SUMMARY of the XRT Stray Light Study -- Measurement and Correction.

Link to the 2015 phase 1 summary page.
Link to the 2015 phase 2 summary.
Link to the 2016 summary.

(20th August, 2014 by Aki Takeda)
(28-Jan-2015: correction factor part added.)
(3-Mar-2015: correction factor polynomial fitting added.)
(22-Sep-2015: plot and fitting of the factor corrected.)

The visible stray light component contaminated in the X-ray filter images were measured during the eclipse season of 2014.

1. Dates of the measurements and purposes.

timeline No. date & time link to the page purposes
1 6-May-2014, 17:50UT to the 1st test page collects light curve, Ti-poly (250msec) at Dick Center
2 15-May-2014, 18:20UT to the 2nd test page light curve, Ti-poly (1.4 sec) & Al-mesh (707msec) at DC
3 27-May-2014, 20:30UT to the 3rd test page light curve, Al-poly (1.0 sec) & C-poly (1.41 sec) at DC
4 3-Jul-2014, 18:30UT to the 4th test page light curve, Ti-poly (1.4 sec) & Thin-Be (2.83 sec) at DC
5 30-Jul-2014, 18:30UT 5th and 6th test page light curve, Med-Al (32 sec) at DC
6 31-Jul-2014, 15:49UT light curve, Med-Be (24 sec) at DC
31-Jul-2014, 17:27UT light curve, Thick-Al (64 sec) at DC
1-Aug-2014, 16:26UT light curve, Thick-Be (64 sec) at DC
7 8-Aug-2014, 18:56UT to the 7th test page light curve, Ti-poly (4.0 sec) & C-poly (4.0ec) at N-pole
8-Aug-2014, 20:40UT light curve, Ti-poly (1.4 sec) & C-poly (1.4 sec) at E-limb
8-Aug-2014, 22:30UT light curve, Ti-poly (4.0 sec) & C-poly (4.0 sec) at S-pole

2. Main results.

3. Stray-light image gallery.

Ti-poly series (click to enlarge):

Disk Center pointingNorth Pole pointing East Limb pointingSouth Pole pointing
XRT20140515_182503.4.fitsXRT20140808_191124.0.fits XRT20140808_205020.5.fitsXRT20140808_222905.4.fits

Movie of all stray-light images in Ti-poly

C-poly series (click to enlarge):

Disk Center pointingNorth Pole pointing East Limb pointingSouth Pole pointing
XRT20140527_204601.0.fitsXRT20140808_191137.1.fits XRT20140808_205031.0.fitsXRT20140808_222918.5.fits

Movie of all stray-light images in C-poly

How to correct an image (example):

IDL> dir_l0='/disk/data/HINODE/xrt/level0/'
IDL> file=dir_l0+'2014/05/15/H1800/XRT20140515_182503.4.fits' ; T-poly stray-light image at DC pointing
IDL> mreadfits,file,index,data
IDL> xrt_prep,index,data,i_stray,d_stray,/norm,/float,/desp

; img_before : an image to be corrected, 1024x1024, Ti-poly, at DC poining, PREPed, intensity normalized.  
; img_after  : an image after correction.
; d_stray    : the stray-light image prepared in the above.

IDL> img_after = img_before - d_stray 

4. Corrected image gallery.

Ti-poly at disk center pointing

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

C-poly at disk center pointing

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

Ti-poly at N-pole pointing

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

C-poly at N-pole pointing

BEFORE [(double) composite image before twilight] AFTER [stray light image (prep-ed) subtracted]

Ti-poly at E-limb pointing

BEFORE [(double) composite image before twilight] AFTER [stray light image (prep-ed) subtracted]

C-poly at E-limb pointing

BEFORE [(double) composite image before twilight] AFTER [stray light image (prep-ed) subtracted]

Ti-poly at S-pole pointing

BEFORE [(double) composite image before twilight] AFTER [stray light image (prep-ed) subtracted]

"Ghost" from 2007 solar eclilpse image (from Kano-san's report 21-Feb-2007 to XRT_COs).
Thin-Be CME watch image (and its enhancement obtained on 1-Dec-2015, processed by Trish Jibben, private communication on 4-Dec-2015).

C-poly at S-pole pointing

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

5. Cosmetic correction of past/future browse images (PNG).

Correction of May-2012 images by 2014 stray-light patern.

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

The stray-light is increasing: the intensity on 9-May-2012 is about half of that on 15-May-2014.
In order to correct the images in the past (or outside eclipse season), we need to know the temporal variation of the stray light intensity (from 9-May-2012 to present).

5.1 Determination of the factor for the correction.

Let's assume that the images are corrected in the following way:
[image_corrected] = [Image_uncorrected] - factor(k) * [stray-light_2014] 
The first rough estimation of the correction factor applied to the initial PNG products was as follows (shown in the green line in the plot below) :
k = 0.5 (9-May-2012 18:00 through 14-Aug-2012 23:59)
k = 0.7 (15-Aug-2012 00:00 through 31-Dec-2012 23:59)
k = 1.0 (1-Jan-2013 00:00 and onward)
More reasonable value of the factor is determined by using intensity correlation between Al-mesh and Ti-poly images. Note that Al-mesh images are not affected by the stray-light.

White: Pixels from 1-May-2014 6UT to 9-May-2014 6UT, masked synoptic images.
Blue line: linear fit (by LADFIT, yy=2.451+0.341*xx).
Red (left): 9-May-2014 18UT image BEFORE correction, Red (right): AFTER correction with the correction factor of 0.5.

The temporal variation of the factor k, determined by the above steps 1-6 are shown with '+' signs in the plot below (see the next subsection for the red and blue lines).

5.2 Fitting the variation of the factor, k.

[NOTE]
This sub-section(5.2) has a new version in which the fitting was re-done using the whole data in phase 1 (9-May-2012 through 14-Jun-2015).
[NOTE]

The above data points are determined for the Al-mesh and Ti-poly pairs, each of the pair image are 'intact' (i.e., with no saturation nor telemetry drop). However, some of the intact Ti-poly images that do not have the Al-mesh counterpart are not included.

Applying the polynomial fit (7th order) to the data points between 9-May-2012 and 31-Dec-2014 (red line in the above plot).

k0 = pl7coef(0)           $    ; pl7coef(0)=   2.1924060 
       +tt*pl7coef(1)     $    ; pl7coef(1)=  -0.040220413 
       +tt^2*pl7coef(2)   $    ; pl7coef(2)=   0.00035595453
       +tt^3*pl7coef(3)   $    ; pl7coef(3)=  -1.4460938e-06
       +tt^4*pl7coef(4)   $    ; pl7coef(4)=   3.1434595e-09
       +tt^5*pl7coef(5)   $    ; pl7coef(5)=  -3.7773749e-12
       +tt^6*pl7coef(6)   $    ; pl7coef(6)=   2.3664218e-15
       +tt^7*pl7coef(7)        ; pl7coef(7)=  -6.0336921e-19
To avoid negative values of the faint structures due to over-correction, the offset value of -0.05(shifting downward) was applyied as the final value of the correction factor (blue line).
k = k0 - 0.05
I made two IDL routines:
1) GET_SLCORFACT.PRO : Calculate the correction factor(k) at an arbitrary time (returns blue line points).
IDL> print,get_slcorfact('9-may-2012 18:00')
      0.50596564
IDL> print,get_slcorfact('15-may-2014 18:00')
      0.98621502
IDL> print,get_slcorfact('1-may-2012 00:00') 
*** the input time before [9-may-2012] set to 0.
2012/05/01 00:00:00.000
      0.00000
IDL> print,get_slcorfact('1-mar-2015 00:00')
*** the input time after [31-dec-2013] set to 1.
2015/03/01 00:00:00.000
      1.00000
2) RD_SLCORFACT_RAW.PRO : Provide raw data points (returns '+' sign points).
IDL> rd_slcorfact_raw, index,factor
% Compiled module: RD_SLCORFACT_RAW.
INDEX           STRUCT    = -> MS_163834261002 Array[1938]
FACTOR          FLOAT     = Array[1938]
IDL> utplot, index.date_obs,factor,psym=1  ; to give the above plot

5.3 The correction applied to the current SCIA PNG products.

The PNG products for May-2012 through Dec-2014 were re-processed using the k value calculated by the GET_SLCORFACT.PRO. (2 March, 2015).

The full-res images (2048x2048) included in the above period were also corrected by sabtracting the half-res stray light image(1024x1024) re-bined to 2048x2048.

The PNG products after Jan-2015 are tentatively corrected with the value of k=1.0. The k value will be updated in the future by a new fitting applied to the new data.