Planetary Observing

Presented by John Bishop

June 2007

This Talk

• Focused on visual observing only

• No planetary mechanics (go to S&T for what’s up tonight)

• Lots of ‘how-to’s

• Some equipment considerations

• There’s also a “human side”

Planets Are Different!

• Small

• Bright

• Detailed

• Individual

Planets are Small

• Neptune is 2 arc-seconds; Venus can be almost 60 arc-seconds during a transit – but that’s still quite small!

• Small means you need to magnify

• Small means seeing is very important

• Small means smoothness (quality) of optics is important

How Much To Magnify?

• Atmosphere limits us to about one arc-second seeing: image blur pixels are thus about one arc-second dots

• Our eyes have pixels that are about one arc-minute dots

• The minimum magnification to see all the detail is thus about 60x.

• But there’s more!

More on Magnification

Use changes in magnification to:• Adjust the brightness to a comfortable

level• Change the visibility of colors (more later)• Change the contrast (more later)• Tease out more detail by matching the

pixels of the contrast to your eye pixels• Prepare your image for the “lucky instant”

of good seeing

Magnification and Tracking

• Magnification magnifies Earth’s rotation as well

• You can hand-track at 300x – but then you can’t do anything else!

• Tracking is a huge help– Equatorial mount– Tracking table

• If you can’t track, use a lower magnification

Planets Are Bright

• Magnitude 8 to -4 (Venus is the third brightest natural thing in the sky!)

• Don’t need to gather lots of light

• Glare (too much light) can be a problem– Color sensed varies by light level– Too much or too little washes out color

• Good news: brightness permits the use of filters

Planets Are Detailed

• They have details going way below 1 arc-second• They have color and brightness variations• They have high-contrast and low-contrast

features• Only Mars (and Mercury, in theory) have a

visible surface; Jupiter has long-lasting cloud features; Saturn has ring features

• They can be oblate; Venus and Mercury can be crescents

Color and Colored filters

• Colored filters can increase contrast at the cost of dimming (use lower magnification)

• Named by “Wratten” numbers (+”A”…)• Light filters are better than dark ones: we’re after

subtle effects, not mood lighting• Dark filters require a bright image (big aperture

and/or low magnification)• There are special “Planetary Contrast” filters; I

haven’t tried them• Try any you have!

The Filters I Use Most Often

• light yellow (#8)

• light blue (#82)

• magenta (#30, “Minus Green”; hard to find, worth finding)

• orange (#21)

• nebula (e.g., DGM VHT, good on Mars if you don’t have magenta)

• neutral (“Moon” filter)

Filters I Have And Don’t Use

• Green (#56)

• Dark Blue (#80)

• Red (#25)

• Dark Yellow (#12 – “Minus Blue”)

• Violet (#47)

Filters Which Might Be Useful

• Yellow-Green (#11)

• Light Amber or “Salmon Pink” (#85)

• “Planetary Contrast”

• Mars “BandMates”,: Mars A, Mars B

• O-III

• H-α

• H-β

Filters I Can’t Find

• “Minus Yellow” – Cyan (?)

• “Minus Red” – Blue-green (#44)

Planets Are Individual

• Each planet is different and should be treated differently

• Some are very rewarding to observe, some are not

• They have history and a connection to myth

• They are well-placed or otherwise!

Rewarding Planets

• The rewarding ones are also the big crowd-pleasers at sky-watches

• Mars: the winner for those who like science fiction

• Jupiter: the winner for those who like moons

• Saturn: the winner for those who like rings

The Planet Mars

• Varies in apparent size up to 27 arc-seconds

• Good “apparitions” about every 2 years

• Lots of surface detail (and it’s a real surface!)

• Cool names (Syrtis Major, etc.)

• Ice cap, clouds vary from day to day

• Sandstorms

Mars Observing How-to

• Yellow, orange or red filters bring out dark areas• Neutral can bring out Hellas basis• Blue can bring out cloud details• Magenta filters are great – close to natural color

and greater contrast of dark areas!• Nebula filter exaggerates colors, makes a bit

blurry• Check ice cap edge – varies as melting happens• Rotates at near-Earth rate – so you’ll see the

same side night after night• Responds well to changing magnification

The Planet Jupiter

• Huge: 40 to 44 arc-seconds

• Many different colors (blue, brown, white, pink…)

• Lots of cloud detail, to limit of resolution

• Rotates very fast – changes as you watch

• Visibly oblate

• Moons add interest

Jupiter Observing How-to

• Try all your color filters!• Try varying magnification• Two big dark bands; look for detail beyond them:

– Zones (white stripes)– Thinner bands (dark lines)– Spots (in zones and bands)– Festoons (diagonal stripes or interrupted lines)– Barges (next level down of detail, I think)

• “Great Red Spot” is a “Pale Pink Spot” these days; look for “Red Jr.”

Jupiter’s Moons

• Move while you watch

• Moon goes behind Jupiter – occultation

• Moon goes into Jupiter’s shadow – eclipse

• Moon in front of Jupiter – transit (hard to see)

• Moon’s shadow on Jupiter – shadow transit (dramatic)

Observing Saturn

• Everyone loves the rings – use yellow filter to bring out Cassini’s Division

• Use blue filter to emphasize ring details

• 1 to 6 moons visible; the inner ones move while you watch

• Planet has subtle banding (use yellow and blue filters)

Not-so-Rewarding Planets

• Mercury

• Venus

• Uranus

• Neptune

• The “rest”

Observing Mercury

• Has phases• Always close to sun and thus usually near

horizon – can use “Horizon Wedge” to see without color blur

• No easily-visible surface detail (like highlands of Moon: all craters)

• “Old guys” thought they saw features and timed Mercury’s rotation – they were wrong!

Horizon “Wedges”

• Atmosphere acts like a prism near the horizon (“atmospheric dispersion”)

• You can get 2- or 4-degree prisms to compensate for atmospheric dispersion

• Get the 2-degree prism; if you want 4-degrees of correction, get two of them

• Require lots of back-focus; may need Barlow-ing to achieve focus

• Big improvement, but can’t completely compensate for dispersion

• Really only good for Mercury or sky-watches when you want to show a planet and the only one around is low

Observing Venus

• Has phases

• Very large at times – good in small scopes

• Usually no surface detail -- violet filters are said to show cloud detail

Observing Uranus

• No detail visible in 4 arc-second disc

• Gorgeous bright yellow-green or green

• Obvious, non-stellar color

• Titania is mag 14, visible in very large telescopes

Observing Neptune

• No detail visible in 2 arc-second disc

• Dark blue – edge indistinct due to limb darkening– Greener in big telescopes

• Obvious, non-stellar color

• Triton visible in larger scopes, mag 13.6

Observing the “Rest”

• Mostly you look at these just to say you saw them

• Pluto – just a mag 13 dot

• Asteriods – brighter dots

• Galilean Moons – 1 arc-second discs but bright; people have seen detail on Ganymede

The Ideal Planetary Telescope

• Right eyepieces – simple, narrow-angle

• Right focal length – long, f/big

• Right aperture – small

• Right design – unobstructed, driven

• The “old guys” weren’t so dumb after all!

The Right Eyepieces

• You want non-coloring, contrast-saving designs– Small number of surfaces is best (less scattering)– Some edge distortion is ok

• Wide angles are definitely not needed• Most eyepiece designs work best between 10

mm and 20 mm• Barlows remove contrast – you want single

eyepieces if possible• Orthoscopics and Monocentrics are the classic

choices; Plossls ok

The Right Focal Length

• You want powers between 60x and 300x

• 300x with a 10 mm eyepiece means a focal length of 3000 mm; 60x with a 20 mm means a focal length of 1200 mm

• You have to make some compromises

• The ideal focal length is at least 1500 mm; 2000 mm is better.

The Right Aperture

• You don’t need lots of light-gathering• You want resolution, but the maximum the sky

supports is about 1 arc-second• 4 inches gives 1 arc-second resolution, 8 gives

½ arc-second• Larger apertures may have more seeing issues• Larger apertures are hard to make high-quality• The ideal aperture is thus a bit over 4 inches: 5

or 6 is great, 8 maybe too much of a good thing.

The Right Design

• Obstructions rob contrast– Under 15% not too bad – planetary-optimized Newtonians– Maksutovs and SCTs are losers here (30% or more)

• Refractors are good if they are long enough– Long focal lengths (4-inch f/10, 6-inch f/20) mean that achromats

will have almost no color error; superior correction of apochromats not needed; short apo-s not long enough

• Unobstructed reflectors have no color problems at all:– Off-axis Newtonian– Schiefspieglers– Yolos– Even-more exotic designs exist (tetra-schiefs...)

• Schupman medials are great (ATMOB has one)• Mounting and tracking a long scope is an issue

If Your Scope Isn’t Perfect

• Non-planetary Newtonian

• Short-focus apochromat

• Short-focus achromat

• SCT or Maksutov

Newtonians

• Use high and low magnifications

• Try an off-axis mask on 12-inch or bigger– Hole should be a convex shape – Position between spider vanes– Re-collimate telescope for best results

• Collimate normally• Add mask• Re-center secondary to point to off-axis section• Re-collimate primary with star collimation

Short-focus Apochromat

• Use Barlow or Powermate to get higher magnifications

• Learn to see details at low magnifications

Short-focus Achromat

• Use Barlow or Powermate to get higher magnifications

• Use “anti-violet” filters to eliminate color fringes at higher powers

• Better – get used to the color-error and train yourself to see “past” it.

• Consider a mask (90%, 80%)– Raises f-number– Reduces color error

Achromat Example

• Rule of thumb is “for an N-inch achromat to have minimal color error, it should be f/3N”

• Orion 120 mm f/8.3 has a 1000 mm focal length• A 110 mm mask produces a 4.3-inch f/9• A 100 mm mask produces a 4-inch f/10• A 92 mm mask produces a 3.6 f/10.8 (fits the

rule)• Trade color error for brightness and resolution

SCT Or Maksutov

• Use very high or very low magnifications to put the contrast frequency of interest in the “sweet” spot (rapid variation)

• Aperture will compensate somewhat for obstruction

The Human Side

• Your eye is not a camera and your brain is very involved with perception

• Your perception can be trained: the more you know, the more you’ll see

• You can accumulate perceptions (even if you can’t accumulate photons)

• If you just look, you won’t observe: engage your mind

• Therefore…

Draw What You See

• Accumulates transitory details– Even “one flash” of detail is probably real

• Accumulates information from all magnifications and filters

• Forces you to really see details (“a blob” vs. “a one-tenth diameter dot two-thirds of the way out from the center towards 2 o’clock”)

• Gives you a permanent record to show other people

How to Draw Planets

• Make a circle 2-inches or more in diameter beforehand

• Sketch in details with pencil as you observe• Talk to yourself and take notes• Use a dim red light to guide your drawing; note

color with labels• Have a solid base for the paper; tape down so

wind doesn’t move it• Finish or copy afterwards using ink and colored

pencils

Notes on Saturn banding

Pencil Sketch of Saturn (31 March 2007)

Mars Rough Sketch (26 Sept 2005)

Mars, based on sketch

Mars, 7 Nov 2005