TIPS/JIM
June 17, 2010
Agenda:
INS Division News (Jerry Kriss)
Status of Testing the NIRSpec Detector Subsystem (Mike Regan)
1-D Flat Fields for COS G130M and G160M (Tom Ake)
STIS CCD: Housing Temperature, Darks, and Charge Transfer
Inefficiency (Michael Wolfe)
Next TIPS/JIM: July 15, 2010
Instruments Division News
06/17/2010
•
Welcome to Paule Sonnentrucker, who recently joined the COS+STIS team as an
ESA/AURA Astronomer.
•
HST news:
o Cycle 18 results were announced last week. We hope to get Brett Blacker
in for a TIPS presentation next month. The proposal statistics are
insteresting: half of the awarded time is for spectroscopy, and half or that
is for grism and prism observations with WFC3/IR and ACS.
o Due to low demand, and to save on resources, there will not be any effort
to restart NICMOS for Cycle 18. We’ll continue to hold it in reserve for
future needs. Thanks to everyone who worked on the numerous restart
attmpts so far. Your efforts have not been in vain since we do have a much
better understanding of how to go about this if needed in the future.
o Bright Object Protection training for Contact Scientists is being held
today, at 2 p.m., in N420.
•
JWST News:
o As Matt Mountain mentioned last Thursday, reviews of budget, schedule
and launch date will continue this summer.
o All the instruments continue in their various stages of engineering and
flight model integration and test. Today you’ll hear about the NIRSpec
Detector Sub-System from Mike Regan.
•
Housing: Our housing outlook is still uncertain, as you heard from Matt last
Thursday. Whatever develops, we will still be squeezed fro 6-9 months from now
until the early part of next year. For at least part of this time, we will need to double
up research staff. I’ve had one WIT-team volunteer so far to pair up with a new
incoming staff member. This could be a good way to help acclimatize the new staff
and help with training. Other possibilities people have suggested is for staff to pair up
with postdocs or RIAs with whom they share a science interest or with whom they
work closely. I heartily invite more volunteers to get in touch with me.
•
I hope you noticed the posting of an ad for the new INS Division Head, to replace me
when I go on sabbatical this fall. Applications are due to HR by July 9.
•
Our monthly INS Division lunch will be next Thursday, June 24, in the boardroom.
•
Next TIPS is July 15, 2010.
TIPS/JIM
June 17, 2010
Agenda:
INS Division News (Jerry Kriss)
Status of Testing the NIRSpec Detector Subsystem (Mike Regan)
1-D Flat Fields for COS G130M and G160M (Tom Ake)
STIS CCD: Housing Temperature, Darks, and Charge Transfer
Inefficiency (Michael Wolfe)
Next TIPS/JIM: July 15, 2010
JWST NIRSpec Detectors
Mike Regan
Eddie Bergeron
The Good, the Bad, and the Ugly
The Ugly
The bad pixels for the two NIRSpec
detectors.
491 has 6.1% bad pixels
492 has 3.1% bad pixels
The upper left corner of 491 shows a very high
density of bad pixels.
What about WFC3?
WFC3 has 2.5% bad pixels
The amplifiers for each strip are reset on
every frame.
The large amplifier jumps lead to a high total
noise when we don’t use reference pixels.
We can reduce the noise by not resetting the
amps but it still doesn’t meet requirements
Teledyne testing (non-ASIC) showed even more
amplifier drift.
The Bad
The devices do
not meet the total
noise requirement.
The Detectors do not meet the QE
requirement.
More fine scale structure appeared in the flat field after the Teledyne testing.
The Good
Even in a 4000 second exposure we are not dark current limited.
The S/N ratio improves at a more at more than the sqrt(time) to 4000 seconds.
Onboard averaging only has a small
effect on the S/N
Only Poisson limited observations receive a significant benefit from not
averaging.
Summary
There are a lot of bad pixels
Summary
There are a lot of bad pixels
The combination of high total noise and low QE
leads to a 10-25% loss of sensitivity.
Summary
There are a lot of bad pixels
The combination of high total noise and low QE
leads to a 10-25% loss of sensitivity.
There is a lot of small scale structure in the flat
field.
Summary
There are a lot of bad pixels
The combination of high total noise and low QE
leads to a 10-25% loss of sensitivity.
There is a lot of small scale structure in the flat
field.
Long exposures can reduce the total noise.
TIPS/JIM
June 17, 2010
Agenda:
INS Division News (Jerry Kriss)
Status of Testing the NIRSpec Detector Subsystem (Mike Regan)
1-D Flat Fields for COS G130M and G160M (Tom Ake)
STIS CCD: Housing Temperature, Darks, and Charge Transfer
Inefficiency (Michael Wolfe)
Next TIPS/JIM: July 15, 2010
1-D Flat Fields for COS G130M and G160M
Tom Ake
TIPS
17 June 2010
Status of COS FUV Flat Fields
•
•
•
•
•
SMOV Program and Results
CALCOS Processing
Generation of 1-D Flats Through Spectral Iteration
1-D Flat Field Evaluation and Achievable S/N
Caveats and Plans
SMOV Program and Results
•  SMOV 11491 mapped science region of
detector with WD0320-539
•  5 cross-dispersion positions with G130M,
and 2 each with G160M and G140L
•  Different cenwaves and FP-POS settings
helped separate spectral and detector features
•  2-D flats were made for each grating and
segment
•  Flats removed prominent dips due to grid
wire shadowing, but induced some structure
due to low S/N
•  1-D correction is somewhat better than 2-D
CALCOS Processing
•
CALCOS was designed to apply a 2-D flat field prior to spectral extraction. A
unity flat is currently implemented in the pipeline.
•
Grid wire shadows are the largest FPN features (20% deep, every 840 pixels)
•  When CALCOS coadded different
FP-POS exposures into an X1DSUM
spectrum, features were reduced in
depth, but appeared in more places
•  For 4 FP-POS steps, 30% of pixels
were affected by grid wires
•  Until we have a flat field, changed
CALCOS SDQFLAG keyword to ignore
grid wires when creating X1DSUM
spectra
Generation of 1-D Flats
•
Spectral iteration of X1D extracted spectra used to create 1-D flats
–
–
–
–
•
Technique had been developed for GHRS
Requires data taken at different grating settings (cenwave and/or FP-POS position)
Iterate between wavelength and pixel space in merging and correcting data sets
Solves simultaneously for the stellar spectrum and underlying fixed pattern noise
Each grating processed separately since spectra fall at different cross-dispersion
locations
PID
11491
11494
11897
11491
11494
11897
Program
SMOV Flat Field
SMOV High S/N
C17 Sensitivity
SMOV Flat Field
SMOV High S/N
C17 Sensitivity
Target
WD0320-539
WD0947+857
WD0947+857
WD0320-539
WD1057+719
WD1057+719
Grating
G130M
G130M
G130M
G160M
G160M
G160M
Cenwave
1291,1309
1309
1291,1309,1327
1600
1600
1577,1589,1600,
1611,1623
FP-POS
1,3
1, 2, 3, 4
3
1, 2, 3, 4
1, 2, 3, 4
3
Spectral Iteration Example - WD
•  Internal calibration system consists of two deuterium lamps
illuminating a flat field calibration aperture (FCA)
–  Light takes nearly the same optical path as an external target
–  Only the science areas of the detectors are illuminated, not the wavelength
calibration region
–  FCA (X=1750 µm, Y= 750 µm) is larger than the PSA (700 µm diameter)
–  Aperture mechanism moves in both dispersion and cross-dispersion
directions
•  External flat field calibration exposures were taken through
the PSA during thermal vacuum tests in 2003 and 2006
–  Preserved internal lamp
–  Allowed characterization of illumination angle dependence between PSA
and FCA
Spectral Iteration Example - Busy Spectrum
Final 1-D Flat Fields - G130M
Final 1-D Flat Fields - G160M
Flat Field Evaluation
Consistency check performed by dividing final 1-D flat into each contributing flat
•  Grid wire shadows are nicely corrected
•  Detector dead spots leave residuals since spectra were taken at different Y position. These
regions were never expected to be correctable and are flagged by CALCOS
•  Long wavelength end of segment A
(X>11000) shows either misalignment
of flats or low S/N effects
G130M Segment Flat Field Statistics
Segment A
With Grid Wires No Grid Wires
Data Set
Total
11897
11494
11491
FLAT
0.050
0.062
0.072
0.050
RESID
0.034
0.046
0.016
FLAT
0.036
0.052
0.064
0.037
RESID
0.034
0.046
0.016
Segment B
With Grid Wires No Grid Wires
Data Set
Total
11897
11494
11491
FLAT
0.052
0.060
0.063
0.052
RESID
0.027
0.033
0.014
FLAT
0.035
0.046
0.051
0.035
RESID
0.027
0.032
0.013
Signal-to-Noise Achieved
•
Distribution of P-flat variations give maximum S/N
without a flat field
–  Histogram of variations in each NUV stripe fit with
Gaussian profile
–  Widths indicate S/N (= 1/σ) ~ 50 per resel can be
obtained without a flat
•  Maximum S/N for single
grating setting (~14 per pixel)
reached at ~700 counts/pixel
•  Current CALCOS
X1DSUM ignoring grid wires
improves global S/N by
smoothing FPN
•
Flat fielding increases S/N
close to Poisson noise for
single exposures. With 4 FPPOS steps, S/N=45 per pixel
possible.
•  Caveat - using same data to evaluate as what went into the flats, although
different targets, various cenwaves, and multiple FP-POS steps were averaged
Conclusions and Plans
•  1-D flats show promise. Need to check against more data
•  Current flats cannot be used to correct old data since the flux
calibration was created without flat fielding
•  Need to investigate why long wavelength side of segment A is so
noisy
•  G140L still to be studied. Criteria for iteration convergence may
need revision since spectrum covers only part of the detector
segments
TIPS/JIM
June 17, 2010
Agenda:
INS Division News (Jerry Kriss)
Status of Testing the NIRSpec Detector Subsystem (Mike Regan)
1-D Flat Fields for COS G130M and G160M (Tom Ake)
STIS CCD: Housing Temperature, Darks, and Charge Transfer
Inefficiency (Michael Wolfe)
Next TIPS/JIM: July 15, 2010
STIS CCD: Housing Temperature, Darks, and
Charge Transfer Inefficiency
Michael A. Wolfe
June 17, 2010
Co-Investigators: W. V. Dixon, P. Goudfrooij, T.
Wheeler
Topics
  Housing Temperature
  Scaling Relation for Darks
 Charge Transfer Inefficiency Extended Pixel Edge Response
Housing Temperature vs. Time Plot
Scaling Relation
Scaling Equation: (dark image)*(1 + slope*(TO - T))
Dark Rate Ratio vs. Temperature Plot Fractional Change in Dark Rate (Old Data) Fractional Change in Dark Rate (New Data)
Histogram of Dark Rate Dark Normalization
•
•
•
•
•
New slope is 0.056/oC ± 0.004/oC
New reference temperature is 22.0 oC
Peak of histogram is -1.8 e-/s/pix
Standard deviation is 0.4565 e-/s/pix
Range of dark current is -2.2865 e-/s/pix to
-1.3136 e-/s/pix
Charge Transfer Inefficiency
•  Extended Pixel Edge Response Test at ≈ 12056.6 e- signal level (Janesick et al. 1991)
Reference
Janesick, J. R., Soli, G., Eliot, T., & Collins, S.,
1991, “The Effects of Proton Damage
on Charge-Coupled Devices”, in Proc.
SPIE, 1447, 87
Extended Pixel Edge Response Test Plot
Temperature/CTI Parameters
 Operating Temperature -83 °C
 Clocking Times
Parallel: 23.2 ms
Serial: 22 μs  E center trap, P-V complex
  -90 °C to -60 °C, 100 μs (parallel)
  Largest slope for STIS is at -80 °C
  -40 °C (serial)
CTI Dependence on Temperature Plot CTI Dependence on Time Plot Future Work
•  Investigate the seeming dependence of CTI on
temperature.
•  Investigate the “settling” of the CCD.
…. just to name a couple.