Refracting telescopes: Composed by an objective lens and an eyepiece.
The use of the eyepiece is to refocus the light rays at infinity (to make them parallel to each other again).
Angular magnification: The combination of objective lens and eyepiece produces angular magnification of the image, according to:
Problems with refracting telescopes:
- To compensate for diminished illumination, a large diameter of the objective is needed (recall that J is proportional to D2/f2), but increasing the size of the objective lens is hindered by practical limitations.
- The lens can only be supported from the edges (light must pass through it), thus as the lens increases, the deformation due to gravity increases. The specific deformation will change as the position of the telescope changes.
- The entire volume of the lens must be nearly free of defects, and both surfaces of the lens must be ground with great precision. Any defects or deviations from the desired shape must be kept to less than about λ/20, to observe light of wavelength λ (e.g. to observe light of 5000 Å wavelength, defects must be smaller than ~250 Å!)
- Thermal response problems: when opening the dome for observations, thermally driven air currents affect the quality of seeing. Also, the temperature of the telescope must adjust to its new surroundings.
- Large lenses introduce mechanical problems due to the large torque generated that requires compensation.
These problems are mostly solved by using reflecting telescopes, and these are the most used today.
The main issue to address in a reflecting telescope is that the prime focus is in the light path. This can be solved by placing a small mirror in front of the telescope that refocuses the light somewhere else:
- Provides a wide-angle field of vision, of several degrees.
- It is designed to minimize spherical aberration (Schmidt corrector plate) and coma (spherical primary mirror).
- Generally used as cameras, with a photographic plate located at the prime focus. To take photographs, a long time integration is needed, which requires the telescope to point to the same area of the sky for some time. This requires the proper mounts for the telescope to follow the astronomical objects carefully.
- It has been used to produce the Guide Star Catalogue of almost 19 million objects as faint as 15th magnitude. This catalogue is used to guide the Hubble Space Telescope.
Mounts: Required to adjust the orientation of the telescope to achieve long time integration.
- Equatorial mount: is adjusted to α and δ. Doing this is expensive and difficult.
- Altitude-azimuth mount: it is an easier solution, but requires constant calculation of A and h. Another difficulty is the continuous rotation of image fields (from a “static Earth” perspective, the sky is not only following a translational motion, but also rotating), which must also be compensated.
Segmented primary mirrors: The mirror’s components are constantly realigned detecting minute changes in position by using laser interferometry (using the interference patterns produced by beams of laser light).
Active optics: The shape of the mirror is automatically modified in response to external influences (wind, temperature, mechanical stress…), using pressure pads on the back of the primary.
Adaptive optics: Perturbations on the wavefront of the light coming from the observed object, for example due to atmospheric effects, are corrected by using deformable mirrors or liquid crystals. The displacement of images of guide stars are analyzed to adjust the shape of the mirror. This is automatically done by the computer without involvement of the observed.
Hubble Space Telescope (HST): A telescope put in low Earth orbit (559 km) in 1990, with a 2.4 m aperture. It of course gets rid of all the seeing problems induced by Earth’s atmosphere. It has an orbital period of 96-97 minutes. Long duration exposures of 18 h or more allow observation of objects as faint as 29th magnitude. It uses a Ritchey-Chrétien optical system (Cassegrain-like), operating from 1200 Å (UV) to 1 µm (IR).
The semiconductor detector in the HST is a charge-coupled device (CCD) able to detect nearly 100% of the incident photons in a wide range of wavelengths. It has a linear response and can detect from X-Rays to IR, differentiating well between very bright and very dim objects.
The CCD collects electrons that are excited to conduction bands by the photons. The number of electrons collected in each pixel is proportional to the brightness. Each pixel is capable of holding up to 70,000 electrons, and the HST has 2 1/2 million pixels.