First Mimir Pupil View Images

12/15/2004 - Flagstaff, Arizona

The first images of the cold pupil in the Mimir instrument were obtained last night by the Mimir team, see image below.

Thumbnail view of pupil image. Click on the picture to see the full-sized image.

How does it work?

Within the Mimir instrument, the first 6 lenses form a collimator that captures the light from the telescope and reforms it into a "pupil" which is essentially an image of the main telescope mirror and the blockage caused by the secondary mirror and its support legs. At thermal infrared wavelengths (that is longer wavelengths than 2 microns), the blockages are "hot" and can dominate the weak sky signal. In forming a pupil image within Mimir, this hot emission can be rejected by a properly constructed mask.

Mimir's pupil viewer mode is achieved by a two-lens simple camera (with internal baffles and a stop) located within the moving camera block. This pupil viewer forms an image of the pupil and its mask onto the array detector. The pupil cold mask currently in Mimir represents a rough "first guess" as to what the secondary blockage would look like at the pupil location. The pupil viewer pictures obtained during this third engineering run with Mimir on the Perkins telescope will guide fabrication of a new mask to reject the hot emission from the secondary blockage and its supports.

What is being shown in the image?

The cold sky seen by the primary telescope shows up in the pupil image as the large, mostly orange and uniform brightness circle that dominates the picture. The hot blockage is seen as the bright, white circle (partially rejected by the central black circular mask) and the thin "spiders" radiating outward along the four diagonals. The cold pupil mask is seen as the black central circle and its four support legs radiating outwards along the vertical and horizontal axes.

The first obvious conclusion is that the current mask's central circle is too small and the spider legs are off by 45 degrees.

Less obvious features include: (1) the extra horizontal black band through the center - this is a feature of the infrared array detector (the "row 512-514 problem") that will be cured in the near future, (2) the bright band around the edges of the bottom and sides of the primary - this represents hot background light originating outside the primary mirror - the pupil mask outer diameter will need to shrink somewhat to mask this in the future, (4) there appears to be a mismatch between the center of the primary/secondary and the occulting mask - this is due either to a collimation (telescope alignment) error or to an instrument tilt offset, (5) the four quadrant crack is visible as the partial parallelogram rising from the left bottom, to upper right, to top, then left top - this is a permanent feature of the array detector, but appears not to be a serious impediment to science, and (6) there is an intermediate brightness circle centered a bit left and lower than the image center - it likely represents a reflection of the secondary blockage emission, but its origin within the instrument is currently unknown.

Update - 12/28/2004

Since the first images were obtained, the instrument rotator was properly locked in place (which reduced play between the telescope and the instrument), and the instrument was tilted by shimming to align its optical axis with that of the telescope. Newer pictures may be found by clicking this link.