Since CaF 2 not only has excellent dispersion properties to correct for chromatic aberration, but also exhibits an excellent transmission beyond the visual wavelength regime, this type of lens is particularly useful for applications in the blue and UV.įigure 12. In order to avoid disrupture of the cemented lens group, some manufacturers have used oil immersion to connect the triplet. The triplet is also difficult to manufacture because of the coefficient of thermal expansion, which is roughly different by a factor of two between CaF 2 and the glass. The manufacture of the CaF 2 lens is critical and expensive, but it provides for an excellent correction of chromatic and spherical aberration of this high quality objective. It is peculiar for its use of a crystal (calcium fluoride, CaF 2) as the optical medium of the central lens. This APQ type of objective is an apochromat, whose chromatic aberration vanishes at 3 wavelengths. The last example is a triplet, employing ZK2, CaF 2, and ZK2. The beneficial use of KzF2 glass is an example of the successful research for the development of new optical media, which was pursued near the end of the nineteenth century in a collaboration between Otto Schott and Ernst Abbé. The lenses show a more pronounced curvature and are more difficult to manufacture. The third example is an improved AS type achromat (KzF2, BK7), which is similar to the E type in that it also possesses an air gap, however with an improved chromatic aberration correction in comparison to the Fraunhofer objective. The objective is critical with regard to the alignment of tip and tilt. As a disadvantage, the two additional glass–air interfaces lead to ghost images (reflected light) and Fresnell losses. The second lens is the Fraunhofer E type, which is characterized by an air gap between the two lenses (BK7, F2), which helps to optimize the image quality of the system, reducing spherical aberration and coma. It is still a useful objective for small telescopes, where a residual chromatic aberration is tolerable. The first lens is a classical C achromat, e.g., using BK7 and SF2 glass, which is essentially the design which was introduced by Hall and Dollond. All lenses are groups 2 or 3, designed to compensate for chromatic aberration through optimized combinations of crown and flint glasses. Figure 10 shows a sequence of lenses, indicating the progress of lens design developments from left to right. Additional steps are then required to produce light curves or absolute fluxes.Refractors are first of all characterized by their objectives. We will see that photometric measurements require that we accomplish only a few steps to provide output flux values.

It will be assumed here that the CCD images being operated on have already been reduced and calibrated as described in detail in the previous chapter. We start this chapter with a brief discussion of the basic methods of performing photometry when using digital data from 2-D arrays. These standard star frames are used to correct for the Earth's atmosphere, color terms, and other possible sources of extinction that may be peculiar to a given observing site or a certain time of year (Pecker, 1970). Some photometric output products, such as differential photometry, require fewer additional steps, whereas to obtain the absolute flux for an object, additional CCD frames of photometric standards are needed. At times, we may be interested in only a single measurement of the flux of some object, while at other times we could want to obtain temporal measurements on time scales from seconds or less to years or longer. Photometry is important for all types of objects from planets to stars to galaxies, each with their own intricacies, procedures, and problems. This process, termed photometry, forms one of the fundamental branches of astronomy. One of the basic astronomical pursuits throughout history has been to determine the amount and temporal nature of the flux emitted by an object as a function of wavelength.

Astrometry versus photometry astronomy