Anomalous scattered light
in ACS/WFC Data
Marco Sirianni
D. Van Orsow, A. Welty, T. Wheeler
R. Gilliland, R. van der Marel
ACS/WFPC2 group
INS Division
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
On Dec 03 two PIs reported an anomaly on their ACS/WFC data
A
B
WFC-1
WFC-2
C
D
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Initial clues:
Both programs used F814W and were executed on Nov 11 2003
The shape of the pattern is the same
The count-rate is similar in different images (~ 0.18 e-/sec)
The photometry of the objects in the contaminated region is consistent with an
additive background.
SCATTERED LIGHT ?
First investigations on all ACS data between Nov 10 and Nov 12:
Not all F814W images were contaminated.
14 contaminated images (only F814W and F606W, no contamination in F555W).
F606W images show a similar pattern but a much lower count-rate
Some images show a lower count rate than others taken with the same filter
LAMP CONTAMINATION ?
VERY RED LAMP ?
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Analysis of telemetry/commanding:
No Anomaly in ACS Lamps
No Lamp Activity in STIS or NICMOS
WFPC2 was taking a series of 1800 sec internal flat fields
(Tungsten lamps + F390N filter) in those days.
The comparison of the ON-OFF cycles of the WFPC2 INTFLAT lamps and the
timing of the ACS anomaly gives a perfect match:
when ACS/WFC images show the bright pattern the WFPC2 lamps are ON
when the count rate is lower (images with the same filter) the lamp has
been turned on during the exposure
(the count rate is uniform if the real exposure to the lamp is taken into account).
The telemetry data allowed us to find more contaminated images
CONTAMINATION FROM WFPC2: INTFLAT+F390N
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Brightness and shape of the pattern are consistent
with the following scenario:
WFPC2 optical configuration:
From OTA
B
Fold
Fold
mirror
mirror
ToToCassegrain
Cassegrain
++CCD
CCD
B
Pick-off
Pick-off
mirror
mirror
SOFA
Lamps SOFA
Lamps
Shutter
Shutter
A
A
Pyramid
Pyramid mirror
mirror
Light leakage from WFCP2
shutter ( 1 %)
to
WFPC2 POM
to
ACS
TO ACS
any other light path (OTA secondary or glint near ACS entrance
aperture) would produce much fainter illumination
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
For a direct path from WFPC2 a simple ray tracing model
predicts a bright image of ACS WFC IM1 mirror
Edges of ACS entrance aperture ?
shifted diagonally toward WFPC2
Edges of IM1 Mirror
Multiple images due to WFC CCD
windows reflections
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Immediate actions:
- All WFPC2 INTFLAT programs on hold
- More detailed investigation of the problem
Occurrence
WFPC2 filter dependence
Are other channel/instrument contaminated?
WFPC2 VISFLAT potential problem?
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Preliminary results:
Problem since SM3B, but it is quite rare circumstance
(~ 40 images so far, mostly parallel, 4 GOs programs)
No contamination in ACS/HRC STIS or NICMOS
No correlation with the WFPC2 Filter ( -> direct leakage)
Clear Correlation with WFPC2 Shutter Blade:
Approximate average count rate e-/sec
WFCPC2 SHUTTER
WFC FILTER
A
B
F606W
0.007
n.a.
F775W
0.080
0.025
F814W
0.180
0.051
Marco Sirianni
TIPS 12/18/2003
Anomalous scattered light in ACS/WFC Data
Summary / Conclusion:
• WFPC2 shutter leakage contaminates ACS/WFC data during INTFLATs
• The level of contamination depends on:
• WFPC2 TUNGSTEN LAMP CYCLE
• WFPC2 SHUTTER BLADE
• ACS/WFC FILTER
• ACS/WFC EXPOSURE TIME
• The PI of the programs with corrupted data will be contacted.
• The problem is fully predictable and therefore can be avoided with
an appropriate scheduling strategy
• WF3 should be a better neighbor
(we’ll plan a specific SMOV test for SM4)
Marco Sirianni
TIPS 12/18/2003