The observatory is equipped with a scientific -grade astronomical CCD camera by Santa-Barbra Instrument Group (SBIG). The camera (ST-7XMED) features a high quantum-efficiency (QE) CCD detector capable of making sensitive images and precise measurements of stars’ brightness. An integrated filter wheel enables measurement of colors and surface temperatures of stars. The very low read-noise of the science-grade detector enables long duration exposures to record faint targets.
Adaptive Optics Module
Astronomical observations require that the target star (or planet, galaxy, nebula etc.) remain motionless in the image-plane during an exposure. Image motion can be tolerated if it is smaller than the fundamental angular resolution of the instrument. This limit is dictated by the telescope aperture, the quality of the optical surfaces (in both the telescope and camera/spectrograph), and by the turbulence of the atmosphere above the observatory. Correcting turbulence and wind-shake is a key technology of modern astronomy, pioneered in part at Gemini Observatory.
The UMass Lowell telescope is capable of delivering a point-spread function (PSF) of 0.55 arcsec FWHM (Rayleigh criterion for 10-inch aperture, wavelength=550nm). However, the atmosphere above Lowell, Massachusetts, will degrade resolution to about 2 arcsec on typical nights. This is a typical figure for sea-level observing sites and is sufficient for the science projects envisaged. This required resolution limit can only be reached after local sources of vibration, shaking, and turbulence are reduced.
Rooftops are subject to shaking motions due to wind and vibrations due to machinery and people inside the building. Local air turbulence is due to air-conditioning outlets, heat-losses, warm air rising, wind gusting/deflecting around the building/urban environment.
The above factors can be effectively diminished by mechanical isolation (passive damping) and by active compensation (fast guiding, active and adaptive optics).
The ST7 camera contains a second CCD detector used to generate guiding corrections by imaging a nearby bright star. The guiding CCD is read-out rapidly (as often as 100 times per second, depending on brightness). The camera’s onboard processor sends signals to the telescope mount and the Adaptive Optics (AO) module to immediately compensate any motion of the star’s image.
The AO module (ST-8 also by SBIG) contains a lens mounted in a cell controlled by piezo-electric actuators. The actuators rapidly (100Hz) tip and tilt this lens, in response to signals from the camera’s guiding CCD, to keep the star centered. AO compensates for wind-shake, low frequency vibrations, and a portion of the local atmospheric turbulence.
Spectroscopy is the technique of dispersing light with a prism or diffraction grating in order to study the overall energy distribution and the absorption/emission lines of elements/molecules present in astronomical objects. A star’s spectrum reveals its surface temperature, chemical composition, magnetic field strength, surface gravity, density, and motion through space, (by virtue of Doppler shifts in its spectral lines). We have selected a spectrograph (the ST-8 by SBIG) that has two different gratings in a remotely selectable turret.
In low-resolution mode, we can obtain the overall spectrum of a star and identify the major absorption/emission lines (e.g. the Hydrogen Balmer series) thus determining the spectral class (O,B,A,F,G,K,M, essentially via the temperature, which is closely correlated with the size, mass and age of the star). In high-resolution mode we can examine the structure of spectral lines, which can reveal more details such as gas density, magnetic field, and surface gravity. Most interestingly, in hi-resolution mode we can determine a star’s radial velocity (towards and away from the earth), and by observing it over many nights, discover the orbits of binary stars, a classic foundation of astrophysics. Perhaps more remarkably, we can measure the redshifts of galaxies, reproducing Hubble’s landmark discovery that led to the big bang theory.
Filters with specially designed transmission curves are used to isolate different parts of the visible spectrum. Images taken through different filters can be combined to produce color images. Photometry, the study of stars’ luminosity, temperature and other gross physical parameters is based on the relative brightness as measured through one of several standard filter sets. The camera is equipped with an LRGB (luminance, red, green, blue) set suitable for color imaging and photometry. Photometry is the study of stars’ brightness in different parts of the spectrum and how they change with time.
A set of narrow-band filters is used to isolate certain spectral lines (OIII, H-alpha, H-beta). While light pollution rejection (LPR) filters, which improve imaging contrast by blocking the spectral regions most-afflicted, LPR filters are essential given the urban setting. Light pollution is the reflection of streetlights and other night-time illumination by suspended aerosol particles in the atmosphere (e.g. water droplets, dust, particulate pollution, etc.). The resulting haze of light obscures the night sky and hides all but the brightest stars from view. Fortunately, light pollution is concentrated in certain well-known regions of the visible spectrum (such as the sodium D-line emitted by streetlights at 589 nm) and can be reduced by the use of appropriate filters.
The telescope is mounted inside a fiberglass dome, which protects the telescope and instruments from the weather, and blocks unwanted light during use. The dome shutter opens and closes by remote control and rotates in synchronization with the telescope.