TIPS-JIM Meeting
19 January 2006, 10am, Auditorium
1.
JWST Overview
Mark Clampin
2.
Peter McCullough
3.
Cerenkov radiation contribution to
the straylight in NIRcam
Removing SAA-Persistent Cosmic
Anton Koekemoer
4.
Rays from NICMOS
Introducing the INS Master Schedule
Wayne Baggett
Next TIPS Meeting will be held on 16 February 2006.
JWST Overview Presentation
is not currently available on-line
TIPS-JIM Meeting
19 January 2006, 10am, Auditorium
1.
JWST Overview
Mark Clampin
2.
Peter McCullough
3.
Cerenkov radiation contribution to
the straylight in NIRcam
Removing SAA-Persistent Cosmic
Anton Koekemoer
4.
Rays from NICMOS
Introducing the INS Master Schedule
Wayne Baggett
Next TIPS Meeting will be held on 16 February 2006.
Cerenkov Radiation
contribution to Stray Light in
NIRCam
Peter McCullough
19 Jan 2006
STScI TIPS
History
Cerenkov radiation was discovered by 30-year old Pavel Cerenkov in
1934 in the USSR. Cerenkov observed the emission of blue light
from a bottle of water subjected to gamma rays*. His parents, Aleksei
and Mariya Cerenkov, were peasants. He received the Nobel prize in
1958. He died in 1990.
*Gamma rays accelerate electrons via the
Compton effect. The relativistic electrons then
create the Cerenkov effect.
nobelprize.org and
Google image search
Some experiences with Cerenkov light
Fly’s eye (George Rieke): detected Cerenkov light in
atmosphere
NIRCam pre-Phase A: Cerenkov glow from lenses not a
problem
HST FOS: stray light
HST STIS near-UV channel: stray light
HST STIS opto-isolators: miscommunication
NIRCam Shortwave Imaging Path
Physics
The number of photons per nm of wavelength per cm of
pathlength of the relativistic particle is
0.46E6 Z2 λ−2 (1-(β n)-2)
Z=1 for the most common cosmic rays (protons)
λ is the wavelength in nm,
β = v/c,
n=1.5 for the glasses within NIRCam.
The coefficient 0.46E6 equals 2 π α times 1E7, the number
of nm per cm, with the fine structure constant α = 1/137.
The quantity (1-(β n)-2) ~ 0.5 for v = 0.9c and n=1.5.
nobelprize.org
Cone of light
Physics applied:
The integrated Cerenkov photon flux within a bandpass filter is
proportional to λ−1 for a filter of a given resolution, such as R=4.
The integral of all Cerenkov photons longer than a given λ (here
in microns) is approximately equal to
230/λ photons per proton per cm of pathlength.
To be both simple and conservative, we assume that every
cosmic ray induces Cerenkov radiation, and the flux equals 5
protons/cm2/s inside NIRCam’s optics located in L2 orbit (cf.
0.25 protons/cm2/s at HST’s orbit outside SAA).
Including Helium cosmic rays will increase Cerenkov
luminosities by no more than a factor of 2.
NIRCam Imaging Triplet
Imaging triplet:
NIRCam’s imaging triplet glows at λ > 0.5 microns with a
luminosity of
L = π/4 * (7 cm)2 * 4 cm * 5 ions/cm2/s * 460 photons/ion/cm
L = 350,000 photons per second.
If they escape isotropically, then the flux at the detector is 400
photons/s/SCA area, or ~1% of the maximum dark current or
~1% of the zodiacal light thru an R=100 filter.
NIRCam FPA Fold Mirror
Luminosity may not be isotropic:
In a special case of a long bar or cylinder, nearly all of
the Cerenkov radiation can be made to escape its ends
due to either mirror coating or total internal reflection.
The latter principle is exploited by Mack (2002) as
shown below:
Cosmic Ray path
Glass
Cerenkov photons
Detectors
Final Fold mirror:
NIRCam’s SW final fold mirror, if it were not light-weighted,
would glow at λ > 0.5 microns with a luminosity of
L = (10 cm)2 * 2 cm * 5 ions/cm2/s * 460 photons/ion/cm
L = 460,000 photons per second,
which is comparable to the zodiacal light collected by one
SCA through a F200W (R=4) filter.
However the mirror is light-weighted and it appears to be
contained within its mount such that glow from its back or
sides cannot reach the detectors.
Other luminescence not addressed here!
“Hence we were inclined to think that this light produced by
the gamma rays was one of the many luminescence
phenomena. Pierre and Marie Curie thought so and they
were incontestably among the first to observe this kind of
light, at any rate under conditions where it was fairly heavily
masked by the ordinary luminescence.” (Cerenkov’s Nobel
lecture; P.R.M.’s emphasis)
Summary
The contribution to stray light in NIRCam due
to Cerenkov radiation is …

negligible from lenses.

(in principle) significant from the final fold
mirrors’ substrates but (in practice) can be
blocked.

Caveat: this author cannot adequately judge
the effectiveness of that blocking from
drawings available to him.

Therefore: designers should be made aware
of the need to contain the Cerenkov glow
emitted by transparent materials within
NIRCam and other instruments on JWST.
See McCullough’s Technical Report, JWST-STScI-000558, and/or the following:
TIPS-JIM Meeting
19 January 2006, 10am, Auditorium
1.
JWST Overview
Mark Clampin
2.
Peter McCullough
3.
Cerenkov radiation contribution to
the straylight in NIRcam
Removing SAA-Persistent Cosmic
Anton Koekemoer
4.
Rays from NICMOS
Introducing the INS Master Schedule
Wayne Baggett
Next TIPS Meeting will be held on 16 February 2006.
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Removing SAA-Persistent Cosmic Ray Flux
from NICMOS
Anton Koekemoer (INS)
with Elizabeth Barker, Vicki Laidler, Eddie Bergeron
Overview of SAA persistence
Algorithm for removing SAA persistent flux
Implementation & testing of “saaclean” task
Future plans
20
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
21
SAA Persistence
Overview of Persistence:




NICMOS HgCdTe/CdTe bulk material contains small flaws that trap
electrons produced by detected photons
The trapped electrons are not read out together with all the other electrons
detected in a given pixel
Thermal excitation causes
(Bergeron & Dickinson, NICMOS-ISR-2003-10)
traps to release the electrons
on longer timescales with a
logarithmic decay
Resulting flux appears in
subsequent exposures, with
a distribution that depends
on the original intensity and
the population of traps
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
22
South Atlantic Anomaly (SAA):




Region in Earth’s magnetosphere where Van Allen belts dip closest to the
surface, resulting from misalignment in Earth’s magnetic axis
HST passes through it ~50% of the time, 8-9 orbits/day
Although NICMOS is powered off, cosmic ray electrons are still trapped
Cosmic ray rate is so high that persistence can increase total noise by 4-5x
in subsequent exposures obtained long after the SAA passage
(O. Lupie, WFC3-ISR-2002-01)
(Bergeron & Dickinson, NICMOS-ISR-2003-10)
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
23
Removing SAA Persistent Flux
Post-SAA darks:

After each SAA passage when NICMOS transitions from SAAOPER to
OPERATE, a pair of dark exposures is obtained:
– ACCUM mode, NREAD=25, EXPTIME=256s
– Darks start 174s and 444s after exiting the SAA


Calibrate the darks to remove other instrumental signatures (pedestal etc)
Create average post-SAA dark image:
– scale the 2nd dark to the level of the 1st dark, using the decay time constant
– average the 2 darks to remove CRs accumulated during the dark exposures
(Bergeron & Dickinson, NICMOS-ISR-2003-10)
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
24
Subtracting the SAA model:



The average of the 2 post-SAA darks is the SAA model, representing all the
CRs accumulated during SAA passage
Scale and subtract from the calibrated (pedestal-corrected) science image
Scale factor is determined iteratively:
–
–
–
–
multiply SAA model image by small scale factors
subtract from science image
measure the resulting noise (FWHM of Gaussian fit to pixel histogram)
Plot noise as a function of scale factor, fit parabola to determine optimal scale
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
25
Fitting for High and Low pixels:




Some CRs during SAA have higher energy and/or occur later than others
Need to fit different scale factors, depending on the level of the residual signal
In practice, divide the pixel histogram into 2 regimes: low and high
Determine separate scale factors, and only apply a correction if the reduction in noise is
> 1%
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
(Barker & Koekemoer, 2005 HSTAnton
Cal Workshop
Koekemoer (INS)
26
Example Results
Before SAAclean
Post-SAA Dark
After SAAclean
SAA CR Persistence model
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
27
Implementation of SAAclean task
Original script:

IDL, provided by E.Bergeron
New version:




Python code, written by Vicki Laidler, following original IDL prototype
Installed as Pyraf/STSDAS hst_calib.nicmos.saaclean
Some slight differences in algorithms (mostly related to IDL fitting
functions that are unavailable or different in Pyraf)
Inputs:
– calibrated exposure (optionally corrected for pedestal effect)
– Post-SAA darks (read from the image header)

Output:
–
–
–
–
calibrated exposure with the SAA model subtracted
SAA model (in off-line version)
Lots of output text containing information about the fitting process
Fitting parameters also stored in header keywords
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
28
Testing Procedures
Compare IDL and Pyraf versions:


run on the full variety of datasets, ranging from extreme SAA events to
datasets that are not impacted
resulting model files and output images compared, examining:
– noise in the images
– individual pixels
– dependence on quality of bad pixel files

Results:
– some differences between IDL and Pyraf; generally only in treatment of bad pixels,
while SAA-impacted pixels showed the same behaviour
Full test of the Pyraf version:


Ran on all SAA-impacted NIC1,2,3 exposures ever obtained (>6,000)
Examined images, as well as the output values of:
–
–
–
–
chi-sq
nhigh/nlow pixels and the threshold value
scale correction factor
noise reduction values
STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
29
Future Plans
Off-line version:



Prototype version was released publicly in STSDAS 3.4 (Nov 1, 2005)
Available for use within STScI and in the outside community; feedback
solicited
Finalize addressing minor issues turned up in very extreme datasets (very
low or very high chi-sq, or low/high pixel regimes not well modelled):
– appears that all of these can be fixed by using the correct bad pixel files


Add some additional parameters to improve flexibility of iterations
Next release likely in Feb/Mar 2006
Pipeline version:





Aim to incorporate all improvements in off-line version by Feb/Mar 2006
Likely release to OPUS in Apr/May 2006
Initially only run on data that is within 1 orbit of SAA passage
Correction only applied if noise reduction is >1%
Continue to solicit feedback from community
TIPS-JIM Meeting
19 January 2006, 10am, Auditorium
1.
JWST Overview
Mark Clampin
2.
Peter McCullough
3.
Cerenkov radiation contribution to
the straylight in NIRcam
Removing SAA-Persistent Cosmic
Anton Koekemoer
4.
Rays from NICMOS
Introducing the INS Master Schedule
Wayne Baggett
Next TIPS Meeting will be held on 16 February 2006.
Introducing the INS
Master Schedule
Wayne Baggett
What Is It?
• The INS Master Schedule
– Captures “project-level” work led by INS
– Includes work by other divisions
– Captures HST observing-cycle activities
• Does not capture every task being done
within INS
– Routine SI calibrations
– Help Desk support
What Will It Be Used For?
• The INS Master Schedule will be used for
– Tracking progress of the work
– Identifying schedule conflicts
– Identifying resource issues
– Assessing resource allocations
– Assessing impact of new work requests
How Was It Produced?
• INS SI Teams and other groups identified
work items
• Work Item Data Sheets written for each
item
– Describes work
– Identifies INS Lead
– Provides priority
– Identifies deliverables
How Was It Produced?
HST INS Work Item Data Sheet
1. SI/Title: ACS/ASCS Study
2. INS Lead: M. Sirianni
3. Description of Work:
Study temperature-dependencies of CTE, QE,
dark current, bad pixel behavior to see if the
ASCS is needed for the best operation of the
ACS. Write one or more ISRs as the formal
report of the study. Calibration data to support
this study will be obtained in November 2005.
4. Schedule Constraints and Dependencies:
An ASCS decision is needed from the HST
Project by early Spring 2006.
5. Risks and Open Issues:
Risk – If the calibration observations do not
correctly complete in November 2005, then the
study will be delayed. Mitigation: repeat any
failed observations in the Spring.
6. Priority: High
7. Priority Justification:
This study could have an impact on the
SM4 manifest.
8. Resources (including estimated calendar
duration for each portion):
a. Requirements
ACS Instrument Scientist (0.21 FTE)
b. Development
ACS Commanding Developer
c. Testing
ACS Commanding Developer
Instrument Engineer
9. Documentation and Deliverables:
Calibration ISR
How Was It Produced?
• Discussed work items with INS Division
Office
• Assigned work items to general time
periods
• Drafted the detailed near-term schedule
• Work(ing) with other divisions to finalize this
schedule
– “If you’re part of the plan, you’re part of the
planning.”
Current Status
• Draft Schedule exists and is starting to be
used for tracking work progress
– ACS!
– STIS"
– COS"
– Cal Planning"
– NICMOS
– User Support
• INS Master Schedule will be a “living”
document
– Updated regularly to reflect progress
– Updated as needed to capture changes to
existing work
– Updated as needed to capture new work
Current Status
• Some Work Items still need to be defined
• Schedule is still in draft form
– Some items are missing
– Discussion with other divisions needs to be
completed
Sample of Schedule
Where to Find It
• Web Site
– http://www.stsci.edu/instruments/ins_mstr_sched/index.html
– Contains
• Link to INS Master Schedule (PDF)
• Instrument and other group pages with Work Item
Data Sheets
– Maintained by W. Baggett
TIPS-JIM Meeting
19 January 2006, 10am, Auditorium
1.
JWST Overview
Mark Clampin
2.
Peter McCullough
3.
Cerenkov radiation contribution to
the straylight in NIRcam
Removing SAA-Persistent Cosmic
Anton Koekemoer
4.
Rays from NICMOS
Introducing the INS Master Schedule
Wayne Baggett
Next TIPS Meeting will be held on 16 February 2006.