XRT Stray Light Study 2 -- Measurement and Correction.

Link to the 1st test page.
Link to the 3rd test page.

(26 May, 2014 by Aki Takeda)

1. Stray light correction project -- the 2nd test.

1.1 Purposes of the 2nd test.

1.2 Test sequence.

The second test was performed on 15-May-2014, 18:19 -- 18:30UT.

2. Result of the test observation.

2.1 Solar condition


2.2 Sunset light curves

Light curve from the 1st test

2.3 Stray light images

Ti-poly stray light images obtained (movie).

Plot from the 1st test

Al-mesh stray light images obtained (movie).

No substantial stray light observed with the Al-mesh filter.

2.4 Image correction.

Correction of the synoptic composite image (Ti-poly) taken 8 minutes before the 2nd test.

BEFORE [synoptic (triple) composite image] AFTER [stray light image (prep-ed) subtracted]

2.5 Comparison of the Stray-light images (1st and 2nd tests).

If the stray light images (taken at the same pointing) do not(or only a little) vary with time, you can use a single stray image to correct images taken outside eclipse season.

The left image is the difference image of the stray-light images taken at the 1st and 2nd test:

dd1: stray light image from the test 1 (6-May-2014, 17:53:55, exp. 256msec)
ii1: index(FITS header) structure of dd1 
dd2: stray light image from the test 2 (15-May-2014, 18:25:03, exp. 1440msec)
ii2: index(FITS header) structure of dd2 

[diff_img] = [dd2] - [dd1 * ii2.exptime / ii1.exptime]

Their difference image is almost featueless, with stripes (as we see in dark frames) and subtle trace of solar disk. Their intensity level on the average is zero with noise roughly ± 20 DN.

2.6 Just giving it a try.

Correction of April 2014 Ti-poly synoptic composite images with the 15-May-2014 stray light image..

About the movie ...

Left images : synoptic composites BEFORE correction.
Right images : [Composite images] - [stray light image]

It is (at least) cosmetically OK to correct Apr-2014 Ti-poly images by stray light image taken on 15-May-2014. (The fudge factor not neccessary. I forgot to normalize the intensity of the stray light image, while the synoptic composites are normalized.)

2.7 Another experiment.

Correction of 9-May-2012 image (first noticed visible light contaminated image). This time, a fudge factor of 0.5 required to be looked decent. Or putting it positively, we can cosmetically correct May-2012 images with May-2014 stray-light image, if you allow a fudge factor of 0.5.

synoptic (triple) composite image stray-light image subtracted stray-light image * 0.5 subtracted