Technical Instrument Report NICMOS 98-001
NICMOS/NCS EMI Test Data Results
Louis E. Bergeron
November 4, 1998
ABSTRACT
In an attempt to better understand how the cryo-cooler induced noise (Schneider, 1998)
would affect future NICMOS calibration, I repeated Glenn’s analysis and looked at a few
additional properties of the test data.
Summary
The observed induced frequencies, including the 60 Hz line noise and the 5-9 kHz
induced signal (with the cooler running, at various turbine speeds) were recovered in the
power spectra as expected. The maximum amplitude of the 5-9 kHz signal is ~7 DN PtoP,
and has an RMS of ~5 DN. This signature is seen in each readout of a MULTIACCUM
sequence, and the pattern and its amplitude remains in a fixed position on the array
through all the reads of a given MULTIACCUM. By performing a 0th read subtraction,
which is the standard procedure in the NICMOS calibration pipeline, the induced signal,
including the 60 Hz line noise and 5-9 kHz noise, is COMPLETELY removed from all
subsequent reads, in both the NCS-ON and NCS-OFF configurations. The pattern in the
image does shift between MULTACCUM sets, but not within each MULTACCUM. Power
spectra of 0th read corrected data confirm that all induced power is removed completely.
It appears that this signal is either imprinted into the array during the array reset
sequence, making it simply a part of the fixed BIAS of all the subsequent non-destructive
array reads, or it is phase-locked to all the reads of the sequence. Given that the 60 Hz
noise is seen to have this behavior even in the NCS-OFF data, the entire induced signal
may just be an artifact of the test environment and may not reflect the true NICMOS/NCS
signature at all.
If this signal did not subtract out with a 0th read correction it would amount to a ~25%
increase in the characteristic on-orbit per-readout noise, given a 35 e- readnoise and a gain
of 5.4 e-/DN:
sqrt(35^2 + (5*5.4)^2) / 35 ~ 1.25
1
Short Analysis Notes
To reproduce Glenn’s power spectra, I repeated his calibration steps exactly. I built a
median “dark” sequence using all the NCS-OFF MULTIACCUM sequences, taking the
median-per-readout, without doing a 0th read subtraction. This is necessary to remove the
shading and amplifier glow signatures from the data before measuring the power spectra.
Subtraction of this dark leaves the quad’s DC levels unbalanced, but quite flat. This is due
in large part to the kTC noise which determines this DC level after each reset (one of the
main reasons for doing a 0th read subtraction is to remove this). Then, the quads were balanced to each other by subtracting their medians. This is not strictly necessary, as each
quad is read-out independently, and a DC does not change the measured power spectrum
in any way.
I then made power spectra for some of the NCS-OFF data, as well as NCS-ON data at
various turbine speeds, and did the same after doing a 0th read subtraction. Power spectra
were measured per-quad by taking the FFT of the sequentially clocked horizontal row,
with the addition of 2 pixels at the end of each 128-pixel row as placeholders for the 21
microsecond vertical line address increment. The power spectra from the 4 quads were
then averaged together to increase the S/N. Once it was found that the pattern was stable
through all the readouts, I medianed all 26 readouts together and made the power spectrum
from the median image of each MULTIACCUM.
To produce images and plots of the induced signals, I applied narrow band-pass filters
to the power spectra, removing the 1/f power contribution in the band, and then taking the
inverse transform. The mean 1/f level was estimated by eye in each band.
Reference
Schneider, Glenn, NICMOS Project, “EMI Noise Properties of the NICMOS Cooling
System as Seen by a NICMOS-3 Flight Spare Detector (or, Turning NICMOS into a
Spectrum Analyzer)”, May 1, 1998
Figures
(all figures and an additional mpeg of the noise are temporarily available on the web
at: ftp://ftp.stsci.edu/outside-access/out.going/eddie/emi/ )
2
60 Hz Signal, zoomed
40
20
20
DN
DN
60 Hz Signal (stdev = 2.25 DN)
40
0
-20
0
-20
-40
0
5.0•103
1.0•104
Sequential Pixel Number
-40
1.5•104
3500
40
20
20
0
-20
0
-20
-40
0
5.0•103
1.0•104
Sequential Pixel Number
-40
1.5•104
3500
40
20
20
0
-20
5000
0
-20
-40
0
5.0•103
1.0•104
Sequential Pixel Number
-40
1.5•104
3500
Actual EMI Test Data (stdev = 12.63 DN)
4000
4500
Sequential Pixel Number
5000
Actual EMI Test Data, zoomed
40
40
20
20
DN
DN
4000
4500
Sequential Pixel Number
Sum of 60 Hz and 5-9 kHz Signal, zoomed
40
DN
DN
Sum of 60 Hz and 5-9 kHz Signal (stdev = 5.43 DN)
bergeron Wed Nov 4 11:45:30 1998
5000
5-9 kHz Signal, zoomed
40
DN
DN
5-9 kHz Signal (stdev = 4.94 DN)
4000
4500
Sequential Pixel Number
0
-20
0
-20
-40
0
5.0•103
1.0•104
Sequential Pixel Number
-40
1.5•104
3500
3
4000
4500
Sequential Pixel Number
5000
bergeron Fri Nov 6 14:01:47 1998
4
Positive Frequency Power
1.2
NCS-OFF
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
Frequency (Hz)
10000
NCS-ON, Turbine Speed = 4500 Hz
1.2
1.0
0.6
5
Positive Frequency Power
0.8
0.4
bergeron Fri Nov 6 14:01:44 1998
0.2
0.0
10
100
1000
Frequency (Hz)
10000
Turbine Speed = 4500 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 4500 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 4800 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 4800 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 4800 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 5000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 5000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 5000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 6000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
10000
Turbine Speed = 6000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Positive Freqencey Power
bergeron Fri Nov 6 11:03:45 1998
10
100
1000
10000
Turbine Speed = 6000 Hz
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
100
1000
6
Hz
10000
Turbine Speed = 4500 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
7000
8000
9000
Turbine Speed = 4500 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 4800 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 4800 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 4800 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 5000 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 5000 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 5000 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 6000 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 6000 Hz
Positive Freqencey Power
bergeron Fri Nov 6 11:01:17 1998
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
Turbine Speed = 6000 Hz
0.700
0.525
0.350
0.175
0.000
4000
5000
6000
7
Hz
Mean of the power spectra of the 4 quads (NCS-ON, single readout)
Positive Freq Power
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
10
100
1000
10000
Each quad’s power spectrum minus the mean spectrum, same scale as above
Positive Freq Power
1.0
0.8
0.6
0.4
0.2
bergeron Fri Nov 6 10:11:12 1998
0.0
-0.2
10
100
1000
Hz
10000
8
Hz
Turbine Speed = 4500 Hz, single readout
1.2
1.0
0.8
0.6
0.2
0.0
10
100
1000
10000
Turbine Speed = 4500 Hz, first difference image (0th read subtracted)
1.2
1.0
0.8
0.6
0.4
bergeron Fri Nov 6 11:43:19 1998
0.2
0.0
10
100
1000
Hz
10000
9
Positive Frequency Power
0.4
10
11
The upper figure shows that the quads, although
measuring the same induced signal, measure it phaseshifted slightly because the quads are not read out perfectly simultaneously. By stepping through a set of lag
times between quads and then measuring the power in
the difference image, one can iterate to determine what
the phase lags are, and thus the relative quad to quad
readout timings. The sensitivity is at the sub-microsecond level thanks to the high spatial frequency of the
induced signal.
Q4 Q3
Q4 = 18.9000
30
Q3 = 17.3250
Q2 = 1.05000
Total power in difference image, 6-9 kHz band
Phase lag from Q1 to the other 3 quads
Q2 Q1
20
10
bergeron Tue Nov 10 17:39:31 1998
0
0
5
10
15
Delay in microseconds
20
25
12
Note that there is a 16 microsecond gap between
when the readout of the lower half of the array is started
and when the top is started. This does not affect the
pixel to pixel timings, its simply the way the quadrant
timings work apparently. There is a 1 microsecond gap
between the left and right-hand quads of a given half.