SOHO EIT light leaks, 1998 February 4
It appears that the
SOHO was struck by a micrometeorite shower in the early
morning hours (UT) of 1998 February 4. As a result, all four EIT sectors
now show light leaks,apprently the result of pinholes in the front filters.
(Formerly, only Fe XV 294 Å and, to a lesser extent, Fe IX, X 171
Å showed light leaks.)
Thermal response
There is no indication that any mechanism overheated during the 03:05 -
03:25 UT period in which the LASCO electronics box (the LEB) stopped
taking images.
![[EIT temperatures plot]](images/eit_temp_19980204.gif)
From this, it appears that the LEB behavior, as well as the appearance of
new light leaks in EIT, are due to external sources, e.g., a micrometeorite
impact.
So where do we go from here?
EIT is equipped with an internal filter wheel that allows us to insert, for
example, an additional thin Al filter to counteract exactly this sort of
failure. This will require exposures 2 - 3 times as long as our current
exposures (wavelength dependent). The greatest challenge will be flat-fielding
the new, dual grid, so for some months, our images will show a (new) grid
pattern.
Quicklook Analysis - Still ongoing...
It is obviously important to understand the calibration between
the Clear and Al+1 filter. Comparisons have been made (both pre-
and post- launch) to obtain the filter ratios. As a double check
we obtained a sequence of Clear and Al+1 exposures (full field, full
resolution and 2x2x binned) in each of the 4 bands. The images
were taken on Feb. 4, 1998 between 15:40 and 17:00 UT.
A quick comparison of their histograms can be viewed at:
Raw 171 Clear and. Al +1
Raw 195 Clear and. Al +1
Raw 284 Clear and. Al +1
Raw 304 Clear and. Al +1
Prep 171 Clear and. Al +1
Prep 195 Clear and. Al +1
Prep 284 Clear and. Al +1
Prep 304 Clear and. Al +1
The relevant parameters are:
| Wave | Filter | Exp. time | Median Bgrd/off limb? |
Median DN/on limb? | DN/sec | Pre-flight Ratio Al +1/Clear
|
|---|
| 171 | Clear | 5.6 | 45 | 170 | 31
|
| 171 | Al +1 | 7.1 | 25 | 105 | 15 | 0.49
|
| 195 | Clear | 9.1 | 40 | 120 | 13
|
| 195 | Al +1 | 12.1 | 25 | 70 | 6 | 0.49
|
| 284 | Clear | 62.1 | 20 | 35 | <1
|
| 284 | Al +1 | 152.1 | 5 | 15 | <1 | 0.33
|
| 304 | Clear | 25.1 | 35 | 240 | 9
|
| 304 | Al +1 | 52.1 | 15 | 140 | 3 | 0.29
|
What does this tell us? This table addresses 3 issues, stray light,
filter ratios, exposure times. First, examining the background levels
we see that the stray light has been reduced with the Al +1 filter. This
is also very apparent in the images. Second we have verified the
filter ratios (factor of 2 for 171, 195 and ~3 for 284, 304) are indeed
correct.
The answer in terms of exposure times is a little more difficult. If one
were looking to obtain the same absolute DN level
with the Al +1 filter as the Clear then we need to increase our exopsure
times. This is not a problem with 171 and 195 but it begins to get
long for 304 and especialy 284. If, however, one cares about obtaining
the same level of signal to noise, then the above exposure times already
achieve this.
We have implemented a new crude grid correction. This is neccessary as
the previous grid correction for the Al +1 filter was computed before
the detector suffered some degradation. The new correction is a
combination of the old correction and an updated correction used for the
clear position. In the coming month we will implement a completely
recomputed Al +1 degrid. Please bear with us.
Web curator:
Joseph B. Gurman
Responsible NASA official:
Joseph B. Gurman,
Facility Scientist, Solar Data Analysis Center
joseph.b.gurman@gsfc.nasa.gov
+1 301 286-4767
NASA Goddard Space Flight Center
Solar Physics Branch / Code 682
Greenbelt, MD 20771
Last revised - J.B. Gurman
