Array Rotation Measurements from 970607n

R. Cutri - IPAC

Final adjustment of the scan dither pattern is being attempted tonight at Mt. Hopkins by rotating the camera to achieve a Ks array rotation of 0.27 degrees with respect to the scan direction. Tom Chester's analysis indicates that for the a scan rate of 56.77"/sec (step size=82.60"), this Ks rotation results in the best compromise dither pattern in the three bands. The goal tonight is to rotate the camera to that angle with the 82.60" step size.

A baseline rotation and step size was obtained from scan 040, taken last night, 970606 UT. That scan was ftp'd from the Observatory earlier today and the array rotation and scan step size extracted. The table below contains a summary of the array rotation angles and in-scan step sizes (in Ks pixels) for all scans measured.

The telescope scan rate was set to 56.77"/sec at the beginning of the night. Raw camera data are being ftp'd from the mountain shortly after acquisition tonight, and immediately processed with 2MAPPS v1.0. The array rotation angles and scan step sizes are measured during position reconstruction, and fed back to the Observatory. The first scan obtained tonight, scan 002, was made without adjusting the camera rotation. As expected, the rotation angles from scan 040 on 970606 and scan 002 on 970607 agree well. The recovered in-scan step size agrees well with the target value.

The first scan transferred to IPAC after rotation of the camera, scan 009, shows a Ks array rotation angle very close to the desired 0.27 degrees. Scans 014, 015, 024 and 025 also agree well with the new value. ( Note that scan 014 is a south-going scan, so the sign of the step size is negative). Scan 16 does not, however, and clearly stands out as aberrant. It is not clear what may have occurred on that scan, but a small RA drift could account for the unusual values. That scan will be investigated soon. The last lines in the table list the average rotation angles in each band and Ks step size after the rotation. Averages are determined with and without scan 016. Even with the unusual result in scan 016, it appears that the rotation has been set very close to desired value. The average Ks step size is 41.65 Ks pixels, corresponding to 82.63".