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Goal: To understand whether or not you will be able to photograph an object

Learn how to increase signal in astrophotography by using a bigger telescope and longer exposure time, while managing sources of noise like statistical noise, instrument noise, universe noise, and moon scatter. Also, understand the impact of light pollution and the limitations of naked eye and small telescopes. Explore the expectations and possibilities of astrophotography with different exposure times.

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Goal: To understand whether or not you will be able to photograph an object

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  1. Goal: To understand whether or not you will be able to photograph an object Objectives: To understand signal To understand noise

  2. Signal • To image an object you need light from that object • The amount of light you collect that comes from the object is called signal

  3. How to increase signal • 1) use a bigger telescope. The collecting area of the telescope determines how much light you collect in a given time. • PLEASE NOTE Area depends on radius squared

  4. If I triple the diameter of a lens • If I triple it how much more light do I collect? • 1) 3 X more • 2) 5 X more • 3) 9 X more • 4) 27 X more

  5. The other way • The other way to collect more light is to have a longer exposure time. • The longer the exposure the more signal you get

  6. Noise • Signal is not enough, your signal needs to be 5 – 10 time higher than the “noise” • There are many sources of noise

  7. Statistical noise • Take the square root of the number of photons you collect in a pixel of the camera. • This is statistical noise. • The more signal you get the bigger your statistical noise

  8. Instrument noise • Even if you took a blank picture you would still have some “counts” • This is current that naturally occurs in your camera • Most modern cameras take this into account in the final image

  9. Universe Noise • No matter what part of the sky you take a picture of there will be other objects bright and faint that will also add their own noise. • Even dust in our solar system emits some light of its own (zodiacal light)

  10. Moon scatter • If the moon is up light from the moon will scatter off of the atmosphere. • While bright objects are largely unaffected objects that are dim (deep sky objects) need to be observed when the moon is not up.

  11. Light Pollution(from NASA)

  12. Naked Eye • 8th Magnitude in country • 5-6th Magnitude in City • Difference of about 10X in brightness • (each magnitude is a factor of 2.5)

  13. Small Telescopes • How deep you can see will depend on: • Where you are • If the moon is up • What part of the sky you are looking at • The exposure time

  14. Limits for naked eye • 6-8” telescope depending on conditions can reach magnitude 13-15 (good enough to barely see Pluto) • The 4” will not see as well, but should get close to 12th magnitude in the county.

  15. Astophotography • Will do better, but there are limits • Longer exposure gives more signal BUT • 1) more noise • 2) if you don’t have tracking the stars will streak, and the further they streak the more the light is spread over many pixels so your signal no longer goes up but the noise still does

  16. Expectations • With exposure times of 10 seconds you should be able to get to about 15th magnitude on a good clear, moonless night, in the country • In the city…. A lot less.

  17. It is that time… • You need to decide your project. • In a moment we are going to follow me to the area where the telescopes and cameras are to hand out. • You need to write on the back of your attendance worksheet your project idea.

  18. Conclusion: • We have examined signal • We have learned about noise • We have decided our projects and now if you will follow me we will pick up the equipment we need.

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