The spectrum you show from Titan was taken using the IRIS spectrometer aboard Voyager 1. Of course Voyager 2 had one as well. A huge amount of work went into developing and optimizing the design in order to develop a precise optical instrument that would survive both the high g-force and vibrations of launch, and the years in a space environment while Voyager brought it to the planets. From Wikipedia:
Early versions of the IRIS were flown on the 1960s Nimbus 3 and Nimbus 4. In 1971, an early prototype was used on Mariner 9 to examine Mars.
From the paper Infrared spectrometer for Voyager discussed more below, some of these improvements are:
- Distances associated with fast flyby measurements are much longer, required a narrow field of view.
- Low level of spectral radiance (SR) required substantial reduction in noise equivalent SR of the instrument.
- The spectrometer had to run at a cold (200 +/- 0.5 K) temperature so that it would not see it’s own IR thermal radiation. It also maintained a temperature stability of 0.1K per day!
- Calibration had to be done in-flight
- Radiation hardening to survive especially the high-rad environment around Jupiter
- weight and power consumption minimization for this deep-space voyage
When we think of spectrographs we usually think of a narrow slit, a diffraction grating to spread the spectrum, and a position sensitive detector like a CCD. Of course for other wavelengths different types of detectors are used, and early spectrographs used photographic film instead.
But Voyager had a spectrometer. As pointed out in this Quora while a spectrograph spreads the spectrum out and measures many wavelengths at once, a spectrometer measures the intensity one wavelength at a time. This can allow for the much narrower resolution necessary to see narrowly spaced spectral lines.
The Voyagers' spectrometers (now both deactivated) are Michaelson Interferometers. There's an illustration of the instrument below. The biggest feature is the large telescope to collect enough light to get a strong signal after dispersing the wavelength so much. You can see an excellent photo of the telescope in this answer, it's beautiful!
The calibration procedure for IRIS is explained in Voyager Infrared Interferometer Spectrometer (IRIS).
I was going to recommend reading Voyager Infrared Spectrometer by Stuart A. Borman. Analytical Chemist, v. 53, no. 13, Nov. 1981 but I now see that it is paywalled. I'll look for a more accessible review.
The Michaelson interferometer is easier to see in the second image of IRIS-D. The idea is that you coherently split a beam of light and then recombine them with a beam-splitter but with one of the paths longer than the other. The interference of the two beams is different for each wavelength. You carefully measure the intensity while moving one of the mirrors, and record a very long series of intensities. Together, this plot of interference intensity versus mirror position is called an interferogram.
Read more in Wikipedia's article for interferometry.
In the IRIS systems aboard the Voyager spacecraft, the mirror moved a total distance 1.58 mm during a scan, at a speed of 0.0351 mm/sec.
The interferogram is stored, and later transmitted back to the Deep Space Network. Back on Earth, scientists then take the Fourier transform of the interference signal in order to reconstruct the original spectrum.
There's a writeup of the IRIS spectrometer in this (paywalled) OSA article Infrared spectrometer for Voyager (R. Hanel, D. Crosby, L. Herath, D. Vanous, D. Collins, H. Creswick, C. Harris, and M. Rhodes, Applied Optics Vol. 19, Issue 9, pp. 1391-1400 (1980) •https://doi.org/10.1364/AO.19.001391)
below: Voyager's IRIS spectrometer, from here.
below: IRIS-D spectrometer for Nimbus-4, from here.
below x2: From Analytical Chemist, v. 53, no. 13, Nov. 1981 Voyager Infrared Spectrometer by Stuart A. Borman.