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

 

DSC_0892

Tele Vue 76

Equinox 80

Equinox 80

Nighthawk

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.