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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

Planetary Observing

Presented by John Bishop

June 2007

this talk
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
Planets Are Different!
  • Small
  • Bright
  • Detailed
  • Individual
planets are small
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
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
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 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
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
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
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
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
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
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
Filters I Can’t Find
  • “Minus Yellow” – Cyan (?)
  • “Minus Red” – Blue-green (#44)
planets are individual
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
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
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
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
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
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
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
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
Not-so-Rewarding Planets
  • Mercury
  • Venus
  • Uranus
  • Neptune
  • The “rest”
observing mercury
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
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
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
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
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
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
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
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
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
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
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
If Your Scope Isn’t Perfect
  • Non-planetary Newtonian
  • Short-focus apochromat
  • Short-focus achromat
  • SCT or Maksutov
newtonians
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
Short-focus Apochromat
  • Use Barlow or Powermate to get higher magnifications
  • Learn to see details at low magnifications
short focus achromat
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
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
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
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
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
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