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Quality Control and
Data Processing

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Trending & QC1
   IRDIS background
QC links:
SPHERE: IRDIS Background
background rate for DBI mode
background rate for DPI mode
background rate for LSS mode
QC1 database (advanced users): browse | plot

Dark frames are taken with INS1.OPTI1.NAME=CLOSED to measure the detector dark current, while IRDIS background calibration frames are acquired with INS1.OPT1.NAME=OPEN to measure the instrumental background.

The background is dependent on the set of used filters:

CPI neutral density filter (IN4.FILT2.NAME = OPEN, or ND_2.0, or ...)
IRDIS broad band filter (INS1.FILT.NAME = B_H or B_Y or ...)
IRDIS dual beam filter (INS1.OPTI2.NAME = CLEAR or D_H12 or ...)

IRDIS background rate


FITS key QC1 database: table, name definition class* HC_plot** more docu
QC.MEDIANMASTER sphere_irdis_background..qc_med product background medianHC [docuSys coming]
[QC procedure] sphere_irdis_background..qc_backrate rate in ADU/sec HC [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.

IRDIS raw background frame with DIT=30 sec, averaged over NDIT=15 sub integrations for the D_K12 dual beam filter. Image cuts -5 and 1000 ADU.


The trending plot shows background rate values for DBI mode only:

  • (1) negligible background in the H-band (no ND filter)
  • (2) stable background for the K-band
  • (3) no background for H-band plus neutral density filters
  • (4) K-band background correlates with the temperature of the neutral density filters. Both temperature values (red and green little dots) are given in [C] and have been scaled via: T(in the plot) = 1.452 T - 1.409 to fit the background rate values given in [ADU/sec].
Note for (4): The neutral density filters (here ND_1.0 and ND_2.0) used for IRDIS are located in the CPI. The same background is measured for both ND filters, hence it is independent on the ND filter throughput. The background is correlated with the CPI temperature.

The background rate (in ADU/sec) is monitored for the DBI modes only (INS1.OPTI2.NAME != CLEAR).

As can be seen on the health check plot , the background in the H-band is stable with about 0.13 ADU/sec (upper left box) and shows a larger scatter for the K-band at 17 ADU/sec (upper right box).

The lower right box shows the K-band background when there is a neutral density filter in the CPI (large back and blue symbols). In this configuration the measured background follows the temperature of the neutral density filter. The black, blue and green symbols show the mesred background rate, while the red small symbols show the scaled temperature sensor value and demonstrates the correlation. The following scaling is used for the temperature sensor INS4.TEMP449.VAL located in the common path interface: i

T(in the plot) = 1.452 * T - 1.409

The background rate is monitored for the less frequently used modes DPI and LSS, but not for CI mode.

Scoring&thresholds IRDIS background rate

The background rates are scored for the DBI mode H and K bands without neutral density filter.


The background is stable within the known temperature fluctuations since begin of operations.

Algorithm IRDIS background rate

The pipeline returns the median level of the raw frame cube. The QC script divides this value by DIT.

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