Transcript Slide 1
EUROPEAN EXTREMELY LARGE TELESCOPE WILL HAVE A 40 METER MIRROR AND WILL GATHER 13 TIMES MORE LIGHT THAT THE BEST CURRENT TELESCOPE – IMAGES WILL BE 16 TIMES SHARPER THAN HUBBLE. WHY CAN’T YOU READ THE WRITING ON A DOLLAR BILL AT 100 METERS? WHY CAN’T YOU READ THE WRITING ON A DOLLAR BILL AT 100 METERS? THE DOLLAR DOES NOT TAKE UP ENOUGH SPACE ON YOUR EYE’S SCREEN (RETINA). THE IMAGE DOES NOT COVER ENOUGH RECEPTOR CELLS (RODS AND CONES). THINK ABOUT A DIGITAL CAMERA THERE WOULD NOT BE ENOUGH PIXELS TO COVER THE IMAGE. WE NEED TO MAKE THE IMAGE LARGER. WE USE TELESCOPES IN ASTRONOMY TO SEE DISTANT OBJECTS. OFTEN THE OBJECTS CAN BE: 1)SMALL – SINCE THEY ARE FAR AWAY 2)FAINT – BECAUSE THEY GIVE OFF LITTLE LIGHT 3)BOTH SO, TO DO THIS WE WILL BE CONCRNED WITH TWO FACTORS IN DESIGNING A TELESCOPE: 1)MAGNIFICATION 2)LIGHT GATHERING POWER THERE ARE TWO KINDS OF TELESCOPE DESIGNS: 1) REFRACTOR TELESCOPES – USE LENSES TO GATHER AND FOCUS LIGHT 2) REFLECTOR TELESCOPES – USE MIRRORS TO GATHER AND FOCUS LIGHT AND LENSES IN THE FINAL STAGE (EYE PIECE) THE OBJECTIVE LENS (FRONT) GATHERS THE LIGHT AND BRINGS IT TO A FOCUS. THE EYEPIECE LENS (OR LENSES) SPREAD THAT LIGHT OUT AND MAGNIFY IT FOR THE EYE. WHEN YOU COMBINE THE TWO, YOU HAVE A TELESCOPE. SO, ON A REFRACTOR TELESCOPE, THE DIAMETER OF THE FRONT LENS WILL DETERMINE THE LIGHT GATHERING ABILITY. A REFRACTOR TELESCOPE IS SIMPLE TO BUILD. YOU COULD JUST USE TWO LENSES. HOWEVER, THERE ARE A COUPLE OF PROBLEMS: 1) ALL OF THE COLORS (WAVELENGTHS) OF LIGHT ARE NOT BENT THE SAME, SO YOU GET SOME FOCUSING DIFFERENTLY, AND THE IMAGE COULD APPEAR FUZZY. THIS PROBLEM CAN BE CORRECTED WITH ADDITIONAL LENSES OR COMPOUND LENSES AND LENS COATINGS, BUT THIS INCREASES THE EXPENSE. 2) WHEN YOU BUILD VERY LARGE TELESCOPES, THE FRONT LENS BECOMES SO LARGE THAT IT WILL DISTORT OVER TIME. THE GLASS WILL ACTUALLY FLOW. THE SECOND TYPE OF TELESCOPE IS A REFLECTOR TELESCOPE. HERE, WE ARE USING A MIRROR IN PLACE OF THE OBJECTIVE LENS. THE LIGHT GATHERING POWER OF A TELESCOPE IS APPROXIMATELY EQUAL TO THE AREA OF THE OBJECTIVE LENS OR MIRROR TO THE AREA OF THE EYEPIECE. FOR EXAMPLE, WITH AN 8 INCH (20 CM) TELESCOPE, THE OBJECTIVE AREA WOULD BE: A = PR2 = 314 cm2 FOR AN EYEPIECE 0.2 cm IN DIAMETER A2 = PR2 = 0.0314 cm2 THE RATIO WOULD BE 314/0.0314 = 10,000 SO, YOU WOULD GATHER 10,000 TIMES AS MUCH LIGHT AS THE EYE WOULD NORMALLY GATHER. THAT’S GOOD FOR STARS, BUT BAD FOR THE SUN. YOU COULD ACTUALLY SHATTER THE EYEPIECE LENS OF A TELESCOPE IF YOU POINTED IT AT THE SUN. NOT TO MENTION WHAT IT WOULD DO WITH YOUR EYE. SUN FILTERS ARE USED ON THE FRONT END OF SOLAR TELESCOPES, AND NOT THE BACK. http://www.stargazing.net/naa/scope2.htm#refractor (GOOD SITE ON HOW TELESCOPES WORK) THE SHORTEST WAVELENGTHS OF LIGHT THAT CAN BE FOCUSED IN A TELESCOPE ARE X-RAYS. CONCENTRIC TUBES WITH CURVED SURFACES ARE USED AS LENSES. THEY HAVE TO BE USED ABOVE THE EARTH’S ATMOSPHERE. THIS TECHNOLOGY IS USED WITH THE CHANDRA X-RAY TELESCOPE. THE CHANDRA TELESCOPE SATELLITE PROBE IS THE LARGEST SATELLITE LAUNCHED BY THE SHUTTLE AND IS 45 FEET LONG. IT WAS LAUNCHED IN 1999 FROM SPACE SHUTTLE COLUMBIA. WE HAVE TELESCOPES AT ALL OF THESE WAVELENGTHS. AT THE OTHER END OF THE SPECTRUM AT LONG WAVELENGTHS ARE RADIO TELESCOPES. THEY WORK LIKE REFRACTING TELESCOPES WITH THE DISK ACTING LIKE A MIRROR. TECHNIQUES ARE AVAILABLE TO LET A NUMBER OF RADIO TELESCOPES WORK TOGETHER AS ONE LARGE TELESCOPE. THAT SAME TECHNIQUE HAS RECENTLY BEEN APPLIED TO OPTICAL TELESCOPES. VERY LARGE ARRAY - NEW MEXICO TELESCOPES OFFER ASTRONOMERS CERTAIN ADVANTAGES: 1. CAN SEE MORE DETAILS (MAGNIFICATION) 2. CAN GATHER LARGE AMOUNTS OF LIGHT TO SEE DIM OBJECTS 3. CAN MAKE TIME EXPOSURES 4. CAN MEASURE LIGHT AT DIFFERENT WAVELENGTHS THE DAYS OF AN ASTRONOMER SITTING FOR HOURS OR DAYS AT A TELESCOPE ARE GONE. TELESCOPES NOW USE ELECTRONIC DETECTORS. THESE, ALONG WITH SPECTROMETERS (TO SEE INDIVIDUAL WAVELENGTHS) ARE CONNECTED TO ALL THE BIG SCOPES. ASTRONOMERS CAN WORK AT COMPUTERS AND DEVELOP SOFTWARE TO HELP THE IN THEIR SEARCHES. AN ASTRONOMER AT HARVARD COULD OPERATE A TELESCOPE IN CHILE.