What
is an APO?
This
article is an expansion of a piece I included with the TV76 review.
APOs,
Achro’s, Apples and Blueberries
Chromatic
aberration dominated astronomy from the invention of the telescope to the early
nineteenth century when Mr Fraunhofer invented the achromatic telescope
objective. At one time, telescopes were hung from masts to make them long
enough to get this optical “fault”, or aberration, under control.
Chromatic
aberration (henceforth “CA”) is the property of lenses that makes
them split the different wavelengths contained in white light into separate
colours, like a prism. In a telescope, this unfortunate property means that
different colours come to focus at different points so that, at high powers,
what you are looking at becomes fringed by false colours, usually blueberry
purple and apple green. At worst, chromatic aberration covers the whole image
in a diffuse haze of purple light, washing out contrast and detail.
A
single lens has the worst CA, whilst a conventional achromat uses two lenses
(called a “doublet”), one convex and positive, the other concave
and negative, to (mostly) cancel the CA out. The problem with conventional
achromatic doublets is that as aperture increases, longer focal lengths are
required to suppress CA to manageable levels, which is why traditional big
achromats were very long (F18 or more). Not only does this make them hard to
mount and use, but gives them a very narrow field of view.
It is
possible to make shorter, better achromats by either using special glasses
and/or by using three lens elements (a so-called “triplet”) instead
of two. At some point, such improvements make the telescope worthy of being
called an “Apochromat” or APO.
Defining
Apochromatic
You
will find many precise definitions of apochromat out there, usually couched in
terms of crossings and airy disk sizes and wavelengths. Unfortunately there
seem to be as many precise definitions as there are definers. The reason for
the ambiguity is that there is really a continuum in refractor telescopes, from
poor correction of chromatic aberration, with a lot of violet and green blur,
to near perfection, as follows:
·
Singlet
telescopes, like the one Galileo used.
·
Fast
big achromats, like the Synta 6” F5 and
specialist comet sweepers.
·
Fast
small achromats like the short tubes 80s and most binoculars.
·
Slower
achromats complying with the 1.22D rule: traditional long refractors such as
those from Unitron.
·
So-called
“semi APOs” which use cheaper types of ED and crown glass.
·
Fast
high-fluoride (like FPL53) ED glass doublets, like the TeleVue
76.
·
Slower
(~F8) FPL53 and fluorite doublets, such as the Takahashi FS series, TV102 etc.
·
ED
glass (and sometimes fluorite) triplet “super APOs”, such as those
from AP, TEC and LZOS; also some 4-element Petzval designs like the TV NP101
and Tak’ FSQ106.
Note
the phrase “near perfection”, it’s significant: no telescope
using lenses can ever be completely
free of chromatic aberration, it’s just a matter of degree. But the very
best have so little you can’t see it, in or out of focus on any object.
Where you need your telescope to lie on that continuum depends on what you want
to do with it. So here’s my (deliberately non-specific) definition of
apochromatic:
“A level of false colour too low to
be noticeable in normal use.”
What
you can expect in terms of CA
When I
talk about CA here, I am referring to CA generated by the objective lens. Many
eyepieces give a lot of CA, but it is mostly seen off-axis as you move your eye
around and at the field-stop. Don’t confuse this with primary CA which
happens across the field and which you can’t get rid of by re-positioning
your eye.
Quantifying
chromatic aberration is difficult on the night sky. My preferred way is to look
at tree branches silhouetted against a bright daytime sky using about 100x
magnification. This way, any false colour evident to the eye will show up (for
aberrations deep into the violet or beyond the visual range you would need to
take CCD images). At the extremes, a simple short focal ratio achromat, like an
F5 Synta ST80, or even an F6 Stellarvue
Nighthawk, shows up significant violet fringing and blur to make the view
unpleasant and lacking detail. Trying the same test on a small triplet super
APO like a LOMO 80/600 reveals no visible false colour at all, in or out of
focus, with a view that is crisp, detailed and contains only natural colours.
Most refractors fall between these extremes.
Would
these differences make much difference in use on the night sky? Well, for
casual viewing at low powers, not much. In fact, given the hype around APOs,
you might be very surprised at the lovely crisp views the Nighthawk delivers on
clusters, even the Moon up to 50x. On the other hand if you want to take images
(either at night or during the day) it will make a big difference. Similarly,
for high power planetary viewing, or daytime spotting, the Nighthawk is very
limited, whilst the 80/600 is superb.
Now let’s
look at that list of refractor types again and describe the level of CA you
might expect in each.
·
Singlet
– a mess of false colour unless the scope is very small in aperture, or
very long, or both.
·
Fast,
big achromat – A lot of false colour, even at low power, so mainly useful
for star sweeping and DSOs.
·
Fast
small achromat – noticeable purple and green fringing on high contrast
subjects, even at low power, but this typically doesn’t spoil the view
unless you are very fussy. At powers over 70x, the false colour becomes a
problem and washes out detail on the Moon and planets; tight doubles are lost
in the violet blur and CCD images show O-B stars bloated with violet blur.
·
Slow
achromat (F ratio at least 1.22D) – Minimal false colour at low powers
and quite useable at high powers on some targets. Views of some planets, such
as Venus and Mars, may be compromised by false colour.
·
Semi-APO
– Much the same performance as the previous group; may even be worse!
·
Fast
ED (FPL 53) Doublet – Minimal false colour, except at high powers in
daytime or on planets when some may be noticeable. Some mild violet
“bloating” of blue stars on CCD images.
·
Longer
focal length (F8) FPL53 and fluorite doublets – CA is almost completely
absent. You have to search for it visually, and it is not intrusive on CCD
images.
·
Triplet
Super APOs – No false colour evident on any target, in or out of focus,
terrestrial or astro, visual or imaging. Some 4-element petzval
designs (effectively very long focal length ED doublets) also come into this
category.
Note that the
final category are mostly triplets, but this doesn’t mean that all
triplets are super-APOs!! In fact, many triplets using cheaper glasses, or
less-than-optimal design and fabrication will fall as far as the Semi-APO
category.
Four 80mm Refractors
Below
are a set of images of the same view on the same day (OK, the birds moved
about!) in four ~80mm refractors: a Takahashi FS78, a Tele Vue 76, a Skywatcher
Equinox 80 and a Stellarvue Nighthawk. The pictures
are in order: from best to worst.
The Stellarvue
is a pure achromat, albeit one of high optical quality. Both the TV76 and
Equinox 80 are FPL53 doublet APOs. The FS78 is a fluorite doublet APO. The
pictures, I think, say it all.
Takahashi FS78
Tele Vue 76
Equinox 80
Stellarvue
Nighthawk
Which
one would you rather be looking or imaging through ??
Summary
If the
message that you have got from all this is that you need a triplet super-APO at all costs and that fast achromats are
spawn of El Diablo and should be ritually destroyed, you would have completely
missed the point. Often optical quality is more important than CA for low power
views. As I said, the Nighthawk delivers lovely views at low power (and costs
about 1/10th as much as the LOMO 80/600). But nonetheless, when you
see the cold, hard images, the differences are pretty clear.
Whether
you need an APO is a matter of need and circumstance. If you want a grab-and-go
scope for star fields and clusters and occasional looks at the Moon, an
achromat is probably all you need. Indeed, a quality achromat may be a far
better (and cheaper) option than a shiny semi-APO with dubious optics that
nobody ever tested before you. If you want a visual planetary scope and happen
to have an old 20 foot copper-domed observatory lying idle in the grounds of
your stately home, buy a big long-focus achromat from the likes of D&G (the
optical company, not the purveyor of handbags). But if you want a portable
scope for imaging and/or critical planetary viewing at high powers, or even for
long-distance birding, you may want a proper APO from a good manufacturer.
Finally,
why haven’t I included reflectors in this discussion? True reflectors
(Newtonians and Cassegrains) by definition do not suffer from CA because
mirrors focus all light to the same point. However, SCTs and Maksutovs can suffer (usually modest) CA, but that is
another topic.