XRT Stray Light Study 7 -- Measurement and Correction.

(14 August, 2014 by Aki Takeda)

1. About the 7th tests (performed in the 2014/08/07 timeline).

Purpose.

• collects light curves of the XRT eclipses.
• first attempt to obtain off-point stray-light images (Ti-poly & C-poly) at N-pole, S-pole, E-limb and W-limb pointings.

Pointing coordinates determination:

• N- and S-pole pointings were chosen to be the same as the HOP81 (Polar Monitoring Campaign). Their pointing coordinates are calculated as: (default value) + (offset) . The default value is (0,900) for the N-pole and (0,-920) for the S-pole observations. The offset is the difference between Hinode and XRT pointings, calculated by Shimizu-san every week using the result of the co-alignment programs. That way, the XRT observes the area centered at (0,900) and (0,-920) year-round during HOP81. For the week of 2014/08/08, the offset value was announced as (11,35), so the polar pointings were calculated as follows.

  N-pole:  (0,900) - (11,35) = (-11, 865)
  S-pole: (0,-920) - (11,35) = (-11,-955)
  

• E-limb and W-limb pointings were determined less attentively. This time, the values used in the HOP130 (Full Disk Slot Scans) was chosen.

  E-limb:  (-936,0)
  W-limb:  ( 936,0) -- not performed this time.
  

Exposure time determination:

• For the polar pointings, the exposure time was chosen to 4096 ms for the both Ti-poly and C-poly filters. This value is used in some of the HOP81 XOBs for Ti-poly.

• For the E- and W-limb pointings, the exosure time is same as the current synoptic XOBs (the longest exposure of the Ti-poly triplet, the longer of the C-poly doublet).

The tests were performed on:

• 8-Aug-2014, 18:56 -- 19:30UT (at N-pole).
• 8-Aug-2014, 20:40 -- 20:50UT (at E-limb).
• 8-Aug-2014, 22:15 -- 22:30UT (at S-pole).
• no good time window found for the W-limb pointing.

Solar condition.

2. Test sequences.

N-pole sequence.

• 18:56:00 -- s/c pointing to the North pole (-11,865)
• 19:03:00 -- test sequence started (until ~19:16).
  • -----------------------------------------------------------------------------------------------------------------------------------------------
  • 1024x1024 pix, 2" resol., Full-Sun, Ti-poly and C-poly long/short exposures.
  • 256x256 pix, 2" resol., every 10 sec on the center FOV, Ti-poly + G-band(VLS close).
  • 1024x1024 pix, 2" resol., every 60 sec, Full-Sun, Ti-poly and C-poly.
  • detailed parameters of the sequence (XOB#1A30).
  • -----------------------------------------------------------------------------------------------------------------------------------------------
• 19:06:00 ------ XRT entering twilight zone.
• 19:15:00 ------ XRT s/c night entry.
• 19:16:00 ------ pointing changed to AR12132.
• 19:22:00 ------ XRT s/c night exit.
• 19:25:00 ------ XRT SAA entry.
• 19:31:00 ------ XRT exiting from twilight.

E-limb sequence.

• 20:40:00 -- s/c pointing to the East limb (-936,0).
• 20:41:00 -- test sequence started (until ~20:54). <--- pointing not stabilized yet.
  • -----------------------------------------------------------------------------------------------------------------------------------------------
  • 1024x1024 pix, 2" resol., Full-Sun, Ti-poly and C-poly long/short exposures.
  • 256x256 pix, 2" resol., every 10 sec on the center FOV, Ti-poly + G-band(VLS close).
  • 1024x1024 pix, 2" resol., every 60 sec, Full-Sun, Ti-poly and C-poly.
  • detailed parameters of the sequence (XOB#1A2F).
  • -----------------------------------------------------------------------------------------------------------------------------------------------
• 20:44:00 ------ XRT entering twilight zone.
• 20:53:30 ------ XRT s/c night entry.
• 20:54:00 ------ pointing changed to AR12132.
• 21:00:30 ------ XRT s/c night exit.
• 21:03:00 ------ XRT SAA entry.
• 21:09:30 ------ XRT exiting from twilight.

S-pole sequence.

• 22:15:00 -- s/c pointing to the South pole (-11,-955)
• 22:19:30 -- test sequence started (until ~22:33).
  • -----------------------------------------------------------------------------------------------------------------------------------------------
  • 1024x1024 pix, 2" resol., Full-Sun, Ti-poly and C-poly long/short exposures.
  • 256x256 pix, 2" resol., every 10 sec on the center FOV, Ti-poly + G-band(VLS close).
  • 1024x1024 pix, 2" resol., every 60 sec, Full-Sun, Ti-poly and C-poly.
  • detailed parameters of the sequence (XOB#1A30).
  • -----------------------------------------------------------------------------------------------------------------------------------------------
• 22:22:30 ------ XRT entering twilight zone.
• 22:32:00 ------ XRT s/c night entry.
• 22:32:30 ------ pointing changed to AR12132.
• 22:38:30 ------ XRT s/c night exit.
• 22:40:30 ------ XRT SAA entry.
• 22:48:00 ------ XRT exiting from twilight.

3. Results of the test observation.

3.1 Sunset light curves

• The same exposure time was used for polar- and limb(equator)- pointings. It turned out that the Ti-poly 250ms was too short for the polar pointings, where no active region included in the FOV. Low signal levels resulted in large fluctuation in the Ti-poly light curves.
• The re-poining schedule was tight on the day of the measurement(8-Aug). The pointing change (to the next observation) turned out a little too early (30 sec to 1 min after the s/c night) and thus the light curves were truncated before the spacecraft completely enters the night.

at the time of the N-pole pointing.

at the time of the E-limb pointing.

at the time of the S-pole pointing.

3.2 Stray light images

• N-pole pointing

Ti-poly stray-light images (4 second exp.) movie.

C-poly stray-light images (4 second exp.) movie.

• E-limb pointing

Ti-poly stray-light images (1.4 second exp.) movie.

C-poly stray-light images (1.4 second exp.) movie.

• S-pole pointing

Ti-poly stray-light images (4 second exp.) movie.

C-poly stray-light images (4 second exp.) movie.

3.3 Image correction

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

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

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

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

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

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