Using optical fibers in astronomy

Question

Why are optical fibers important in astronomy? I have read on the internet that they find applications in many fields including astronomy and this intrigued me and I would like to know what they are used for

Answer 1

There is already a good answer that has been upvoted and accepted. I also upvoted that answer right after I posted. But maybe I can make a helpful contribution.

In an answer I posted here, I showed how my homemade echelle spectrograph (a type of spectrometer) could be used to obtain spectra of various light sources. In each case, I used an inexpensive fiber optic cable (Thorlabs silica 50 micron step index multimode) to get light into my spectrograph.

In response to comments, here is a diagram of my echelle spectrograph and an annotated photograph:

I could not find a mount for the echelle grating that would let me have two, more or less independent, degrees of freedom for inclining and rotating the grating, so I made my own:

Also, since the spectrograph photo was taken, I have also tried two new telephoto lenses: a Samyang 135 mm f 2.0 and a Rokinon 85 mm f 1.4. They all work great and the two new ones are manual focus only. Right now, the Rokinon lens is installed.

I do not have a good telescope or good sky viewing from my small yards, but I have a good friend (a fellow retired chemistry professor) who has an 8 inch Dobsonian telescope and unobstructed sky that is reasonably dark. So on March 10, I took my echelle spectrograph to his place and we hooked it up to the eyepiece on his telescope.

He has one of those adjustable devices that allow users to attach a smart phone to the telescope’s eyepiece. I made a plastic dummy smart phone that had a small collimating lens and SMA fiber optic coupler. So the input to the little collimator is positioned where the smart phone’s camera lens would be. This is very crude compared to what astronomers and astrophysicists do, but you have to start somewhere.

The next two photos show that contraption with a fiber optic cable attached:

The other end of the fiber optic cable provides input light for the echelle spectrograph. You can read more about how the spectrograph works by reading my answer that I linked above.

My friend aimed his telescope at the moon, which was about hall full and almost directly overhead, and made sure we stayed on it. The next photo is the color echellogram we obtained:

and here is the same photo in grayscale:

We used a 130 second exposure with ISO 800 and 85 mm f 1.8 telephoto camera lens. The color moon echellogram was cropped down from 6000 by 4000 pixels and lowered in resolution for posting here. You can see the Fraunhofer lines in absorption, e.g., H alpha (the red Balmer line), sodium D lines, the magnesium triplet, etc. This is a continuum spectrum with missing dark spots due to absorption of specific wavelengths: the Fraunhofer lines.

Using fiber optic cables to get light into a spectrometer or spectrograph has advantages and disadvantages. For my purpose, doing this just for fun, convenience is desirable.

For example, suppose I want to get light from an ordinary white light compact fluorescent lamp (CFL) into my spectrograph. Then this is the simple interface I used to get the CFL echellogram in my linked answer:

and this is a slightly newer echellogram, taken with a Rokinon 85 mm f 1.4 telephoto lens on the camera:

Another example: one of those inexpensive plasma globes. I think the fill gases are argon, neon and nitrogen, but maybe not. The interface is simply the small collimator with SMA fiber optic connector:

The resulting echellogram is

The echellogram was a 30 second exposure at ISO 1600 with an 85 mm f 1.8 telephoto lens. So now I get to try assigning the spectral lines and see what the fill gases actually are.

Update: I finally got around to examining the light from the plasma globe more carefully and find that the fill gases are neon and xenon, but no argon:

Live and learn! I also upgraded the echelle grating mount to use a modified Thorlabs rotation mount. Much better, but no need for another photo.

One more thing. I started working on this because a person I met on the chemistry stack exchange (M. Farooq) wanted to make an echelle spectrograph that could resolve the sodium D lines and fit in a shoebox-sized footprint. Neither of us knew much about the little adventure we were to embark on. So we read answers here and on the physics stack exchange, rounded up lots of publications and read them over and over, ransacked the web for helpful information, etc. We have exchanged hundreds of e-mails to date, sent lots of files, and failed over and over. We made some dumb mistakes and went down some blind alleys. So we just kept encouraging each other and kept working. I took the lead on actually making the spectrograph because I had some optical mounts, breadboards, optics, etc. This is just for fun.

Answer 2

You can link telescopes to spectrographs by optical fibre. See "Using Fibres to link Telescopes to Spectrographs. from the ESO.

The advantage is that you don't need your spectrograph to be physically attached to the telescope. If the spectrograph is large or heavy, that could be a problem. However, there are issues with this. Some light is absorbed by the fibre, and the set-up can be fragile. You can read many more details at The Caos site on optical fibres:

Optical fibres are widely used in professional observatories in order to detach spectrographs from telescopes. The advantages are indisputable but at expense of a reduction of light flux available to the spectrograph. In amateur astronomy, observers are confronted with the practical ways to prepare the fibre link between the telescope to the spectrograph. A careful choice of the type of fibre and a suitable optomechanical design are required for each particular case. In addition, their preparation and installation require special tools and skills. [lightly edited for grammar]

In addition, and more mundanely, optical fibres make good high-speed data networks, though this isn't specific to astronomy.

Answer 3

Two main uses:

1. A fibre-fed spectrograph can be much more stable, because the spectrograph can be mounted solidly in a pressure- and temperature-controlled environment, away from the telescope. Being away from the telescope minimises mechanical movement; environmental control minimises temperature-dependent expansion and movement. Fibres can also "scramble" the light, which is a huge advantage for precise radial velocity measurements compared with slit-fed spectrographs, where the detailed shape of the image falling onto the spectrograph can wobble about with the seeing (atmospheric turbulence), causing small changes in the measured velocity. All the top planet-finding spectrographs are fibre-fed.

2. To feed multiple images to the same spectrograph - multi-fibre spectroscopy. The "heads" of the fibres are positioned (usually by some sort of robot) in the focal plane, according to a set of given stellar (or galaxy) positions. The other ends of the fibres are brought together, passed through the spectrograph, and the spectrum of each object is then imaged on a CCD camera, stacked at right angles to their dispersion direction. In this way you can obtain the spectra of hundreds or even thousands of objects in the same field of view at the same time.