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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

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Planetary observing l.jpg

Planetary Observing

Presented by John Bishop

June 2007

This talk l.jpg
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”

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Planets Are Different!

  • Small

  • Bright

  • Detailed

  • Individual

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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

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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!

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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

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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

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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

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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

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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!

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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)

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Filters I Have And Don’t Use

  • Green (#56)

  • Dark Blue (#80)

  • Red (#25)

  • Dark Yellow (#12 – “Minus Blue”)

  • Violet (#47)

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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

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Filters I Can’t Find

  • “Minus Yellow” – Cyan (?)

  • “Minus Red” – Blue-green (#44)

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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!

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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

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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

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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

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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

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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.”

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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)

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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)

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Not-so-Rewarding Planets

  • Mercury

  • Venus

  • Uranus

  • Neptune

  • The “rest”

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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!

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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

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Observing Venus

  • Has phases

  • Very large at times – good in small scopes

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

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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

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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

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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

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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!

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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

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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.

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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.

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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

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If Your Scope Isn’t Perfect

  • Non-planetary Newtonian

  • Short-focus apochromat

  • Short-focus achromat

  • SCT or Maksutov

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  • 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

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Short-focus Apochromat

  • Use Barlow or Powermate to get higher magnifications

  • Learn to see details at low magnifications

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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

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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

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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

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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…

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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

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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