XRT SOT Alignment DeLuca With comments from Tarbell & Metcalf 21-Jan-2006

Assumptions - XRT NOTE: This presentation only addresses the XRT to SOT alignment. Co-alignment with EIS is not addressed here. Short time scale “jitter” of the XRT image plane will be remove on the ground by processing of the signals from the fine sun sensors. Large amplitude and long time scale thermal motions of the image plane will be removed on the spacecraft. The residual thermal motions will be roughly 10% (TBR) of the original motions in amplitude and will vary with a long time scale (orbital timescale). These are the motions that combined XRT and SOT observations need to be able to correct.

Assumptions - SOT For SOT, short time scale jitter will be removed by the Correlation Tracker and Tilt Mirror. However, these will introduce additional slow drifting motions unique to SOT which will add to the thermal drift of the S/C + SOT + FPP; and there will be occasional resets of the Tilt Mirror which will introduce a step function into the FPP pointing. All of these image motions in SOT/FPP will be measured accurately in time using filter images in photospheric wavelengths. Either sunspots and pores (if any) or normal granulation provide suitable objects for co-alignment. Spectra from the SP are not suitable, so every observing program will include filter images for tracking & alignment as well as scientific interest, taken every few minutes. This does not represent any change or upscope to the SOT observing plans.

Basic Procedure: Observations Synoptic full sun images in visible light and X-rays. –Provides absolute pointing, detail co-alignment of VLI and X-ray images. Is there a star tracker to provide roll information? During science operations –VLI & X-ray full FOV images roughly once an orbit. SOT provides simultaneous G-band images –X-ray images of limb regions as needed to monitor the motion of the XRT image plane. –SOT image plane motion is determined by correlation tracker data and regular filtergraph data.

Observations

Basic Procedure: Analysis Synoptic observations define the XRT plate scale for VLI & X-ray images, and it defines the heliographic coordinate system. Full FOV VLI & X-ray images during science operations (ie SC pointed at arbitrary position on solar disk) define the location of a photospheric feature relative to the visible and X-ray limb. X-ray limb observations during science operations show the deviation of the XRT image plane during the orbit. This data can be used to correct image plane location for intermediate images

X-Ray Limb Observations Thin Al full resolution images at the limb. Image on the right has been shifted by (2,4) pixels to align the limb. Only integral-number shifts were attempted. Long vertical and horizontal lines are shown as curves on the next page. Short lines indicate bright corona above the limb, shown on next page.

Intensity Variations Across X-ray Limb

Blink Movie

Discussion Using a simple discrete shift on Yohkoh full resolution limb observations we can align images such that the limb is stationary when blinked. Cross-correlation analysis does not work since the bright x-ray features change and rotate, while the limb remains fixed. Limb observations at high latitude will constrain the y-shift of the focal plane, lower latitude observations are required to accurately constrain the x-shift.

Next Steps - Software We have demonstrated that X-ray limb observations can be used to identify and correct focal plane motion. To meet the co-alignment requirements we need sotware tools that can routinely remove the image plane jitter, and fit the X- ray limb to remove long time scale orbital drift. IDL jitter removal code needs to be written. This code may be used by both XRT and EIS. The SXT FIT_LIMB.PRO IDL procedure needs to be modified for use by XRT data. This procedure was used to fit the limb automatically for SXT full disk images. The SXT full disk limb fits need to be examined to determine the accuracy of the fits in pixels. Was sub-pixel accuracy achieved with the above procedure? Can 1/2 pixel accuracy be achieved.

Additional Steps Estimates of the amplitude and time variations of the residual thermal/orbital motion of the focal planes are needed. Corrections of EIS slit positions need to be addressed.