Design principles

The primary driver that motivates the development of a new gas cell design is the prospect of very long absorption pathlengths. Following the principles discussed in Chapter 10, the

distance available for the beam of light to traverse a reference gas shapes the line depths of the spectral imprint by the absorbing gas at a given density (Lambert-Beer law). The line strengths, in turn, define the fidelity of the lines and wavelength positions of the reference absorption lines can be determined with (among other parameters such as line sharpness, line density, line blending; cf. Sec. 10.1). Strong line depths (avoiding saturation, of course) are thus one of the crucial ingredients to achieve high-precision wavelength calibration. Con-sequently, the pathlength through an absorption gas is an important parameter in the opti-mization process of a gas cell calibration strategy. Leaving the pathlength as a free parameter is beneficial for two major reasons (see Sec. 10.3).

1. The choice of the reference gas is not limited to strong absorbers (implying short path-lengths), hence gases and wavelength ranges with weaker lines become usable.

2. Lower gas pressures can be realized, minimizing the perturbing effects of pressure broad-ening on the wavelength calibration quality (sharper reference lines).

The new gas cell concept developed in this work thus strives to maximize the achievable absorption pathlength.

The concept is developed with the following basic design principles:

• The calibration cell is to provide simultaneous wavelength calibration by means of a transmission spectrum, superimposed onto the stellar observations. The reference gas is thus to be placed into the telescope beam, so that the spectral reference imprint and the stellar light share the same optical path through the spectrograph. Both the stellar spectrum and the transmission spectrum are thus co-located on the detector.

• The pathlength of the gas cell shall be longer than that of a conventional gas cell.

This means that the limitation in length of a conventional approach shall be overcome, enabling long pathlengths.

• At the same time, the mechanism is to fit into the availabe space along the optical beam, between the telescope and the spectrograph. Hence, the mechanical setup of the telescope and of the instrument is kept unchanged, so that no relocation of the instrument is necessary.

• The calibration cell shall be optional during operations of the spectrograph. The cell thus shall be designed so that observations with the cell taken out of the optical beam are feasible. Taking out the cell shall not require modifications to the instrument.

• Concurrently, the cell mechanism shall not alter the telescope beam properties entering the spectrograph. The optical input properties for the instrument shall thus remain unchanged.

The constraints imposed by these design principles render an application of such a cell at a fiber-fed instrument unfeasible, in which the cell would be placed behind the telescope focal plane, as in the latter the fiber head is mounted to couple light into a fiber. For such a setup, the gas cell is illuminated by the fiber, coupled into the cell on its way to the spectrograph.

No simple removal of the cell device is then feasible, unless the cell is bridged by some sort of other fiber feed, or unless the complete fiber line from fiber head in the focal plane to

11.1. Design principles 107

Figure 11.1.: Concept for a conventional long-path gas cell design by means of a collimation line. The telescope beam with a given focal ratio is collimated at a convenient location after the telescope intermediate focus (IF) by a mirror (M1). The collimated beam (at a convenient diameter) can extend over a long distance as needed to insert a tube-like gas cell, given enough space. A second mirror (M2) re-focuses the beam into the focal plane with the desired focal ratio (back to f/15 in this example). In practice, such an idealistic paraxial approach requires very slow focal ratios and minimal field of view to manage aberrations, eg. an optical fiber as a point source input in the IF.

the instrument is switched to an additional, continuous connection.¹ In any case, a fiber-fed gas cell concept suffers from the limitations discussed in Sec. 10.2.2. We will later see that for the case of fiber based instruments, much simplified gas cells can be conceived. As a fiber connected cell can be easily displaced from the telescope beam (it can be located in any convenient place), physical space limitations do not strictly apply, and a conventional cell approach appears reasonable (with the respective fiber coupling).

The case of a fiber-fed gas cell is thus not further considered in the present development.

However, it is beneficial to place a gas cell into the optical beam, before the stellar light is coupled into a fiber (cf. Sec.10.2.2). The new gas cell concept already comprises such a case.

The principles above envision an application of the new gas cell design primarily at existing facilities. This is because for newly designed instruments, the principal goal—an increased optical pathlength of the gas cell—can be realized, in most cases, more conveniently by implementation of a large distance between telescope focus and spectrograph entry in the first place. This distance can be easily employed to accomodate a gas cell with the respective long pathlength, if the instrument design considerations allow.

As an example, given enough space availabe, collimation of the telescope beam with suc-cessive transmission through an arbitrarily long, conventional tube-like gas cell (followed by some re-imaging unit), is an obvious solution but requires that the telescope focus and the instrument can be designed and placed accordingly. Fig. 11.1 illustrates such a case. I will revisit this example in more detail in Sec.11.3to highlight selected design difficulties for the application scenario at existing, immutable instrumentation.

The major challenge in the design of a long-path cell for present-day spectrographs in operations is thus to beat the restrictions imposed by the (non-)available space in the optical layout. A solution to the problem must, at the same time, consider reasonable complexity and managable cost.

1In reality, most fiber-fed spectrographs employ additional devices between the telescope focal plane and light injection onto the spectrograph slit, eg. a fiber scrambler and -shaker. Any such device further necessitates a changing connection once the gas cell apparatus is taken out.