The Carnegie Supernova Project, located at Las Companas Observatory in Chile, aims to take light curves and spectrophotometry of ~220 nearby Type Ia, II, and Ibc supernovae over a 5 year period.
We built a dome flat spectrophotometric calibration system for the Swope and the Dupont telescope used for this project. This system allows us to measure the color transfer functions for the entire optical path of the telescope (mirrors, filters, detector QE) over a wavelength range from 350nm to 1800nm (U, B, V, g, r, i, H, J filters). We do this by illuminating the telescope aperture with a uniform, quasi-monochromatic light source of known intensity. This calibration system consists of a tunable light source, a fiber delivery system, a screen and a photodiode monitoring system. Each part of the system is discussed briefly below:
Tunable light source:
The light source consists of a Xenon broadband light source that is coupled into a monochromator that selects a narrow bandwidth of the white light source. The wavelength is changed by turning a grating and the bandwidth can be changed by varying the slit size. We typically use a bandwidth of 1 to 3 nm.
Fiber delivery system:
The quasi monochromatic light is coupled to a special broadband fiber bundle that illuminates the screen from the top of the telescope.
Screen - Kris Cabral:
The screen reflects the light towards the telescope. It needs to be highly reflective over the whole wavelength range, reflect the light uniformly even while illuminated at an angle and face the telescope directly.
We built a screen prototype for preliminary testing at the TAMU Observatory. Many design considerations were made when building the mounting system for the flat field screen. The mounting system must enable the user to easily hang the screen from the observatory rafters. It must also allow the user to change the angle of the screen in relation to the telescope face. The screen itself must be uniformly rough to diffuse incoming light. To satisfy these parameters the screen was mounted on a set of hinged arms that allow it to be swiveled about its center. A locking arm holds the screen at any angle from vertical to 30 degrees from vertical. Hooks were also mounted to the arms using a set of hinges to allow the square frame to be mounted to the round rafters of the dome. To diffuse the light the screen surface was sandblasted and coated in white paint that has a extended reflectivity in the UV. In addition to the solid screen, a screen cut into quarters was also used to test for any difference that grooves, visible bolt faces, and gaps may have on the flat field calibration process.
Photodiode monitoring system:
A series of calibrated photodiodes, placed at strategic points in the illumination system will allow us to know what power should be read by the CCD.