When scientists first began to examine the spectra of different stars, how were they sure that they were seeing the spectrum for that one star and not a composite of all the stars in the telescopes FOV?
Early stellar spectroscopy merely put a prism in the light path, after it was focused by the primary mirror, but before it reached the focal plane (where the photographic plates/film is).
This produces a star field, like you normally observe, but each star is smeared out into it's rainbow spectrum instead of being just a pin point of light. Each star had it's own spectrum, right where that star normally appears, so there's no chance of mixing up any spectra, unless you have very closely spaced stars where the spectra overlap...in which case you skip those until you can get a higher magnification that seperates them out.
Newer approaches use CCD cameras. One I'm familiar with actually involves making a physical plate, with holes cut out where the stars would appear on a normal photographic plate. Each hole is then fitted with a fiber optic cable that then feeds the light from that single star through to the instrument package for automatic computer analysis.
OK so if I wanted to see the spectra of a star I was observing, do you thing it would be possible to attach a prism to the eyepiece socket (Im not sure if an eyepiece should be used, but it would be a short order to remove it) and project the spectrum onto a sheet of paper? Or would it have to go where the secondary is and then have the spectrum refracted off to the side?
Just curious.......I read about all the things people like Newton did, and want to experience some of that for myself. No need for high tech,(not now, not on my budget) creative thinking may be able to overcome the pricetags of some of that expensive equipment. (I hope)
Just holding a prism behind the eyepiece gets to be very clumsy, because the spectrum comes out side ways. Another way of getting a spectrum is use a diffraction grating. These are either transmission or reflection grating. The cheapest are the "spectrum glasses" (which have a thin plastic grating (it has a lot of parallel grove etched on one side). Education suppliers have these as well as sheets of the plastic grating material (make your own). Again you'll need to look side ways and would only work on the brightest stars.. Fireworks, street lamps look cool through these. The grooves are easily rubbed out by clumsy thumbs.
The spectrum is actually images of the object at each wavelength.
Normally a slit in front of the spectrograph is used to isolate the star from others in the FOV. So the spectrum is a set of images of the slit at each wavelength. At wavelengths where a star does not emit light a dark line appears; hence name spectral lines.(absorption line in previous case because the background light from deep in the star is absorbed by the cooler outer layers. The reverse can happen where the object (planetary nebulae are a good example) emit light only a specific wavelength so get a emission line spectrum; or without a slit a series of separate images.