TIPS/JIM
January 20, 2011
Agenda:
INS Division News (Danny Lennon)
Gain Sag in the COS FUV Detector (Dave Sahnow)!
Observing Scenarios for NIRSpec (Dave Soderblom)!
Next TIPS/JIM: February 17, 2011
INS Status
TIPS-JIM Meeting
January 20, 2011
New arrivals in INS
Roberto Avila (RIAB)
Changes in INS
Max Mutchler is the new RIAB Lead
Annoucements
INS Lunch this month will be organized by the WIT thanks to Harry Ferguson, Deepsahri Thatte
and Bill Blair
A number of new reports have been published on the INS web pages. All staff are encouraged to
read these and the accompanying management responses.
The DCRWG helped draw up a list of best practices for holding meetings, which will be
published on INS 'Orientation' web pages.
TIPS/JIM
January 20, 2011
Agenda:
INS Division News (Danny Lennon)
Gain Sag in the COS FUV Detector (Dave Sahnow)!
Observing Scenarios for NIRSpec (Dave Soderblom)!
Next TIPS/JIM: February 17, 2011
Gain Sag in the COS FUV
Detector
David Sahnow
20 January 2011
1 Microchannel Plates 101 Wiza, NIM 162 (1979)
2 Pulse Height and Gain •  The MCP gain is the number of electrons output for each input photon. Typically ~107 for Delay Lines. •  The Pulse Height of an event is the (5-­‐bit) digiHzed value of the gain. It is saved with the photon posiHon in TIME-­‐TAG mode. •  The Pulse Height DistribuHon (PHD) is the distribuHon of pulse heights, and is typically characterized by its modal gain and width. •  Gain is a funcHon of high voltage on the MCPs, properHes of the glass, number of MCPs, etc. •  PHD and electronics were matched before launch for the best overall performance. 3 XDL Anode •  Not a CCD (not even a MAMA). •  PosiHon of photon event is determined by the Hme it takes for the event to propagate along the anode, which means it is dependent on: –
–
–
–
–
ProperHes of anode Aging of electronic components Temperature Size of charge cloud etc. •  Requires correcHons for geometric, thermal (and other?) distorHons. •  Analog process 4 XDL Anode 5 Segment A Cumula=ve Image 6 Segment B Cumula=ve Image Back
7 Segment B Cumula=ve Image 8 Cumula=ve Counts •  Ly-­‐α airglow can appear at 20 different X posiHons on Segment B. •  CumulaHve exposure at any given posiHon depends on graHng, central wavelength, FP-­‐
POS, exposure Hme, etc. 9 Integrated Counts Back
10 Types of Gain Sag •  Short-­‐term –  When counHng at high rates, electrons cannot be replenished fast enough, and the number of electrons per event decreases. •  Long-­‐term –  Exposure to photons leads to a decrease in the secondary emission coefficient of the glass, and a drop in the gain. 11 A Tale of Two (Super)pixels CI
Int
12 Modal Gain 13 Gain vs. Exposure 14 FUSE Gain Sag 15 Effects on the Data •  Loss of events –  Loss of Photons •  Recently adjusted lower pulse height threshold from 4 to 2 •  Plan to add a posiHon-­‐dependent threshold –  Detector background •  Add a Hme-­‐dependent scale factor to the background in calcos •  Y Walk –  Flat Field •  Add a Y walk correcHon to calcos –  Spectroscopic Target AcquisiHon •  Recommend other ACQ types •  Correct on board •  X Walk (Maybe) –  Decrease in Resolving Power •  Add an X walk correcHon to calcos 16 Holes Gain sag holes
Blue ! PHA=[2,30]
Red ! PHA=[4,30]
G160M/1577 data from program 12424, obtained on
Dec 22nd 2010.
Back
17 Dark Rate Back
18 Y Walk Back
19 What Can We Do About It? •  Change the lower pulse height threshold (Done, 21 December 2010) –  Advantages: •  Quick and easy –  Disadvantages: •  Possible change in flux calibraHon •  Increased background •  Live with the holes –  Advantages: •  No changes to operaHons, etc. –  Disadvantages: •  CalibraHon becomes increasingly difficult 20 What Can We Do About It? (2) •  Increase the High Voltage (Did a test last month; did this regularly on FUSE) –  Advantages: •  RelaHvely quick to change •  Not much addiHonal calibraHon required –  Disadvantages: •  Very likle ground tesHng done at higher voltage levels •  Possible increase in HV Transients •  Move to another lifeHme posiHon (Did a parHal test in March) –  Advantages: •  Four (or so) more posiHons available –  Disadvantages: •  Requires a test to determine where to move •  More extensive calibraHon may be required 21 HV Test 22 TIPS/JIM
January 20, 2011
Agenda:
INS Division News (Danny Lennon)
Gain Sag in the COS FUV Detector (Dave Sahnow)!
Observing Scenarios for NIRSpec (Dave Soderblom)!
Next TIPS/JIM: February 17, 2011
NIRSpec observing scenarios:
the short report
David Soderblom and the NIRSpec team
TIPS: 2011-01-20
A suite of scenarios
Science observing scenarios selected to exercise full range of
NIRSpec capabilities and likely science needs.
Not typical as an ensemble, but representative in their range.
Some scenarios use original NIRSpec justifications (e.g., high-z galaxies)
Goal was to identify systemic obstructions and inefficiencies (in
time and mechanism usage).
Crowded fields (multiple target sets, multiple dithers)
Very bright sources and high-dynamic-range scenes
Very faint sources (tens of integration hours)
Critical timing and ToOs
Mosaics and large fields
Multi-instrument usage
NIRSpec programs
Program
Title
Author
Mode
Features
200
Kinematics of stars in the Galactic Center
Valenti
MSA
crowded field;
multiple pointings
201
Evolution of ices in star-forming environments
Beck
MSA
202
YSO jets near IRS-1 in NGC 2264
Karakla
MSA
extended objects;
bright sources
203
Massive star-forming regions in the Milky Way
Muzerolle
MSA
very crowded region;
large brightness range
204
First-light galaxies in the Hubble Ultra-Deep Field
Soderblom
MSA
very long integrations;
very faint sources
205
Carbon abundances in
Omega Centauri
Tumlinson
MSA
extremely crowded field
207
MSA spectroscopy
of a very extended object
Keyes
MSA
coverage efficiency
230
NIRSpec follow-up of Gamma-ray burst afterglows
Tumlinson
Fixed Slit
Quick turnaround TOO
231
Exoplanet atmospheres
Valenti
Fixed Slit
high signal-to-noise;
critical timing
261
Atomic hydrogen filaments in Perseus A (NGC 1275)
Beck
IFU
mosaic, large field
502
MIRI/NIRSpec IFU observations of extragalactic H II
regions
Gordon
IFU
multi-instrument
Example 1: Galactic center kinematics
Science rationale:
R = 2700 spectra for large number of giants in Galactic center to
determine kinematics
Measure black hole mass
Separate populations
Features and concerns:
Crowded field, multiple pointings
Background measurement problematic
NIRSpec, MSASPEC, G235M+F170LP, NRSRAPID
Galactic center kinematics (cont.)
LGS-AO image (Lu et al. 2009);
arrows show PMs;
red = “disk” members.
K mag = 9 to 15
Green slitlets to scale; note
lack of clean background.
Magenta slitlets denote a
second target set, with MSA
reconfig.
In green set, dither down
0.45 arcsec.
Note difficulty of visualizing
MSA observations.
Template parameters
Spacecraft pointing and orientation:
Five sets of [MSA RA, MSA Dec, MSA Orient] values, one per FOV
Target acquisition:
Filter, MSA configuration, and readout pattern (Ngroup = 3, NINT = 1,
subarray = FULL).
Filter = F110W, readout = NRSRAPID.
Acquisition reference stars list (RA and Dec): 5 sets.
Ten MSA configurations for acquisitions; two per target set.
Dither pattern...
Grating+filter list: confirmation images, spectrum parameters
Observation notes
Visits are very short, so group into single observation.
Target sets:
Some targets too bright for confirmation image.
Initial pointings for each set have slight offset:
Fraction of shutter to align target set in shutters
Integer offset to minimize effects of stuck-closed shutters
Four dither positions in 2x2 block
Need to fill wavelength gap to get good velocities
Target acquisitions:
Pointings for all target sets and all dithers within a visit are within 5 arcsec
Only one TA per visit
Configure MSA during TA to block bright stars
Exposures:
One grating for all exposures
NRSRAPID used when NRS would yield < 6 groups
Targets in one set so bright that only 3 groups possible; set NINT = 3 to allow CR rejection
Visit breakdown
Identified concerns:
Need for multiple TAs within a visit:
Re-acq after 10,000 sec
Re-acq after slew > 5 arcsec
Programs may have varying requirements on pointing tolerance
Need for confirmation image for each target set within a visit.
Workaround is to break into separate visits, but with much wasted time
for GS acqs.
Not clear if a single set of acq parameters works for all target
sets in a visit.
Example 2: GRB follow-up
NIRSpec, FIXEDSLIT, G235M+F170LP, NRS
Quick turnaround ToO; faint sources, fixed slit (1.6 arcsec)
JWST must be able to support ToOs
Regular ToOs do not interrupt; done within 2 weeks
Rapid ToOs are much faster:
Must be able to update Observation Plan within 24 hours of receipt of updated Phase II
24 hours consists of 22 hours for PPS to process new Phase II (and H&S checks) plus 2
hours for plan to be staged by FOS for uplink.
Time budget:
User completes new Phase II: 2–4 hours
PPS processing: < 22 hours
Staging and upload: < 2 hours
Wait for next JWST ground contact: typically ~6 hours but up to 12
Likely shortest time to respond is 32 hours, with a maximum of ~40.
This is still interesting for a GRB.
GRB operations notes
Science goal is good spectrum from 1–5 microns at good S/N.
No preliminary image possible.
Questions:
Can a satisfactory acq be achieved in the large FS (1.6 arcsec)?
Would that need S/W like the COS or FOS PEAKUP command?
Is the ground system adequate for ToO science needs (think LSST)?
Will there be “taxes” assessed on rapid ToOs?
Conclusions 1: Template limitations
Single visits may need multiple target acquisitions
If visit > 10,000 sec or dither > 5 arcsec; both likely to be common.
At present only a single confirmation image is obtained. Multiple
TAs mean multiple images. The work-around (multiple visits) is
very inefficient in time and usage.
Some scenarios require no interruptions for a single target set
(“NON-INT”).
Different dither locations may need different exposure
parameters; current assumption is the same at all dither points.
Templates (cont.)
NIRSpec usage ordinarily assumes NIRCam or similar preliminary image for
astrometry, but ToOs can’t fit that model. 1.6 arcsec aperture with peak-up
capability would suffice.
Time-critical observations (e.g., planetary transits) may come with high
overheads so they execute at correct time.
Exo-planet transits need high S/N, with need for sub-arrays for the bright
targets. May also need very large number of short integrations; this may
force breaking up into several exposures to work around constraints, but
with accurate timing to ensure data continuity.
Very high S/N may benefit from suppressing re-acquisition of guide stars
(minimize movement).
2: User information and APT
Users need better info on overheads that go with changing guide stars or
large dithers (to cross the gap).
Programs that combine dithers with multiple gratings can be done many
ways. Which is most efficient? Which conserves resources best?
Simulations are needed.
Bright sources falling on the MSA are a problem if they fall on a low-contrast
shutter. Such shutters need to be identified and tracked in APT. Templates
may include standard procedures for measuring persistence (e.g., take data,
close all shutters, then retake).
Multiple targets sets will be needed in the majority of cases; information and
guidance is needed.
To deal with inherent target dynamic range
Avoid overlapping spectra
Target positions relative to shutter grid
User info and APT (cont.)
Detector readout schemes (NRS vs. NRSRAPID) not well described. How
does the observer choose?
The S/W for optimizing target centering in shutters needs to distinguish
between point sources and others.
Exoplanet hosts are very bright, making accurate astrometry difficult
(shortest NIRCam image will saturate). A peak-up procedure that uses the
target itself would be beneficial.
Overheads for changing guide stars not clear, as when dithering across gap.
Observations of very bright sources may be limited by small number of
groups, hence poor cosmic-ray rejection.
Can APT handle IFU mosaics? With multiple guide stars?
How to choose IFU over MSA?
Next steps
Refine contents of scenarios as NIRSpec capabilities defined
better.
Use scenarios for notional science proposals and programs to
test PPS and operational systems.
Add scenarios in areas not covered (esp. moving targets).