1. Compact X-ray structures selection for determining SXT PSF shape
Szymon Gburek
Janusz Sylwester
Space Research Centre, Polish Academy of Sciences
Wrocław
Multi-Wavelength Observations of Coronal Structure and Dynamics - Yohkoh 10th Anniversary Meeting
September 17-20, 2001 in Kailua-Kona, HI
(Postponed)

2. ABSTRACT
The performance and speed of blind deconvolution algorithms (BID) for restoration of SXT images depend on a good initial guess for PSF function shape. From the analysis of several compact flare kernels we came to conclusion that a good guess for PSF can be provided directly from images of X-ray compact structures observed by SXT. Recently, we conducted extensive mission-long searches for compact structures through entire database of SXT full resolution frames. The searches returned plenty of compact structures which my serve to construct an initial approximation of the PSF for BID restoration method. We show a selection of the most compact structures found and their location on SXT CCD detector. Using observation for the selected set of structures we construct constraints for Al12 PSF shrouds and compare them with ground calibration data.

3. Introduction
The main purpose of our contribution is to find possibly the best approximation for the core part of the SXT telescope (Tsuneta, S., et al ) point spread function (PSF) that can be gained from images collected in flight and to compare them with ground calibration data.
We decided to deal with the SXT partial frame images (PFI) taken in Al12 filter. PFI images comes from telemetry at highest possible SXT resolution and therefore are particularly useful for finding fine sources which can then yield appropriate PSF core approximation. From the searches that we performed on the entire PFI images archive we recognised that the thick aluminium filter was one of the most frequently used. Hence, the choice of Al12 data allows for obtaining a representative image sample from the whole PFI database. On the other hand majority of the SXT ground calibrations in soft X-rays were performed in aluminium K spectral line which lies close to the maximum of effective area for Al12 filter. This coincidence makes possible the most thorough cross-comparison between flight recorded and ground based data. Because one can not exclude that PSF would change in time, it seems to be important to look on PFI data in relatively short time interval, say one year, in comparison with Yohkoh mission duration. That is why we focused our research on period covering the year 2000.

4. SXT Data Selection
The first important question that arises here is in which part of the CCD one can determine approximation for SXT PSF from selected Al12 PFI data. The answer comes from analysis of situation of PFI frames within CCD detector for the year It turns out that they are not evenly spread all over the CCD surface. Vast majority of them comprise an about 1000x650 pixels belt only, little bit shifted from CCD centre to its upper left corner.
To the left, a coverage map of the CCD detector surface by full resolution SXT frames. Gray intensity says how many times a given pixel was captured within a full resolution frame during year To the right a shaded surface for the coverage map (Log10 scale).

5. We found that the compact sources which we identified in SXT data for 2000 pretty well cover the belt of PFI data occurence shown above. All of the sources are structures with signal rapidly vanishing with distance from the peak and good signal to noise ratio. Special care has been taken, during searches for these sources, to not mistake them with ordinary spikes. Search criteria and result are discussed in detail in (Gburek S., Sylwester J., sent to Solar Physics, 2001). In total, we detected above 104 SXT compact source images in flare and quiet instrument operation mode for year Their location on CCD detector is shown to the right. We decided to remove from our selection of compact sources these of them for which a rgular data reduction is questionable due to dark current saturation effects.
Map of all compact sources found in full resolution SXT frames during year 2000 (in CCD co-ordinates)

6. Dark current subtraction and warm pixels treatment
During each SXT exposure the unwanted dark current signal accumulates in the CCD image along with the X-ray photon signal. Specialised data reduction routines have been incorporated in order to calculate dark field for SXT telescope images. These routines use two dark current reference frames taken with short and long exposure times and linearily interpolate them for exposure time of a given SXT X-ray image. Obtained in this way dark current component is then subtracted from the original image to regain genuine X-ray signal up to shot noise. For PFI images, the dark current reference frames are 1024x512 images taken commutatively from upper and lower half of the CCD with exposure times almost exclusively of 8 ms and 30 s. The average signal in dark reference frames ranges from about 13 to 50 DN. This average values depend mainly on frame exposure time. It has been also observed that dark current averages tends to grow as Yohkoh mission time elapses.
Exposures 30s
exposures 7ms
Average dark current signal [DN]
time UT
Full resolution dark current frames

7. Certain amount of the SXT CCD pixels do not follow mean trends of the entire pixel population. In these pixels, often called warm or hot pixels, signal exceeds many times the average DN values for dark frames. Such tendency depends on the particular pixel properties and may be temporal or years long lasting. Warm dark pixels may corrupt the shape of compact structures we found for approximating PSF. In particular, these of them which get saturated during dark frames acquisition falling into non-linear regime of the CCD operation. Similarly like average dark current value the number of saturated warm CCD pixels increases with exposure time. Dark saturation happens rarely for 8 ms frame exposures but can be well observed for exposures as long as 30 s. It also depends on the position of the dark frame in CCD. (Data analysis show that Lower half of the CCD is more vulnerable for saturation during dark frame generation). Eventually, we removed from our Al12 data selection selection these compact structures which had its maximal value within 7x7 pixel square vicinity of any saturated pixel we found in dark frames.
lower half of the CCD
Full resolution dark current frames
exposure 30s
number of saturated CCD pixels
upper half of the CCD
time UT

8. Below, a map of CCD rejected area is shown in grey
Below, a map of CCD rejected area is shown in grey. The area consist of dark current saturated pixels and their 7x7 pixel square vicinities.

9. SXT Ground Calibration
Extensively ground calibration tests of SXT telescope were performed at High Energy Laser System Test Facility at White Sands Missile Range (WSMR) in New Mexico Four months before launch of Yohkoh satellite. (Martens, P. et al 1995). The X-ray tests objectives were to check the SXT mirror parameters, fix focus position and collect material for determining telescope blur. During the WSMR X-ray calibration tests SXT CCD detector was lit up by a microfocus source beam in different energy ranges and positions relative to telescope axis. The Al-K line (1.49 keV) was used in the majority of calibration test exposures in soft X-rays. For comparison of ground calibration data with in flight recorded AL12 images we have chosen bx01_apr23 series because
a) it was taken in AL.-K line which lies near maximum of the SXT effective area
curve for Al12 filter,
b) the SXT focus was set in the flight position during bx01_apr23 frames acquisition,
c) calibration beam equispatially dots the entire CCD area.
The above mentioned properties of bx01_apr23 allows for most thorough comparison of in-flight data with ground test data.

10. WSMR ground calibration bx01_apr23 series
Images of bx01_apr23 ground calibration series at its location on the CCD are shown above, The peak position and serial number of an image in a series are given below each peak. To the left, shaded surfaces showing the enlarged calibration beam images of serial numbers 0 and 24 in their 11x11 pixel peak vicinity. The calibration beam images have form of symmetrical sharp peak in central portion of the CCD detector. A strong distortion and peak broadening is observed when approaching edges and corners of the CCD.

11. Peak 11x11 pixel portions of all of bx02_apr23 calibration series images. Conturs of 0.1, 0.2, 0.4 and 0.8 maximum value are superimposed on each image. The elliptical distortion of SXT PSF is clearly seen on the edges and in the corners of the CCD. In the central parts contours are more symmetrical

12. PSF approximation and comparison with ground calibration data
The selected AL12 images form an interesting observational material in which various X-ray structures can be found. Among them, flaring sources with significant emission localised in only several pixel vicinity of signal maximum - almost point like ones,
as well as compact strong emission sources superimposed on non-uniform extended background like flare kernel or foot-points,
or limb flares and X-ray sources partly occulted by solar limb.

13. Inspection of collected SXT Al12 data for year 2000 shows many very well localised X-ray sources. It could be rather hardly expected that a single image can yield a good estimation of the SXT PSF however. We have checked that to achieve improvement in PSF shroud determination it is better to work with several to several tens of images from approximately the same CCD region simultaneously. In the approach presented here, we took square sub-arrays of all images centred at the maximum, normalised them to [0, 1] signal range and stack them onto a data cube. Then we construct a surface of the same size as the sub-arrays taken but with signal value at each pixel equal to minimal value we found at relative pixel position along the data cube of normalised images. Such a surface is considered as a final PSF approximation in or method. We constructed our PSF approximations in neighbourhoods of peak observed in calibration data from bx02_apr23 series. Peak positions for in-flight data which we selected and calibration data are shown on left.
Compact sources images found in Al12 data (small dots) and WSMR peak positions on CCD surface (crosses).

14. To the left, a comparison of PSF approximation constructed for central part of CCD detector (neighbourhood of peak position of image 24 in bx02_apr23 series). In the upper-left panel the peak of calibration image 24 from bx02_apr23 is shown. The PSF approximation constructed from 85 images, closest to the location of image 24 peak in CCD, is displayed in the upper-right panel. Contours above both shaded surfaces shows that approximation obtained is still slightly broader than the calibration peak. Below, x-cross-section (right) and y-cross-sections (left) of approximated PSF and the mentioned calibration image are compared. Cross-sections of PSF approximations are indicated in dots. Thick and thin solid lines show absolute error values estimated from calibration beam images 16, 17, 18, 23, 24, 25, 30, 31, 32 of bx02_apr23 series. The cross-sections show that the approximation obtained is pretty good in the core portion of the SXT PSF.

15. Conclusions
The approximation for PSF of SXT telescope from in flight data has been determined for full resolution images taken in Al12 filter.
A good agreement of the determined PSF approximation and calibration data has been found for core part of the PSF. The obtained PSF profiles are in general slightly broader, however.
Future improvements of the obtained Al12 PSF shape is expected from BID deconvolution of the Al12 SXT data.
A preparatory material has been collected for BID deconvolution purposes in the CCD area where SXT full resolution images were acquired in year 2000.
References,
Gburek S., Sylwester J., Search for Compact X-ray Sources in SXT Observations,
sent to Solar Physics, 2001
Martens, P. C. Lemen, J. R.,  and  Acton, L. W. 1995, Solar Phys., 157, 141
Tsuneta, S., et al. 1991, Solar Phys., 136, 37