Quality Control: DISTORTION frames

SINFONI Quality Control:
distortion and slitlet distances

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general information | parameters trended


QC PLOTS
DISTORTIONS:	CURRENT	HISTORY
average slitlet distance (all bands)
average slitlet distance (J, H, K, H+K)
distortion shift at 5 detector positions
distortion coefficient C00
distortion coefficient C01
distortion coefficient C10
distortion coefficient C11
distortion coefficient C20
distortion coefficient C02
distortion coefficient C21
distortion coefficient C12

QC1 database (advanced users): browse \| plot
Click on CURRENT to see the current trending (Health Check). Click on HISTORY to see the historical evolution of the trending.

top General Information

Distortion frames are used to determine the spatial distortion on the SINFONI detector and determine the distances between the individual slitlets. This calibration is made once per month, as well as immediately following an intervention. It consists of about 75 FIBRE_NS frames, a FLAT_NS (a lamp-on/off pair), and a WAVE_NS (arc lamp on/off pair). Each fibre frame consists of an illuminated fibre imaged along each slitlet position and moved, from image to image, perpendicularly to the slices. When these frames are coadded they create uniformly illuminated slits across the full detector (see Figure below). The arc lamp frames are used to determine the positions of the edges of each slitlet. The distortions are only obtained at the 25 mas pixel scale. Due to the inherent stability of SINFONI, the distortions are only obtained once every 30 days.

The background-subtracted stack of 75 FIBRE_NS frames. The uniformly illuminated centers of each SINFONI slitlet is clearly visible. The slitlet numbering is shown at the bottom.

To make the effects of changing distortion coefficients more easily visible, the optical distortion is applied to five different positions on the detector. Shown as green dots, these positions are at pixels: (512,512), (512,1536), (1536,512), (1536,1536), and (1024,1024). This is trended in the distortion shifts.

top DISTORTION parameters trended

QC1 plots

The QC1 plots for the SINFONI distortions show:

average relative distance between all 32 slitlets (all bands shown together) and the associated rms (in pixels)
average relative distance between all 32 slitlets (J, H, K, and H+K gratings shown separately) and the associated rms (in pixels)
distortion shift at five different detector positions (in pixels)
8 plots with each distortion coefficient and for each grating (J, H, K, and H+K)

The two dimensional polynomial fit to characterize the curvature of the 32 spectral slitlet traces is given by:

distortion(x,y) = C00 + C10*x + C01*y + C11*x*y + C20*x² + C02*y² + C21*x²*y + C12*x*y²

QC1 parameters trended

Page	Parameter	Table: Name (QC1 database)	Description of Procedure
1	average slitlet distance and rms (all)	sinfoni_dist: qc.sl.distavg & qc.sl.distrms	SINFONI pipeline: The ~75 frames consisting of a single fibre light source moved perpendicular to the 32 pseudo slit slices are coadded to produce an image with all slits illuminated. The intensity maximum of each column is determined and a least-squares Gaussian fit is performed to locate the fibre's flux and locate the slitlet center. For this parameter the relative distance between each trace is averaged. The distortions are done at the 25 mas pixel scale. All grating points are plotted together. The optimal value is 64 pixels.
2	average slitlet distance and rms (J, H, K, and H+K)	sinfoni_dist: qc.sl.distavg & qc.sl.distrms	SINFONI pipeline: As above, but with the J, H, K, and H+K average slitlet distances shown in different colours. The optimal value is 64 pixels.
3	distortion shifts	sinfoni_dist: qc.shift.cc, qc.shift.ll, qc.shift.lr, qc.shift.ur, & qc.shift.ul	SINFONI pipeline: Since the distortion coefficients are relatively abstract, the optical distortion is computed at five positions on the detector. The X-axis (along a row) shift that results from the distortion correction is shown at these five positions for each grating. X-axis shifts range from about -10 to +4 pixels.
4 to 11	distortion coefficients	sinfoni_dist: qc.coeff.ij with ij = 00, 10, 01, 11, 20, 02, 21, and 12	SINFONI pipeline: The distortion function characterizes the curvature of the 32 spectral traces on the coadded fibre images. For each slitlet the centroids of the fibre traces are detected and traced along the curved spectra. Two 2-d numerical grids are constructed; one on the distorted fibre positions and the other on the optimal expected fibre positions. The above 2-d polynomial function is fit to transform the positions from the raw to undistorted coordinates.

Trending and Issues

Based on the inherent stability of the SINFONI optical path, the measurement of the optical distortion on a monthly basis is justified. The only potential issue is that the distortion calibration template will, occasionally, produce an incomplete set of frames. This can include either missing FLAT_NS frames, missing FIBRE_NS frames, or frames without flux from the fibre. In these cases the distortion calibration will be repeated when this problem is detected by Garching, QC.

History

The optical distortion measured on the SINFONI detector is quite stable over long time periods. Since the beginning of operations (more than 3 years ago), the average distance between slitlets has been constant at the level of better than 1/20 th of a pixel (3 sigma).