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

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 cant 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

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 CCDs (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)

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

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

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 dont 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

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 CCDs (charge coupled devices) –silicon wafer with a two dimensional array of elements called pixels

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 CCDs 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 1980s –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