Can We Observe x tonight

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Can We Observe x tonight?

• Where is it?
• When is it?
• How bright is it?

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What does brightness mean?

• What is light?

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Single Packet of Lightis a photon

• Speed wavelength x frequency
• Speed of light is 3.00x108 m/s
• Energy of photon h x frequency
• h 6.63 x 10-34 Js

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Frequency Color
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Spectrum
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Photon vs. Energy Flux
Number
Energy
Frequency
Frequency
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Cameras Record Total Photons
Each dot has a spectrum under it
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Spot You See is Accumulation

• Over color/frequency
• Over time

But brightness, like beauty, is also in the eye
of the beholder
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Sensitivity is Crucial

• Your eye is sensitive from
• f7.5x1013 Hz (blue) to
• f4.2x1013 Hz (red)
• Colors outside that range you dont see
• Infrared, Ultraviolet, Radio, X-ray, etc.
• Source could be putting out millions of photons,
  but if you cant see them, its not bright

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Mathematics of Sensitivity
times
equals
Sensitivity Curve
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Low Light Camera Red-Sensitive
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A Potential Problem
Intensity
Intensity
Source Spectrum (Blue Star)
Source Spectrum (Red Star)
Same Number of photons in each curve!
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Add a filter

• Doesnt make the problem go away, but does reduce
  its effect

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Some Standard Filters
Johnson-Cousins Filters
Stromgren Filters
Spectrum of star Vega
See also Filter web page
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Images are still Black and White
http//actions.home.att.net/Astronomy_Tools.html
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To get image

• Source Spectrum
• Times filter
• Times sensitivity
• Accumulated over frequency
• Accumulated over time
• Equals brightness!

Almost
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More Issues to Deal With

• Point Spread Function (PSF)
• Saturation
• Bleeding
• Background
• Extinction

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Now what?

• Youve picked out all the emission from your star
• Youve subtracted the background
• Your result is useless by itself
• No two telescopes will get same number for same
  star
• But ratios between two stars should be the same!

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Magnitude Scale
Note that magnitude is filter-dependent!
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