Telescopes for Planets

Astro Physics 130 EDT – an absolute planetary classic.

A Manifesto for Planetary Astronomy

My main (though certainly not only) interest in Astronomy is and always has been the Moon and Planets. To some this is the hallmark of a newbie, which always makes me laugh because it is an attitude hangover from the middle of last century when planetary astronomy was making no progress and was deeply out of fashion. In fact, the Moon and planets make fascinating subjects for both the observer and the imager.

The Moon has an endless array of interesting detail and is a whole other world of mountains, cratelerlets, domes, rilles and rays to explore. People used to think the Moon was static and unchanging; reporters of “Transient Lunar Phenomena” were regarded as cranks. Then someone a few years imaged flashes from meteorite impacts…

Meanwhile, it is a constant challenge to pick out and perhaps sketch planetary detail, whilst the planets change in a way stars and deep sky objects do not. Mars has clouds and dust storms and the albedo marking show subtle variations; the ice caps grow and shrink with the seasons. Mars periodically suffers global dust storms which blot out all surface detail.

Jupiter has frequent changes in its cloud belts and spots, whilst its moons change position, occult and show shadow transits. Saturn is more stable, but the rings change angle and you might be lucky enough to see a new white spot (the original was found by an amateur – Will Hay). Occasionally something really dramatic happens, like the impact of a comet into Jupiter which has happened twice in the last few decades and left dark marks visible with even a small telescope: I vividly recall seeing the wield left by Shoemaker-Levy on Jupiter with my old C8.

Then again, a view of Saturn on a fine night through a big scope is just unforgettably beautiful and emotive.

You might think that, given all the probes out there, that NASA would be the first to notice big things happening on the planets. Not so. The cameras of space probes (and for that matter of Hubble and professional Earth-bound observatories) tend to be pointed at small areas for short periods because observing time for specific investigations is precious and limited. Nowadays, as always, if something happens there may be no professional looking.

Which Telescope?

So if you want to explore the Moon and planets yourself, what kind of equipment do you need?

To stand a chance of revealing detail on the planets visually, a telescope needs to deliver:

· High resolution

· High contrast (which isn’t the same thing)

· Good sharpness at high power

· Precise and stable focusing

An accurate focuser, like this Feather Touch from Starlight Instruments, is vital at high powers on planets.

These essentials depend heavily on the quality of the optics, but also on the focuser, which needs to be accurate and have minimal image-shift to achieve critical focus at high power. The mount is important, too, but I’ll get back to that. There is also some dependency on the type of telescope. I’ll look at that next...

Refractors

The traditional planetary telescope is the long-focus refractor. Refractors tend to work well on planets, if they are good optically, because of the following factors:

· No central obstruction (secondary mirror) means better contrast for a given aperture.

· Refractors don’t need such high optical quality to work well (see section on optical quality).

· Lenses scatter less light than mirrors.

· Refractors typically suffer less from collimation and cool-down problems.

· Small, unobstructed apertures often perform better in mediocre seeing.

None of these factors make refractors definitively better than reflectors, but size for size a good refractor will always outperform a good reflector, though by how much depends on the design and quality of the reflector – an optically-perfect long-focal-length Newtonian can get quite close to a similarly sized refractor.

Does a refractor need to be an apochromat? No, but if it’s an achromat it needs to be a long focal length one (F12-F15 plus) to minimise false colour. Chromatic aberration in significant quantities will kill planetary detail and contrast.

Apochromats don’t have to be long focal length, but it can give some advantages:

1) Long focal lengths are easier to figure well and likely to be of higher optical quality

2) Aberrations are easier to control in longer focal lengths

3) Longer focal ratios make precise focus less critical and easier to obtain

4) May work better in poor seeing

Triplet or doublet apochromat?

Doublets are easier to manufacture, cool more quickly and have fewer optical surfaces to scatter light or become miscollimated. Long focal length doublets (F8 or more) are often a better choice. Triplets can work at shorter focal lengths, but only if they’re of high quality.

However, many doublets generate too much residual chromatic aberration to be ideal for planets. Many doublets are poorly corrected at the red end of the spectrum too (so are relatively poor for Mars). An exception is the long focal length fluorite doublet which can be an ideal planetary instrument, especially the older ones that are optimised for visual (many modern ones are better corrected for digital imaging, the FC-100DZ shown below is an exception).

Takahashi’s F8 fluorite doublets make excellent planetary scopes: recent FC-100DZ on the left, classic FC-100 on the right.

Reflectors

If you opt for a reflector, choose the following characteristics:

· Small central obstruction

· Good baffling

· The best optical quality possible

The situation to avoid, as Suiter identifies in Star Testing Astronomical Telescopes, is when optical problems overwhelm the image like a wobbly stack of filters that finally falls over. This can happen, for example, when a reflector has barely acceptable optics, a large obstruction, is slightly out of collimation and has a sloppy focuser. Such a telescope will probably show planets as nothing more than bright, blurry blobs.

Classical Cassegrains and variants like this Takahashi Mewlon Dall-Kirkham can work well for planets.

This certainly doesn’t exclude reflectors as planetary telescopes, it’s just that they typically have a few more filters on that wobbly stack. In fact, the view of Saturn through a 10” Newtonian belonging to my school physics teacher was what got me hooked on planetary observing in the first place. A Newtonian of good optical quality can make an ideal planetary instrument, but particularly one with a long focal length. Why?

1) A longer focal length makes the primary mirror easier to figure to a high standard

2) A smaller central obstruction is possible with a longer focal length, improving contrast

3) A longer focal length makes collimation easier; critical collimation is vital for use at high powers

The main problem with long Newtonians is the difficulty in mounting them, given their eyepiece position.

The other main type of reflector is the Cassegrain (and its variants). Classical Cassegrains are rare because the optics are difficult to figure, but can make fine planetary scopes. Much more commonly encountered are Dall Kirkhams – these can be good too, but tend to have a very long focal length (so a small field of view) and a lot of coma off axis, so are quite specialist. All Cassegrains tend to be much more expensive than Newtonians of the same aperture.

A number of companies now produce Ritchey-Chretiens, another variant of the Cassegrain. These are excellent flat-field astrographs but are typically poor visually due to a large central obstruction.

In general no reflector with a central obstruction above 35% is going to be better for planets; anything above 40% will be poor. A large central obstruction damages contrast, but also seems to work less well in mediocre seeing.

Central obstruction – both the secondary and baffles - on the Mewlon is a moderate 31%.

SCTs and Maksutovs (catadioptrics)

Maksutovs can work well, but they take a long time to cool and the optical quality will need to be high (many fast food ones aren’t good enough). In my experience many SCTs suffer too many compromises (poor optics, large obstruction) for planets, but examples that are fine optically may work well if properly collimated and cooled. The problem is simply that these more complicated designs place more filters on that wobbly stack. Astrographic designs with multi-element correctors pile on yet more.

My personal planetary favourites are all either refractors, Cassegrains or Newtonians.

How Big?

Unfortunately, there is no easy answer to this – it will depend on where you live. Why? In a word, seeing.  

In parts of the world where seeing is generally good (i.e. the atmosphere is stable; we’re not talking about transparency which doesn’t matter much for planets), a larger telescope will show the maximum planetary detail because the resolving power of a telescope increases in proportion to its aperture. However, for visual use, even in places with great seeing – deserts and mountains - a ten or twelve inch telescope of high quality will probably deliver as much detail as the atmosphere will usually allow. In exceptional seeing, larger sizes may reveal more.

In areas with poor seeing, even smaller apertures work better because the image tends to wobble rather than break up and blur; this has to do with the size of turbulent air cells. It’s an effect you can readily see if you compare a big dob’ and a small refractor on a night of mediocre seeing.

On this test night, with mediocre seeing, the 60mm APO and 90mm Mak delivered the best view of Jupiter; on a night of perfect seeing the Dob’ was in a different league.

So for planets there isn’t much point in going above twelve to fourteen inches aperture, even if you live on top of Kitt Peak or Mauna Loa. If you live in Britain, you may well find that a good quality scope of 6-10 inches aperture is the maximum you’d need.

That’s for visual use. If you want to image the planets using a web-cam then going up to a 14-16 inch scope might be worthwhile. Whilst we are on the subject, don’t expect to see detail like you get on the best web cam images – you never will, even looking through the same scope on a night of near perfect seeing. The reason is that the stacking technique used enhances contrast in a way your eye can’t. So for web cam imaging, image brightness and scale is more important than straight contrast delivery because the stacking process will boost contrast anyway. So though many of the finest web cam images were taken with big SCTs, these are often a fairly poor choice for visual use: they are too large for the prevailing seeing and often deliver relatively poor visual contrast.

This tiny (50mm) Takahashi refractor has perfect optics and gives better planetary views than some larger telescopes.

At the other extreme, if you are used to big scopes, you may be surprised at how much planetary detail a small perfect aperture can deliver. Whilst I generally reckon a 100mm (4”) refractor or 150mm (6”) reflector is the minimum for a dedicated planetary scope, I have a classic Takahashi FC-50 refractor with perfect optics that delivers surprising planetary views, including detail on Mars. Which brings us to the question of how good do your optics need to be for planetary use?

Optical Quality

Suitor describes the effect of optical quality on different types of telescope very well by defining a measure of contrast-transfer called EER that equates to the Strehl ratio you often see quoted as a measure of optical quality. Even perfect reflector optics with a 33% obstruction (a quartz Questar, for example) have an equivalent EER of about 0.79 that roughly equates to a Strehl of 79% in a refractor. Such a refractor would be a poor example – most quality APOs have Strehls above 95%.

Reflectors with smaller obstructions fare much better, so that a perfect F8 Newtonian with an 18% obstruction will have an EER of 93% and will be truly ‘refractor like’ for planetary use.

In other words, reflectors with big obstructions need to have superb optics to match even a very mediocre refractor of the same size for planetary contrast.

This has some interesting implications:

· All things being equal, refractors can ‘get away’ with poorer optical quality than reflectors.

· All things being equal a quality ten-inch Newtonian will meet or exceed a six-inch refractor for a much lower cost (but you have to find a way to mount it for high powers).

· In very good seeing, a big reflector with decent optical quality will equal or exceed a perfect smaller refractor – which is why I got one of my best views of Saturn with a Celestron C11 on Mauna Kea, but not here in England.

In testing I reckon a near-perfect 90mm with a 33% obstruction (i.e. a Questar) performs for planets at roughly the level of a perfect 60mm APO, a 2/3 proportion that applies to medium sized apertures as well.

90mm Mak and 60mm APO – similar planetary performance.

Finally, having said that high optical quality is less vital for a refractor, can you really tell the difference between a good diffraction-limited refractor of perhaps 90% Strehl and a “perfect” one of 98%+ Strehl?

I have found that in refractors Strehls above about 95% aren’t visibly better. And 95% Strehl was the minimum requirement (to the maker, Canon/Optron) for Takahashi’s class FS-series planetary fluorite doublets. For reflectors, higher may be ideal due to that wobbly stack effect (see above).

To summarise: I have had good planetary views with all the major types of telescope discussed, but all the reflectors (at least) were of high optical quality.

Eyepieces

I personally think eyepieces are less important than the other factors discussed here. In my opinion, many premium quality eyepieces will work well for planets. One small caveat is ghosting. Planets are small and bright and can cause ‘ghosts’ in some eyepieces – distracting blobs that seem to shoot randomly around the field of view as you move your eye.

If you are on a tight budget, quality Plossls (e.g. Tele Vue) or Orthoscopics  (e.g. Circle-T)  are a fine choice for planets. For the purist, very high quality Orthoscopics from the likes of Zeiss or Pentax may give a tiny edge over other types. In all cases, choose lengths that will give you about 40x-50x per inch of aperture initially (you’ll rarely need more). A quality barlow lens to multiply magnification of lower power eyepieces may be a good alternative.

Specialist eyepieces – like these Takahashi Hi-Orthos – can work well for planets.

Overall, get your scope and mount right before worrying about specialist eyepieces.

For a full discussion of whether you need special eyepieces for planets, I have written a separate article on the subject, here.

Diagonals

A diagonal can only ever detract from your view. It’s another dusty, light-scattering, perhaps miscollimated filter on that wobbly stack. Japanese observers often view straight through and my own experience is that this does make a small difference for planetary observing (but it’s also typically uncomfortable). Try it!

When you go back to using a diagonal due to that crick in your neck, get a good one. Most people regard a 2” as better, because the central section of any mirror is usually better than the edge. As to prism versus mirror and dielectric versus single coating – the arguments rage on. It does seem that introducing a glass-path (i.e. a prism) into the equation may actually improve correction for some refractors.

Mounts

One of the most important factors in seeing planetary detail is relaxing at the eyepiece and waiting for those moments of fine seeing: a good mount helps a lot, a bad one makes it nearly impossible.

Alt-az mounts are great for quick looks and low-medium power sweeping, but my experience is that they are less than ideal for planetary observing. At the high powers needed for planets you get almost no time to actually look at the planet before you are nudging the mount to track it. Often this nudging results in losing the planet altogether, or at best inducing vibes that last almost until it’s time to nudge again; frustrating stuff. Wide field eyepieces with perfectly corrected fields, like Naglers and Ethos, help a bit here, but I overwhelmingly prefer driven equatorial mountings for planetary use.

A common misconception is that mounts only matter for long exposure imaging, but this couldn’t be further from the truth. For high-power planetary viewing and imaging you need an accurately aligned mount with good tracking just as you would for astrophotography, because otherwise the planet is forever drifting out of view or off that tiny web-cam chip. What’s more, the mount needs to be especially stable and vibration free, as high powers magnify any vibrations as well.

I recall a recent incident viewing Jupiter with a high-quality small refractor. I had always used the scope on small wobbly mounts, but decided to piggyback it on my big permanent mount for testing. The seeing was pretty typical, but on a vibration-free mount with perfect tracking I was able to make out a level of Jovian detail way beyond anything I had managed with that scope before – a level of detail I didn’t know it could deliver.

A solid, tracking equatorial mount makes a big difference at high powers.

Finally, do you need GOTO? Well, not between Venus and Saturn at least! But for Mercury and the outer planets it can be a help.

Summary

There is no magic bullet for seeing planetary detail (apart from moving to Mauna Kea summit), but you need a high quality set of optics of medium aperture on a good mount which lets you relax at the eyepiece and just look, or which can give a stable, well focused image at large scale on a webcam chip. Worry less about your eyepieces than the quality of your telescope and mount. Finally, rather than worrying about your gear, go out and observe or image! That way, you’ll stand the best chance of catching those moments of fine seeing when Planetary detail really stands out.