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Hubble Space Telescope
Motion of the Coronographic Hole Over Time

One effect of the so-called "dewar anomaly" on the coronagraph was the apparent drift of the location of the coronographic hole relative to the fixed acquisition aperture on NIC2. This motion is presented below in three formats: an animation, plots, and tabular form.


This animation shows the motion of the coronagraphic hole over time. The hole is the dark "spot" in the upper left quadrant. These images are lamp-flats made with the F110W filter. Since the NICMOS has no shutter, lamp-flats are technically "external" exposures, and sometimes contain external sources (stars). In this case, only one flat was taken at each epoch to track hole motion, so the sources were not removed.

The left-hand panel shows the flats with their exposure times normalized to each other. Early in SMOV the detectors were operating colder than their nominal operating temperature (the instrunment was launched colder). As they warmed up, their QE also increased.

The right-hand panel shows the same sequence but has been "flattened" using a flat-field image from ground testing. One can see the QE increasing early on (dark images, rising to grey images), and then remaning stable throughout the remainder of SMOV. Also, notice the vignetting edge which moves up from the bottom as the hole moves upward, and then moves back down in conjunction with the hole. The quadrant-based background pattern is a dark subtraction effect in the thermal-vac flat.

The "grot," which is believed to be small paint-chips dislodged from the forward light baffle at the time of the thermal short, can be seen as small black specks that begin to appear in the second frame, when the hole begins to move. Some of these have bright "rings" around them.


The same time period covered in the above movie is reflected in these plots


tabulated version of the (x, y) position of the coronagraphic hole as a function of time from Feb. 1997 - Oct. 1998 is also available.