Big Bear Solar Observatory

Why a New Telescope?

When it was added to the Big Bear Solar Observatory in 1973 our 65-cm vacuum reflector was a "state-of-the-art" solar telescope. It provided a large enough aperture to allow diffraction-limited observations under the very best observing conditions possible at Big Bear. It also provided enough light to feed even the narrowest-band observations of the time with short exposure times.

However, in the 1990s several changes in observing technology arrived which changed everything. Foremost of these was adaptive optics, which allows atmospheric seeing to be corrected to the diffraction limit for any size telescope (given enough computer capability, etc.). Second of these were computer systems capable of running much more sophisticated instruments such as Fabry-Perot interferometers. Third was the increasing importance of observations in infrared wavelengths.

Adaptive optics allows us to image the actual photospheric flux tubes, at about 100 km resolution, if we have a telescope large enough. The size required is around 1.5 meters diameter. Such a size requires very different engineering solutions from previous solar telescopes. The basic problem is that while previous solar telescopes solved the problems of atmospheric distortion inside the telescope by building a vacuum system, that solution will not work for telescopes larger than about 1 meter aperture. It's simply not possible to build a window with sufficient optical quality larger than that size. Therefore solar telescopes larger than 1 meter must be open to the air.

Being open to the air means that to prevent "tube seeing" we must control the temperatures of the mirror surfaces and the air inside the telescope very precisely. We do this with:

Active cooling of the primary mirror to keep it close to the ambient temperature.
Active cooling of the air above the mirror surfaces (air knives) to prevent turbulence.
Active ventilation control of the dome to keep a steady air flow inside the dome and maintain the dome interior at the ambient temperature.

In addition to improve the contrast and reduce stray light we are using an off-axis primary and secondary mirror configuration.

Our chief science objectives include:

High-resolution, high-cadence studies of solar flares.
Study of the structure and evolution of magnetic fields in flaring active regions.
Study of the dynamics of the photospheric kilogauss flux tubes.
Study of magneto-convection in sunspots.
Study of the heating of the upper solar atmosphere.

For these studies we will be using a newly upgraded instrumentation package:

New imaging polarimeters for visible and infrared wavelengths.
Real-time image restoration.
A new spectrograph system, the FISS.
Adaptive optics systems, both our current system and a new system to take full advantage of the 1.5 meter aperture.