TIPS/JIM
November 19, 2009
Agenda:
INS News (Jerry Kriss)
Webb Flux Calibration Status (Karl Gordon)
JWST FGS Guide Star Studies (Sherie Holfeltz)
Breathing and Focus Field Variations for NICMOS (Deepashri Thatte)
Next TIPS/JIM: December 17, 2009
Instruments Division News
11/19/2009
•
•
•
•
•
•
HST news:
o The STUC was here November 12 and 13. They were impressed with our
accomplishments following SM4. They gave strong support to the plan to
maximize science in the 5 years following SM4 as our primary goal.
o Yesterday we hosted a successful SMOV Closure Review for SM4. The
presenters did a marvelous job at staying on time and in focus.
Congratulations to all the HST teams and all the institute staff who made
SMOV successful.
o We’ve had two more SI C&DH lock-ups. Recovery this time was quick--less than 2 days down each time. However, NICMOS is now warming,
and the project is considering doing a purge of the cooling system before
cooling again.
JWST news:
o The Optical Telescope Element (OTE) passed its CDR in October.
Members of the Telescopes Group supported the review.
o The new JWST Advisory Committee, JSTAC, met here for the first time
on Nov. 4 & 5. The experience of the panel showed, and they made
several valuable recommendations to Matt Mountain to assist in JWST
science readiness.
The Future of the Workplace Committee gave a very positive verbal report from their
recent visit. They saw the high morale following SM4 and SMOV, and they see
continued progress on climate and the environment. Written recommendations are
still to come.
Remember: Personal Benefit Elections must be completed by this Friday, November
20.
We are beginning to organize a committee to plan a calibration workshop in the
summer of 2010. Sessions will cover both HST and JWST. Please let me, Danny
Lennon, or your team lead know if you are interested in helping with the
organization.
We are planning our division holiday party for Thursday, December 17. So far I have
several volunteers for helping out. Please let me know if you are interested.
TIPS/JIM
November 19, 2009
Agenda:
INS News (Jerry Kriss)
Webb Flux Calibration Status (Karl Gordon)
JWST FGS Guide Star Studies (Sherie Holfeltz)
Breathing and Focus Field Variations for NICMOS (Deepashri Thatte)
Next TIPS/JIM: December 17, 2009
Webb Flux Calibration Plan Status
Karl D. Gordon & Ralph Bohlin
WIT Team Meeting
STScI
17 Nov 2009
Goals



Enable the highest accuracy possible flux calibration for all Webb
instruments
−
Need a sample of stars with well known fluxes from 0.8-28 microns
−
Nominal requirement is 2% absolute flux accuracy
Push for a uniform Hubble/Spitzer/Webb calibration
−
Allows for comparison of results between telescopes
−
Multi-wavelength astronomy!
Interface with Instrument Team efforts
−
Mainly MIRI & NIRCam
Flux Calibration Basics

Absolute Flux Calibration = knowledge of conversion between DN/sec and
physical units for the range of possible fluxes

Need:


−
Source with known flux at the wavelengths of interest
−
Measure the DN/sec with your instrument
−
Divide the known flux by DN/sec → calibration factor
Sources with known fluxes (reference standards)
−
Referenced to laboratory calibrated black bodies
−
Vega – good for V, K, & 10 micron only
−
MSX 8 micron measurements
Secondary sources
−

Established via relative measurements to above reference standards
Use models to interpolate/extrapolate to wavelengths of interest
JWST Absolute Flux Calibration I.
Proposed Primary Calibrators


Set of 14 stars with high quality Hubble/Spitzer data
−
4 white dwarfs (including 3 primary calibrators for Hubble)
−
6 A-stars (primary Spitzer/IRAC calibrators)
−
4 solar analogs (including NICMOS primary calibrator)
−
Use different types to control systematic uncertainties
How do these stars map to the Webb instrument sensitivities?
−
Maximum flux: saturation in the smallest subarray
−
Minimum flux (imaging): S/N=200 in 1 hour
−
Minimum flux (spectroscopy): S/N=50 in 1 hour (resolution element)

Gordon, Bohlin, Fullerton, Beck, & Robberto

JWST-STScI-001855
Sample
NIRCam

Good coverage for
bright fluxes

Need more
standards at faint
levels to check for
flux nonlinearities
NIRSpec

Good coverage for
bright and faint
fluxes
FGS/TFI

Need more
standards at
bright and faint
levels
MIRI
Imaging

Need more
standards at
bright levels
MIRI
Coronagraphy
&
Spectroscopy

Need more
standards at
bright levels
JWST Absolute Flux Calibration II.
Expanded List of Calibrators



Metric on what is needed per instrument capability
−
How many stars?
−
How many of different types?
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Range of brightnesses?
Add more bright/faint stars
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MIRI George Rieke's work
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NIRCam solar analogs in clusters NICMOS work
−
Suggestions for other targets?
Write report
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Nominal due date is Jul 2010
Webb/Spitzer/Hubble Cross-calibration

Program started by Ralph & Jerry K. for Webb/Hubble
−


Continued in cycle 17 with more STIS observations
Spitzer observations added to complete the set
−
Cycle 5 DDT program
−
Was accepted as part of the regular Spitzer calibration program
Team: Gordon, K. D., Bohlin R., Rieke G., Carey, S., Armus, L., Ardila, D., NoriegaCrespo, A., Deustua, S., Engelbracht, C., Meixner, M., Flanagan, K.

Stated plan is to write a single paper establishing a common
Spitzer/Hubble calibration (Bohlin et al.)
−

Assuming both Spitzer and Hubble calibrations need to be slightly adjusted
Status
−
Spitzer photometry done (some cleanup needed)
−
Hubble/STIS cycle 17 spectroscopy in progress
Prediction with Spitzer Photometry
Predicted/Observed
Summary

“JWST Absolute Flux Calibration I. Proposed Preliminary Calibrators”
−


Gordon, Bohlin et al. (2009, JWST-STScI-001855)
Hubble/Spitzer cross-calibration program
−
Spitzer cycle 5 DDT time + archive data
−
Hubble cycle 17 time + archive data
−
Plan is for a refereed paper (Bohlin, Rieke, Gordon, et al.)
cal.stsci.edu
−
Twiki site for Webb calibration efforts
−
Goal is to have a place for internal and external collaboration
−
WebbFluxCal: Assembling the set of calibration stars (reports I & II)
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AbsLevel: Setting the absolute flux level
−
HubbleSpitzerCal: Hubble/Spitzer cross calibration
−
Others? (astrometry, wavelength, etc.)
MIRI Sensitivities
Predicted
Spectra

Most of sample
Predicted/Observed (sigma)
Calibration Challenges

How does one derive the absolute calibration for an instrument?

How does this differ between photometry and spectroscopy?

How does this differ between point and extended sources?

What is a good absolute calibration goal (5%, 1%, or 0.1%) ?

−
Think science instead of requirements.
−
(But also good to make sure we reach our requirements.)
How many objects is a good number to avoid systematic biases?
−
Think Vega = pole-on rapid rotator with a disk!

What is the difference between calibration and characterization?

How do Hubble and Spitzer differ in their calibration strategies?
−
Hubble = UV/Opt community, Spitzer = IR community

What has already been done for JWST (by instrument)?

JWST will necessarily use indirect calibrators.
Calibration versus Characterization

Calibration is to get the conversion between instrument and
physical (Jy) units

Characterization is the process to make sure this calibration
applies to the range of observations that can be taken
−
Range of fluxes (bright to faint)
−
Different exposure times/readout patterns
−
Subarrays
−
Targets of different colors (blue/red - asteroids, QSOs, etc.)
−
Extended sources
Calibration in the IR




IRAS
−
Stars and asteroids
−
Indirect calibration
−
Stars and asteroids
−
Indirect calibration
−
Large calibration effort = NIST blackbodies in space!
−
Direct calibration
ISO
MSX
Spitzer
−
Stars and asteroids (MIPS 160um only – blue leak)
−
Indirect calibration
Absolute Physical Calibration in the Infrared

Rieke, G. et al. 2008, AJ, 135, 2245

Consistent calibration of A dwarf and solar analogs (1.5%)

Based on direct calibrations in the infrared

−
Ground-based at 2.2 and 10um
−
Space-based at 4-21um (MSX)
−
Used to be done by extrapolating optical direct calibrations
Fluxes quoted in the “Vega” system
−
−


Mythical star with Kurucz 1993 model spectrum of an A0 star (Kurucz 2005) with Teff =
9550, log g = 3.95, log z = -0.5
Normalized to corrected (for debris disk) direct Vega flux measurements
Solar analogs confirm “Vega” results
−
Solar spectrum (0.2-2.5 um) from Thuillier et al. (2003, satellite)
−
Sparser measurements and models for longer wavelengths
−
Solar analog star color scatter checked with 2MASS/IRAC/MIPS
Large sample of A dwarf and solar analog stars measured by IRAC/MIPS
used to generate the zero points for IRAC/MIPS
Sample
Next Steps

Can all the Webb instruments observe the set of primary calibrations
defined by the Webb/Spitzer/Hubble program?

What are the existing instrument team plans for calibration?

Are there supporting observations needed prior to Webb?

Are there modeling efforts needed prior to Webb?

How do we bootstrap the calibration to fainter Webb levels?

And we shouldn't forget
−
Astrometric calibration (Jay)
−
Commissioning (Carl)
Status of Webb Calibration Plans
Karl D. Gordon
Space Telescope
Science Institute
WIT Calibration Kickoff
STScI
17 Dec 2008
TIPS/JIM
November 19, 2009
Agenda:
INS News (Jerry Kriss)
Webb Flux Calibration Status (Karl Gordon)
JWST FGS Guide Star Studies (Sherie Holfeltz)
Breathing and Focus Field Variations for NICMOS (Deepashri Thatte)
Next TIPS/JIM: December 17, 2009
JWST FGS Guide Star Studies
TIPS/JIM
Nov. 19, 2009
Sherie Holfeltz
with
Ed Nelan & Pierre Chayer
JWST Guide Star ID & ACQ
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•
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•
•
The following steps are part of the JWST guide star identification
and acquisition process:
The ground system must provide a local catalog containing the
positions and expected electron count rates, as measured by
the FGS, for the guide star and several reference objects
An Identification image is taken and the observed scene is
pattern-matched to the uplinked catalog
If ID or ACQ fails for one guide star candidate, try the next
guide star candidate and its reference objects. Up to 3 guide
star candidates may be tried
Several studies have been undertaken, touching on many aspects
of the guide star ID process
The Uplinked Local Catalog
•
•
•
JWST will use GSC-II + 2MASS for guide stars
– Deepest all-sky survey available
– Optical catalog (B, R, I λeff= 0.47, 0.64, 0.85 µm)
FGS operates from 0.6 - 5µm
For stars without 2MASS mags, GSC-II mags will be transformed into
NIR
Flux of M0 Star at
J = 18.5
GSC-II pass bands
2MASS pass bands
FGS pass band
Magnitude Transformations
Transformations derived for stars at high galactic latitudes (|b|>40o)
•
cross correlated GSC-II with 2MASS
•
Fit polynomials to color-color diagrams
(B-R)
(B-I)
(R-I)
J
H
K
Testing The Magnitude Transformations
•
•
•
Tested by comparing predicted NIR
mags to observed mags from CFHT
& UKIDSS/LAS
Allowed testing of GSC-II down to
the faint limit of the FGS
Transformations sufficiently accurate
when applied to stars; mean Δmag ~
0 and 1-σ errors < 0.4, except for Ksband predictions when GSC-II lacks
I-band magnitude
Predicted NIR Magnitudes
•
•
GSC-II matched to UKIDSS/LAS
GSC-II optical mags
median B,R,I = 19.5, 18.1, 17.0
faint cutoffs for B,R,I = 22.5, 20.8,
18.5
•
•
GSC-II optical mags → NIR
Predicted compared to observed
magnitudes
•
Predicted:
median J,H,K = 16.4, 15.8, 15.7
42% predicted JAB < 18
Observed:
32% observed JAB < 18
•
Pattern Matching in the ID Image
•
•
FGS identifies the guide star
in its FOV by matching the
ground supplied predicted
scene (red) with the
observed scene (blue).
Challenges include:
– pointing error
– s/c jitter and drift
– cosmic rays
– missing objects
– surprise objects
– mis-classed objects
– catalog contamination
Missing Objects
•
•
~10% of GSC-II objects are artifacts, not real objects
– No match in SDSS or UKIDSS and are not seen in HST images
– Tendency to be detected in only one GSC-II pass band
– Usually near (faint) plate limit
– No way to identify them as artifacts based solely on GSC-II data
Faint blue objects may drop out in NIR
B,R,I = (18.6, 18.1,17.9)
SDSS objects
SDSS objects
B,R,I = (20.5, 18.8, 17.1)
B,R,I = (20.5, 18.8, 17.1)
B,R,I = (21.0,19.1,18.7)
B,R,I = (21.0,19.1,18.7)
B,R,I = (18.6, 18.1,17.9)
I = 18.7
(no B,R photometry)
GSC-2 ID: N52L007908
I-Band Image
R-Band Image
Surprise Objects
•
GSC-II is ~69% complete down to JAB < 19.5 for both stars and nonstars, based on cross correlating with ~400 deg2 of UKIDSS/LAS
catalog at high galactic latitudes. For every two objects predicted to be
in the FGS FOV, there will be one additional surprise object
•
Objects classified as non-stars in GSC-II out number the stars by
2 to 4 times at high galactic latitudes (|b| > 30°)
•
These objects are bright enough to be seen in the guide star ID image
•
If not accounted for in the predicted scene, the FGS may fail to identify
the guide star due to pattern match confusion
GSC-II Non-Stars
This study had two goals:
1.
2.
Characterize the GSC-II objects classified as non-stars to evaluate
their affect on the FGS’s ability to identify the guide star
Evaluate whether or not the nature of the non-stars could be
understood in a meaningful way on an object-by-object basis using
only GSC-II parameters
Characterizing GSC-II Non-Stars
•
•
•
•
•
•
Archival HST (ACS/WFC) images used to study the non-stars
Objects detected in ACS images were matched to GSC-II
catalog
Sizes and shapes from the GSC-II catalog were compared to
those measured from the HST images
GSC-II sizes and shapes were not found to be predictive of nonstar characteristics:
– GSC-II size was strongly correlated with brightness
– Dispersion of GSC-II eccentricity strongly correlated with
faintness
Size and shape distributions similar for stars & non-stars; most
objects of both types have small size measures
Non-stars should be used as reference objects for guide star ID
pattern match
Catalog Errors
•
•
•
•
Contamination, e.g., binary stars
– JWST will use GSC-II at its faint end, where catalog contamination
rate is estimated to be ~12 - 15%
Object type mis-classification
– GSC-II mis-classifies ~25% of its stars as non-stars based on
SDSS and UKIDSS/LAS data cross correlated with GSC-II
– A visual inspection of GSC-II objects matched to HST images (at
high galactic latitudes) estimated that up to 20% of both GSC-II
stars and non-stars are mis-classed
– Mis-classifications more prevalent at the faint end of the catalog,
where JWST will be operating
Catalog artifacts
– ~10% of GSC-II objects are artifacts
Errors in optical-to-NIR magnitude transformations
– Garbage in, garbage out
– Non-preferred transformation methods
Up to 3 Guide Star Candidates?
•
Allowing 3 candidate guide stars (if available), each with an 85%
success rate, yields a combined success rate of 99.7%
•
Will we routinely have 3 candidate guide stars available?
•
Guide star availability studies
– Confirmed previous assumption that the Poisson distribution is a
reasonable approximation to the distribution of stars in GSC-II over
local regions of the sky
– # of GSC-II guide stars per FGS FOV at high galactic latitudes
– # of 2MASS guide stars per FGS FOV at low galactic latitudes
GSC-II Guide Star Availability
at High Galactic Latitudes
•
Virtual FGS FOV scanned over ~170 deg2 of GSC-II at |b| ≥ 45°
•
•
# stars / FGS FOV: min = 0, max = 12, mean = 2.7, median = 3
FGS has two FOVs
2MASS Guide Star Availability
at Low Galactic Latitudes
•
•
•
Virtual FGS FOV scanned over ~103 deg2 of 2MASS
at |b| ≤ 30°
# stars / FGS FOV: min = 0, max = 377, mean = 28.2, median = 15
(preliminary results)
FGS has two FOVs
Mitigating Guide Star Failures
•
•
•
•
•
Allowing up to 3 candidate guide stars improves success rate
Including non-stars as reference objects enhances the
probability of success
Candidate guide stars with I-band photometry should be chosen
preferentially over those lacking I-band photometry
Flight software ID algorithm should be tested against realistic
conditions
Most JWST GSC-II work to date focused on high galactic
latitude fields. Availability and selection of guide stars in other
areas needs to be studied:
– in the disk of the galaxy (2MASS; underway)
– optically opaque star forming regions
– crowded fields
– near bright (V<6) stars that may be targets for coronagraphy
Relevant Reports
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•
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•
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The Areal Density of the 2MASS Catalog at Low Galactic Latitudes
Holfeltz, Nelan, Chayer 2009, in progress
Comparison of GSC2.3 and UKIDSS LAS at High Galactic Latitudes
JWST-STScI-001668, SM-12
Holfeltz, Chayer, Nelan 2009, in review
Characterizing Non-Stars in GSC2.3
JWST-STScI-001641, SM-12
Holfeltz, Chayer, Nelan 2009, in review
The Distribution of Stars in GSC2.3 at High Galactic Latitudes, Part 1
Holfeltz, Chayer, Nelan, 2009
Algorithms for Transforming GSC-II Magnitudes into the NIR
JWST-STScI-001410
Chayer, Nelan, 2008
TIPS/JIM
November 19, 2009
Agenda:
INS News (Jerry Kriss)
Webb Flux Calibration Status (Karl Gordon)
JWST FGS Guide Star Studies (Sherie Holfeltz)
Breathing and Focus Field Variations for NICMOS (Deepashri Thatte)
Next TIPS/JIM: December 17, 2009
Breathing and Focus Field
Variation for NICMOS
Deepashri Thatte
Tommy Wiklind
TIPS November 19, 2009
NICMOS PAM
•Pupil Alignment Mechanism (PAM) for
adjusting focus.
•Adjustable mirror moving ±10mm about
mechanical zero.
•NIC1 and NIC2 share an intermediate
focus. Best focus for NIC3 is currently
beyond the maximum range setting of
the PAM.
NICMOS Instrument Handbook, 2009
•PAM is also used to correct for misalign-ment between HST exit pupil and
NICMOS entrance pupil.
NICMOS Focus Monitoring
• Every two months for NIC1 and NIC2, every six
months for NIC3.
• The optimal focus position has not changed
notably since 2002.
• Focus is monitored in order to detect sudden and
significant deviations from optimal position as
well as long term trends.
Encircled energy method
•
Series of exposures of a
crowded star field at
different PAM settings.
•
The total number of
counts in a fixed aperture
is measured for several
stars at each PAM
position.
•
The PAM position
corresponding to the
maximum count rate is the
best focus.
•
Note:- All the stars are normalized
to the same flux level
Focus History
Factors affecting focus and image quality
• Breathing
• Focus Field Variation
• PAM tilt
Breathing
• Breathing:- Short term
variations in HST focus
on orbital time scale
• Caused by thermal
expansion/contraction of
the HST optical telescope
assembly (OTA) as the
telescope warms up
during its orbital day and
cools down during orbital
night.
• Also depends upon
orientation of the telescope.
HST Thermal Breathing Model
The thermal model of the breathing effect relates the
HST secondary mirror position to temperature variations.
∆SM ~ 0.7 (T – <T>) µm
10 microns of the breathing defocus at the HST
Secondary mirror is equivalent to ~1 mm of PAM
movement.
∆PAM = (1.171)(0.1)(∆SM) mm
( for NIC1)
New PAM = old PAM (given by NPFOCUSP)
+ ∆PAM
Di Nino, D., Makidon, R. B., Lallo, M. et al. ISR ACS 2008-03
Suchkov, A., Bergeron, L., Galas, G. ISR NICMOS 98-004, STScI
Breathing correction
(a) No breathing correction
(b) With breathing correction
Focus Field variation
• The NICMOS foci vary spatially across the
detector's field of view.
• The magnitude of the effect for NIC2 and
NIC3 is up to ~1.5mm in PAM space.
• The focus determined for a star at a given
position on the detector needs to be
corrected for the FFV.
The curvature was determined by
Suchkov & Galas by plotting PAM
positions for best focus as a function
of rows and columns separately.
g(x,y)= a1x + a2y + a3x2 + a4y2 + a5
xy
f(z) = a0 exp (-[z + g(x,y)] –z0)/w)2
zo : PAM position for best focus at (0,0)
w and a0 : width and amplitude
z : PAM in mm
f(z) : Normalized encircled energy
(a) Fits to individual stars in regular
focus monitoring run.
(b) Corrected for focus field variation
(c) Corrected for focus field variation
and breathing.
NIC1 focus history
No correction
FFV only
Breathing
Breathing & FFV
2.19 ±0.31
2.30 ±0.31
2.21± 0.41
2.32 ±0.40
NIC2 focus history
No correction
FFV only
Breathing
Breathing & FFV
0.37 ±0.46
0.35 ±0.45
0.33± 0.55
0.31±0.54
NIC1 and NIC2 PAM
Variations in focus position for NIC1 & NIC2
• Not attributable to breathing, FFV or detector temperature
• NIC1 and NIC2 variations correlated
• Breathing and FFV only changes the average level
• Amplitude of variations is large (a few tenths of mm in
PAM space)
• Time scale is months – year
The cause of these variations remains unexplained but
could be due to annual changes in solar input
Remains to be done: correlate focus variations with aftshroud temperature
Correction to PAM using aft-shroud temperature
SMOV4 focus test
NIC1 PAM 2.43 ± 0.07 mm
NIC2 PAM 0.77 ± 0.06 mm
• Both NIC1 and NIC2 appear to have shifted approximately
+0.6mm relative to the average focus position in the period 2002-2008.
• Consistent with the secondary mirror move of +2.97 micron on July 20
2009.
The SM move corresponds to +0.4mm for both NIC1 and NIC2.
Old values
PAM mm
New values
PAM mm
Best Focus for NIC1
1.8
2.3
Best Focus for NIC2
0.2
0.7
SMOV4 PAM tilt results
X-coma & y-coma were measured for all tilt
positions.
Total coma = (x2coma + y2coma)1/2
Acknowledgements
Matt Lallo
Pey-Lian Lim