More Optical Telescopes

1. More Optical Telescopes • There are some standard reflecting telescope designs used today • All have the common feature of light entering a tube and hitting a primary mirror, from which light is reflected towards the prime focus • One design has instruments placed at the prime focus itself. • Very hard to mount large intruments like spectrometers or large cameras • usually light is reflected by a secondary mirror to a more convenient place to mount big measuring instruments. • Another design, called a Newtonian telescope, has light deflected 90° prior to prime focus by a secondary mirror • a very popular design for amateur telescopes • light is usually deflected to an eyepiece • other possibilities for instruments are cameras or photometers (a device used to measure integrated light levels) • A traditional design used by professional astronomers is the Cassegrain telescope • light bounces from primary mirror to a secondary mirror mounted in front of prime focus, and then reflected back towards the primary mirror, where it exits through a hole

2. More Optical Telescopes • large intruments can be mounted on the back of the telescope • the point beyond the mirror where the focus lies is called the Cassegrain focus • still a very popular design (KPNO and CTIO 4-m, Palomar 5-m) • Still other designs involve extra mirrors to guide light to various measuring instruments • one or more mirrors placed in a Cassegrain telescope causing light to reach a focus down a tube aligned along the north pole to large intruments in a separate room (called a Coude’ focus) • allows for very finely tuned instruments that can’t possibly be mounted onto the telescope itself • multiple reflections = higher light losses, something that must be accounted for in instrument design • image never moves, only rotates • All modern telescopes can be configured to any of these setups • Light blocked by the secondary mirror is minimal • The Schmidt telescope has a an unusual design • light enters through a thin lens called a correcting plate before reaching the primary mirror, which is spherical • the lens bends light not exactly parallel to the axis of the telescope so that the spherical mirror can focus large images

3. More Optical Telescopes • results in a curved image at a kind of prime focus where a photographic plate or mosaic of detectors lies • For those who care --- Many amateur telescopes (Celestron and Meade) use a combined design of Schmidt-Cassegrain • has a correcting plate and spherical mirror with a Cassegrain focus • secondary mirror mounted on the back of the correcting plate • light can bounce back and forth multiple times between primary and secondary • extremely compact and portable design, but thickness of correcting lens is too much for professional telescopes • A variety of instruments can be used on optical telescopes • can be used as a giant camera to take images of objects • images can be recorded on film or on electronic detectors called CCD’s (charged couple devices) • usually done at prime or Cassegrain focus in order to minimize light losses • can use filters to limit which part of the EM spectrum is in the image (how most pictures are taken)

4. More Optical Telescopes • can measure levels of integrated light intensity with a photometer • usually done at Cassegrain focus because of ease of use • usually performed over specific parts of EM spectrum to determine the temperature, but can just give information on time variations in brightness • can measure the spectrum (usually only small pieces) with a spectrometer • can be done at any focus, depending on size

5. Telescope Size • Size determines the light gathering power of a telescope • light gathered is proportional to the collecting area of the telescope • this refers to the refracting lens or primary mirror • an example: if telescope #1 is 10 times the diameter of telescope #2, telescope #1 will collect light at a rate 100 times that of telescope #2 • can also think of light gathering power in terms of time • if telescope #1 is ten times the diameter of telescope #2, telescope #1 will collect the same number of photons 100 times faster than telescope #2 • telescope observations are given in terms of the exposure • Size also determines the resolving power of the telescope • the ability to form distinct images of neighboring objects is called angular resolution • diffraction provides the lower limit for angular resolution • light waves entering a telescope always undergo some degree of diffraction, which introduces fuzziness in images

6. Telescope Size • angular resolution depends both on wavelength and size • this is also called the diffraction-limited resolution • the human eye has an angular resolution of 0.5’ • in practice, telescopes don’t reach this limit due to refraction in the atmosphere by turbulence (seeing) • Modern telescopes have employed new engineering techniques • large primary mirrors have a list of problems • usually made of glass (pyrex) or quartz and need very low thermal expansion properties • hard to manufacture large pieces of glass • only the 6-m telescope in the Caucasus, the 6.5-m Gemini telescopes and the 8-m MMT mirrors have been built since the Palomar 5-m in 1948 • new large optical telescopes (like Keck in Hawaii, and HET in Texas) use segmented primary mirror designs • hexagonal mirror segments separately controlled by motors • individual mirros have common focus by use of laser sighting • can be very expensive (Keck ~ 140 million) or very inexpensive (HET ~ 15 million) depending on telescope function

7. High Resolution Astronomy • The useful resolution of a telescope is determined mostly by the quality of the image transmitted by the atmosphere • individual turbulent motions cause a random mixture of refractions which add a fuzziness to images • this is called the seeing • this is extremely dependent on local atmospheric conditions • for instance: the McDonald Observatory 2.7-m has a diffraction limit of about 0.04”, yet the average measured image is about 1-2” • this property was one of the primary reasons for the development of HST (no atmosphere = diffraction limited resolution) • Image processing plays a big role in astronomy • historically, plate film was used to record images • hard to maintain, develop film, provide quantitative data and store • almost all data is recorded on CCD’s (charge coupled devices) • silicon wafer with a two dimensional array of elements called pixels

8. High Resolution Astronomy • when a photon hits a pixel, free electrons multiply (by the photoelectric effect) • after a prescribed exposure time, the number of electrons as a function of position on the CCD are measured by a computer and converted to intensity • much more efficient than film • instant digitization of data for easy storage and analysis • can manufacture CCD’s to have peak efficiencies in different parts of the EM spectrum • New technology is playing a big role in image processing • online adjusment of the telescope mirrors to compensate for atmospheric seeing conditions would hopefully result in difraction limited images • would reduce the need for expensive space-based platforms • this technique is called adaptive optics and is an offshoot of the Star Wars (SDI) program of the 1980’s • in active optics, individual actuators deform the mirror to subtract the distortion by the atmosphere. Amount of atmospheric distortion is measured by an artificial star created by a laser which can only penetrate to the Na layer of the atmosphere • in passive systems, laser guide star measurements are subtracted out by a computer from images after the image is taken