Shutter

The LBC shutters were designed to provide exposure uniformity over a wide field of diameter 130 mm (LBCB) or 160 mm (LBCR) down to 0.1 seconds. They contain two blades mounted on three teflon pads which slide in the same direction, with a delay set by the commanded exposure time. This means that for very short exposures times, a thin slit passes over the field and the field is not exposed simultaneously. In consecutive exposures, the blades move in opposite directions and this is indicated in a comment field in the header (Shutter: motion A or Shutter: motion B).

During instrument commissioning, the shutter opening time was determined to be linear from 0.1 seconds, although below ~1 second there was more scatter, some of which could be attributed to a difference in timing when the shutter blades both moved from left to right vs right to left, some to scintillation of the light source, and finally some to a residual influence of the 60 Hz AC current.

The exposure time is measured by the passage of small neodynium magnets that are glued to the teflon pads past a Hall sensor mounted to the chassis. During commissioning, a offset was found between the requested and actual exposure time. For LBCB, this was 0.07 sec and for LBCR, 0.21 sec. Currently, the offsets are, for LBCB, 0.269/0.231 (A/B),  and for LBCR,  0.237/0.240 (A/B) seconds. The header keyword TEXPTIME represents the requested exposure time plus this offset.

 

Figure: The images of the LBCB and LBCR shutters were taken from Speziali et al. 2010, Proc SPIE, Vol 7014. The LBCB aperture is 130 mm while the LBCR one is 160 mm.