3/7/2018. Light and Telescope. PHYS 1411 Introduction to Astronomy. Topics for Today s class. What is a Telescopes?

Transcription

1 PHYS 1411 Introduction to Astronomy Light and Telescope Chapter 6 Topics for Today s class Optical Telescopes Big Telescopes Advances in Telescope Designs Telescopes Mountings Problems with Mirrors and Lenses Astronomical Detectors CCD Camera Spectrometer What is a Telescopes? A Telescope is an instrument that collects light of a certain wavelength. Optical Telescope work in visible waves (light). Radio Telescope work in radio waves Many Space Telescope work in wavelengths that never reach us on Earth. Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The Powers and Limitations of Telescopes (cont d.) Light-gathering power: depends on the surface area (A) of the primary lens or mirror, proportional to diameter squared LGP ~ A = (D/2) 2 The larger the telescope, the more light it gathers. 1

2 Refracting/Reflecting Telescopes Focal length Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal Focal length plane Almost all modern telescopes are reflecting telescopes. Secondary Optics In reflecting telescopes: Secondary mirror, to redirect light path towards back or side of incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics. Magnifying Power Ability of the telescope to make the image appear bigger Depends on the ratio of focal lengths of the primary mirror or lens (F p ) and the eyepiece (F e ): Field of View in a Telescope M = F p /F e RocketMime A larger magnification does not improve the resolving power of the telescope! Skyandtelescope.com Total amount of sky you can see with an eyepiece Reducing the aperture (diameter) of eyepiece increases magnification but also decreases the field of view Resolving Power Minimum angular distance min between two objects that can be separated. D is diameter of mirror or lens min = 1.22 ( /D) For optical wavelengths, this gives min = 11.6 arcsec / D[cm] 2

3 Hobby Telescopes Eyepiece Modern Optical Telescopes Schnmidt - Cassegrain Focus Cassegrain Focus Prime Focus Refractor - Lens Jtwastronomy.com Reflector - Mirror Astroshop.eu Modern Optical Telescopes (cont d.) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona) Advances in Modern Telescope Design Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror 3

4 Examples of Modern Telescopic Design Examples of Modern Telescopic Design (cont d.) Funding Federal Government (DOE FIPSE) Tarleton State University Manufacturer Astronomical Consultant and Equipment Inc.(ACE) 32-inch (0.8m) Reflector f/ratio= f/7 Remotely Control ACE Software * Observatory CCD Camera Finger Lakes - FOV = 17 x 17 Resolution /pixel Imaging and Photometry UBVRI, Hα, OIII and Neutral Density Telescopes Mounts Telescope require mounts that allow rotation Popular Equatorial Mount No counterweight needed counterweight GeekWrapped.com Starizona.com 4

5 Comparing Refracting Telescopes telescopes/telescope10.html LGP ~ A = (D/2) 2 M = F p /F e min = 11.6 arcsec / D[cm] Lenses and Mirrors have Inherent Problems Correcting for Spherical Aberration Spherical Aberration Both Lenses and Mirrors Chromatic Aberration Found in Lenses Openphotography.com Researchgate.net amazingspace.com Phys.apps.org Amazing-space.stsci.edu Example of Spherical Aberration Disadvantage of Refractors Before After Tube very long for big telescope Lenses are expensive compared to mirrors More Engineering Maintenance Alt-Az mounting not Suitable Image not remarkably better than mirrors Chromatic Aberration Clspectrum.com 5

6 Correcting for Chromatic Aberration Example of Chromatic Aberration Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be Cengage Learning supported 2016 at the edges Vimeo.com Astronomical Detectors How to Measure and Record Light The Eye Limited ability The Photographic plate now obsolete Photomultiplier Very limited in capability The CCD (Charge Couple Device) Making Digital Image The Spectrograph Making a Spectrum - Measuring; Chemical Composition Temperature Velocity CCD Camera Makes Digital Images How does a CCD work? It is a imaging device 6

7 How does a CCD work? A CCD Image and Pixel Values Mathworks.com Astrophotography Science from multiple wavelength CCD Images NASA NASA Science from a CCD Image Spectrographs/Spectrometer Analyzing a sequence of such image gives us this light curve Brightskies.us It is an instrument that allows us to see to colors of white light coming from a star or a hot source 7

8 Spectrographs/Spectrometer How to make a simple spectrometer A CCD spectrometer has a CCD camera and a Spectrometer A CCD image shows the spectrum Analysis of a Spectrum tells us chemical composition and temperature Popularmechanics.com Spectra of Some Astronomical Objects Spectra of Chemical Elements Spectra can give us velocity information 8

9 Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 9