Optics Calibration Facility

The Center for Space Physics at Boston University (BU) has constructed an optics calibration facility which is used for the evaluation of mirrors, gratings, filters, detectors, and other space flight hardware. The full facility is 1200 square feet and includes a 200 sq. ft. darkroom, a 200 sq. ft. class, 10,000 clean room, two vacuum tanks (the prototype and the main system), and 250 sq. ft. of work space.

The Small Vacuum Tank

The prototype ultraviolet calibration facility is a horizontal cylinder 60 inches long and 22 inches in diameter. One end of the cylinder extends into our clean room. The door at this end of the cylinder is opened to install the instrument or component to be tested in the chamber. The door is attached to the cylinder and holds the electrical, mechanical, and plumbing feed throughs that support the instrument being tested. There is a cap at the other end that holds the more permanent equipment. The pumps, the monochromator (which feeds light into the system) and the NIST reference photodiode all mount at this end of the tank.

The photodiode is the calibration standard against which all absolute measurements are made. It is mounted on a rack and pinion track which can be extended into the optical path to measure the light intensity and removed to allow the light to continue on into the tank. The diode reads out through a Keithley electrometer mounted outside the vacuum tank. The electrometer is equipped with an IEEE-488 bus for computer readout.

Light for the system is generated by a hollow cathode discharge lamp powered by a constant current high voltage power supply. The hollow cathode source uses high voltage (V ~ .5-2 kV) to excite emission lines in a low pressure (P ~ 30-100 mu) gas. The gas enters the system through a metering valve which controls the pressure. It passes through the light source and is removed from the vacuum system by a rotary vane pump. By changing the type of gas used, different wavelength emission lines can be generated. We currently use helium (e.g. lambda = 304,584 angstrom), nitrogen (e.g. lambda=1134,1085A), or oxygen (e.g. lambda = 1304,989,834A). As other lines are needed we can also use hydrogen (e.g. lambda=1216,1025A), argon (e.g. lambda=1048, 919A), neon (e.g. lambda= 734,460A), or any other gas.

A particular emission line is selected for use by a normal incidence monochromator. The small tank’s monochromator is a normal incidence f/4.5 Rowland configuration. It uses a movable 1200 g/mm iridium coated grating and fixed entrance and exit slits to select the desired wavelength of light. In addition to the primary entrance slit the monochromator has an auxiliary entrance slit. A flip mirror, which can be inserted and extracted from the beam while the system is evacuated, directs the light from the second entrance slit onto the grating. The second slit which is covered by a quartz window is used to direct alignment lasers and light from mercury penrays into the system for alignment and longer wavelength tests. The monochromator is equipped with a IEEE-488 computer interface which will allow us to adjust the monochromator through the lab’s computer.

The monochromator is held at low pressure by a 150 l/s turbo pump backed by a small rotary vane pump. The main tank is pumped by a 350 l/s turbo pump with a somewhat bigger rotary vane pump backing it. Additional vacuum hardware includes a cold cathode gauge on the main tank, thermocouple gauges on all of the chambers and forelines and a course vacuum gauge. The vacuum system is controlled through panels on the front of the assembly which also provide a limited safety interlock system.

The tank contains a stainless steel optical breadboard which can be rolled out of the tank to install components and to set up experiments. Once the set-up procedure is completed the breadboard can be rolled into the tank and aligned with the incoming light beam. We have the capability of moving the experiment while the system is evacuated. This will be accomplished by a pair of Klinger rotational stages. The stage controllers are equipped with an IEEE-488 interface and complete computer control will be forthcoming.

Large Vacuum Tank

Vacuum tank being lifted by crane into the CAS building

Vacuum tank being lifted into the CAS building

As the name implies, the main tank is very similar to the smaller system also in use. The vacuum chamber is a 5 foot diameter 6 foot long horizontal cylinder with a light source and photodiode at one end and a hinged door extending through the cleanroom wall at the other.

The monochromator for this system is a scanning grazing incidence Rowland system for use for wavelengths from 1304A down to wavelengths below 100A. The light source is a hollow cathode source similar to that of the smaller system. For the shortest wavelengths this is supplemented by an electron impact soft x-ray source. As with the other system, the monochromator is evacuated by a turbopump.

The main tank is evacuated with a cryopump rather than a turbo. A cryopump offers higher pumping speed and less contamination to compromise the instruments being tested. The main tank manipulator is able to adjust the position of a full sounding rocket payload to allow end-to-end calibration of a sounding rocket payload-sized instrument.