Solar Telescope Design Details

Boston University’s Solar Telescope was designed and constructed by Jeffrey Baumgardner in 1977. It consists of three primary parts, the heliostat, the telescope, and the spectroscope. The heliostat tracks the sun, or any observable object, and directs the light to the telescope. The heliostat consists of a two flat mirrors, one 10” and an 8”; one mirror tracks and the other redirects the light into the telescopes 6” f/15 objective lens. it can be sent to a wall projection, a tabletop projection, or the spectroscope.

The wall projection uses a piece of IRUS screen (from Draper Labs) placed in the wall of the classroom to be visible from the hallway. This allows for a white light image of the sun to be viewed from the hallway by passing students.

The tabletop projection casts a 6” image of the sun onto the table top where students can plot the positions of the sunspots.

When the light is sent to the spectroscope it is redirected via mirrors to fall onto a slit of variable width, the entrance slit. The light from the entrance slit is collected by a 4” f/15 off-axis parabolic mirror and sent to a 600 lines/mm reflection grating. From the grating the light is sent to another 4” parabolic mirror and up to the exit slit (in spectroheliograph mode). From the exit slit the light is collimated by a lens, redirected via mirror, and made available for imaging; the bandwidth of these images are determined by the exit slit’s width. The light can be picked off by a mirror before it reaches the exit slit and imaged; these images are of large portions of the spectrum.

The solar telescope can also act as a spectrohelioscope. When used as a spectrohelioscope the entire sun can be viewed at any visible wavelength. This is accomplished by scanning the image of the sun across the entrance slit of the spectroscope while viewing a scanning image of the exit slit.. The scanning takes place by reflecting the incoming and exiting light from the spectroscope across a mirror that scans back and forth (vibrates) along one axis. Since both the incoming and exiting scanned light are incident on the same mirror the scans are in phase with one another. A more narrow entrance slit results in finer resolution of the spectrum, but a cost of brightness. As the exit slit becomes more narrow, the bandwidth viewed becomes more narrow, but this also comes at the cost of brightness.