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]

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:
WaveFilterExp. timeMedian Bgrd/off limb? Median DN/on limb?DN/secPre-flight Ratio Al +1/Clear
171Al +17.125105150.49
195Al +112.1257060.49
284Al +1152.1515<10.33
304Al +152.11514030.29

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
+1 301 286-4767

NASA Goddard Space Flight Center
Solar Physics Branch / Code 682
Greenbelt, MD 20771

Last revised - J.B. Gurman