Hinode/XRT stray light correction 2021

XRT Stray Light Study 2021 -- phase 4 (after 29-May-2018).

Link to the 2014 summary page.
Link to the 2015 phase 1 summary. (after 9-May-2012 ← 1st pre-filter failure event)
Link to the 2015 phase 2 summary. (after 14-Jun-2015 ← 2nd failure event)
Link to the 2016 page and 2017 phase 2 page.
Link to the 2017 phase 3 page (after 27-May-2017 ← 3rd event).
Link to the 2018 phase 4 page (after 29-May-2018 ← 4th event).
Link to the 2019 phase 4 page.
Link to the 2020 phase 4 page.
Link to the 2022 phase 4 page.

(26 Aug, 2021 by Aki Takeda)
(14 Mar, 2022 C_poly image correction added)
(28 Mar, 2022 a note for xrt_synleaksub.pro added)

1. Purposes of the stray light measurement in 2021.

While this year's hard eclipse (with night transition) season started on 5-May, the start the stray light measurements was delayed until 5/11 because of the HOPs to support and the CCD bakeout.

Judging from the intensity level of G-band images and FLD images, we have not seen anoher increase of stray light level since the start of the phase 4 (Aug-2018). The 2021 measurements are to confirm this by taking real stray light images. We newly took the stray light images for the Al_poly/Ti_poly filter combination. The stray light images with thick filters were forgone because the twilight period was not long enough for using long exposures.

2. Summary of Results.

It was confirmed that there was no chanage in the stray light level of the measured filters compared with those at the start of the phase 4.
[Al_mesh] No change in the stray light component (pattern & intensity level) since the beginning of the phase 4 (Aug-2018).
[Al_poly] No change in the stray light component either, since Aug-2018. The West-pointing anomaly (significant amount of visible light component, ~40 DN/sec, noticed near West-limb pointing) was reproduced.
[Al_poly/Ti_poly] It was found that the stray light is in the negligible level for this filter combination.
[thin_Be] Stray light component is still in a negligible level.
[med_Be and med_Al filters] No change noticed, still negligible.
[C_poly] The current level of stray light (at phase 4) is roughly 3.3 times larger than that of phase 2 (Aug-2015). No measurement was performed during the phase 3.
[thick_Al and thick_Be filters] No measurements this year.

3. Measurements 2021 at a glance.

ID No.	date & time (UT)	XOB	purposes (link: obtained plots & images)	memo
1	11-May-2021, 17:12:30	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) at DC.	previous bakeout + 0 day.
2	12-May-2021, 17:48:00	1CAE	light crv., med-Al (32.0s) at DC.	prev. b.o. + 1 day.
3	12-May-2021, 21:04:30	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on AR.	prev. b.o. + 1 day.
4	13-May-2021, 15:06:30	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) at N-pole	prev. b.o. + 2 days.
5	13-May-2021, 18:23:30	1CB0	light crv., med-Be (22.6s) at DC.	prev. b.o. + 2 days.
6	14-May-2021, 17:18:30	1CAF	light crv., Al-poly/Ti-poly (2.0s) and C_poly (177ms) at DC.	prev. b.o. + 3 days.
7	15-May-2021, 16:18:01	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) at S-pole	prev. b.o. + 4 days.
8	15-May-2021, 19:34:30	1CB1	light crv., thin-Be (11.3s) at DC.	prev. b.o. + 4 days.
9	16-May-2021, 18:32:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) at W-limb.	prev. b.o. + 5 days.
10	17-May-2021, 17:29:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) at E-limb.	prev. b.o. + 6 days.
11	17-May-2021, 20:46:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on AR.	prev. b.o. + 6 days.
12	18-May-2021, 23:00:00	1CB2	light crv., Al-poly/Ti-poly (4.0s) and Al-poly (2.0s) on AR.	prev. b.o. + 7 days.
13	19-May-2021, 12:07:00	1CB2	light crv., Al-poly/Ti-poly (4.0s) and Al-poly (2.0s) on CH.	prev. b.o. + 8 days.
14	19-May-2021, 20:18:30	1CB2	light crv., Al-poly/Ti-poly (4.0s) and Al-poly (2.0s) on W-limb.	prev. b.o. + 8 days.
15	20-May-2021, 12:42:30	1CB2	light crv., Al-poly/Ti-poly (4.0s) and Al-poly (2.0s) on S-pole.	prev. b.o. + 9 days.
16	20-May-2021, 14:21:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on HOP412 pnt.	prev. b.o. + 9 days.
17	20-May-2021, 15:59:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on HOP412 pnt.	prev. b.o. + 9 days.
18	20-May-2021, 22:32:30	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on HOP412 pnt.	prev. b.o. + 9 days.
19	21-May-2021, 00:11:00	1CAD	light crv., Al-mesh (1.4s) and Al-poly (2.0s) on HOP412 pnt.	prev. b.o. + 10 days.
20	21-May-2021, 01:48:30	1CB2	light crv., Al-poly/Ti-poly (4.0s) and Al-poly (2.0s) on N-pole.	prev. b.o. + 10 days.
21	21-May-2021, 18:11:00	1CB3	light crv., Al-poly/Ti-poly (2.0s) and C-poly (177ms) at DC.	prev. b.o. + 10 days.

4. Stray light images 2021, at a glance.

Al_mesh filter

No change in the stray light component included in the Al_mesh filter (no new pattern found). Observed no increase of the stray light level since the 2018 July data. Comparison of the stray light images taken before and after CCD bakdout shows ~2% increase of the stray light level in 14 days.

- DC pointings
[NOTE] pnt. indicates the satellite pointing at the time of data acquisition, i.e., (index.xcen, index.ycen) corrected by the the coalignment DB.

Al-mesh, 20210511(1.4s) [pbo+0d] pnt.(-16.0,13.7)	Al-mesh, 20180712(1.4s) [pbo+2d] pnt.(-22.1,19.8)	After PREP comparison 1 comparison with 2018

- limb pointings

Al-mesh, 20210513(1.4s) [pbo+2d] pnt.(-43.4, 876.3)	Al-mesh, 20210515(1.4s) [pbo+4d] pnt.(-34.7,-942.6)	Al-mesh, 20210516(1.4s) [pbo+5d] pnt.( 883.3, 24.4)	Al-mesh, 20210517(1.4s) [pbo+6d] pnt.(-917.0, 12.2)

- Other pointings

Al-mesh, 20210512(1.4s) [pbo+1d] pnt.(-144.1, 304.5)	Al-mesh, 20210517(1.4s) [pbo+1d] pnt.( 763.0, 278.9)	Al-mesh, 20210520(1.4s) [pbo+9d] pnt.( -37.5,-661.2)	Al-mesh, 20210520(1.4s) [pbo+9d] pnt.( -39.0,-361.5)

Al-mesh, 20210520(1.4s) [pbo+9d] pnt.(-42.9, 428.9)	Al-mesh, 20210521(1.4s) [pbo+10d] pnt.(-44.1, 728.9)

Al_poly filter

No change in the stray light component included in the Al_poly filter. No change in the stray light intensity since 2018. The stray light intensity before the CCD bakeout turned out ~4% smaller than the image taken after the bakeout, indicating that there are other factors that determine the stray light intensities of Al_poly (e.g., slight change of pointing) than the growing CCD contamination layers. The West pointing anomaly (significant amount of stray light intensity near West-limb pointing, ~40 DN/sec) was reproduced.

- DC pointings

Al-poly, 20210511(2.0s) [pbo+0d] pnt.( -16.0, 13.7)	Al-poly, 20180712(2.0s) [pbo+2d], pnt.(-22.2,19.8)	After PREP comparison 1 comparison with 2018

- limb pointings

Al-poly, 20210513(1.4s) [pbo+2d] pnt.(-43.4, 876.3)	Al-poly, 20210515(1.4s) [pbo+4d] pnt.(-34.7,-942.6)	Al-poly, 20210516(1.4s) [pbo+5d] pnt.( 883.3, 24.4)	Al-poly, 20210517(1.4s) [pbo+6d] pnt.(-917.0, 12.2)

Al-poly, 20210520(1.4s) [pbo+9d] pnt.(-36.1,-961.4)	Al-poly, 20210521(1.4s) [pbo+10d] pnt.(-45.3, 929.2)	Al-poly, 20210519(1.4s) [pbo+8d] pnt.(882.2, 27.4)

- Other pointings

Al-poly, 20210512(2.0s) [pbo+1d] pnt.(-144.1, 304.5)	Al-poly, 20210517(2.0s) [pbo+6d] pnt.( 762.9, 278.6)	Al-poly, 20210520(2.0s) [pbo+9d] pnt.( -37.5,-661.2)	Al-poly, 20210520(2.0s) [pbo+9d] pnt.( -39.0,-361.4)

Al-poly, 20210520(2.0s) [pbo+9d] pnt.( -42.9, 429.0)	Al-poly, 20210521(2.0s) [pbo+10d] pnt.( -44.1, 729.0)	Al-poly, 20210518(2.0s) [pbo+7d] pnt.(-865.4, 308.2)	Al-poly, 20210519(2.0s) [pbo+8d] pnt.( 395.7, -91.7)

Al_poly/Ti_poly filter

Stray light level in the Al_poly/Ti_poly images (including W-limb pointing) is in negligible level. This filter combination will provide better(almost stray-light free) quality images for the targets near W-limb, in morphological point of veiw. However, the quantitative point of view, the correction of effective area functions will include more uncertainty.

- DC pointings

Apol/Tpol,20200514 (2.0s), pnt.(-17.7, 15.9)	Apol/Tpol,20200521 (2.0s), pnt.(-18.5, 24.1)

- Limb pointings

Apol/Tpol,20200519 (4.0s), pnt.(882.3, 27.6)	Apol/Tpol,20200520 (4.0s), pnt.(-36.1,-961.5)	Apol/Tpol,20200521 (4.0s), pnt.(-45.3, 929.1)

- Other pointings

Apol/Tpol,20200518 (4.0s), pnt.(-865.4, 308.2)	Apol/Tpol,20200519 (4.0s), pnt.( 395.7, -91.7)

C_poly filter

The current level of stray light (at phase 4) is roughly 3.3 times larger than that of phase 2 (Aug-2015). No measurement performed in the phase 3. This is the same level as the stray light of the Ti_poly images at the phase 2 (with ~2000 DN/sec at the peak).

- DC pointings

Cpol,20210514 (177ms), pnt.(-17.7, 15.9)	Cpol,20210521 (177ms), pnt.(-18.5, 24.1)	Cpol,20150811 (2.048s), pnt.(-19.3, 0.26)	After PREP comparison comparison with 2015

- Image correction

(a) 20210514	(b) 20210521	(c) 20210514	(d) 20210521

(a) : C_poly composite image (86ms + 1.41s) 
(b) : C_poly composite image (86ms + 1.00s) 
(c) : [image (a)] - [stray_light_image at 20210514_172607 (prep-ed)]
(d) : [image (b)] - [stray_light_image at 20210514_172607 (prep-ed)]

Thicker filters

Stray light included in thin_Be, med_Be and med_AL images is the same level as the previous measurements and still in negligible level.

- DC pointings

thin-Be,20210515(11s) pnt.(-20.6, 16.8)	med-Be,20210513(23s) pnt.(-17.3, 15.5)	med-Al,20210512(32s) pnt.(-17.0, 14.4)

5. Software for stray light subtraction.

An IDL procedure, xrt_synleaksub.pro was created to subtract stray light component from XRT synoptic composite images. Note that this procedure assumes Open/Al_mesh or Al_poly/Open full-Sun composite images (stray light subtraction was done for Open/Ti_poly SCIA images).

The procedure was submitted to SAO on Feb-2022, delocalized by L. Guliano, and released under the $SSW tree on 11-Mar-2022.

 LOCATION: $SSW/hinode/xrt/idl/util/xrt_synleaksub.pro
 EXAMPLES: (basic usage)

       IDL> file='./comp_XRT20200220_061539.6.fits'  ; sample Al_mesh image
       IDL> read_xrt, file, in_idx, in_da
       IDL> xrt_synleaksub, in_idx, in_da, out_idx, out_da