Binospec is an imaging spectrograph with dual 8’x15’ fields of view, and a very compact layout for excellent stability. It will be installed on the 6.5 meter MMT telescope in Arizona. The instrument was designed by Daniel Fabricant’s group at the Smithsonian Astrophysical Observatory (SAO). Our lab is involved with the calibration portion of the instrument. Based on a design by SAO, we are fabricating and assembling the retractable calibration screen and derotator assembly.

Figure 1
Layout for the calibration portion of the instrument

The role of the calibration screen assembly is to project light from calibration lamps on a rear projection screen that then illuminates the focal plane of the spectrometer uniformly. This allows the user to perform wavelength and flat calibration of the science detectors. The calibration system is integrated to the instrument and the calibration screen is on rails and can be moved in and out of the optical path to allow calibration without having to move the telescope or close the dome.

Figure 2
Calibration screen on its cart, located below the MMT primary mirror
Figure 3
The image above shows the uniformity of the illumination in the Binospec focal plane provided by the upper calibration assembly. The illumination uniformity is better than +/- 5% in the two rectangular regions, where the instrument slit masks will be positioned. The data was taken using a photodiode mounted on an XY translation stage. A pinhole was placed in front of the photodiode to limit the field of view of the photodiode to f/5 to mimic the instrument field of view.
Figure 4
Figure 5
Metrology of the instrument was performed with a Faro coordinate measurement arm during assembly to ensure that the instrument meets the specifications.
Figure 6
The (almost) complete Calibration Unit on its shipping cart, getting ready to be shipped to Harvard-CFA for final integration with the main Binospec instrument.
Figure 7
Initial measurements of the throughput of DECam performed by DECal. The DECal calibration system is completely automated and can measure the throughput of one filter per ~2 hours. With a repeatability and noise level well below 1%, the system is also used to detect light leaks outside the filters transmission bands. It will be used along with Skyflats to improve the photometric accuracy of DECam to 0.5%.
Figure 8
Spatial information measured by DECal. These 6 plots show the net transmission of each filter with the contribution of the CCD removed. Our system is stable enough to enable the measurement of fine structure in the transmission of the filters.
Figure 9
J-P and Emily posing in front of the assembled flat field screen before installation on the dome roof.

For more information, visit the Binospec page at the Harvard-Smithsonian Center for Astrophysics.