Abstract
For our first spectrograph we had been thinking about a fiber feed. But we had no idea about that and we could only imagine having problems. Therefore, we installed the spectrograph at the Newtonian focus, fed through a small prism. We did not use fibers. The first measurements delivered nice spectra but strangely enough, the orientation of the spectrum on the chip was not parallel to the CCD pixel rows. And when the telescope was moved along its two coordinates, the spectrum moved somewhere, but not orthogonally to the chip. Everything was somehow wrong. Unfortunately, we had forgotten that the prism had to feed the light parallel to the optical axis of the spectrograph but we had more focused on a good-looking orientation when mounting the device. In any case, the beam was not parallel to the telescope. We were very sure that we had understood the optical geometry and had already completed all mechanics. Who needs fiber optics anyway?
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Notes
- 1.
This applies to the present consideration of step-index fibers with discretely different refraction indices for core and cladding. For the so-called mono-mode fibers with core diameters of the order of the light wavelength and for gradiant-index fibers with a continuous refraction index profile, we refer to the corresponding literature.
- 2.
In the following we only consider incident rays inside the acceptance angle. All other beams leak through the fiber cladding.
- 3.
Fiber optics at the ESO VLT F/13.41 Cassegrain focus without adjusting optics would make any reasonable measurement impossible.
- 4.
The color-dependent transit time differences additionally introduce a reduction of signal quality and reduced bandwidth (modal dispersion). This can be avoided by the use of gradient-index fibers. Their fiber core has continuous refractive indices decreasing towards the cladding. However, these effects can be neglected in astronomical spectroscopy.
- 5.
Here again, in spectroscopy the overall system and the harmonic tuning of all parameters is essential.
- 6.
The internal transmissivity is usually well over 90 % in the visual wavelength range.
- 7.
At this point we neglect the possibility of guiding the telescope with external guiding optics.
- 8.
Given the very small fiber aperture, the pinholes potentially act as light tunnel introducing potential vignetting when tilted. The thickness of the pinhole plate should be thinner than the diameter of the pinhole.
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Eversberg, T., Vollmann, K. (2015). Remarks on Fiber Optics . In: Spectroscopic Instrumentation. Springer Praxis Books(). Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-44535-8_11
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