 1200mm/ FL EP 12mm = 100x
OR Magnification EP = FL Ts
1200mm/FL EP 6mm – 200x . The take-home message from this? The
shorter the focal length of the eyepiece – the higher the magnification you
will get; which is ideal for close-up views of planets. Longer focal
lengths are better for wide-field observations.
3.
Concept 3 – ‘useful magnification’. Not all telescopes work well at extreme
ranges of magnification! 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. How do I know this? Well, a
simple rule which goes like this - the useful magnification on a telescope
is x2 the aperture of your telescope.
So, if you have a 150mm aperture telescope, like me, – then 2 x 150 =
300x. 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.
If you are unsure about the useful magnification and eyepiece
suitability for your new telescope, talk to your dealer. Have I said, it
pays to buy your gear from a reputable dealer?
Watching an excellent webinar
last year by Tom Kerss for ‘The Sky at Night’, I was struck by one of his
observations about magnification and eyepieces:
“As a beginner don’t fall into
the trap of buying eyepieces that would give you a greater magnification than
the ‘useful magnification’ limit of our telescope. Here in the UK, our
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”.
A couple of final points about
the concept of magnification before I move on – firstly, you increase magnification
on a telescope to bring closer to you a deep sky object; thus, you see less of
it in the field of view. Magnification DOES NOT increase the brightness or
detail of that object. Some DSO’s such as galaxies and nebulae may get far
dimmer as you increase magnification; double the magnification and the image
becomes four times fainter. So be warned!
Secondly, the more you magnify an
object, the shorter time it stays in your field of view because the earth is
spinning on its axis – so the more telescope knob ‘fine adjusting’ you have to
do to keep the DSO in your field of view.
Oh, and a third point – sorry!
Just to complicate things further, different DSO’s can be best seen at
different magnifications – so, remember where I said earlier I was advised to
start with three eyepieces of different magnifications/powers? I have 8mm, 12mm
and 25mm. If I was looking to extend this collection – a 15mm would be useful.
An experienced astronomer I know has four eyepieces she uses most frequently – 6mm, 10mm, 15mm and 25mm – which seem to cover
most of her observing requirements.
Towards the end of this blog post
I will briefly summarise some eyepiece focal length categories and what DSO
they can be used on.
4.
Field of view – we have mentioned it a
couple of times already. Concept 4! After an eyepieces focal length,
this is probably the most important follow up concept. FOV will be marked
somewhere on an eyepiece barrel. Two ideas to consider here:
a.
apparent field of view (AFOV) -
the apparent angular width of the sky offered to your eye (usually ranging from
40 to 100 degrees). A large AFOV eyepiece shows you more sky for a particular
magnification. Brilliant for a single view of large galaxies and nebulae. Not
so great? Such eyepieces are normally very large and very expensive.
b.
The true field of view (TFOV) - the
important concept to grasp. Another calculation - 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 won’t be seeing any detail). I’d like to say I understand
this fully but I don’t. However, to try and put this concept 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 50degrees, 68degrees
and 82degrees 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.
Yeah!
I’m not sure I understand this either. Ho hum! It’s a learning journey and
there were bound to be tricky bits! Essentially, I think ‘The true FOV’ is
the important number as this tells you exactly how much sky you can see through
the eyepiece.
5.
If you wear glasses – you need to consider the ‘eye-relief’
of your potential eyepiece. Concept 5! This is the
maximum distance the eye needs to be held behind the eye lens of an eyepiece to
see images and the entire FOV properly
through it. Someone told me if you wear glasses – 15 – 20mm is generally
best and generally a larger relief is a better viewing experience. Long focal length eyepieces generally have
good eye relief as well.
6.
Concept 6 - Exit Pupil size . It’s
the diameter size of the image that comes out of the eyepiece and projects onto
the entrance pupil of your eye. Did you know a human eye’s dark-adapted pupil
copes with an image around 5 – 7mm? Nope, neither did I – go figure. For an eyepiece of a particular focal length
to work well with my telescope I need to determine the exit pupil size. 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 size no larger than 6mm and no smaller than
2mm, for best effect. So, using a 32mm eyepiece on my 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; so, a 32mm eyepiece is just over giving me
that maximum 6mm exit pupil size measurement.
At
this point, would it help if I told you my head really hurts? I struggle to
retain all this. Maybe its just me!
Let’s try to return to some normality here, as I may well be
losing the will to exist!
Another question beginners need to ask: “What
different types of eyepieces are there?”
Hah! Yes, there are different types of eyepieces and I have
tried to summarise their characteristics below:
1.
Plossl pieces – a popular option
for beginners due to affordability and versatility. They come with
a.
Wide FOV (around 50 degrees) so good for both
planetary and deep space object viewing
b.
A problem of ‘short eye relief’ especially with
focal lengths <12mm
c.
two
back-to-back lens systems
d.
Sharp images
e.
Varying prices from very cheap to very expensive
2.
Radian pieces: These have a
a.
FOV similar to Plossl above
b.
different eye relief – much bigger – so
great for glasses wearers
c.
User friendly factor
d.
Good range for medium and higher
magnifications to get plenty of planetary detail
e.
6 or 7 lens element construction that have
very short focal lengths
3.
Nagler pieces:
a.
Very large FOV e.g. 82 degrees
b.
6 or 7 coated lens elements to increase the
amount of light that travels through eyepiece
c.
Very heavy
4.
Orthoscopic eyepieces - Effectively
replaced by Plossls, but still good eyepieces that come with a
a.
Four element optical system
b.
Good eye relief
c.
FOV 40 –
50 degrees
d.
Good for observation of moon and planets
Slightly different to an eyepiece but just as important are Barlow
lenses.
A Barlow lens doubles your eyepiece collection. Great for
saving you money then. Not an eyepiece as such, but its optical elements work
with an eyepiece to immediately make the eyepiece x2 or x3 longer than it is.
E.g. an 8mm Ep becomes a 4mm one or a 32mm one becomes a 16mm one.
How do you use it? Insert your eyepiece barrel into the
Barlow and tighten it up. Put the combined system into your focuser or
telescope diagonal piece. Job done!
“How should we look after our eyepieces?”
Very carefully. Treasure them! My quick answer! You will
have spent a lot on them! That long term investment collection, remember? Basics
then:
·
Protect from dust and moisture – keep them in a
protective box – with a desiccant sachet to absorb any moisture
·
Handle them carefully to avoid scratches or
smudges on lenses
·
Clean them gently with a proper cleaning
microfibre cloth
So, final bits and pieces. I promised early on this post to
look at suggested 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. So here they are:
Best uses
3 – 6mm
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 – 13mm
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 – 18mm
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 – 24mm
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
24mm
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 – 30mm
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 – 40mm
Well suited for shorter FL telescopes – extended
views and large starry fields, nebula with star fields etc.
And let’s finish with some tips, which we can glean
from above:
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.
Remember maximum magnification is limited by
your eye and exit pupil size of an eyepiece.
3.
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
4.
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.
5.
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)?
6.
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.
7.
Buy a range of eyepieces over time. A
range allows you to observe a greater number of different objects during any
one-night session.
8.
Organise your eyepieces outside on an
observation by focal length order
9.
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.
10.
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.
11.
Non-reflective ‘fully multi-coatings’ or ‘multi-coated’
on your eyepieces are a must
12.
A good starting eyepiece collection would
include eyepieces of 6mm, 10mm, 15mm and 25mm – these will cover most
observing situations.
13.
A Barlow lens double your eyepiece
collection immediately. Mine turns my two accompanying eyepieces into four –
the 25mm becomes a 12.5mm; the 12mm becomes a 6mm. So, I’m almost there!
14.
If the maths is causing you some confusion –
lets face it, it does with me, but then I am mathematically challenged at the
best of times – anyway perhaps this very useful calculator will help take some
of the pain away. https://skyandtelescope.org/observing/telescope-calculator/
If you have managed to read the whole blog post and haven’t resorted
to alcohol to blot out the pain, then well done and thank you. I hope that this
has helped you better understand the importance of correct eyepiece choice when
starting out in astronomy.
As always, if I have missed something or got something
wrong, drop me a comment below so I can correct things. Similarly, any further
tips - pop me a comment so I can acknowledge it and amend the post.
It only remains for me to wish you clear skies and fantastic
viewing opportunities wherever you may be. Take care, have fun and stay safe out
there
Steve
References and acknowledgements: much of my reading came
from these sources
https://www.skyatnightmagazine.com/
