CCDs and Remote Observing Why?

1. CCDs and Remote Observing Why? Dan Lester, University of Texas 7-21-09 dfl@astro.as.utexas.edu

2. Robotic astronomy?

3. Electronic imaging sensors and remote control of observatories have revolutionized the way we do astronomy. We used to do it this way. But that didn’t give an unambiguous record of our observations. Which was sometimes a problem.

4. Photographic plates were nice, but … • • Sensitivity was poor (see faint things) • QE <10%! • • Very limited spectral response • • Linearity was poor (measure brightness accurately) • saturation, blooming • • Resolution was mixed (granularity) • • Processing was long, wet, smelly … • • Shareability was … US Mail • + Big advantage was LOTS of “pixels”

5. Photographic plates were “grainy”. Photographic plates were a “dark art” involving potions and appearances.

6. Space is a FANTASTIC place for astronomy. But things get tricky using photographic plates there. But astronomers don’t work there … so you have to do it by remote control.

7. You could just take pictures and return the photographs for processing. As the DOD did in early recon satellites.

8. Even trickier when you couldn’t easily get the film back Lunar Orbiter 1966-7

9. So everyone is so happy that we now have CCD imagers, and we will use those for taking pictures on MONET. CCD (charge coupled device) imagers are electronic substitutes for photographic emulsion. Info is electronic!

10. The charge coupled device architecture on the MONET camera is actually very similar to that in consumer digital cameras! The MONET CCD has lower electronic noise, and is more optimized for high sensitivity. Also, it is functionally just a light detector, and doesn’t in itself measure color. The image is sampled electronically, and digitized. Information returned in “pixels” or “picture elements”.

11. Within each single CCD pixel, a photon of light is converted Into a free electron. That produces a charge, which is retained in a “potential well” that is kind of a mini-capacitor. Each pixel samples a different piece of the focal plane of the telescope. Each pixel sees a tiny piece of sky with the light collecting power of the whole telescope.

12. We can only measure one pixel at a time, so we shift the charges electronically in a bucket-brigade fashion. So one 2-D images is read out as a long string of voltages.

13. Astronomers like inverse images. Stars are black!

14. “Bayer filter” But you might want to know how your digital camera sees color. It does it by having little color filters over all the pixels, and recombining nearby pixels as color info. Why don’t we do this in astronomy? How could astronomers get color info from their cameras?

15. CCDs and astronomical progress Since the CCD was first developed, we’ve increased the number of pixels by a million!

16. “Hendy’s Law” for consumer digital cameras tracks technology.

17. … which looks a lot like “Moore’s Law. Why?

18. “Richards Law” In other parts of the spectrum, even the sensitivity of individual pixels is increasing dramatically!

19. But remote observing. What’s so special about that? • Sure, it’s nice to sit in a comfortable place when you’re observing, and not having to travel. • Sure it’s nice to be able to observe in the daytime while it’s nighttime somewhere else. But this is *precisely* what we do for all of our observing in space. This is what makes space astronomy possible! What would space astronomy be like if we couldn’t do remote observing?

21. Sun as seen by our eyes Sun as seen from space Sun as seen from surface of the Earth There is a lot of light from the Sun we can’t see from the surface of the Earth.

22. The spectrum of the universe is what we see from space. What would our understanding of the world be like if we could see only one color? opaque The importance of robotic telescopes is that we can observe in space without being there!

24. Robots? Telerobotic -- Virtual Presence -- Remote control Autonomous robotic