TIPS Meeting
19 June 2003, 10am, Auditorium
1. NIRSPEC Operations Concept
`
Mike Regan
2. WFC3 Optical Alignment
Massimo Stiavelli
3. Increased Thermal Background
for the post-NCS NICMOS
Megan Sosey
Next TIPS Meeting will be held on 17 July 2003.
NIRSpec Operations Concept
Michael Regan(STScI), Jeff Valenti (STScI)
Wolfram Freduling(ECF), Harald Kuntschner(ECF),
Robert Fosbury (ECF)
Ops Concept: What is it?
• Explain how the instrument will be used
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What are the observing modes?
What types of calibrations are required?
How are observations planned?
What are the actions required to perform an observation?
Which actions are performed in flight software, flight
hardware, ground planning software, and ground pipeline
software?
– How often will mechanisms be used?
NIRSpec Optical Layout
Filter
Wheel
Pick-off
Optics
Micro-Shutter
Array Grating/Prism/Mirror
Wheel
Fore-optics
Collimator
Camera
Detector
Array
Target Acquisition
• Need to have maximal light from science targets
going through all the slits formed by shutters
– This requires getting both the correct pointing and the
correct roll
– After acquisition both the pointing and the roll must
be held relatively constant throughout the observation.
Target Location Tolerance
• Assure that the ensemble throughput is not
reduced by more than 10% for 95% of the
observations
– Leads to a two sigma error of 25 mas.
– Therefore, one sigma we must be within 12 mas of
desired location.
– Both pointing and roll errors contribute to this error
How do they interact?
• Sin(roll_error) < sqrt(12mas2-pointing_error2)/100”
Roll Angle Acquisition
• User will be given a range of roll angles after visit
has been preliminarily scheduled
– User will select a roll and design their shutter mask
– Chosen roll angle and shutter mask will be put into visit file
– Spacecraft will use star trackers to move telescope to
required roll angle
Positional Acquisition
• Uncertainties in the locations of stars in the
GSC2 are much larger than the required
(<10mas)
– Have to take acquisition image to get an offset to the
correct location.
Microshutter Grid and Point Source
Location
• Microshutter grid will lead to
biases in the centroid of a point
source ~14mas.
– More sophisticated
algorithms can reduce this
• Only by dithering one source or
using multiple reference objects
can this be averaged out.
• With 9 targets get final error of 5
mas.
Roll requirement
• With a 5 mas positional uncertainty
– Allowed roll error is ~15 arcseconds
• Even with perfect positional accuracy
– Allowed roll error is ~20 arcseconds
• Note that this error includes the user’s
uncertainty in being able to determine the
required roll angle
• Therefore, for now, we are assuming
that roll will need to be adjusted.
Image Stability
• Around 1/3 of the science will be one day per
grating selection
– Need to be stable on this time scale
– Otherwise, will have to reacquire and recalibrate
– Spacecraft roll about FGS star will need to be stable
to within ~1.7” per day
• Smaller due to larger radius to FGS star
Steps in a Target Acquisition
• Assume wheels at home locations or move them:
– filter wheel at closed location
– grating wheel at mirror location
• Turn on calibration lamp
• Take image of MSA plane (uncertain mirror location)
• 1D – Centroid each fixed slit
– Store away the difference between expected and
actual position
• Turn off lamp
• Open all MSA shutters [except those around bright
objects in field]
• Move filter wheel to requested acquisition filter
• Take acquisition images and centroid
• Find ∆x, ∆y, and ∆roll
• Offset pointing and roll to correct location
Correcting for MSA and Cosmic Ray
Effects
• Two method for getting required positional precision
– One object, multiple dithers, (slower but more flexible)
– Multiple objects, one dither, (faster but more restrictive)
• At each location take acquisition image (up-the-ramp,3)
– Form two differences (1-0, 2-1)
– Flat field two differences?
– Take minimum pixel value (CR reject)
• Add mirror offset and acquisition offset
• Pass delta pix to FGS
• Repeat if accuracy is not good enough.
Contemporaneous Calibrations
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After target acquisition
Switch to a long pass filter
Configure MSA for observation
Take a short direct image
– This will help pipeline processing
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Switch to requested grating/prism
Switch to closed filter wheel
Turn on emission line lamp
Take a wavecal image
Turn off emission line lamp
Switch to filter wheel long pass filter
Begin science exposures
Detector Operations
• NIRSpec will be detector noise limited in
R>1000 modes
• Up-the-ramp/Multiaccum sampling has been
shown to be better than Fowler for detector noise
limited observations
• In addition, up-the-ramp sampling is more robust
against cosmic rays
Signal Level
Baseline Readout Mode
Reset
Samples
T2
T2
T2
T2
T2
Groups
TIME
Signal Level
Alternative Readout Mode
(depends on noise characteristics of
flight electronics & detector)
Reset
Samples
T2
T2
T2
T2
T2
Groups
TIME
Readout Parameters
• Time between storing of groups on SSR = 50 sec
• Samples per group = 1,4
• Number of groups = exposure_time/50
Other parameters
• Sub-array readout
– Minimum 12 second exposure time is too long for
many sources
– Sub-array readout will be needed
– Only one sub-array at a time
– Readout time = 12*(number of pixels in subarray/8
million) seconds
Electronic Gain
• Goal is to have only one gain setting for
NIRSpec
– Maximum gain is set by Nyquist sampling single
sample read noise (~9e-) or ~4 e-/ADU
– Would like to be able to use entire full well ~90K -200K e– 16 bit A/D values lead to 64K dynamic range
– Saturated values can be reconstructed from early reads
in up-the-ramp.
– A single gain of 1.5 e- to 2.5 e- will work
Calibration
• Assumptions
– NIRSpec will have internal line and continuum
sources
– Line sources will reach required S/N is a 60 sec
exposure
– There will be NO parallel calibration
• Although it should not be ruled out
– Wavelength zero point calibration are required every
time the grating wheel is moved.
– MSA-to-detector calibration is required every time the
mirror is moved in.
Monitoring Calibrations
• Two types
– Parallel Capable (do not require dedicated visit)
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Dark current/read noise/gain
Hot pixels
Shutter throughput
Fixed slit throughput
Small scale flat field variations
– Dedicated (frequency depends on stability of detectors and
geometry of optical bench)
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Linearity
Persistence
Geometric distortions
Large scale flat field
Wavelength solution
Mission Lifetime Usages
• Assumptions
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5 year lifetime
NIRSpec is used 50% of the time
70% of the time we are doing science
All observations are multi-object MSA spectroscopy
Type of projects
Type
Fraction
of
NIRSpec
Time
Average
Time for
one visit
(sec)
Number
of visits
per year
Short
0.10
3K
470
Medium
0.60
20K
430
Long
0.30
100K
43
Usage for each visit
Type
Filter
Wheel
Grating
Wheel
MSA
Lamp
Short
7
4
8
4
Medium
6
3
4
3
Long
5
2
4
2
Total Usage
Type
Filter
Wheel
Grating
Wheel
MSA
Lamp
Short
5*370*9
5*370*4
5*370*8
5*370*4
Medium
5*330*7
5*330*3
5*330*6
5*330*3
Long
5*33*5
5*33*2
5*33*4
5*33*2
Total
29K
13K
25K
13K
Optical alignment and image
quality testing at GSFC
• Test lead: Bill Eichorn, GSFC
Test conductors: George Hartig, Sylvia Baggett, Massimo
Stiavelli
• Full suite of alignment and image quality tests performed
twice: before and after vibe test. 1st epoch : 21 Mar-18 Apr
’03, 2nd epoch ongoing
– WFC3 with non-flight detectors (IR MUX, UVIS surrogate) and
flight POM installed in RIAF with OS (CASTLE) providing OTAlike point source illumination at 633 nm (only)
– During 2nd epoch flat fields in F336W, F439W, and F625W were
also obtained
– Over ~4000 images obtained on both IR and UVIS channels
May 22, 2003
WFC3 Report at the June 03 TIPS
1
Optical alignment and image
quality testing at GSFC
• Test objectives:
– Demonstrate Optical Stimulus (CASTLE) performance over field;
develop techniques and procedures
– Obtain pre-vibration (of OA) alignment baseline and compare it
with post-vibration data
– Compare with results obtained at Ball with Mini-Stimulus
– Measure residual surrogate detector alignment offsets for use in
flight detector alignment
– Assess image quality and wavefront performance of the completed
optical assembly
May 22, 2003
WFC3 Report at the June 03 TIPS
2
Optical Stimulus (CASTLE)
Rm 150
Bld 29 Clean room
May 22, 2003
WFC3 Report at the June 03 TIPS
3
CASTLE Schematic
M4 Steering Mirror
Precision Encoders
M3 Steering Mirror
Precision Encoders
• Inverted Cassegrain with
46% Central Obscuration
M1
Stimulus
Relay
Optics
• Images positioned
over field with 2
gimballed flats produces pupil rotation
M2
Switchin g Mirror
Slides in plane for
Extended Sources
Integrating
Sphere
Target
Plane
Upper Reticle
Point
Sources
WFC3 POM
• Point source XYZ
stage permits accurate
focus adjustment
Fiber Feed
RIAF
OTA Latch Plane
ABC Latches
Lower Ret icle
OTA Sta 198.440
Alignment
Monitor
May 22, 2003
Flux Monitors
WFC3 Report at the June 03 TIPS
4
CASTLE/RIAF in SSDIF
May 22, 2003
WFC3 Report at the June 03 TIPS
5
CASTLE Test Image Field
Locations
• Measurements made at 16-17 points, uniformly-distributed
over IR and UVIS fields (4-5 used at Ball)
May 22, 2003
WFC3 Report at the June 03 TIPS
6
IR MUX Image at Field Center
May 22, 2003
WFC3 Report at the June 03 TIPS
7
UVIS Image Near Field Center
May 22, 2003
WFC3 Report at the June 03 TIPS
8
UVIS alignment focus scans with
WFC3 corrector and CASTLE
May 22, 2003
WFC3 Report at the June 03 TIPS
9
IR channel focus scans feasible
with CASTLE only
May 22, 2003
WFC3 Report at the June 03 TIPS
10
Phase Retrieval
• Phase retrieval (PR) analysis measures low-order aberration
content of the system at the detector, including focus
• Sets of monochromatic images obtained at multiple focus
settings, on both sides of best focus, and fit simultaneously
to remove ambiguity
• IDL PR routines developed by Burrows & Krist (ca 1990),
used for COSTAR, STIS, NICMOS, and ACS optical
verification and alignment, and adapted for WFC3
• Only 13 lowest-order terms in Zernike expansion used for
wavefront fit; generally provides good indicator of
alignment state and final optical performance
May 22, 2003
WFC3 Report at the June 03 TIPS
11
IR Channel Alignment Phase
Retrieval
Pupil/WFE map
(cold mask)
Measured MUX
image
Modeled (fit)
image
Weighted
difference image
May 22, 2003
WFC3 Report at the June 03 TIPS
12
UVIS Channel Alignment Phase
Retrieval
Pupil/WFE map
(OS obsc/spiders)
Measured MUX
image
Modeled (fit)
image
Weighted
difference image
May 22, 2003
WFC3 Report at the June 03 TIPS
13
Optical alignment summary
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Phase retrieval and focus scan (CASTLE fiber or WFC3 corrector) techniques
yield similar, repeatable results
Residual detector tip/tilt after alignment at Ball corroborated with CASTLE
measurements using alternative focal surface alignment technique
– Alignment of both detectors should be improved with shim offsets as flight detectors
are aligned in transfer fixture at Ball
•
UVIS detector roll off nominal by 0.8 degrees, position off by ~50 pixels
– Set incorrectly at Ball, will be compensated for flight detectors
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IR detector roll nearly perfect (<0.1 degrees from nominal)
IR focus is essentially identical to measurement at Ball
UVIS focus differs by ~0.5 mm (STOPT) from Ball measurement
– Believed to be due to offset in model positions for Mini-stimulus
•
Vibration testing did not alter alignment or image quality apart from a small shift
– identical in both channel – probably due to latches tolerance
May 22, 2003
WFC3 Report at the June 03 TIPS
14
UVIS Encircled Energy
Measurements vs. Model
• CASTLE EE measurements require correction for 0.46 central
obscuration ratio (OTA is 0.33), spider width and other
differences from OTA, as well as limited measurement radius,
to predict on-orbit EE performance.
• CEI Spec: EE(.25 arcsec diam.) > 0.75 (goal 0.80) at 633 nm
UVIS,??=633
MS (Ball)
OS (GSFC)
OTA
Model: Perfect
? =0.015 ? =0.25
0.67
0.86
0.58
0.85
0.65
0.84
Model: 0.03? RMS
? =0.15
? =0.25
0.63
0.84
0.55
0.83
0.63
0.82
Actual Measured
? =0.15
? =0.25
0.62-0.66 0.82-0.85
0.55-0.58 0.80-0.83
?SMOV? ?SMOV?
• EE spec will readily met, goal will likely be achieved onorbit, if flight detector performance and alignment are as
expected.
May 22, 2003
WFC3 Report at the June 03 TIPS
15
Flat Field Analysis
C-D amp
May 22, 2003
WFC3 Report at the June 03 TIPS
16
Flat Field Analysis
• Many negative pupil images seen in the flat fields. They do
not depend on the filter and have sizes compatible with
what would be espected from dust particles on the surrogate
UVIS detector.
• Vignetting has been found on one corner of the field of
view. It is a triangular area about 400 pixels long and 30
pixels wide at the largest point. Over this area the flux rolls
down by more than a factor 2.
– Not caused by filters, pickoff or channel select mechanism: too
sharp (pickoff) and unaffected by rotating filters/CSM to offnominal positions
– It may be caused either by baffles close to the detector or by the
camera head itself (currently at an incorrect position)
May 22, 2003
WFC3 Report at the June 03 TIPS
17
Flat Field Analysis
A-B Amp
May 22, 2003
WFC3 Report at the June 03 TIPS
18
Conclusions
• The various sets of measurements are consistent
• The data will allow us to position the flight detectors at the
correct position
• Vibration testing did not significantly alter alignment or
image quality
• Small amount of vignetting in UVIS channel (still to be
investigated)
• EE field center goal on UVIS should be met over the
whole field
May 22, 2003
WFC3 Report at the June 03 TIPS
19
Increased Thermal Background
for the post-NCS NICMOS
TIPS – June 19, 2003
Megan Sosey
NICMOS
Cycle 11 Calibration Plan
♣ Parallel observations in NIC3:F222m,
NIC2: F222m & F237M were crafted to
re-measure the thermal background for
cycles 11 and beyond
-See proposals 9269 and 9702
♣ Accounting for DQE increase, the
average thermal background was
predicted to be approximatly 20%
higher than in Cycle 7
So, Where’s the extra signal?
♣ NO correlation was found between the
variation in the thermal background and
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telescope pointing
sun angle
prime instrument
time since SAA passage
or spacecraft orientation
♣ The T-1-1 temperature sensor shows the
detectors are stable under operation of the
NCS and do not change by more than 0.1K on
long time scales
T-1-1 Temperature Sensor vs. Time
Telescope Warm
Season
9/10/2002
Commanded Setpoint change
-
5/30/2003
Thermal Stability of the NICMOS
Enclosure and Telescope Assembly
♣NICMOS elements which affect the TB:
re-imaging mirror (RM), FOM, PAM, filters,
FDA(imaging mirrors), cold mask, bend
mirrors, baffles, pupil
♣HST optics which affect the TB:
primary mirror, secondary mirror,
spiders,pads – these play significant roles
in the TB because of their high
temperatures (~290K)
Representation of Cold Mask
Misalignment
See Robberto, Proc. SPIE v4013, p.386 and Krist, NICMOSISR-00-011 for more details on the cold mask alignment
Rough Optical Path Diagram for the
NICMOS Fore-Optics
Thermal stability of the HST
optics and aft shroud
♣The primary and secondary have remained
thermally stable, introducing no extra thermal
emission - however, the aft shroud has increased
approximately 10K since Cycle 7
♣There are multiple thermistor sensors (thermally
sensitive resistors) in the aft shroud which are used
to monitor temperature
-TAFTBULK is a weighted average of the internal sensors
located on the aft bulkhead
-TASINAFB is an effective sink measurement from all the
internal sensors, including some of the ones from the TAFTBULK
average
Thermal Environment of the HST Aft
Shroud around NICMOS 2001-2002
Thermal Environment of the HST Aft
Shroud around NICMOS 2001-2002
Aft Temperature Comparison of HST and
NICMOS
naftbtmp
NIC3,
F222m
HST AFT
TASINAFB
TAFTBULK
C7 & C11 NICMOS Aft Temps
NICMOS Temperature Variations
Avg. C7
Temp
Avg. C11
Temp
Delta T
Key
Descrip.
nfob2tmp
-5.82 C
-0.48 C
+5.34 C
ndosftmp
Fore-optical
brkt.
Dewar Fore
-7.27 C
-1.21 C
+6.06 C
ntrs3tmp
Truss Aft
1.22 C
7.52 C
+6.3 C
ndosatmp
Dewar Aft
-1.60 C
5.30 C
+7.24 C
naftbtmp
Aft NIC Encl.
8.5 C
17.5 C
+9.0 C
taftbulk
HST Aft
Shroud
-17 C
-7 C
+10.0 C
♣The NIC fore-optics swing in temperature in the
same manner as the aft end, but with a smaller
amplitude
Direct plot of thermal background vs fore-optics
temperature
Does this explain the increase?
♣The previous plot indicates that a 2K
variation in temperature, translates to a
1.5DN change in the thermal background
♣An increase of 5K between Cycle 7 and
Cycle 11 yields an additional 3DN of
thermal background signal! - this makes up
the difference between the predicted and
observed thermal background
Simple blackbody comparison of HST and NICMOS
optical surfaces using NIC2 Optical Parameters
Comparing to the NICMOS Thermal
Background Code Estimates
Conclusions
The overall temperature environment of NICMOS
and the operating temperature of the
detectors has changed since cycle7 resulting
in:
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•
•
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Increase of ~17K in detector temperature
General temp. increase in the aft HST
Increased temperatures in the aft NICMOS
Increased thermal background in long
wavelength, or extra wide, camera 2 and
camera 3 filters
• Thermal background exposures are still
affective in removing the excess signal