NACO: Flat fields and calibration lamps
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HC PLOTS |
fixed pattern noise: |
![](/observing/dfo/quality/ALL/img/sq1.png) |
lamp flux: |
![](/observing/dfo/quality/ALL/img/sq1.png) |
flat counts: |
![](/observing/dfo/quality/ALL/img/sq1.png) |
flat gradient: |
![](/observing/dfo/quality/ALL/img/sq1.png) |
QC1 database (advanced users):
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The LAMP FLAT frames are obtained within the NACO daytime calibrations every
night the science data are taken. Each LAMP FLAT consists of 6 raw frames (3
pairs lampON/lampOFF) obtained with the instrumental setup matching the one
used for science observations. Only the short wavelength filters are supported
(J, H K regime). For the long wavelength (L-band) the "flats on sky" are
taken.
The LAMP FLATs are supplied for both imaging and spectroscopy but only the
imaging lamp flats are processed by the NACO pipeline.
Example master flats are from 2016-11-26
![](../img//NC_ICLF_161028A_DR_H_S13.png) |
LAMP FLAT frame - H, S13, Double_Read
This is an example of the master calibration LAMP FLAT frame taken with filter H and camera S13. The Double_RdRstRd read-out and HighDensity detector modes were used. It is a pipeline product of the "naco_img_lampflat" recipe. The bright spot in the upper left quadrant of the frame is a feature characteristic to the camera S13.
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![](../img/NC_ICLF_161028A_FS_Ks_S27.png) |
LAMP FLAT frame - Ks, S27
This is a master calibration LAMP FLAT frame taken with filter Ks, camera S27. The Fowler sampling read-out detector modes were used. It is a pipeline product of the "naco_img_lampflat" recipe.
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Fixed pattern noise
Normalized master lamp
flats contain the pixel-to-pixel gain variations, (the gain map, or fixed
pattern noise) as an instrumental signature which is removed from the science
observation by dividing the dark subtracted science frame by the master lamp
flat.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[derived from QC procedure] | naco_lampflat..fpn_norm | relative fpn (=fpn/flat_med_max) | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The term noise in 'fixed pattern noise' is misleading,
because FPN is not defined for a single pixel, while
detector read noise and photon noise are defined for a single pixel. In a
well exposed single flat frame and even better in a frame
combined of many flats with a high sign-to-noise ratio,
the signal between different pixels
represent the difference of responses or gains. A robust
rms over a larger area of detector pixels is called the
fixed pattern noise, or the RMS of the gain map. The fixed
pattern noise is sensitive to illumination
gradients, vignetting, persistence
and light contamination. The left trending
plot covers the setup DoubleCorr, HighDensity,
filters J, H, Ks, camera S13 and DIT=15sec, the right
plot covers setups FowlerNsamp HighSensitivity, filter J,
H, Ks, camera S13 and DIT=15sec..
![](/qc/NACO/reports/2015/LAMPFPN/trend_report_LAMPFPN_2015_3.png?57306.8019) |
FP noise values trended in the left box for DoubleCorr read mode in different filters and in the right box for FowlerNsamp read mode.
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Scoring&thresholds Fixed pattern noise
the fixed pattern noise derived from the lamp flats is not scored.
History
After the detmon recipe has been put in operations in 2016,
the FPN derived from lamp flats serves only as a fall back solution.
While the
FPN derived from the detmon recipe covers four read modes, camera=S27, and filter=NB_1.64,
the lamp flat FPN
covers a larger variety of cameras.
Algorithm Fixed pattern noise
The pipeline delivered lam flat QC.FPNOISE parameter is divided by the raw frame counts
to become insensitive to the actual counts in the flat.
Flux of the flat field lamp
The lamp flux, measured in ADU/sec from the difference frames: LAMP=ON minus
LAMP=OFF, is to monitor aging effect of the flat field lamp. The flat field
lamp is used for filters the wavelength bands J, H and K.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.LAMPFLUX | naco_lampflat..lamp_flux | lamp flux in e-/pix/sec | CAL | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The lamp flux is monitored for two read modes, three
broad band filters and two cameras separately.
Scoring&thresholds Flux of the flat field lamp
The lamp flux [ADU/sec] is not scored. The counts [ADU] in the flats are scored instead.
History
Before 2015, another detector was in operation.
Algorithm Flux of the flat field lamp
Median flux of the difference between the first lamp ON and
the first lamp OFF raw frames of the lampflat, divided by DIT; calculated by
the naco_img_lampflat recipe.
Counts in the flat frames
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[derived by a QC procedure] | naco_lampflat..flat_med_max | median of the highest flux raw frame | CAL | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The flat counts are monitored for two read modes, three
broad band filters and two cameras separately.
Scoring&thresholds Counts in the flat frames
The flat counts are scored to obtain the quality of the
calibration product. The lower threshold is set to 3000
ADU, that the master flat, based on three raw frames
with 9000 ADU, corresponds to a signal-to-noise ratio
of 95. Most of the flats show about 4000 ADU resulting
in a SN-ratio of ~100.
According to the detmon results the non-linearity is about
18% at 8000 ADU for DoubleCorr and
14% at 3000 ADU for FowlerNsamp. Operational thresholds
of 6000 ADU and 3000 ADU flags flats with more than ~15%
non-linearity.
History
Before 2015, another detector was in operation.
Algorithm Counts in the flat frames
Median counts of the first raw flat of the sequence.
Illumination gradient
Imperfect alignment of the calibration unit and the calibration path can result in an inhomogeneous
illumination of the detector. The illumination gradient, as a first order deviation from a
balanced illumination for lamp flat fields is parameterized and monitored.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
[derived from QC procedure] | naco_lampflat..xgradient | x gradient: relative counts over 1024 pixel | ENG | | [docuSys coming] |
[derived from QC procedure] | naco_lampflat..ygradient | y gradient: relative counts over 1024 pixel | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
Introduced in 2015 after some issues with the calibration
unit, this parameter measures the large scale gradient
over the field of view in two directions. The gradient is
stable since then within each read mode and optical setup.
Scoring&thresholds Illumination gradient
The gradient of the flat field is scored. The thresholds are set to +-5%.
History
Before 2015, another detector was in operation.
Algorithm Illumination gradient
To measure the illumination gradient over the FOV, four pixel-windows have
been defined on the master lamp flat product, each one 100x100 large, and with
a distance of about 100 pixel from the edge of the chip with centers at
- x=150,y=512 (left)
- x=850,y=512 (right)
- x=512,y=150 (bottom)
- x=512,y=850 (top)
Based on these four windows one flux gradient in x and one in y is calculated.
The flux gradients are normalized to the size of the FOV (Delta x = 1024
pixel). E.g. xgradient = -0.2 means, that the counts are 20% less on the right
rim than on the left rim of the detector. The gradient monitors a symptom, not
a cause. Note that the lamp flat FOV illumination gradient is different
between detector read modes since of a contamination issue.
Gain (from lamp flats)
Before the detmon template was put in operation in 2016, the detector gain
has been derived from lamp flats. Note that the gain derived from the detmon template
via the photon transfer curve is much more precise and unbiased.
QC1_parameters
FITS key |
QC1 database: table, name |
definition |
class* |
HC_plot** |
more docu |
QC.GAIN | naco_lampflat..fpn_norm | relative fpn (=fpn/flat_med_max) | ENG | | [docuSys coming] |
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one. |
Trending
The lamp flat gain is monitored for two read modes.
Scoring&thresholds Gain (from lamp flats)
The gain derived from the lamp flats is not scored.
History
The gain is stable. The two detectors which replaced older ones were/are operated with the same gain values.
Algorithm Gain (from lamp flats)
The pipeline subtracts two raw flats of the same
exposure level to eliminate fixed pattern noise
(gain map contributions). The residual statistical
noise is composed of mainly photon noise and smaller
contributions of detector read noise. The gain is derived
numerically from the counts and the photon noise.
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