Mimir Capabilities
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Mimir operates in three main modes: Imaging, Spectroscopy, and Polarimetry. In addition, there are three distinct "cameras" that may be selected for use within Mimir: the wide-field (F/5) camera; the narrow-field (F/17) camera; and the Pupil Viewer camera. In spectroscopy mode, there are 13 different slits, opens, and darks that may be inserted at the first focus. A review of the Mimir optics is found by following this link. The Mimir detector is a 1024 x 1024 InSb Aladdin III device which is sensitive to light from 0.6 to 5.6 microns wavelength. It features 17.8 electrons read noise, a conversion gain of 8.21 e/ADU, and well depths of 62,000-98,000 electrons per pixel. The dark current is about 10 electrons (1 ADU) per second per pixel for long exposures (100s). ImagingIn the imaging mode, the slit and decker cars are withdrawn from the beam, providing a full open field for the 10x10 arcmin square (3.66 x 3.66 inches) entrance to the collimator unit. Filters are selected from the filter wheel list either using available commonly-used scripts or by direct selection. Filter bandpass plots and data are also posted. Finally, the final plate scale and field of view are selected by commanding either the F/5 or F/17 cameras into the beam. The capabilities for these two plate scales are summarized in the following table:
At the present time, L' imaging is not commissioned for the F/5 camera and M' imaging is not commissioned for the F/17 camera. SpectroscopyIn spectroscopy, one slit is selected, the decker is centered to select only the chosen slit, filters and grism dispersing elements are selected, and a camera is selected. With three different grisms, each with multiple orders, two cameras, and many filters the number of distinct spectroscopy options is quite large. Some of the most common and most efficient options using the F/5 camera are listed in the table below. A more complete spectroscopy option list is also posted. Note that all slits are oriented North-South and instrument rotation is not allowed.
Detector maps and efficiency plots are also posted. Spectroscopic efficiencies (throughputs) and example spectra will be calculated over the next few months and posted here. PolarimetryIn imaging polarimetry mode, the POL wheel is moved to put the rotateable H-band Half-Wave (HWP) into the beam, the Molectron Wire Grid is moved into the beam, the slit and decker plates are moved out of the beam, and the H-band Barr filter and PK50 long-wavelength suppressor are put into the beam. The HWP modulates the plane of linear polarization while the wire grid performs the analysis. Obtaining polarimetric data consists of obtaining a set of images, each with a unique position angle for the HWP. The HWP will introduce an intensity modulation with a frequency four times the HWP rotation frequency (a "4-theta" modulation). Extraction of the polarimetric signal, and calibration against known standards, is handled by our custom software (see software page and polarimetric software page). In this polarimetric imaging mode, the entire 10x10 arcmin field is available when using the F/5 camera or the entire 3x3 arcmin field when using the F/17 camera. Scripts to obtain polarimetry data are efficient and robust. The HWP position angle steps between inages by 10.8 degrees, resulting in 16 image-HWP positions, covering a bit more than one full HWP rotation per polarimetric data set. Alternatively, a faster U, Q chopping mode collects 8 U-Q sets of data per HWP rotation. This U, Q chopping mode is encorporated into a 6-position dithering script that collects all polarization data while performing small motion (~15 arcsec) dithering on the sky. This plot shows the measured intensity modulation versus position angle on the sky from Mimir polarimeter measurements of two polarimetric standard stars (from the list by Whittet et al. 1992). The upper plot is for the 4% polarized star Elias 25; the lower plot is of for the 0.3% polarized star HD147648. The 4-theta polarimetric signal dominates the upper plot.
Polarimetric efficiency has been measured to be 91.2+/-2.5%. The following plot shows how well Mimir polarimetric values match to the values of Whittet et al. standards. The line has unity slope and zero offset.
Instrumental polarization is low (~0.35+/-0.02%) on average, but has a field dependence, as shown in the plot below. The central contour is at 0.2% instrumental polarization and the contour step is 0.2%.
The plot below shows the polarimetric uncertainty versus H-band magnitude for 3 second polarimetric integrations toward star fields in the Galactic Plane Infrared Polarization Survey Pilot region.
DPC 20080808 |
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