TIPS Meeting
15 May 2003, 10am, Auditorium
1. FGS Redesign Study; Motivation
for 95% success
Ed Nelan
2. New Apertures and CTE
Corrections for STIS
Paul Goudfrooij
3. Non-Standard Gains for NICMOS
Daniela Calzetti
Next TIPS Meeting will be held on 19 June 2003.
Motivation for a 95% Guide Star Acquisition
Probability with JWST
Ed Nelan
With help from:
Jerry Kriss
Wayne Kinzel
Peter Stockman
JWST
95% Guide Star Acquisition Probability
James Webb Space Telescope
↔ What does this mean?
JWST Level 2 Requirement 3.2.15.2.2 states:
“The Observatory shall have a greater than 95% probability of acquiring
a guide star and maintaining pointing stability for any valid attitude
within the instantaneous field of regard”.
In other words:
There must be a 95% probability that a star suitable for guiding will be in
the FGS field of view for any pointing of the telescope.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
2
James Webb Space Telescope
JWST, FGS, & GSC-2: a brief history
↔ Originally, NIRCam was to provide the guide function. The FOV in
which to find a guide star was expected to be ~16 arcmin2 (the
Yardstick NIRCam)
•
A. Spagna (2001) concluded that the well calibrated portion of
GSC-2 (JB < 19.5, RF < 18.0) should be a suitable source of guide
stars for NGST (assuming the 16 arcmin2 “FGS” FOV).
•
Guide function subsequently moved from NIRCam to a dedicated
FGS to be provided by CSA.
↔ CSA proposed an FGS with an 8.4 arcmin2 FOV (summer 2002)
•
Two units for redundancy, one operating, the other in “cold
storage”
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
3
JWST, FGS, & GSC-2: a brief history
James Webb Space Telescope
↔ Smaller FGS FOV prompted two studies:
•
Assuming GSC-2 is inadequate, what are the operational work
arounds for getting guide stars (Nelan et al.) ?
• Not pretty
•
Can JWST meet the 95% GS Acq. probability using GSC-2 if the
FGS FOV is 8.4 arcmin2 (Kriss & Stys)?
• Yes, but need entire catalog (down to plate limits in BJ and RF)
& an FGS with the sensitivity to use stars down to JAB < 20.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
4
JWST, FGS, & GSC-2: a brief history
James Webb Space Telescope
↔ Smaller FGS FOV prompted two studies:
•
Assuming GSC-2 is inadequate, what are the operational work
arounds for getting guide stars (Nelan et al.) ?
• Not pretty
•
Can JWST meet the 95% GS Acq. probability using GSC-2 if the
FGS FOV is 8.4 arcmin2 (Kriss & Stys)?
• Yes, but need entire catalog (down to plate limits in BJ and RF)
& an FGS with the sensitivity to use stars down to JAB < 20.
↔ May 9, 2003 CSA proposed a different FGS. The new design may
reduce total FOV, sensitivity, and redundancy, putting the 95% GS
acquisition requirement at risk of not being met.
•
A NASA
Origins
Mission
6/3/03
time to re-examine our motivation for achieving a 95% guide star
acquisition rate with JWST.
TIPS
Ed Nelan
5
James Webb Space Telescope
Requirements and the FGS design
↔ Achieving a 95% guide star acquisition probability requires that the
FGS FOV is large enough to contain, on average, three stars (from
GSC-2) for even the most sparsely populated regions of the sky.
•
Having 3 stars “on average” implies only a 5% chance of having
none if the stars follow a Poisson distribution.
•
The surface density of stars at “high” Galactic latitudes (|b| > 45o)
constrains the minimum size of the available FGS FOV.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
6
James Webb Space Telescope
Requirements and the FGS design
↔ Achieving a 95% guide star acquisition probability requires that the
FGS FOV is large enough to contain, on average, three stars (from
GSC-2) for even the most sparsely populated regions of the sky.
•
Having 3 stars “on average” implies only a 5% chance of having
none if the stars follow a Poisson distribution.
•
The surface density of stars at “high” Galactic latitudes (|b| > 45o)
constrains the minimum size of the available FGS FOV.
↔ Why not just increase the size of the FOV?
•
by increasing the pixel scale
• decreases the accuracy with which guide stars can be centroided, resulting in
degraded guiding.
•
add additional detectors
• exceeds cost constraints.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
7
James Webb Space Telescope
Galactic Disk (l,b) = (0,30)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
8
James Webb Space Telescope
North Galactic Pole (l,b) = (180,80)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
9
James Webb Space Telescope
North Galactic Pole (l,b) = (180,80)
spare FGS
(redundancy)
2.1’ x 4.2’ FGS FOV
(summer 2002)
JAB ~ 18.5 (F4 at 10 kpc)
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
10
James Webb Space Telescope
FGS & TF Science Instrument
revised FGS (May 2003)
A
B/C
D
three 2.3’ x 2.3’ FOV units
• A & B are dedicated guiders
• C is LW/TF (dichroic, same FOV as B)
• D is SW/TF and guider w/10% through put
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
11
James Webb Space Telescope
FGS & TF Science Instrument
revised FGS (May 2003)
A
B/C
D
Achieving the 95% guide star acquisition rate requires
any two of units A, B, or D to be operating.
• Redundancy against single unit failure is preserved.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
12
James Webb Space Telescope
The 95% requirement
Why is it so important to achieve a 95%
guide star acquisition probability?
• Scientific motivation
• Operational impacts if we fail to do so
“Motivation for Meeting the 95% Guide Star Acquisition Rate
with JWST” (Nelan, Kriss, Kinzel) STScI-JWST-TM-2003-0007A
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
13
James Webb Space Telescope
Scientific Motivations for a high
probability of access to guide stars
↔ Given JWST’s discovery potential, astronomers will want to observe
objects with more than one (perhaps all) of the Observatory’s Science
Instruments. Each visit will have a different bore sight pointing and
spacecraft roll angle. Different guide stars are needed for each.
↔ JWST will mosaic regions of the sky that are perhaps several times
larger than the FGS FOV. To avoid gaps, guide stars are needed for
all of the tiles.
↔ Long term monitoring of targets (e.g., high redshift SNe Ia) requires a
high probability of guide stars being available at every orientation of
the telescope for a given pointing.
↔ Targets of opportunity must be observed in a timely fashion. Can’t
wait for the date when a particular roll range(+/- 5o off nominal) allows
access to a guide star. TOO success rate is closely linked to the 95%
objective.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
14
James Webb Space Telescope
Scientific Motivations for a high
probability of access to guide stars
↔ NIRSpec observations with the Micro Shutter Array (MSA) will be
highly roll constrained to optimally place the target field in the array.
Optimal orientations can be used routinely only if there is a high
probability of having access to guide stars at all orientations.
↔ It is best (but not absolutely necessary) to execute large dithers (~20”)
using the same guide star, especially for NIRSpec MSA observations
(avoid complicated SI target acquisitions and wavelength calibrations),
and perhaps coronagraphic observations (MIRI target acquisition).
•
The higher the GS Acq probability, the more likely same GS can
be used for large dithers (generally, more than one GS will be in
the FGS FOV to choose from).
•
Conversely, the lower the GS Acq probability, the less likely a
guide star will become available in the small amount of new sky
that enters into the FGS FOV if its needed.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
15
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV is
high, more than one is typically available.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
16
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV is
high, more than one is typically available. Select the one
that works for dithers!
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
17
James Webb Space Telescope
Dithering and GS Availability
A
B
If the probability of having guide stars in the FGS FOV
is low (loose a guide channel, e.g.), more vulnerable to
problems with dithers.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
18
James Webb Space Telescope
Dithering and GS Availability
A
If the probability of having guide stars in the FGS FOV
is low (loose a guide channel, e.g.), more vulnerable to
problems with dithers. Probability of new GS
becoming available when needed is low.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
19
James Webb Space Telescope
Operational Motivations for a high
probability of access to guide stars
If there is a high probability that guide stars are available for any pointing
and orientation:
↔ vast majority of proposals will schedule without guide star problems.
•
•
minimizes the need for schedulers to iterate with the GOs
important for proposals with large dithers, mosaics, NIRSpec MSA
↔ proposers will not need to be concerned about guide star availability.
•
•
STScI will not need to provide GOs with guide star selection rules.
STScI retains the responsibility for selecting guide stars, and
therefore the scheduling of proposals
↔ proposals that are not roll or time constrained can be scheduled when
best for LRP and the Observatory’s efficiency.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
20
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
↔ Risk from catalog contamination. As overall the guide star availability
declines, the percentage of visits that are scheduled with just one
candidate GS increases. These are vulnerable to acquisition failures
due to contamination of GSC-2 (~10%).
acquisition
probability
visits with
one GS
visit failure
rate
95%
15%
1.5%
90%
25%
2.5%
85%
33%
3.3%
80%
40%
4.0%
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
21
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
↔ Increased risk for successfully completing multi-visit programs that
use different pointings and/or orientations. The probability of being
able to mosaic a field of area AM using an FGS with FOV AFGS with
out gaps is:
Pm = Pon
where Po is the probability of having a guide star for a single visit, and
n = Am / AFGS
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
22
James Webb Space Telescope
Consequences of Declining Guide Star
Availability
↔ The table below shows the probability of being able to mosaic a 7’x’7’
field without gaps as a function of guider FOV. It is assumed that the
guider can use stars down to JAB < 20, except for the SW/TF
channel (JAB < 17.5).
FGS FOV (arcmin2)
Per-pointing GS Acq
Probability
Probability of gap-free
mosaic
10.8
99 %
95 %
8.4
98 %
89 %
10.8*
95 %
69 %
5.4
88 %
32 %
* using channel A or B with D (SW/TF)
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
23
James Webb Space Telescope
Conclusions
Satisfying Level 2 Requirement 3.2.15.2.2 is a very important objective.
↔ GOs can use observing strategies to optimize scientific returns
without constraints imposed by guide star availability.
↔ Simplifies operations, keeps cost down.
↔ Facilitates the generation of an LRP that makes the most efficient use
of the Observatory.
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
24
James Webb Space Telescope
Conclusions
Satisfying Level 2 Requirement 3.2.15.2.2 is a very important objective.
↔ GOs can use observing strategies to optimize scientific returns
without constraints imposed by guide star availability.
↔ Simplifies operations, keeps cost down.
↔ Facilitates the generation of an LRP that makes the most efficient use
of the Observatory.
↔Don’t forget! Total lunar eclipse tonight!
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
25
James Webb Space Telescope
Guide Star Acquisition Probabilities
A NASA
Origins
Mission
6/3/03
TIPS
Ed Nelan
26
SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
TIPS: STIS Report
Paul Goudfrooij
1. Unusual Target ACQ Failures: Update & Resolution
2. Calibration of CTE loss in Spectroscopic Modes
•
Full story available under
http://www.stsci.edu/hst/stis/training/team/activities/lectures.html
•
Also STIS ISR 2003-03
3. New “Pseudo-Apertures” (if time available)
Recent Target ACQ Failures
(with L. Dressel, R. Pitts, T. Wheeler)
•
Two recent ACQs failed (March 2, April 6) due to No Flux in the Lamp Image
1
2
1
2
3
3
Π
Ο
•
All mechanisms show nominal telemetry
•
ACQ macro used 3.8 mA setting for HITM1 lamp, much below ‘default’ 10 mA
3.8 mA setting originally put in place to allow wavecals for the most
sensitive MAMA settings, and to save lamp life time
Contacted manufacturer + their consultant
– Sputtered material forms a ring inside glass envelope around cathode. If
set at (too) low current, electrons may flow to sputtered ring rather than to
cathode
Conclusion: Lamp did not fire
Since 5/12/03, ACQs use 10 mA setting. All ACQs taken so far are OK.
–
•
•
•
TIPS Presentation
May 15, 2003
Paul Goudfrooij
2
STIS CCD:
Nominal Readout Direction
Sensitive Region
(1024x1024 pix)
• 4 Readout Amps (1 / corner)
• Bi-directional Clocking yields
CTI ≡ 1 – CTE:
1 δ(fluxD / fluxB)
CTI =
δY
2
Measured using
“Sparse Field Tests”
TIPS Presentation
Amp
D
Nominal Clocking Direction
Amp
C
Axis2 (Y)
Correcting CCD Spectroscopy
for CTE Loss (with R. Bohlin)
Parallel (virtual) overscan
Amp
A
Serial
overscan
May 15, 2003
Axis1 (X)
Serial
overscan
Paul Goudfrooij
Amp
B
3
“Sparse Field” Tests
•
•
Sparse fields to ensure that sources do not overlap, in
which case (e.g.) PSF wings could fill traps for sources
along the readout direction
Two varieties:
(i) “Internal”
Sparse Field
Test
–
–
Annual series of lamp images through narrow slits,
projected at 5 positions along columns (or rows)
Designed to represent “worst–case” point source
spectroscopy (should be no background to fill traps)
TIPS Presentation
May 15, 2003
Paul Goudfrooij
4
“Sparse Field” Tests
•
(ii) “External” sparse field test (annually)
– A. Imaging:
¬
¬
¬
–
Sparse outer field in NGC 6752
CVZ target (‘cheap’ observing time;
yields range of backgrounds)
3 exposure times; 50CCD mode
B. Spectroscopy:
¬
¬
¬
¬
Young open cluster NGC 346, in
nebulosity
CVZ target
Slitless; 3 exp. times; G430L
[O II] λ3727, Hβ, [O III] λ5007 lines in
nebulosity provide three convenient,
~constant “sky” levels per spectrum
TIPS Presentation
May 15, 2003
Paul Goudfrooij
5
CTI Parametrization:
Imaging vs. Spectroscopy
•
Dependence on signal & background levels to be done
separately for imaging and spectroscopy
Spectroscopy
Imaging
CCD Column Number
TIPS Presentation
CCD Row Number
May 15, 2003
Paul Goudfrooij
6
External Sparse Field Test:
Imaging CTI Analysis
Clear dependence on background level (“sky”)
• Slope
systematically flatter
with increasing flux
• “Sky” presumably
fills traps in bottoms
of potential wells,
mostly affecting
transfer of small
charge packets.
• Suggests CTI
bck α
∝ exp –
signal
TIPS Presentation
May 15, 2003
Paul Goudfrooij
7
The Strong Effect of Background:
Gain=1 vs. Gain=4
•
Background level in spectroscopy mode typically low,
dominated by dark current
–
Also need to account for spurious charge of the STIS CCD
CC
DR
ead
CC
TIPS Presentation
May 15, 2003
out
s
D flu
h
Paul Goudfrooij
8
Functional Dependence on Signal
and Background Levels
•
Iterative Process for Spectroscopy
– Parameter space covered by ESF test at a given epoch is limited
– Sensitivity monitor: good coverage of signal levels, but not of sky
¬
G230LB data allow suitable cross-comparison with MAMA G230L
AGK+81D266,
G230LB
TIPS Presentation
May 15, 2003
Paul Goudfrooij
9
Time Constant of CTI Evolution
•
•
Need several datasets, each with same signal & background level
Need datasets covering long baseline in time ⇒ ISF data
Have to correct for signal & background dependence prior to fitting
60
e–
–
CTI = CTI0 + { 1 + 0.243 [± 0.016] (t – t0) }
120
180
500
3400
TIPS Presentation
(with t in yr)
CTI data points from Tom Brown
May 15, 2003
Paul Goudfrooij
10
Final CTI Correction Formula
(For Point-Source Spectroscopy)
•
Define background (sky) and epoch parameters:
yr = (MJD – 51765.25) / 365.25
(i.e., relative to 2000.6)
bg = max(BACKGROUND,0) + 0.5 for CCD Gain = 1
+ 5.0 for CCD Gain = 4
•
Functional form producing best fit to the data:
(
CTI = 0.0467 GROSS – 0.720 ∗ exp –3.85
•
bg
GROSS
0.17
)∗ (1 + 0.243 yr)
Implementation into the pipeline:
¬
¬
Formula parameters into CCD table reference file (new columns)
1-D extraction step (x1d) will correct for CTI by default for CCD data
(CTE correction step switchable)
• For Cycle 12 Phase II, provided downloadable IRAF script to
calculate correction factor for a given net & background level.
TIPS Presentation
May 15, 2003
Paul Goudfrooij
11
Quality of CTI fit
CTI Correction good to ≤ 7% ⇒ Spectrophotometry good to ≤ 1%
TIPS Presentation
May 15, 2003
Paul Goudfrooij
12
New “Pseudo-Apertures”
•
•
•
•
FUV-MAMA first-order spectroscopy at detector
location with low dark
– ~ 2’’ above bottom of detector
– Reduction of dark current by factor of 5
– 52x0.05D1, …, F25QTZD1
Improvement of Fringe Flats at E1 positions
– Important to align fringes in flat with those in
target spectrum
– 52x0.1 slit (best for defringing) location is
offset in dispersion direction from wider slits
– New ‘E2’ positions will place target slightly offcenter in slits ≥ 0.2 arcsec wide
New WEDGEA0.6 position for 50CORON
Provide POS TARGs to GOs in Phase-II Update;
Apertures to be implemented in next APT build.
TIPS Presentation
May 15, 2003
nominal
new
Paul Goudfrooij
13
Non-standard Gains for NICMOS
Daniela Calzetti
(In lieu of Torsten Boeker)
TIPS May 15th 2003
NCS/NICMOS Temperatures
Control Loop
Dewar
Eric Roberts
NCS Compressor Speed
B, I, Hα
J, H, Pα
Eric Roberts
The Case for Non-Standard Gains
Xu & Boeker, ISR, 2003 + Noll
θ NICMOS implements a single gain (X2), which corresponds
to ~ 6 e-/DN (5.4 e-/DN in NIC1 and NIC2, 6.5 e-/DN in
NIC3);
θ However, NICMOS was designed with multiple gain
capability, which were never fully implemented;
θ The NICMOS A/D converter has a 16-bit accuracy, and
therefore a dynamic range of 65,536;
θ Least significative bit in NICMOS A/D converter is unstable,
which adds an extra 1 DN uncertainty (digitization noise) to all
NICMOS signals;
θ 1 DN digitization noise should be compared with 4.2 DN of
readout noise in NIC3.
θPrompted by the former IDT, we investigated the pros and cons
of the other two gains present in the NICMOS design (X4 and
X8), evaluating:
θ Actual overall noise reduction;
θ Requirements for additional calibrations if offered.
Data from Program # 9324 (in SAA-free orbits):
Note:
X4 ~ 3 e-/DN
X8 ~ 1.5 e-/DN
¬Gains X4 and X8 are almost exactly _ and _ of X2.
¬ Reset voltage increases (imposes use of NSETVLT/NOPRVLT FS
macros in conjunction with the NDETGAIN macros)
DNs
DNs
Pixel
Pixel
¬Structure of darks and FFs is unchanged;
¬Noise properties of darks will be different if darks are
readout noise-limited (use ‘superdarks’ as a solution).
¬Effective noise reduction is ~10%-15% (3-4 e-) for both
gains (X4 and X8) relative to standard gain.
‘Advantages’ of non-standard gains:
¬ No need for separate calibrations (caveat on darks!);
¬ Noise reduction of ~15%; this implies a 0.11 mag ‘extra
depth’ in a 1,000 sec exposure with NIC3/F160W;
¬ A potential improvement in the discrimination of low-level
CRs is also possible (hard to quantify with current datasets).
But… non-exhaustive list of work to do:
¬ ‘Activation’ of flight software macros (NDETGAIN and
NSETVLT/NOPRVLT);
¬ ADCZERO and ADCGAIN science header keywords need
to be properly populated (calnica currently cannot work with
non-standard gains);
¬ Update of linearity correction reference files (calnica);
¬ ETC, APT, etc.
Conclusion:
Main advantage of non-standard gains is a general readnoise
reduction of ~10-15%.
However, in order to make non-standard gains supported
modes, some work on the FS and ground systems is
necessary.
Incremental increase in sensitivity does not warrant effort to
support non-standard gains. Might be made available on a
case-by-case basis.