TIPS/JIM
August 21, 2008
Agenda:
INS Division News (Stefano Casertano)
The status of COS Flat Fields (Tom Ake)
HST Focus Status Entering SMOV (Matt Lallo)
Next TIPS/JIM: September 18, 2008
1
INS Division News - 8/21/2008
Stefano Casertano
* Launch invitations have been sharply limited across the project. Only a small fraction
of the invitations requested by STScI personnel have been issued. The cause for this
decision is not clear at this time. Management is working to understand and, if
possible, change the situation. Some of our colleagues are developing plans to travel
to the Cape and watch the launch from open sites.
* The Division continues to grow. We welcome new staff that joined over the last
month: Jason Tumlinson (WIT), Christine Chen (WIT), and Derck Massa (COS/STIS).
In addition, Keith Noll (WIT/ERO Scientist) and Bill Sparks (WIT) returned from their
sabbatical. Over the next several weeks we will welcome Tiffany Borders, Rachel
Osten, Rémi Soummer, Jason Kalirai, Brian York, and Abhijit Rajan.
* The Diversity, Culture, and Respect Working Group is rotating its membership to
ensure that more of the Division be represented. As announced by Kevin Lindsay,
four new members, Russ Makidon, Misty Cracraft, Aparna Maybhate, and Tracy Beck
have joined the DCRWG on August 1. Kevin Lindsay, Alessandra Aloisi, Linda Smith,
and Dixie Shipley will step down January 1, 2009. We wish to renew our thanks to
Kevin and the whole DCRWG for their great service and enormous role in improving
the working atmosphere and relationships within the Division and between us and the
rest of the Institute.
* The INS Mentoring Program was announced a few weeks ago. About a dozen people
have expressed interest thus far in entering a mentoring relationship. We expect to
start the program within the next two weeks. Please keep in mind that everyone is
encouraged to take advantage of this opportunity, and that there is no deadline for
expressing your interest.
* The ACS-R Thermal Vacuum test is ongoing.
* WFC3 passed its final inspections at the Cape and was installed in its protective
enclosure (see photo in the slide package).
TIPS 21 August 2008
1
TIPS/JIM
August 21, 2008
Agenda:
INS Division News (Stefano Casertano)
The status of COS Flat Fields (Tom Ake)
HST Focus Status Entering SMOV (Matt Lallo)
Next TIPS/JIM: September 18, 2008
1
The Status of COS Flat Fields
Tom Ake
TIPS
21 August 2008
Status of COS Flat Fields
• Material covered today
–
–
–
–
COS Description and Signal-to-Noise Requirements
NUV Ground Calibration and SMOV4 Plans
FUV Detector Characteristics
FUV Ground Calibration and SMOV4 Plans
• Most of the results are from COS IDT analyses
– Thermal Vac 2003 at Ball Aerospace
– Thermal Vac 2006 at GSFC
COS Detector Overview
• COS comprised of two spectrograph channels with two
different types of micro channel plate (MCP) detectors
COS S/N Requirements
•
Designed to obtain S/N = 30 per resel routinely, 100 per resel with
special effort
•
S/N requirements achieved by various techniques
– Flat fielding to divide out small scale fixed pattern noise
– FP-POS grating steps to average out small scale fixed pattern noise
– Pulse height amplitude (PHA) screening (FUV time-tag only)
•
As a performance metric, the S/N ratio is taken to be the reciprocal of
the RMS scatter around a smooth fit to the data
Example of Detector Artifacts
•
Features correctable by
flat fielding
–
–
–
–
•
Hex pattern
Moiré pattern
Divots/blemishes
Grid wire shadows (on
FUV only)
Uncorrectable features
– Dead spots
– Hot spots
•
Spectrum location not
known until SMOV4
alignment
Flat Field Calibration
• Internal calibration system consists of two deuterium lamps
illuminating a flat field calibration aperture (FCA)
– Light takes nearly the same optical path as an external target
– Only the science areas of the detectors are illuminated, not the wavelength
calibration region
– FCA (X=1750 µm, Y= 750 µm) is larger than the PSA (700 µm diameter)
– Aperture mechanism moves in both dispersion and cross-dispersion
directions
• External flat field calibration exposures were taken through
the PSA during thermal vacuum tests in 2003 and 2006
– Preserved internal lamp
– Allowed characterization of illumination angle dependence between PSA
and FCA
COS Optical Layout
NUV Ground Calibration
•
•
G185M grating used since D2 lamp throughput has
peak continuum flux there
Internal and external exposures taken at various
FCA Y positions to paint science area
– Internal data - 210 counts/pixel in 65 Ksec
– External lamps - 8700 counts/pixel in 25 Ksec
•
Superflat constructed from all data
– Polynomial fits performed along dispersion for Lflat
– Total counts high enough to yield pixel-to-pixel
variations (P-flat)
– S/N = 95 from photon statitsics
NUV Flat Field Assessment
•
Distribution of P-flat variations give maximum S/N
without a flat field
– Histogram of variations in each NUV stripe fit with
Gaussian profile
– Widths indicate S/N (= 1/σ) ~ 50 per resel can be
obtained without a flat
Penton
•
NUV high quality test spectrum obtained
using external D2 lamp through O2 absorption
cell
– Extracted spectrum with slit 25 pix high in
cross-dispersion direction
– Compare run of S/N with count level
– S/N ~ 100 realized with FP-POS and flat
fielding
Penton
NUV SMOV4 Calibration
•
Since MAMA pixels are mapped to physical anode
wires, expect ground superflat will be valid
– Flat field will be used by CalCOS
•
Baseline plan is to replicate TV2003 internal lamp
exposures (60 Ksec with G185M grating)
– Only nominal Y location of FCA
– Mapping in Y performed by using different gratings
•
•
On-orbit data will achieve S/N ~ 15 per
pixel (~ 45 per resel)
New P-flat will be compare to ground flat
to verify it has not changed
– If there is a difference, another set of
exposures taken in SMOV4
– If necessary, more taken during Cycle 17
calibration
FUV Detector Characteristics
• The FUV XDL detector is inherently different from the
NUV MAMA
– Photon locations are defined by the difference in time it takes for
the MCP charge cloud pulse to reach the ends of the delay lines
– Positions needs to be thermally corrected using stim pulses and
geometrically corrected to equalize pixel area
– COS team prefers the term “detector element” to “pixel” since the
pixels are not physical
– In time-tag mode, PHA data provide measure of charge cloud
produced by the photon, which is a function of the gain
distribution at each pixel
FUV Detector Gain
•
Detector gain can be important for flat fields
– Photon position errors can arise from pulse shape
differences at different PHAs
– Detector structures can be different
– Noise reduction by eliminating low and high
PHAs, which are not likely real photons
•
For COS, pulse height is digitized to values
between 0-31
•
Detector gain is not the same as sensitivity
– Since detector is photon counting rather than
charge integrating, intensity of charge cloud
unimportant as long as event can be
discerned above the noise
FUV Ground Calibration (TV2003)
•
The two segments of the FUV are treated separately
– More spectral features in D2 occur in FUV region
– To avoid D2 structure, G130M was used for FUVA, G160M for FUVB.
This doubled the amount of exposure time needed.
•
95 internal D2 lamp exposures were taken at all central wavelengths
and FP-POS positions
– 19 Ksec per segment
– Yielded median counts of 276 (FUVA) and 296 (FUV B) per pixel
– S/N ~ 18 per pixel (~130 per resel )
•
External lamp data not useful due to low lamp throughput, so no
supplemental data used to create a ground flat
FUV Flat Field Assessment (TV2003)
•
As with NUV, L-flats and P-flats constructed
for each segment
– S/N ~ 5/6 per detector element
– Maximum S/N without a flat field estimated
to be ~20 per resel
Penton
•
FUV high quality spectrum test with CO
absorption cell and Kr lamp
– S/N >30 obtained with FP-POS technique
– TV2003 flat did not improve data beyond
what could be achieved with FP-POS
– Plateau in S/N reached at ~ 2000 counts
Penton
FUV Ground Calibration (TV2006)
•
In TV2006, the calibration system delivered more light from the
external D2 source than the TV2003 set up
– External source produced continuum only > 1600 Å, so long wavelength
portion of segment A analyzed
– Series of tests conducted to investigate S/N characteristics from a 1-D flat
field
•
Twenty high S/N exposures acquired to simulate a point source in the
PSA
– G160M grating used at 5 central wavelengths, 4 FP-POS settings each
– Divided data into two sets of 10 exposures and used one set to flatten the
other
FUV Ground Calibration (TV2006)
Penton
FUV Flat Field Assessment (TV2006)
•
1-D flat fields created and one group used to
flatten the other
– Data co-added in detector space and
normalized by linear fit
– Data aligned in wavelength space for S/N
evaluation with and without flat fielding
Penton
•
Penton
S/N close to photon statistics achieved with
flat fielding and FP-POS merging
– Photon limited result factors in quality of the
flat (estimated to be 3%)
– No plateau in the S/N distribution, so
maximum S/N achievable is higher
– External flat should be obtained on-orbit
since illumination appears to be important
FUV SMOV4 Calibration
• No on-orbit D2 lamp exposures planned for SMOV4
• FASTEX standard observed in PSA with all gratings
– Each grating has slightly different Y location (G140L top, G130M
middle, G160M bottom of science area
– Use Y POS-TARG steps in PSA to ±1.2” with 0.6” spacing to
increase area covered
– Spectra shifted to four locations in X by combination of central
wavelength and FP-POS selections
FUV SMOV4 Calibration
• Chose high declination DA white dwarf , WD0320-539 (V=14.9)
– Provides count rate near time-tag limit (~ 20,000 counts/sec) so we can
maximize counts and still obtain PHA data
– Exposure times chosen to achieve S/N = 35-45 per resel
– Will take 11 orbits
• Data will be combined to create 2-D flat field using an iterative
technique
– Methodology first used with GHRS and described in STIS ISR 98-16
(Gilliland)
– Iterate between wavelength and pixel space in merging and correcting data
sets
– Solve simultaneously for the stellar spectrum and underlying fixed pattern
noise
COS Flat Field Summary
• NUV flat field will be in good shape after SMOV4
– Enough counts obtained in TV to create P-flat at pixel level
– Expect on-orbit flat field to be consistent with or correctable to ground flat
– CalCOS set to perform flat field correction as default
• FUV flat field requires more work
– Investigating P-flat at resel level or 2-D S-flat
– Uncertainty about flat field changes with detector aging
– CalCOS currently set NOT to perform flat field correction, but can be
turned on with a switch
– Best technique for improving S/N is through an FP-POS strategy
TIPS/JIM
August 21, 2008
Agenda:
INS Division News (Stefano Casertano)
The status of COS Flat Fields (Tom Ake)
HST Focus Status Entering SMOV (Matt Lallo)
Next TIPS/JIM: September 18, 2008
1
TIPS/JIM
21 Aug 2008
TEL Group
HST Focus Status Entering SMOV
M. Lallo for Telescopes Group
TIPS/JIM 21 August 2008
HST Metering Truss
TIPS/JIM
21 Aug 2008
TEL Group
Long-Term Contraction
TIPS/JIM
21 Aug 2008
TEL Group
Shrinkage of OTA Metering Truss over Mission Life
• 0.15 millimeters over 5 meters
and 15 years!
• Earlier followed simple function
of exponentials. Rate in recent
years slower but less
predictable.
• Typical temperature-driven
variations in OTA focus over
timescales of hours/days is a
few (~5) microns.
• 1micron at secondary induces
rms WFE ~1/80 wave (V-band).
• Secondary Mirror Adjustments
are periodically made to
maintain despace within ~4
microns or ~1/20 wave.
1/01/02
Trend since SM3B
Accumulated Secondary Mirror Despace (Microns)
15
TIPS/JIM
21 Aug 2008
TEL Group
HST
Refocus
July
2006
HST
refocus ,31
+5.3
microns
+5.3
microns
31 July
2006
10
5
0
-5
-10
-15
-20
1/01/02
1/01/03
1/01/04
1/01/05
1/01/06
1/01/07
1/02/08
1/01/09
Focus Maintenance since SM3B
TIPS/JIM
21 Aug 2008
TEL Group
Focus measured with ACS/HRC < Jan '07 and with PC > Jan '07
15
HST Refocus 02 Dec 2002
+3.6 microns
HST Refocus 22 Dec 2004
+4.2 microns
HST Refocus 31 July 2006
+5.3 microns
Microns of Secondary Mirror Despace
10
5
0
-5
-10
-15
1/1/02
Orbital range of HRC focus measurements
Orbital mean of HRC focus measurements
Orbital range of PC focus measurements
Orbital mean of PC focus measurements
HST Secondary Mirror Moves
1/1/03
1/1/04
12/31/04
1/1/06
1/1/07
(Thanks to Russ Makidon for phase retrieval during recent years.)
1/1/08
Focus Entering SM4
TIPS/JIM
21 Aug 2008
TEL Group
As inferred from the previous plot, we will enter SM4 with OTA
well focused for the existing complement of SIs.
This means that if the locations of the focal surfaces in ACS, STIS,
and NICMOS have not been altered, then the COS and WFC3
focusing activities during SMOV should result in good confocality
among all the SIs without any additional coordination. Note that
past Servicing Missions have had trivial impact on focal lengths of
existing SIs and OTA.
We are currently seeing the most stable period of OTA focus to
date, with no sign of negative trending after 2 years, which if
continues would also simplify any subsequent tweaking of the
optical alignment of COS and WFC3 after SI settling and
desorption.
Focus Monitoring in Cycle 17
TIPS/JIM
21 Aug 2008
TEL Group
For Cycle 17 we plan to continue routine monthly monitoring of HST focus (to be presented to
HSTMO in OTA Cal Plan, Sept. 25th). Phase Retrieval will be performed on WFC3/UVIS along
with ACS/HRC and STIS/CCD images (if SIs are viable) taken in parallel or contiguous in time. As
in the past, multiple SIs are used to establish OTA-level variations as well as any SI-specific
behavior. However....
What is our response to the monitored focus trending out of acceptable range in Cycle 17+?
•
WFPC2 has no internal focus adjustment. OTA focus has been maintained for best PC
image quality. Other SIs have focused as closely as practical to this.
•
With the removal of WFPC2, all Cycle 17 SIs will have the capability of individual
focusing.
•
Assessment of effort and engineering analyses of risks associated with refocusing at the
SIs vs. Secondary Mirror compensation will need to be vetted among the INS teams,
HSTMO, and probably HST-Project. Guess is we will likely continue with status quo:
- STIS & ACS focus mechanisms infrequently used with little desire to exercise more
than necessary.
- HST Secondary move of pure despace well understood and consistently executed.
- Less effort by STScI to move a single OTA mechanism than five SIs.
- Effect of accumulated OTA defocus at FGSs?
- Given recent rates of OTA trending, we may have zero to perhaps ~3 occasions to
refocus for the remainder of expected HST mission life (compare with 21 past
Secondary Mirror adjustments).