WFPC2 94-02
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From: John Fitch
To: Disuibution
Date: April 13, 1994
Subj: Observed WFPCll Bias jumps
Several unexplained jumps in the WFPCll bias levels have been observed since launch. These jumps can
be observed in the image data as well as in the .xOD files (12 words of bias data). An example of how a typical
event appears is shown in figure 1.
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Figure 1- Bias Jump Example from XOD file
The events to date are listed in table 1 below. If the event has an associated number, it has been confirmed
to have had a bias jump and is included in the summary plots below. If no number is associated with an event, it
has been identified as having a possible bias jump by either OSS personnel or Christine Ritchie, but I have not yet
been able to confIrm this in the image or the XOD files. As such, they are not included in the summary plots.
There have been 23 confirmed bias jumps to date. An additional 4 have been identifIed, but have not yet been .
reviewed.
For the 3.75 years on orbit with twice as many detectors, only 7 events were seen for WFPCI. No
conclusive theory was determined for what caused the WFPCI events, or what is causing the WFPCll events.
Figure 2 shows the time history of events relative to the number of observations taken with WFPCII.
I
-Number
30
1--11-
'
Total Observations
Bias Jumps vs date vs total exposures
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Figure 2: Time history oC observed bias jumps
From figure 2, the rate of events appears to be approximately 5 per 1000 readouts of the CCDs (Note that
the readouts as defined are for all 4 CCDs, so the rate for a single CCD readout is roughly I to 2 per 1000 reads).
None of the events cross chip boundaries, and none of the events appear to have any affect on subsequent
readouts of the same chip. Events occur approximately twice as many times in the 7e-IDN readout bay as in the
l5e-ION bay, however the 7e-ION readout is used approximately twice as frequently.
There is a chip to chip distribution, which is shown in figure 3. Note that the events are distributed
roughly equally among the PC1, WF3, and WF4. No events to date have been recorded in WF2.
Figure 4 shows the magnitude of the observed events as a function of readout bay. It can be seen from this
plot that the total change in electrons seen in both bays is approximately equal. That is the range of observed
jumps in the 7e-ION bay is roughly twice as large as the range of observed jumps in the 15 e-IDN bay. This
would argue that the events are independent of readout bay and are associated with the common signal chain prior
to the individual bays.
During TVAC, bias jumps were seen whenever the major frame was crossed during a readout I have
requested a detailed explanation of this phenomena from JPL. The on orbit events however are not associated with
major frame boundary crossing. All of the WFPCII readouts to date have been contained within a single major
frame.
The analysis and documentation of these events are continuing. It appears likely that the vast majority of
events are being identified by either OSS or Christine Ritchie. Future analysis will include an attempt to correlate
these events with potential sources of interference from the SIC.
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Bias jump levels versus readout bay.
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sOGsm
Observed
Exposure
CCQ !Jain Biil� Jllmllm�l Il(�
EIIJlQ�lln: Slm:llim�
4
�llml:!�[
15
0.0
ext
15
0.0
ext
1
15
0.50
ext
JanI2211994 22:19:00
1
15
0.0
ext
u22u030bt
Janl24119949:37:00
1
15
0.50
ext
2
u25d0301t
Febl5/1994 12:49:00
3
7
0.50
ext
3
u27zOn02t
Febl16/1994 16:39:00
3
7
0.50
ext
4
u26q0503t
FeblI9/19944:53:00
3
7
-0.50
ext
5
u26kim01t
Feb/211199421:37:00
1
7
-1.2
ext
6
u2630406t
FebI241199422:35:00
3
15
-0.40
ext
7
u2840806t
Feb/2711994 17:40:00
1
7
-1.5
ext
8
u29q0207t
Febl28119942:40:00
1
7
-1.5
ext
9
u2328wOlt
Mar/1l199421:03:00
1
15
0.50
visflat
10
u27r4jObt
Mar/31199418:07:00
3
7
1.0
ext
11
u26xOa03t
Marnll99415:23:00
3
7
0.60
ext
12
u27rba02t
Mar/19/1994 0:00:00
1
7
-1.5
ext
13
u27z6v02t
Mar/l9!I99416:42:18
4
7
0.50
ext
14
u21ylc04t
Mar/21119947:08:00
4
15
0.0
bias,data drop
u28x7107t
Mar/22119940:11:01
4
15
-0.20
ext
u29n0505t
Mar/22119949:10:00
4
15
0.0
bias
u27r910lt
Mar/25/19942:38:17
4
7
0.50
ext
16
u2820604t
Mar/26/199419:57:17
4
15
-0.20
ext
17
u2820606t
Mar/261199420:02:17
4
15
-0.20
ext
18
u2820607t
Mar/26/199420:05:17
4
u2820803t
Mar/29/199415:30:17
4
15
-0.20
ext
19
u29n0802t
Apr/1l199413:18:00
1
7
-1.5
Bias
20
u2713r03t
Apr/4/199412:23:17
3
7
1.0
ext
21
u28x9104t
Apr/5/19945:57:17
3
15
0.50
ext
22
u28x9108t
Apr/5/19946:12:17
1
15
0.0
ext
u28x9109t
Apr/5/19946:14:17
1
15
0.0
ext
u28x9305t
Apr/8/199420:48:17
3
15
0.30
ext
u26xOo03p
Apr/9/19949:51:18
u26xOo05p
Apr/91199411:28:18
u26xOo07t
Apr/9/199413:04:18
u231Of04t
Dec/24/1993 13:24:00
u22p01Owt
Decl29/1993 11:40:00
u2311b02t
JanlI9/199413:49:00
u22t02Oft
Table
1.0
-
Observed Bias Jumps since launch
1
15
23
,
,
,