Data pre-processing

Understanding the contaminations in the shutterless mode of EIT

(David Berghmans, October 26th, 1998)

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This text discusses a few technical aspects of the shutterless mode of EIT. My discussion is based on the shutterless mode sequence of May 13, 1998. Specifically, I have a look at instrumenal effects such as the 'smearing' of the image due to reading out the CCD with the shutter open and skipping ccd clears. To conclude I give a few recommendations for new shutterless mode runs by EIT and for future EIT-like instruments.

The May 13, 1998 shutterless mode sequence (efz19980513.173209) had the following setup (see Fig. 1):

For the following discussion we also note the following images on the same day:

filename image type filter
efz19980513.165311FOV test imageAl sup
efz19980513.170145195 fdfrAl +1'
efz19980513.183451195 fdfrAl +1'

General procedure

After being exposed, a shutterless mode image of the subfield 'FOV' is transferred to the readout port. According to Delaboudiniere et al (SolPhys 1995, 162, p. 304) reading out a subfield takes the following three steps:

It is important to note that, since we do not do any 'ccd clears', this contaminated smear out image remains in the fov when the exposure of the next image starts.

Figure 1.

Estimation of the contamination

So, to summarise a subfield 'image' consists of the following contributions:

Finally, these 4 contributions are being transferred another 341-128=213 lines (which takes 213*0.480 ms = 0.10224 s) and are read out in 2.6624 s. So they spend T_o= 2.76464 s under the occulting mask. This value for T_o can be confirmed by noting (see Fig.2) that the average intensity of a shutterless mode image scales as the the time interval between the observation of that image and the preceding image plus an offset:

average_intensity(i) ~ obstime(i)-obstime(i-1) - ( 2.71 +/- 0.05 s)

Since we do not do any ccd clear the total exposure (t1+t2+t3+t4) time together with the time T_o spent under the occulting mask must equal the time interval between the observation of the preceeding image and the present image:

obstime(i)-obstime(i-1) = t1+t2+t3+t4 + T_o

This allows us to derive the effective exposure time t3 of the true solar image in the fov:

t3 = obstime(i)-obstime(i-1) - t1-t2-t4 - T_o
= obstime(i)-obstime(i-1) - 5.75488 s
= obstime(i)-obstime(i-1) - 5.76 s

Let us now, starting from the nearby full disc image efz19980513.183451 try to estimate what the combination of contaminations 1,2 and 4 look like. Taking the appropiate integrations from this full disc image and using the above deduced 'effective exposures' for each contamination, we sum up the

Summing these two contributions gives the total contamination template which has the following statistics expressed in DN:
Minimum = 4.5, Maximum = 58.8, Average = 19.4+/- 11.0

Verification of the deduced total contamination template

Figure 3.

Let us now try to find observational evidence of the above scenario. Fig 3A shows a subfield extraction of the fdfr image efz19980513.170145 and Fig 3B shows the subfield test image efz19980513.165311. Both are taken with the normal operational mode of shutter opening and closing. Their difference, shown in Fig. 3c, are due to

It's important to note that the difference in filter only produces vertical stripes, indicating that the two corresponding gridpatterns are well aligned in the y-direction but somewhat offset in the x-direction. A fortunate consequence is that although efz19980513.18345 has a different grid pattern than the shutterless mode sequence, we can still use it to estimate the smear out contamination since horizontal differences are absent and vertical ones get smoothed anyway when smearing.

In the next line, Fig. 3d shows a subfield extraction of the fdfr image efz19980513.183451 (Al + 1) taken with the normal operational mode of shutter opening and closing, while Fig. 3e shows the last but one image in the shutterless mode sequence (BLOCK_EAST). Their difference (Fig. 3f) shows all features allready seen in Fig 2c, but due to shutterless mode operation we now have an additional left-bright right-dark trend. We have enhanced this trend by applying horizontal smoothing to remove the grid modulation and median filtering to reduce the noise. This gives Fig. 4a which can be compared to the derived contamination template Fig. 4B. (shown in the same grey scaling). The bright white regions in Fig. 4a are regions where the shutterless mode image has a lower value than the fdfr subfield image, due to intrinsic solar changes.

On a global scale, Fig. 4a and Fig 4b show the same left to right dimming pattern. This becomes especially clear when plotting in Fig. 4c the vertical average of both Fig. 4a (solid line) and Fig. 4b (dotdashed line). Both curves have the same amplitude and the same trend. The largest differences (approx 5 DN, see Fig. 4d) are seen in between column 50 and 100, which is probably due to solar variability. Also several of the horizontal stripes in the derived contamination (Fig. 4b) are recognisable in Fig. 4a: in Fig. 4e we a vertical cut through Fig. 4a (solid line) and Fig. 4b (dotdashed line) averaged over the first 10 columns at the left. The profile of both curves is clearly similar, though the derived contamination apparently underestimates the observed difference by roughly 5 DN. Nevertheless this difference can be compensated by assuming that the exposure time mentioned in the fdfr image header was slightly too high (only 1.5 %).

By comparing the contamination template calculated from the 18:34 fdfr image with templates based on the 13:14, 17:01 and 19:13 fdfr image we conclude that the temporal variations of the total contamination template are typically less than 10% on the timescale of 1 hour which in absolute numbers means an uncertainty of only 2 DN.

Figure 4.

Conclusion and Recommendations

With the above analysis we have shown that due to the shutterless mode operation in combination with the absence of any ccd clears during the sequence:

Since this contamination can be removed relatively well, it is shown that skipping all CCD clears is an allowable option to enhance the observation cadence. For future shutterless mode runs on EIT, we recommend to add full disc images as close as possible before and after the run, both in the Al +1 filter as in the blockeast (or west) filter. This will allow an even better estimate of the contamination template and its evolution during the sequence. In case no ccd clear are programmed during a shutterless mode run, the occulting filter (block east or block west) should be choosen, not according to the selected read out port, but depending on the relative brightness of the far west or the far east of the full disc image.

For future EIT like instruments we propose to consider the implementation of 'partial ccd clears' which would allow to fine-tune the removal of contamination from the fov (and only there) without degrading excessively the observation cadence. In case of demanding mass/budget restrictions one could also consider to remove the occulting half masks completely, since our analysis shows that the resulting smear-out contamination can be well estimated and removed to within a few percent.